What would be the temperature of earth if there was no atmosphere?

According to Wikipedia an approximate average surface temperature for a bare earth is 274.5 K. This scenario is quite reasonable in my opinion as stripping the atmosphere without changing much else would (on a geological timescale) rather quickly result in a bare earth without ice caps or vegetation, causing circumstances quite close to those on the moon. (I assume the Earth's magnetic field protecting both the atmosphere and life below it has disappeared as well)

This is estimated by comparing the black body radiation of the Earth and Moon, which is then corrected for albedo (fraction of incoming radiation that is reflected) and emissivity (ability of a material to emit radiation), which are properties of a material. Since the Earth and moon are both at the same distance to the sun and made up from the same material on average, measurements of the albedo and emissivity of the moon can than be used as estimations of these properties for the Earth.

The black body radiation of the sun is calculated with the Stefan-Boltzmann law:

$$P_{\text{S,emit}} = 4\pi R_S^2 \sigma T_S^4$$

$P_{\text{S,emit}}$ is the emitted energy by the sun, $R_S$ is the radius of the sun, and $T_S$ is the temperature of the sun. The fraction of this energy recieved by the Earth is then proportional to the circular surface area facing the sun and the energy density at the distance $D$ between the Earth and the sun.

$$P_{\text{SE}} = P_{\text{S,emit}}\left(\frac{\pi R_E^2}{4\pi D^2}\right)$$

$R_E$ is the Earth's radius. Using albedo $\alpha$ the absorbed energy can be calculated:

$$P_{\text{E,abs}} = (1-\alpha)P_{\text{SE}}$$

Applying the Stefan-Boltzman law to the Earth, corrected for the emissivity $\overline{\epsilon}$, the emitted energy is then:

$$P_{\text{E,emit}} = \overline{\epsilon} 4\pi R_E^2 \sigma T_E^4$$

Assuming energy equilibrium $P_{\text{E,abs}} = P_{\text{E,emit}}$ we can now calculate $T_E$:

$$\begin{aligned}
\frac{(1-\alpha)4\pi R_S^2 \sigma T_S^4\pi R_E^2}{4\pi D^2} & = \overline{\epsilon}4\pi R_E^2 \sigma T_E^4 \\
T_E^4 & = \frac{(1-\alpha)4\pi R_S^2 \sigma T_S^4\pi R_E^2}{\overline{\epsilon}4\pi D^2 4\pi R_E^2 \sigma} \\
T_E^4 & = \frac{(1-\alpha) R_S^2 T_S^4}{ 4\overline{\epsilon}D^2  } \\
T_E & = \left( \frac{(1-\alpha) R_S^2 T_S^4}{4 \overline{\epsilon}D^2 }\right)^{\frac{1}{4}} \\
T_E & = T_S \left( \frac{(1-\alpha) R_S^2}{4 \overline{\epsilon} D^2 }\right)^{\frac{1}{4}} \\
T_E & = T_S \sqrt{ \frac{ R_S \sqrt{\frac{1-\alpha}{\overline{\epsilon}}} }{2 D } }
\end{aligned}$$

Finally we only need to insert the correct values:

$R_S = 6.96\times 10^8$ m
$T_S = 5778$ K
$D = 1.496\times 10^{11}$ m
$\alpha = 0.1054$ (assuming value of the moon)
$\overline{\epsilon} = 0.95$ (assuming value of the moon)

This gives us a temperature of 274.5 K.

Note that there are many factors that can cause local and temporal variations. For example, incoming radiation varies with latitude and season, and if the removal of the atmosphere would be caused by a dying sun that grows to engulf the earth temperatures would be much higher than this. All in all, to account for all those factors a very large model must be made that can analyse the influence of each factor, including the decrease in temperature of a dying sun etc., but that would be nearly impossible to build if only for the resources it would take to do so.

Since one of the most contested factors is the albedo after the atmosphere is removed the following graph shows how the average surface temperature changes with albedo. At an albedo of zero all incoming solar radiation is absorbed, while at 1 all radiation is reflected. Note that the temperature of 0K is an effect of the assumed equilibrium between incoming and emitted radiation, which will not hold at that point. As said above, the albedo for a bare earth will be approximately 0.1, while current values on average range from 0.3-0.4, largely contributed to by clouds. An average for the albedo of the Earth in its current vegetated state, but without clouds I haven't been able to find.

As stated by @ardie-j in his answer, another possible fate of the Earth could be that it gets covered in ice, as another Snowball Earth Event. In that case the albedo would rise to levels ranging from 0.4-0.9, resulting in a drastically cooler Earth.


Time of concentration - Difference between large and small basins

This is a difficult question since time of concentration ($t_c$) can in theory,  vary substantially, for two basins of equal size.

Off the top of my head, I can imagine several other variables that would be necessary to determine $t_c$: drainage density, basin relief, basin length, surface roughness (Mannings coefficient), and rainfall statistics. There's certainly others too.

I found a paper titled The characteristic time scale for basin hydrological response using radar data is freely available. There is a discussion of $t_c$ beginning on page 93 at bullet point number 2. They cite other papers in their discussion that would be worth tracking down and reading.

Note they present an empirical equation for $t_c$:

$$t_c=5.4\left(\frac{L}{S^{0.5}}\right)^{0.75}$$

L is basin length, and S is slope/relief. It's weird, yet I'm not surprised since it's empirical. Evidently the 5.4 has dimensions of $time$ and $length^{-0.75}$. It's hard to imagine that this equation would actually be useful.


Palaeontologists use lots of methods to investigate past life.

Like most sciences these day, palaeontology is a growing field. More scientists are applying more methods in more sophisticated ways. The literature — what we 'know' — grows and grows. So palaeontologists use hundreds of different methods and tools today; indeed there's a whole journal dedicated to the subject!

But like a lot of other geological disciplines, it's more or less impossible to test a hypothesis rigorously, because there are no actual dinosaurs, they died out ages ago, and so we cannot know everything about them. So we have to get creative.

For example, a palaeontologist interested in how dinosaurs or early insects walked might...

Make detailed drawings of some fossils of the organism.
Use X-ray tomography to see through a specimen and look at a fossil in 3D.
Make a 3D computer model to look at how the animal might have walked.
Examine museum collections to look for evidence of a hypothesized gait, eg in field photos or in casts from actual outcrops — perhaps there's a track out there somewhere!
Look at the chromosomes of similar modern animals to determine lineages and 'shared DNA', and compare walking styles.
Build or 3D-print an actual model and see if it actually does what the scientist thinks.
Dress up like a dinosaur or dragonfly and walk around on a beach or mudflat to study the footprints. (I'm sort of but not really joking.)

(Google around dinosaur locomotion for more!)

I realize you can't do all of this sort of thing for a science fair, but hopefully it illustrates that, when it comes to hypothesizing about the past, we have to come at problems from a few different angles, using high and low technology.

Hope this helps a bit, good luck with the project!!


What's the diference between claystone ,silt,mud, mudstone ,mudrock, clay ,shale and siltstone?

Clastic sedimentary rocks are classified by size of the sediment particles making up the rock.  Particle size descriptions like sand, silt, and clay have specific meaning in geology and engineering. (see chart below).

Shales, mudstones and claystones are rock types that are very similar to each other.

Siltstone - greater than half of the composition is silt-sized
particles.
Claystone - greater than half of the composition is
clay-sized particles.
Mudstone - hardened mud; a mix of silt and clay
sized particles.

The difference between mudstone and shale is that mudstones break into blocky pieces whereas shales break into thin chips with roughly parallel tops and bottoms.

The terms shale and claystone are sometimes used interchangeably.

References:
Mudstones and shales
Fun rock charts on pinterest


It is continental crust which hs the greater buoyancy, so when it meets another plate of continental crust neither can subduct. Instead, they collide, crumple and fold, making them thicker and higher. An example of this is the collision between the Indo-Australian plate and the Eurasian plate, which has formed the Himalayas. Had the Indo-Australian plate been oceanic plate, which is thinner and less buoyant, it would have been subducted.


How is the vertical velocity defined in sigma-pressure-coordinates?

There is a misconception in your assumption:
$\omega = \frac{Dp}{Dt} = 0 $ means that the mass above the air parcel stays constant.

Proof: Assume quasistatic eqn:
-$\frac{1}{g}\, dp=\rho \, dz$;
apply operator of horizontal mean: $\overline{(\cdot)}= \frac{\int (\cdot)  \, dxdy}{\int dxdy}=\frac{\int (\cdot)  \, dA}{\Delta A}$
-$\frac{1}{g}\, \overline{dp} \, \Delta A=\rho \, dxdydz= \rho dV = dM$
Integrating between arbitrary pressure levels gives mass between those levels.

(1) You correctly estimated $\partial p_s/\partial y <0 $ in order to have wind to the north, but
(2) you also set $\partial p_s/\partial t =0 $ in your derivation (to replace total derivative with advection of $p_s$). This means that the pressure field is constant in time.
Result
(1) and (2) yield to lower pressure in the north ($dp_s/dy<0$) and decreasing surface pressure following the parcel ($dp_s/dt<0$)
This means that an air parcel has to descend (in terms of $\sigma$) in order to satisfy the condition of constant mass lying over it ($\omega = \frac{Dp}{Dt} = 0 $):
$d\sigma/dt = a \, dp/dt- b\, dp_s/dt$; (with a,b const.)
=> $d\sigma/dt >0$
Example
If your parcel starts at 950 hPa, travels to north with falling surface pressure, condition $\omega=0$ means it stays at 950 hPa, but with (1) surface pressure drops to 950 hPa as the parcel continues north and the parcel miraculously is found to be at ground level.
Conclusion
As $\sigma$ depends on pressure and surface pressure, so does its time derivative.


Was the Southern Hemisphere cooler during the Younger Dryas?

According to the paper The Younger Dryas Climate Event (Carlson, 2013), the Younger Dryas was a period of warming in the extratropical Southern Hemisphere, and quite a significant one as well, as, from the article:

  SST records show a warming
  of 0.3–1.9 C from the southeast Atlantic to New Zealand

This is supported by

  Speleothem and pollen records from New Zealand and
  pollen records from South America confirm that the Younger
  Dryas was generally a period of warming

Citing glacial retreat measured in places such as New Zealand, the article Glacier retreat in New Zealand during the Younger Dryas stadial (Kaplan et al. 2010) hypothesised the mechanism to be due to

  extensive winter sea ice and curtailed meridional ocean overturning in the North Atlantic led to a strong interhemispheric thermal gradient8 during late-glacial times, in turn leading to increased upwelling and $\mathrm{CO}_2$ release from the Southern Ocean, thereby triggering Southern Hemisphere warming during the northern Younger Dryas.

Research on the deposits observed in the Australian tropics of the age in the article Late Pleistocene record of cyclic eolian activity from tropical Australia suggesting the Younger Dryas is not an unusual climatic event (De Deckker et al. 1991) revealed that the southern tropics experienced a drying phase, that fits well with a cyclic climatic phase.

In terms of the southern tropics, the article Tropical climates in the game of two hemispheres revealed by
abrupt climatic change (Ledru et al. 2002) suggest that there were significant variations, as recorded in pollen samples from the South American tropics, leading the authors to suggest that

  One consequence of the Younger Dryas changes would be the location of the Intratropical Convergence Zone in a southern position, so that even tropical regions would have been under Arctic influence.


Slowing the planet would have major consequences. Stopping the planet would have major consequences (here are some: Effect on sea level if the Earth stopped rotating).

Doing either of these things too quickly would have even greater consequences, but trying to predict these consequences and the effects for life on earth would be (a) a major primary research project, and (b) perhaps impossible. The answer is that we just don't know, and I doubt that anybody is about to fund the research ;-)

How to do it is, as I suspect you realise, also a very big question. I mean, if for some reason you wanted to. The difficulty is conservation of angular momentum: for the planet to have stopped turning, you need to have shifted that angular momentum somewhere else. That probably means firing stuff into space, or using tidal mechanisms (as somebody noted re the Moon), or maybe building a giant / massive flywheel on the Earth's axis and spinning it really fast the other way..... all of these are, of course, well into the realm of science fiction.


https://en.wikipedia.org/wiki/Martian_regolith_simulant#MGS-1 has a link to a standard for Martian regolith that includes recipes. It will, though, not be trivial to replicate the grain size from the minerals. Silicate stuff is hard, one might need an expensive mill or grinder to produce it, or a friend who works at a stonecutter's workshop. Perchlorates are not included in the standard, yet, but maybe its healthier to leave that unimportant detail (slight sarkasm :-)) away.

Mind potential health risks !

Suggestion: maybe it would suffice for an A level project to visit a site where one could take away a basalt, grind it somewhere and sterilize it (heat it to >100°C for an hour or so).


Marine microfibres: less plastic than predicted

They don't mention it explicitly. But they don't classify further than "natural" and "synthetic". And, may I say, animal husbandry as well as agricultural use of the producers of such natural fibres (e.g. jute, wool) are under human control as well. They are introduced into the ocean via waste-water, rivers, wash from the continents.

The study used samples from surface waters, if I understand it correctly from reading superficially. They say that too many fibres are prematurely seen as "microplastic" while they may actually be natural.

Other studies have come to the conclusion that the missing plastic-fibre in surface waters rests deeper in the water column, or may come in small(er) sizes than sampled.


Are beaches found all along the coast? If not, what is required for them to be formed?

Not all coastlines have beaches. Some have cliffs, such the Great Australian Bight, which forms a significant part of the southern coast of Australia. The limestone cliff portion of the Bight is over 1000 km in length and between 60 and 120 metres in height.

Similarly, in the Otway region of the Australian state of Victoria forested hills form the coast in regions. As can be seen when travelling along the Great Ocean Road.

In Western Australia, the Zuytdorp Cliffs form 150 km of the coast. Like the Great Australian Bight, these cliffs are formed formed of ancient limestone. Some of the cliffs rise to 170 m in height.

Additionally, many of the fjords of Norway and Fiordland in New Zealand also have coastlines without beaches, where hills & ranges meet the sea.

Beaches are only formed when land gently slopes into the sea. They

  typically occur in areas along the coast where wave or current action deposits and reworks sediments.

The sediments can be sand, pebbles, or biological materials such as shells and coralline algae.

In addition to forming on ocean coastlines, some beach for on lake shores.


Fast exhumation is not necessary to retain blueschist mineral assemblage. This is the classical difference between prograde and retrograde metamorphism, and it's not limited to blueschists. Why do we have eclogite on Earth's surface? What about granulites? Amphibolites? In fact, why do we see any metamorphic rocks on the surface instead of just various clays and quartz?
There are two main reasons for that.

Prograde metamorphism results in dehydration and loss of water(1). Most of the original water content of the rock was in various hydrous minerals, and as you progress in your metamorphic path you lose more and more water to dehydration reactions. While it is true that in blueschist waster can be retained in minerals such as lawsonite, phengite, epidote and glaucophane, most of it is still lost. If you want to reequilibrate the rock to its previous mineral assemblage, you need to reintroduce the water. This can occur occasionally, for instance in this classic example from Norway:

source: http://www.geol.ucsb.edu/faculty/hacker/geo102C/lectures/part11.html
...where a granulite was rehydrated along a joint to form eclogite, but it's usually very localised and uncommon.

Most prograde metamorphic reactions are exothermic, i.e. they release heat. Retrograde reactions are usually endothermic. They require heat in order to proceed, but in the event of exhumation, temperatures are decreasing. There is simply not enough heat around to support the retrograde reactions. Furthermore, you have to remember that lower temperature also lowers the reaction kinetics.

This doesn't mean that fast exhumation cannot occur...
There are pieces of evidence that in some cases exhumation can be even faster than the erosion rate(2). This suggest that orogenic events may also result from simple uplift and not only compression and folding. What causes the uplift? This may actually be the result of crustal thinning and extensional stress regimes, as in the examples of Liaodong(3) and Brittany(4). This phenomenon is also known as a core complex, where extensional tectonics result in low angle normal detachment faults that results in rapid unroofing and exposure deep seated rocks. You may be familiar with oceanic core complexes, so this processes is quite similar.

(1) Forgive me for not including the zeolite facies in my definition of metamorphism.
(2) Rubatto, D., & Hermann, J. (2001). Exhumation as fast as subduction?. Geology, 29(1), 3-6. 10.1130/0091-7613(2001)029<0003:EAFAS>2.0.CO;2
(3) Yang, J. H., Wu, F. Y., Chung, S. L., Lo, C. H., Wilde, S. A., & Davis, G. A. (2007). Rapid exhumation and cooling of the Liaonan metamorphic core complex: Inferences from 40Ar/39Ar thermochronology and implications for Late Mesozoic extension in the eastern North China Craton. Geological Society of America Bulletin, 119(11-12), 1405-1414. 10.1130/B26085.1
(4) Brown, M., & Dallmeyer, R. D. (1996). Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France. Journal of Metamorphic Geology, 14(3), 361-379. 10.1111/j.1525-1314.1996.00361.x


Could the LA County Museum of Art fall into the LaBrea Tar pit during an earthquake?

The two geological faults that may be responsible of large earthquake in proximity of the museum are:

Hollywood fault;
Santa Monica fault (actually many branches, but they all go together as Santa Monica fault).

The museum seems to be enough far away from the fault trace (the surface expression of the fault itself), so it is unlikely that there will be a major shift in the soil.

However, the museum may sink in the soil due to the earthquake mobilizing the soil, especially if composed of loose sand.

The phenomena responsible of the museum "falling in the soil during an earthquake" is liquefaction. The geological service of California provides you an interactive map to check if a certain address may be under liquefaction hazard https://www.americangeosciences.org/critical-issues/maps/interactive-map-california-earthquake-hazard-zones and ...

at the moment the interactive map is not accesible to me. Sorry :)

If liquefaction is possible, either the tar may quickly move and sputter on/in the museum, or the museum may sink in and later tar may infiltrate slowly in the building.


Project Mogul: Does upper atmosphere indeed have a sound waveguide channel?

More than a year later, I'm doing a preliminary literature survey (noting that infrasound propagation is used in nuke test ban treaty verification):

https://dx.doi.org/10.1016/0012-8252(77)90063-0 W.L. Jones. Waves in the atmosphere; Atmospheric infrasound and gravity waves, their generation and propagation. (1977)
https://dx.doi.org/10.1111/j.1365-246x.2011.04975.x Green et al. Infrasound radiated by the Gerdec and Chelopechene explosions: propagation along unexpected paths. (2011)
https://dx.doi.org/10.1111/j.1365-246x.2010.04925.x Groot-Hedlin, Hedlin, Walker. Finite difference synthesis of infrasound propagation through a windy, viscous atmosphere: application to a bolide explosion detected by seismic networks. (2011)
https://dx.doi.org/10.1029/2011jd017014 Evers et al. Anomalous infrasound propagation in a hot stratosphere and the existence of extremely small shadow zones. (2012)
https://dx.doi.org/10.1121/1.1903454 Liszka. Long-distance propagation of infrasound from artificial sources. (1974)
https://dx.doi.org/10.3390/ani3020349 Kelley, Garstang. On the possible detection of lightning storms by elephants. (2013) (courtesy of casey)

... and a ton of others. When I have enough time, will study. Others may surely use the leads I collected to post an answer before that.

Things to keep in mind: anomalous propagation, whether acoustic or electromagnetic, below the surface or above it, is based on speed inversions. Whenever there are countervailing trends in salinity (for water)/water content (for air), density, and temperature, natural waveguides may exist.


what is the best way to detect spatial and temporal pattern of precipitation in mountainous region?

This is a non-trivial problem.  Short answer:  I don't think it can be done with monthly satellite data to a useful precision.

Here's where I started:

https://earthobservatory.nasa.gov/Search/index.php?hq=site%3Aearthobservatory.nasa.gov%2FFeatures%2F&q=measuring+rainfall

They now have a tropical rainfall project.

https://earthobservatory.nasa.gov/Features/TRMM/

It uses space borne precip radar.  In the article above I didn't find specs on the resolution.  Looking elsewhere, it's temporal resolution is 1 month, and horizontal is .25 degree (about 18 miles) If your study zone has latitude under 35 degrees, it may be of use, but overall it seems too coarse.  I would expect a montane environment would require a resolution on the order of a kilometre.

Even a simple rain gauge is problematic.  Go look at the siting requirements.  I suspect that parking one on a cliff at 6700m is going to give you non-conformant data.

I think synthetic aperture radar is being used to determine snowpack depth.
In the bad old days they hired people to go measure snow packs.

Using corner reflectors on the surface where they would be buried in snow may give you better results.  You measure the attenuation from the snow on top of them.  Take substantial time and effort to calibrate. Big win:  You are measuring the mass of ice in the column, not the depth.

Rainfall is much more difficult.  Water doesn't pile up and wait to be measured.
Or maybe you could stack it:  Consider a corner reflector at the bottom of a cone.  Choose a frequency that is somewhat absorbed by liquid water.  The cone is deliberately angled so that reflections off of it miss the transmitter.  So the corner reflector's signal is attenuated by the pile of water on it.  The cone magnifieds the water depth.

Weather radar can determine precipitation intensity, but using it in the mountains would mean getting returns of the mountain too.  Hard to sort.

Possibly put the radar on top of a mountain, and give it zero angle of elevation. In essence you would be measuring the amount of water passing through a disk. I don't think these instruments come in crackerjack boxes.

However, you said satellite.

If you have known chokepoints in stream channels, you can monitor them using very high resolution imagery -- in effect you are using a natural weir.  However you need resolution in inches. I'm skeptical about the folks at Langley lending you their satellites.

Might be possible to monitor the rise and fall of lake and ponds.  This requires a precision that I don't think is possible right now.

Might be possible using a pair of corner reflectors.  One is fixed to the ground, the other floats in a plastic open top barrel.  Bounce microwave pulse and measure the difference in time.  This requires accurate timing.  A nanosecond is about 9 inches at speed of light.  You want to reliable measure 1/10 of this.  Centimeter band microwave should be able to do this.  You still have to place the instruments, and I suspect you will need to visit them to clean out dead leaves, bird nests, and replace instruments that have been damaged by moose and woodpeckers.  You will also need to do calibration measures to compare what the instrument in the open measures compared to ones under the tree canopy.

Your upper levels are well above tree line, which means that as often as not, precipitation travels horizontally.


Has Earth's atmosphere lost mass throughout the geological ages due to biological $\small\sf{CO_2}$ fixation?

Yes, the Earth's atmosphere has thinned in the past few billion years, and biological fixation of CO2 was one of the main causes. It also gave us our present abundance of oxygen (21%). Enormous quantities of CO2 were used in the form of CaCO3 by foraminifera, corals etc to make their hard skeletons, and the resulting sediments formed limestone. Other elements formed compounds with carbonic acid (CO2 + H2O) to create other carbonate rocks.

In addition there is the more obvious sequestration of CO2 which was taken from the atmosphere by photosynthetic organisms and buried underground in the form of fossil hydrocarbons (oil, coal, gas, lignite etc). It was this which gave us our all-important oxygen. So the answer to your question is that biological activity has done both. It has hugely reduced the mass of the Earth's atmosphere, and at the same time greatly altered its composition.

It is also worth noting that the Earth loses substantial quantities of atmospheric gases into space, mostly hydrogen, but other gases as well. The bombardment of the outer atmosphere by ultraviolet rays, cosmic rays and the solar wind plays a part in this.


Was rural visibility (air transparency) longer in pre-industrial times?

Pre-industrial would mean that a lot of energy use would be the in-efficient use of wood burning.  I recall that part of J. S. Bach's payment at one of his posts was firewood provided by his employer -- something like "8 cords of birch wood, 2 cords of Alder, split for kitchen use....  I think the list was about 15 cords of wood for a year.  Bach had a big household, but even so.

Flip side:

Certainly the use of coal at the start of the industrial age clouded up the air substantially.  See records of London 'pea soup' fogs.  Coal was used on farms too, but the spread out nature of farms would dilute it somewhat.

I live in a rural area where many people heat with wood in winter, and some with coal.  At this point with an average of one household per square mile it makes little impact.  6 miles away is a coal fired power station.  On a really bad day I can just barely see it's plume.  The dust collection is pretty good.

In pre-industrial times, farms averaged about 5 acres per family unit, about 100 times the current Alberta farm density, and 1000 times Saskatchewan's farm density.

Ok:  What are the contributors to visibility reduction:

Plant transpiration -- water vapour is a big one, but plants also put significant amounts of complex organics into the air.  Most of the 'Blue Ridge" mountain haze is the result of plant activity.  This will change depending on ground cover.  Deciduous trees are more prone to this than grasses and forbs.
Dust from natural activities.  Only occurs in climates dry enough that ground cover is incomplete.
Man made dust.  Tillage in agriculture is the main cause of this, but watch any gravel road too.
Smoke from combustion.  Much depends on how stuff is burned.  Large particulates (e.g. diesel soot) fall out of the air column fairly quickly.

My wild ass guess is that average visibility hit a minimum somewhere between 1930 and 1970 when tillage and coal use was at it's peak, but has since increased due to both less coal being burned, and better combustion design, along with increasing use of zero or minimal tillage in agriculture.

You could get one check on this by looking at particulate measures in glacial ice.


How do scientists determine the absolute or 'chronometric' date of fossils?

Biostratigraphy - observing which fossils were present in which stratigraphic units, and using minimally-deformed sequences to determine age order - gives us relative ages of fossils.

Absolute ages, however, rely on geochemical chronometers: just like dating the reversals in the Earth's magnetic polarity by establishing how old the oceanic crust that records them, or working out when volcanic eruptions happened. Which chronometer you use depends on exactly what you're working with, but what they all have in common is that they use radiometric dating: the "ticking clock" is radioactive decay, which proceeds according to a radioactive isotope's half-life. Radiocarbon dating can be used for the most recent of these, but the example I'm going to use is uranium-series decay in corals - even though it's a weird one, because unlike most systems it doesn't measure parent-daughter ratios, but rather how closely the system has approached secular equilibrium.

Uranium is soluble in natural waters at conditions found on the Earth's surface; thorium, however, is not; so corals (predominantly calcium carbonate), while growing, continuously incorporate uranium but not thorium. Once they stop growing, this is no longer true - the amount of uranium incorporated in the coral is now fixed. Uranium-234 decays to give thorium-230, which is itself radioactive, with a significantly shorter half-life. At secular equilibrium, the rate of thorium-230 production (from uranium-234 decay) equals the rate of thorium-230 decay, so 230Th reaches steady state. Provided secular equilibrium has not been reached and the concentration of 238U (234U's parent) is also known, it is possible to calculate how long ago the coral stopped incorporating new uranium - became a "closed system". The mathematics of U-series secular equilibria are well understood but somewhat tedious, and are freely available.


Fjords are formed as U-shaped valleys by the extreme erosion of glaciers. The glacier erodes large quantities of material that is transported by the ice.

The glacier can erode deep under the sea level (remember how an iceberg only has 1/10 of its mass above the surface) and if the postglacial rebound is limited, the fjord will remain as an estuary.

When the glacier slows down and withdraws, the sediments are dumped at the front of it to form a terminal moraine or sill. This is particularly pronounced if the front of the glacier had shrunken, growing again before melting due to environmental oscillations.

The sills are usually deposited at the mouth of the fjord, before the water depth increases. However, the biggest part of the glacial debris is deposited at deeper water.

You might also have shallow water in front of rias, were the flow slows down and sediments are deposited, but they will rarely form obvious sills as in a fjord.


Let's first consider the scenario in the absence of free radicals that can act as catalysts of $O_3$ destruction.
In such scenario, added to the absence of UV radiation, the photolysis of ozone ($O_3 + UV → O_2 + O$) would not be possible. Therefore, the only way to destroy ozone would be by the reaction

$O_3 + O· → 2 O_2$

But the lack of UV radiation would also stop the production of atomic oxygen by photolysis, therefore only the preexisting atomic oxygen would be available for that reaction.
Then, for simplicity we can consider two extreme scenarios:

All the atomic Oxygen forms $O_2$ and the Ozone level remains constant at the same level it has prior to the shut down of UV radiation.
All the atomic Oxygen is used to destroy Ozone

In the second scenario the final concentration of $O_3$ will depend on the initial concentration of it and atomic oxygen. But as seen in the following image, the stratospheric concentration of atomic oxygen is much lower than the one of $O_3$, therefore the ozone levels would never fall to zero.

(Figure taken from here)

Let's now consider the more realistic case, on which there are other "polutants" that can break down ozone. The moste common ones are hydroxyl radical (OH·), nitric oxide radical (NO·), chlorine radical (Cl·) and bromine radical (Br·) (source), and the first two are the only ones naturally occurring. Therefore, the ozone remaining from our first scenario would be reduced to zero by reactions like:

$OH + O_3 → {HO}_2 + O_2$

or

${NO}_2 + O_3 → {NO}_3 + O_2$

Among others.

The exact time that will take will depend on the elevation, as the abundance of ozone as well as the radicals can vary orders of magnitude trough the stratosphere. It will also depend on the exact spectrum of your "sustitute Sun", as some of those reactions are also mediated by light and might not happen in the absence of light of the right wavelength.

I know this answer might be disappointing, as it seem that a model would need to be run to really say how long it would take to destroy all stratospheric ozone. But still I thought would be an informative answer after 11 month without one. I hope, it motivate a better answer or the improvement of this one.


How to identify chemical property and structure of rocks?

What you are asking about is mineralogy,

a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical) properties of minerals and mineralized artifacts. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization.

It is an essential part of all geology courses. Allied to this is study of how rocks form, petrology.
There is a lot to learn within mineralogy and mineralogists are a specilized group of geologists.


How much will sea level rise if all the polar ice melts?

Using the latest numbers from the 2013 IPCC report (Ch. 4, the Cryosphere), Antarctica contains 58.3 m of sea level equivalent (sle) and Greenland 7.36 m sle. Remaining glaciers provide an additional 0.41 m sle. In total and adding very minor contributions from permafrost etc. the total comes out to approximately 66.1 m sle.

EDIT: Just to be complete: If one by "pole ices" also includes sea or other floating ice apart from glaciers terminating in the sea, the contribution from those components to sea level is zero.


For what percentage of the earth's history has there been permanent ice?

As has been abundantly said in the comments and on meta, there is no proper answer to this question in the current state of knowledge.
But indeed we can at least check if we can come up with a rough estimate (and thus try to see from where this 20% estimate comes from).

Let's first look at the main known glaciations:

The Current one: there is indeed a permanent ice sheet on the East Antarctic continent since, presumably, the early Oligocene (i. e. 35Ma, see e. g. DeConto & Pollard 2003). There is considerable dispute on the exact timing of the onset of the EAIS but we're talking here about differences of <10Myrs, i. e. <0.25% of Earth history, so we can ignore for now this difference.
The Karoo ice age: the Carboniferous-Permian glaciation is thought to have spanned from the Mississipian (ca. 360Ma, Buggish et al. 2008) to the Sakmarian (ca. 290Ma; Korte et al. 2008), giving us a ca. 70Myr interval. However some studies have shown (e. g. Fielding et al. 2008) that it was most probably an alternance of glacial and nonglacial climate rather than a continuous glaciation, meaning that permanent ice was probably present for way less than 70Myr.
The Ordovician glaciation: it is thought to have started between the late Katian and the early Hirnantian (ca. 445Ma) and to have ended either before the Silurian (see e. g. Finlay et al. 2010) or at the beginning of the Silurian (ca. 440Ma; see e. g. Cooper & Sadler, 2012). Hence an interval of 1 to 5 Myr. This one has been studied extensively as it is thought to be associated with one of the 5 major mass extinctions of the Phanerozoic (see the whole Sepkoski corpus).
The Cryogenian: two glaciations are thought to have marked the Cryogenian (see e. g. Shields-Zhou et al. 2012), the Sturtian (ca. 720 to 660Ma) and the Marinoan glaciations (ca. 650-635Ma), thus accounting for 75 Myr.
The Huronian glaciation lasting presumably from 2.45Ga to 2.22Ga (see a small review in Van Kranendonk, 2012). The issue here being that the accuracy of paleoproterozoic event datation does not really allow to go much more into the details as this (both because of the extreme scarcity of the data and the relative low resolution of the dating techniques). Leaving us with a glaciation (or more probably a series of glaciations) that could have lasted up to 230Myr.

To this, we can add various minor glaciations that are still highly debated such as the Late Devonian or the Middle to Late Jurassic (e. g. Dromart et al. 2003). Whether of not they indeed occur, they hardly account for much of Earth History when aggregated so we can ignore them safely for the purpose of a rough estimate.

This leads us to 35 + 70 + 5 + 75 + 230 = 415 Myr which equates to ~ 9 % of Earth History. Which is still far off 20% and I doubt that the minor glaciations put together account for 500Myr.

Buggish, W., Joachimski, M. M., Sevastopulo, G., Morrow, J. R., 2008. Mississippian $\delta ^{13}C_{carb}$ and conodont apatite $\delta ^{18}O$ records — Their relation to the Late Palaeozoic Glaciation. Palaeogeography, Palaeoclimatology, Palaeoecology, 268: 273-292.
Cooper, R. A., Sadler, P. M., The Ordovician Period. In Gradstein et al. 2012, The Geologic Time Scale 2012: 489-523.
DeConto, R. M., Pollard, D. 2003. Rapid Cenozoic glaciation of Antarctica induced by declining atmospheric $CO_2$. Nature, 421: 245-249.
Dromart, G., Garcia, J.P., Picard, S., Atrops, F., Lécuyer, C., Sheppard, S.M.F., 2003. Ice age at the Middle–Late Jurassic transition? Earth and Planetary Science Letters, 213: 205-220.
Fielding, C. R., Frank, T. D., Birgenheier, L. P., Rygel, M. C., Jones, A. T., Roberts, J., 2008. Stratigraphic imprint of the late Palaeozoic ice age in eastern Australia: A record of alternating glacial and nonglacial climate regime. Journal fo the Geological Society, 165: 129-140.
Korte, C., Jones, P. J., Brand, U., Mertmann, D., Veizer, J. 2008. Oxygen isotope values from high-latitudes: Clues for Permian sea-surface temperature gradients and Late Palaeozoic deglaciation. Palaeogeography, Palaeoclimatology, Palaeoecology, 269: 1-16.
Shields-Zhou, G. A., Hill, A. C., Macgabhann, B. A., 2012. The Cryogenian Period. In Gradstein et al. 2012, The Geologic Time Scale 2012: 393-411.
Van Kranendonk, M. J., 2012. A Chronostratigraphic Division of the Precambrian. In Gradstein et al. 2012, The Geologic Time Scale 2012: 299-392.


Young researcher hoping to volunteer for Antarctica fieldwork

Try the long game:

Start reading up on the research that is being done.  For the interesting ones, start corresponding with the authors asking questions about the work.  They have to be good questions, showing that you know what you are talking about.
Look at who funds each researcher.  Check the papers for those 'thank-yous' that give credit to various individuals and institutions.  Check the institutions to see how they hire support personnel.  If you can't get a job as a researcher, look at getting a job just as a tech.
Most researchers are connected with one more more institutions.  Some of these specialize in polar science.  Some may supply support crews that span scientific disciplines.  E.g. They may have a pool of snow-cat operators that any of the scientists can draw upon to move equipment and personnel.  This is more likely with the permanent bases.
Can you speak well?  Do you have a large grasp of polar science in many fields?  Look at being a guide on the cruise boats that go to the arctic.  If you are quick witted, and can reduce difficult topics to digestible chunks, this is a possibility.  Train for it by writing a science blog.
Write a polar science blog.  Turn dry science into good clear interesting commentary about what's happing, how it happens, why it's important.  This becomes something to put on your resume, and if you get a decent following may itself get you there as a science journalist.
Prepare yourself. Learn how to dress and take care of yourself in Arctic regions.  Someone they don't have to train as much is cheaper for them.
Consider having your appendix removed.  At one point this was recommended as an appendectomy in Antarctica is double plus un-fun.


Can hippopotamus fossils be found all along the Nile?

They most probably can, but there are two things to be borne in mind. The vast majority of animal corpses never fossilise, as fossilisation requires special conditions. This is especially true of animals from aquatic or very damp environments. Of those that do fossilise, only a tiny proportion are found and excavated.  Some weather out of the rock and crumble away on the surface without ever being noticed by an expert. Many hominid fossils have disappeared in this way.

The other thing required by hippos if they are to flourish and eventually fossilise is a source of vegetable food, sometimes from the river itself, but more usually from grazing on the nearby river bank. Much of the lower Nile today passes through cataracts and deserts, but six or seven thousand years ago the climate was moister, and much of the eastern Sahara was similar to East Africa today, and had similar animals and vegetation. It is quite feasible that in those days there was enough grazing on the banks of the lower Nile to support a population of hippos. They were certainly found in the Nile delta.

There are short stretches that pass through cataracts or strong rapids. It is unlikely that these stretches were ever inhabited by hippos. The current is too strong and food sources difficult to reach, even if they were fairly nearby. Many hippopotamus carcasses are dismembered by crocodiles, which further reduces the likelihood that they will ever be found as fossils. Hippos don't like to stray more than a couple of hundred yards from water, and if there is plenty of aquatic vegetation they will seldom venture onto dry land.


Could the extraction of energy by wind turbines be harmful to the environment?

The short answer is yes, it's possible. And it can also reduce harm.
Summary of the changes
Here's a brief summary of the changes that happen when electricity is generated from a wind turbine, rather than from a fossil or nuclear plant:

energy is taken out of the wind further upwind than it would otherwise be
there is more turbulence just downwind from the wind turbine
less low-grade heat is put into the environment around the fossil or nuclear plant that has been turned down, or off, because the electricity is supplied from wind now.

Global impacts
As far as we can tell it's unlikely that there would be global effects at any plausible levels of global deployment (let's say, up to terawatts of mean power, but below tens of terawatts). But a single row of turbines can be enough to cause very local effects, and those could be positive or negative.
There is an open discussion in the literature about what the impact of removing terawatts of wind power would be.  But it's a theoretical discussion, and not really meaningful, because it would absolutely depend on where the power was extracted.
What happens to the wind's energy?
Ultimately, pretty much all energy in the wind ends up as low-grade heat. And that's regardless of whether or not it gets there via a wind turbine or not. If not, it will eventually dissipate due to friction, where it becomes ambient low-grade heat. If it is, then it will get converted into electricity, then some energy service (e.g. lighting), and then ambient low-grade heat.
So deploying lots of wind turbines just moves where that energy gets taken out of the wind, but not how much.
An unshrouded turbine can extract at most 16/27ths (59.3%) of the wind's kinetic energy - that's the Betz limit. In practice, it's more like 20-50%. Here's the power curve for an E82-Enercon 3MW turbine, from the Enercon brochure:

The lighter curve, plotted against the vertical axis on the right, shows the proportion of the wind's kinetic energy that is converted to electricity. At around 9 m/s wind speeds, that proportion maxes out at about 50%. A further small (~2) percentage goes to internal conversion losses within the turbine.
What are the energy changes, and where?
The full picture is more complicated. Whether switching from fossil/nuclear to wind would change patterns of energy service demand, and energy efficiency, is an open question, so there are plenty of ripple effects. But, putting those aside to simplify things, just switching from fossil/nuclear to wind wouldn't create new sinks for the low-grade heat: lighting would carry on getting used at the same time and place as it does at the moment. What would happen is that energy would be extracted from the wind in new places: at turbines, rather than further downwind where the energy would be dissipated by friction. The other thing that would change is that there would no longer be additional sources of low-grade heat at all those fossil and nuclear power stations. Now, they're pretty intense sources of heat: nuclear or coal plants typically push out 150% - 200% as much energy as local heat, as they do electricity. So that's a significant input of heat into a local weather system, which would no longer be there when they're turned off because the electricity is being supplied from wind instead.
A quick romp through the literature
Smith, Barthelmie & Prior used In situ observations of the influence of a large onshore wind farm on near-surface temperature, turbulence intensity and wind speed profiles. They found that in the daytime, there was no observable change in temperatures once they were more than 2km downwind from turbines. However, there was an increase in night-time temperatures at 2.5m above ground (hence the use by farmers of wind turbines for frost prevention)
On top of what I've written above, we've also got a bunch of modelling in the literature. Now, this is largely an argument between different assumptions: pick the answer you want, and tune your assumptions accordingly. However, these papers are written by people with different underlying positions, so you'll find a range of possible outcomes, and you can decide for yourself which assumptions are more or less likely.
Mark Z Jacobson wrote the book on the Fundamentals of Atmospheric Modelling, and tends to build up from the micro level. A recent paper of his shows potential benefits of reducing the energy in hurricanes that reach the shore, and we know that the first-order impacts of that would be positive.
Roy & Traiteur looked at Impacts of wind farms on surface air temperatures and found significant very local effects, and options for mitigation.
Wang & Prinn looked at potential long-term global impacts of multi-terawatt-scale installations and found that massive onshore wind turbines might cause some warming over land (though less than the coal & gas plants they would replace), and massive offshore wind turbines might cause some global cooling. The modelling was done by assuming the turbine interactions would behave like a generic increase in surface roughness; the other key assumption is that the loss of detail from meso-scale modelling did not significantly impact the results.
Kirk-Davidoff and Keith, like Wang and Prinn, used coarse meso-scale modelling by adding anomalies to surface roughness coefficients. With continental-scale windfarms, they found very little change in global temperatures, but local changes could be up to 2 Kelvin.
Rooijmans' Masters' thesis (Universiteit Utrecht, 2004) looked at the impact of a huge (9 000 km2) offshore wind farm on patterns of rainfall, using a mesoscale circulation model, and found that the distribution of precipitation could change, with more rain at sea, and less onshore. That modelled windfarm would be about four times the size of the largest envisaged offshore wind farm.
Consequences and conclusions
Distributions of heat affect weather and climate. So, deploying lots of wind turbines can change local patterns of evaporation and rainfall. They can also change patterns of frost: the turbines increase the turbulence in the flow, close to the ground, reducing frosts: there are farms that deploy turbines in order to harness that frost-prevention: you can even (HT scruss) buy wind machines dedicated to producing that turbulence in no-wind conditions (these wind machines, ironically, are typically powered by fossil fuels).
So that's the ultimate change: energy would be extracted from the wind further upstream than it was previously; and there's less heat put into the system from the cooling systems at fossil and nuclear power stations. These are all local weather effects, and could be beneficial, harmful, or neutral.


These globular structures are called varioles, and there is not complete agreement as to how they are formed. The most likely theory is that the basalt has been hydrothermically altered by millions of years of water infiltration bringing in tiny amounts of other minerals, but the jury is still out.


T-phigram satellite derived sounding vs model output question

As already commented; a good indicator for rain in soundings is that the lower troposphere is saturated by water. Individual saturated levels would imply moisture, but if the moisture doesn't reach the surface then it maybe just clouds or rain that evaporates before reaching surface (virga). The deeper the saturated layer, the better indicator for rain it is. The dew point and ambient temperature doesn't necessarily have to be exactly the same, but very close to each others.

You pointed out cin and cape, but they are really just indicators for forcing convection and aren't necessarily present when it already rains.


Gnomonic projection of a curves with constant latitude

Circles!

Well, some of them, anyway.

You can plot any azimuthal projection in polar coordinates, where the radius is a function of the angular distance from the center point.

So where two points' angular distances are the same, their radii are the same, and the projection of any locus of constant angular distance from the center point is a circle, if it appears at all.

Since you said that the tangent point is a pole, that means that parallels (lines of constant latitude) are those lines of constant angular distance. Ergo, Northern Hemisphere parallels appear as circles.

(Wikimedia Commons)

Since the Gnomonic projection can show only a single hemisphere, the Equator and parallels in the Southern hemisphere can't appear at all -- they would be an infinite distance from the center point.

The polar orientation of the projection even takes away  complications resulting from the earth's not being a perfect sphere -- the radius formula changes a little, but the equal results from the same input remain.


What are the differences between geology, petrology, and lithology?

The basic differences are:

Geology

study of rocks and minerals: the study of the structure of the Earth or another planet, especially its rocks, soil, and minerals, and its history and origins
structure of area: the rocks, minerals, and physical structure of a specific area

Petrology

study of rocks: the study of sedimentary, igneous, and metamorphic rocks with respect to their occurrence, structure, origin, history, and mineral content

Lithology

description of the physical characteristics of a rock unit or rock formation, and actually not a scientific discipline as the two above


Which chemicals in smog obscure visibility?

Winter and Summer smog

There are two types of smog:

winter smog:

high emissions of soot and of $SO_2$ through heating with wood, coal, etc.
$SO_2$ reacts to sulfuric acid => formation of particulate matter => reduced visibility

summer smog (photochemical smog):

strong sunlight and emissions of $NO_X$, $CO$ and VOCs
emissions of primary particulate matter not necessary
reaction of these air pollutants to other gaseous air pollutants (PAN, $O_3$, $HNO_3$) through photo-oxidation
photo-oxidation of various VOCs => lump together to ultra-fine particles (secondary PM) => particles grow

Species that impact visibility

We 'need' the gaseous air pollutants to form secondary aerosol particles. The reduced visibility during winter and summer smog events is caused by these secondary aerosol particles (they scatter the light and, by this process, reduce visibility). The gaseous air pollutants might contribute to reduced visibility but their contribution is negligible compared to the aerosol impact.

Contribution of different aerosol species

The composition as well as the size distribution of the aerosol particles is important to determine the aerosol's optical properties. If you look into mass concentrations, a given mass of fine particles extincts more light than the same mass of coarse particles. If you look into composition (inorganic components), nitrate extincts more light than sea salt and sulfate. The extinction of organic aerosol depends on their composition (I have no idea which factors/species are relevant there).

If you are interested in further literature on the impact of the particle composition on the light extinction you might look into Pitchford et al. (2007). I didn't find a publication which quantifies the contribution of different particulate compositions/species to light extinction for smog. However, Pitchford et al. (2007) deals with light extinction for calculating the optical thickness from model data.


How to learn the different types of rock

The best way to learn about rock types is to handle rock specimens guided by an experienced geologist.

By handling rock specimens you get to feel the weight of the rock, its roughness or smoothness or if it feels slippery, soapy, glassy, firm or crumbly. Is it weak or strong? You will also be able to better see the colours in the rock, the sizes of the grains of minerals, the way the rock breaks, its cleavage. You can test its hardness. If you have a piece of unglazed white porcelain you can do a streak colour test.

All these sorts of things can help you identify different rock types.

Also, the same rock type can look slightly different in other parts of the world, because of geological and environmental processes that rocks experience in different parts of the world. This is particularly so if the rock is deeply weathered.

Learning the different rock types, alone from books and pictures is a difficult way to learn, befriend a geologist, you'll learn more quickly and it will be more enjoyable.


What is the name for the forested areas in mountains below the treeline?

Forests directly below the alpine tree line are high-elevation or Montane forests.  See more about Montane ecology here.

  Image from http://www.nature.com/scitable/knowledge/library/global-treeline-position-15897370


"New" cloud types: Can they be used as markers for climate change observations?

My answer is no. The reclassification of clouds is not affected by climate change. It is more of our understanding of how they work, formation mechanisms, etc. Cloud classification can generally be broken down into altitude and shape (cirrus-type, cumulus-type, and stratus-type). The subtypes of those describe the behavior, and may be combinations of types (such as cirrostratus, pyrocumulus and cumulonimbus). There are other types, such as the new asperitas cloud (formed by gravity waves), the Kelvin-Helmholtz instability cloud (formed by the Kelvin-Helmholtz instability), contrails (formed by jet exhaust), fog (formed by either advection or radiation), etc.

That is not to say that clouds don't affect climate. For example, cirrus clouds, and other high clouds, are optically thick, preventing the low temperature from dropping. It has been debated what the net effect clouds have on the environment, newer research seems to indicate net warming (see latest IPCC report), despite the traditional thought being cooling. Being that water vapor is a greenhouse gas with generally greater concentrations than the famed CO$_2$, it is extremely important to consider in climate discussion.


Correct phrase for a 'forgetful' natural system?

How would you call a system that displays such 'forgetfulness'?

In one word, dissipative. In multiple words, a system whose autocorrelation function tends to zero as time tends to infinity.

Note that the autocorrelation function may not exist; this concept implicitly assumes a linear (or approximately linear) system. Consider a river. Adding a small impulse of water upstream (e.g., a not-too-large downburst) results in the river flow increasing briefly but eventually stabilizing to some nominal value. Adding a slightly larger impulse results in a similar but slightly larger response. But adding a huge impulse of water upstream (e.g., a very large and rapid snowmelt) may cause the river to carve itself a new channel. With this huge impulse, the flow at some downstream point will increase initially but then drop to zero and stay that way because the river no longer flows past that point.


The simple answer is that you just subtract the nearest value from the glacial isostatic adjustment (GIA) model. The values provided by Peltier in your netCDF file are in a grid that is 1 degree x 1 degree. So with the lat/lon position from the tidal gauge sea level data, you choose the closest value to your station and apply the correction. In the PSMSL GIA correction page they already explain that you should be using the centered value (±250 years). If you decide to use the ASCII files, those are already interpolated to the tide gauge station locations. You have additional information in the U. of Colorado GIA websites(1, 2) and in the Peltier (2001) reference.


Why are there both climate regions from axial tilt and weather cells?

Generally speaking, the tropics are a geographical term. It's the area on the surface of the planet where at least once a year the Sun passes directly overhead.
It has relation to climate, but even then within the tropical region you have a variety of climate zones from rain-forests all the way to savannas and deserts. Being in tropical region only ensures that it's generally warm all year round, other parameters define the actual climate type. Take a look here at the range of climate types available in the tropics.
Now, coming to Hadley cells and how they relate to tropics - they are not directly related to one another, i.e. one of them is not used to define the other.
In Hadley cell the humid air rises and moves north or south depending whether it started north or south of the equator and then sinks to the surface at around 30 degrees north and south of equator. So they are both dependent on the same factors, namely the simple fact that it's generally hot in the tropical region.
What happens between 23 and 30 degrees really depends on where in the world you are. If you're between 23 and 30 degrees in North Africa, it's very unpleasant, because Sahara is right there. Other places are different depending on their own geographical and climate parameters.


The Earth's core is both solid and liquid. The outer core is molten and produces currents of molten nickel/iron which generate the Earth's magnetic field in a dynamo effect, while the inner core is solid. The reason the inner core is solid is that it is under tremendous pressure, more so than the outer core. Scientists know this from the analysis of seismic waves which pass through the Earth.


Making sense of the math of climate change

I was able to dig up this (although without being able to find full sources):

It shows a pattern where the temperatures are dramatically lower where ice sheets have built up relative to today. Interestingly, a lower overall temperature means weaker mixing across latitudes, so some areas are actually warmer.  Boston gets chilled by -20 according to the map.

Also note the altitude effect. If the surface is raised by 1 kilometer, then due to the lapse rate, the temperature at the new surface will drop by 6.5 degrees. This is a very important factor in stabilizing large ice sheets.


Why are there no hurricanes in the southern Atlantic basin?

There have been tropical storms and hurricanes in the south Atlantic, with, according to NOAA's webpage Subject: G6) Why doesn't the South Atlantic Ocean experience tropical cyclones?, with a hurricane forming in the south Atlantic making landfall in Brazil in 2004 and a strong tropical depression/weak tropical storm that formed off the coast of Congo in 1991 - but these are exceedingly rare.

The reason why these storms generally don't occur in the south Atlantic, according to the Penn State webpage Upper-level Lows as being:

  There are two primary reasons why tropical cyclones are rare in the south Atlantic basin. First, vertical wind shear between 850 mb and 200 mb is typically greater than 10 meters per second (check out the long-term average of vertical wind shear between 850 mb and 200 mb). To make matters worse, westerly shear dominates over latitudes where tropical cyclones would be most likely to form. Second, easterly waves from Africa do not form south of the equator (the MLAEJ is a northern hemispheric singularity.

Further, from the NASA News page The Nameless Hurricane, they provide an extension to the explanation with

  Vertical wind shears in the south Atlantic are too strong for hurricanes," Hood explains. Winds in the upper troposphere (about 10 km high) are 20+ mph faster than winds at the ocean surface. This difference, or shear, rips storms apart before they intensify too much

An article The first South Atlantic hurricane: Unprecedented blocking, low shear and climate change (Pezza and Simmonds, 2005) suggest that the implications of the southern hemisphere hurricane presents

  evidence to suggest that Catarina could be linked to climate change in the SH circulation, and other possible future South Atlantic hurricanes could be more likely to occur under global warming conditions.

Catarina refers to the southern hemisphere hurricane

SH = Southern Hemisphere

edited to add an NASA Earth Observatory satellite image of the hurricane:


Is the lack of aerosols increasing temperatures in 2020?

Depending on type and size of aerosol there can be scattering, (Mie- and Rayleigh- scattering), atmospheric windows filter radiation, we have absorption, transmission and transformation of wavelengths which all influence the amount of energy that's trapped or reflected back.
The lockdown has led to a significant reduction of aerosols and drastic improvement of air quality in some regions (meaning not everywhere), but reliable studies seem to be rare atm. Evaluation is underway, but i think we must wait until a season has passed and activity has returned. A huge effect on temperature has afaik not yet been observed and described and if, it was towards a local decrease in temperature, not an increase. Anyway, the positive effects of lower economic activity are ceasing as no or little attempts are being made to use the reset for a transition to more sustainability.
Two examples I found, so far:
South-East Asia from satellite observation
https://www.sciencedirect.com/science/article/pii/S0048969720331788

... enforced lockdown at different degrees to contain the spread of the disease, which has brought some positive effects on natural environment.

Morocco, a slight (-0.3°C) reduction in temperaure, but higher variablity https://www.sciencedirect.com/science/article/pii/S0048969720330588
... not much for day temperatures for beyond 40°C in the area.

... the emissions from vehicle exhaust and industrial production were significantly reduced, which contribute to the decrease in the concentrations of the studied pollutants ... local emission reductions related to the lockdown were overwhelmed by the contribution of long-range transported aerosols outside areas.

tl, dr:
No, probably not, it may locally temporarily lower the temperature by a tiny bit, but overall this effect seems to be shadowed. But future work will show, it is too early to be definitive because the story still develops.


What is the importance of oxidation process?

Perhaps this is better put on the Chemistry SE page since oxidation is much more important than beyond earth sciences, but I'll give my take on it from an atmospheric sciences perspective.

If oxidation isn't allowed to take place, then the hydroxyl radical ($\ce{^*OH}$), nicknamed the detergent of the atmosphere, cannot remove pollutants from the atmosphere. I think it is evident why removing pollution is a good thing.

This is just one of many uses that oxidation has.


The Rossby radius of deformation tells you the distance when the Coriolis effect becomes just as important as the buoyancy or gravity.

Let the Rossby radius of deformation be denoted as $L_R$. According to Holton's Fifth Edition of "An introduction to Dynamic Meteorology,"  $$L_R=\frac{NH}{f_0}$$ where $N$ is the Brunt-Vaisala frequency, $H$ is the scale height, and $f_0$ is the Coriolis parameter.

For phenomena with length scale $L$, buoyancy can be neglected if $L<<L_R$.

In statically stable equatorial flows, $$\lim_{f_0\to 0} L_R=\infty$$

In certain coastal flows, $L \approx L_R$

So the author is saying that in equatorial flows, buoyancy can't be neglected, since the Rossby radius "blows up" and for coastal flows $L\approx L_R$


Two severe cyclones connected by a monsoonal trough

Let us state the obvious reasons first:

Warm SSTs along the Timor Sea as well along the Queensland coast favour the formation of Tropical Cyclones -

The following two URLs show the SSTs along the Northern as well as Eastern Coast of Australia for the referenced periods.
a) http://earth.nullschool.net/#2015/02/15/2100Z/ocean/primary/waves/overlay=sea_surface_temp/orthographic=136.27,-24.99,512

b) http://earth.nullschool.net/#2015/02/20/2100Z/ocean/primary/waves/overlay=sea_surface_temp/orthographic=136.27,-24.99,512

The threshold SST required for tropical cyclogenesis is 26.5 C as shown in The Threshold Sea Surface Temperature Condition for Tropical Cyclogenesis
The role of equatorial waves in tropical cyclongenesis and in this case interaction of equatorial waves with the monsoon trough. Results of filtered OLR as shown in the image below shows the presence of an Equatorial Rossby Wave over those regions and the presence of blue shades indicates negative OLR (enhanced convection).

Source - Tropical Wave Monitoring

This reference The Role of Equatorial Rossby Waves in Tropical Cyclogenesis. Part II: IdealizedSimulations in a Monsoon Trough Environment clearly states that

  " When just the MT is simulated without the ER wave anomaly fields,
  the MT remains quasi-steady and TC genesis does not occur. It is only
  when the ER wave is imposed on the MT that TC genesis is initiated.
  The results imply that the ER wave–MT interactions produce more TCs
  than would otherwise occur if no such interactions took place."

A question that can be raised  is that  we do know that presence of MJO also promotes tropical cyclongenesis as shown in this reference Diagnosis of the MJO Modulation of Tropical CyclogenesisUsing an Empirical Index. So how can does one state unequivocally that it is a Equatorial Rossby Wave and not the MJO's interaction with the MT that favours tropical cyclogenesis ?

If we look at the strength of the MJO signal during and after the formation of TC Lam & Maria

as shown in these set of images

The amplitude of MJO signal is currently weak (less than 1 being present inside the circle) and is unlikely to be a significant contributor to the formation of tropical cyclones.

This conclusion can be enhanced by looking at the filtered OLR contributions from MJO -

The presence of positive OLR(in yellow over the Northern/Eastern part of Australia indicating suppressed convection.
Both the MJO as well as tropical waves favour the formation of twin cyclones as shown by references Dynamical Aspects Of Twin Tropical cyclones  Associated with the Madden Julien Oscillation  and A Case Study of an Outbreak of Twin Tropical Cyclones

Update
In an earlier question that I had asked Relation between direction of shear and cyclone formation one can clearly see a working example of easterly shear favoring the formation of tropical cyclone. So easterly shear as shown in the map below is defined as the difference in the zonal winds at two different pressure levels (850 hPa and 200 hPa). Since by convention easterly winds are noted to be negative increasing positive values of shear is easterly shear(as one travels vertically) and the opposite is westerly shear. The following shear plot is a mean of several days and one can see the easterly shear over the Australian regions where the cyclone formation took place.

$zonal\_shear = U_{850} - U_{200} $

The data is from NCAR NCEP reanalysis.


The Great Lakes contribute to a huge thermal inertia, which will even out short-term temperature fluctuations, and delay the smoothed seasonal temperature peak. On the other hand, more sinuous looping of the jet stream  is likely to have a comparable, if not larger, effect upon the temperatures in Buffalo. The process is this: As global warming progresses there is a disproportionate warming towards the polar regions. This results in a reduced temperature gradient with respect to latitude. It is this gradient which is largely responsible for the jet stream intensity. As the jet stream weakens it becomes more sluggish and sinuous, resulting in regional hot and cold anomalies. If Buffalo happens to get caught in one of these jet stream loops, it is quite possible to have periods of anomalously low temperature, even though there is an overall North American warming. It is also possible for Buffalo to get caught in a negative loop, and hence experience anomalously hot temperatures.


Carbonaceous aerosols are formed by a mixture of substances with different chemical, physical, and optical properties.

Certain organic substances are mostly transparent to sunlight and therefore do not contribute to Earth warming or cooling. Other substances are mostly reflective, therefore their presence in the atmosphere contributes to Earth's cooling. Yet there are substances that absorb sunlight thus warming the atmosphere.

Light absorbing carbonaceous aerosols do not form a uniform group either. They include soot particles, smoke, organic compounds such as levoglucosan and the spectre of humic substances all with different optical properties. The figure below shows schematically the absorption spectrum of two compounds representative of different groups of light absorbing materials.

One compound labeled B1 has a more uniform distribution of absorption coefficient across the visible spectrum where the absorption coefficient at the red edge is about half of that at the blue edge. Another compound labeled B2 has a much steeper slope at the blue edge. It would have a yellowish or light brown color. Think of the yellowish fumes that can be seen when a piece of wood is ignited.

Assuming uniform absorption spectrum for light absorbing organic carbon can lead to significant overestimation of light absorption by atmospheric carbon.

On the other hand estimating the carbon content of the atmosphere via measurements in the green or red parts of the spectrum would lead to a significant underestimation of light absorption in the blue and ultraviolet parts of the spectrum.

To reduce the error caused by the non-uniform absorption spectrum of some of light absorbing carbon we could instead think of light absorbing carbonaceous aerosols as two separate groups namely Black Carbon with uniform light absorption spectrum and Brown Carbon with steep increase in light absorption towards the blue edge.

In our example B1 would be assigned to Black Carbon group and B2 would be assigned to the Brown Carbon group.


Do we know anything about the nature of Earth's core that hasn't come from magnetic or seismic measurements?

Short answer: yes, most of our knowledge is not from seismics and geomagnetism, but from experiment and measurements, deductions from geochemistry and -physics, material science and physics in general (besides the already mentioned measurements of earth's gravity, but the variations are rather connected to mantle convection, water storage, etc. than the core). But all in all those weigh much more than "painstaking" seismics and geomagnetism.

One experiment to replicate pressure/temperature (p/t) conditions is the diamond anvil cell and its cascaded version in combination with laser heating of the probe, to obtain data about mineral and cristalline phases and transitions under conditions in a planet's core.

Wikipedia: Diamond Anvil Cell
Teaching Mineralogy; Topical Resources The Diamond Anvil Cell (DAC)

And some random links on the outcome of the matter:

Reviews of Geophyics: High-Pressure Experiments and the Phase Diagram of Lower Mantle and Core Materials
Nature Communications: Implementation of micro-ball nanodiamond anvils for high-pressure studies above 6 Mbar
Earth and Planetary Science Letters: The Earth's core composition from high pressure density measurements of liquid iron alloys
Geophysical Research Letters: Iron-Nickel alloy in the Earth's core

Laser pulses is a developing technique to reach very high p/t conditions, higher than diamond anvil cells can, but they do not last that long

Science Advances: Crystal structure and equation of state of Fe-Si alloys at super-Earth core conditions


You're right. Water has a much higher specific heat capacity than air, so will change temperature much more slowly. This means that,

the sea surface tends not to reach the same extremes as the air (hence the seasonal effect that you mentioned)
the sea tends to lag behind the air in seasonal changes
the air readily heats and cools with day and night, while the sea does this much less; so which one is warmer can vary on a diurnal as well as a seasonal basis.

All this can then be greatly complicated by sea and air currents, especially vertical circulation in the ocean - but you have the right idea.


Is there a formula to compute the destructiveness of an earthquake?

The 'destructiveness' is a function of local geology and hydrogeology, type of earthquake, depth of earthquake, local population density, and whether the local builders paid attention to the appropriate building codes. So no, there isn't an equation for destructiveness. However, you can estimate the local intensity (at the surface) from the magnitude. The mathematical approach is given in the APPLICATIONS OF EXPONENTIAL AND LOGARITHMIC FUNCTIONS for earthquakes.


How can isostatic compensation of the crust be reconciled with a relatively strong upper mantle?

Because it is not the strong portion that gets displaced. The mantle just below the crust bends, just like the crust above it, together they make up the lithosphere,  It is not the thing being displaced it is the thing doing the displacing. The asthenosphere is being displaced.

Your image is missing  something important.

Images courtesy of http://www.physicalgeography.net/fundamentals/10h.html


Can cooled/hardened lava rock return to a molten state?

Yes, at least partially.

It is important to remember that a "rock" usually does not have a single melting point. It melts over an interval. Igneous (magmatic) rocks are made of combination of minerals (for example, quartz, olivine, pyroxene, feldspar) that each will melt at a different point. When a rock is heated up, the minerals with the lower melting point melt first, followed by the higher melting point minerals with progressive heating.

Some rocks are composed mostly of easier-to-melt material (for example the stuff that came out of Mt. St Helens, mostly granitic composition) and others are made of harder-to-melt material (for example Hawaiian basalts). Taking a a granite and throwing it into a basalt will melt most of it for sure, if not all. Not the opposite though.

Even if you do not completely melt the rock, you will disintegrate it enough that the remaining harder-to-melt minerals will just drift away and mix with the lava, making it impossible to distinguish the original rock any more.

Likewise, when hot lava flows on easy-to-melt material (for example clay-rich soil), some of it will melt or vitrify (turn to glass).

I will finish by emphasising that all lavas, every single one of them, were originally solid rocks that melted at depth by some kind of process. So yes, it is possible to melt rocks.


Effect of urbanization on weather events

I did my undergraduate capstone on observational moisture changes around cities.  It's an interesting field, though it certainly seems fairly challenging to draw out and explain changes given the incredible spatial and temporal variability.  So that's definitely one of the challenges to it.  Another is the current observational density, as there are very few micronets setup and satellite resolution probably isn't helpful to diagnose variations on small time/distance scales.

But, while it definitely doesn't have near the attention (or money) that climate modeling itself does, there are a few sharp folks working in the field at least.  From my experience, having dabbled on the topic but having incomplete research familiarity particularly over the past decade, it would seem that theoretical microscale modelling might be the most fruitful research direction.  Computing power and models continue to step up, and there's much better room for experimentation.  But then, it'll still be very hard to demonstrate that those idealized simulations match reality, and impacts to connect changes definitively to theory/causes.

Indeed, if somehow sharp links can be made that are powerfully convincing, then work could be directed into reshaping urban planning to address such changes.


How and why did the oceans form on Earth but not on other planets?

Your assumption that there is not a lot of water elsewhere in the solar system is incorrect.  According the this article on NASA's website;

Missions in recent years have overturned our view of a dry solar system, returning mounting evidence of ample water from a vast array of locations.
Comets from the remote corners of our solar system are made of water and other ices. Orbiters, landers, and rovers reveal Mars as a watery world in the distant past -- a world that today may contain entire underground oceans of frozen water.

It also points out that

Jupiter's moon Europa has a frozen crust of water, covering a thick global ocean. By current estimates, it has twice as much water as all of Earth's oceans and rivers!

The perception that the rest of the solar system does not have a lot of water is probably due to the fact that the Earth is in the 'Goldilocks zone' where water can exist in all three phases at the surface of the planet.  The temperatures on the rest of the planets in the solar system are either so hot that it can only exist in a vapor phase or so cold that it can only exist in ice at the surface or liquid beneath.


When I need data about past and future projected land use change, I usually refer to the global dataset that was developed for the global climate simulations CMIP6.
The dataset is the Land Use Harmonization V.2 LUHv2 and it was developed by researchers at the University of Maryland.

Reading the supporting document of the dataset, among others, the following characteristics are reported:

  global domain;
  850‐2100 annual land-use states, transitions, and gridded mgt layers;
  0.25 x 0.25 degree spatial resolution;
  12 possible land-use states including separation of Primary and Secondary natural  vegetation into Forest and Non‐forest sub-types,
  Pasture into Managed Pasture and  Rangeland, and Cropland into
  multiple crop functional types

The dataset is composed of a set of simulations using different land surface models:

  These datasets are being developed as a contribution of the Land-Use
  Model Intercomparison Project (LUMIP) to the Forcing Group for CMIP6

Data can be downloaded from this page.

Of course, the layers you find here are "just" models' results, but as much as past observations made a good calibration possible, the variations reported are likely be the most accurate information about the evolution of the world forested areas currently available.


What is the error in bathymetric observations in the presence of vegetation?

As noted in the comments, this is actually a much more complex question than it might seem at first. Answering fully would require a very involved and technical discussion of single-beam surveying's inherent errors and their causes (water velocity, side slope reflection, highly transmissive hydrocarbon-rich anthropocene silt, and other such complications). I will skip those topics—thankfully, I think—and simply focus on the OP's question of what errors Posidonia sea grass might introduce to a hypothetical singlebeam survey. As such this may be more of a partial answer.

Let's say you're using a singlebeam instrument with a fairly narrow pulse angle, like 3º. (Your vessel should not roll too much with this narrow a beam, but it will give higher resolution). You're surveying over a colony of Posidonia on a flat seabed. Posidonia leaves have a density fairly similar to that of the water column. Depending on the frequency of sound you are surveying with, you may get a ping with a slightly different shape from surveying over an area of "bare ground". This is, at least in theory, due to the slight variation in density between the water column and Posidonia leaf blades causing small amounts of reflection and refraction to the sound wave before it hits the root mat and the top of sediment. Just for imagination's sake, I've included the conceptual "bare ground" model below.

  Behavior of P-waves and S-waves at a impedance change boundary. Image reproduced from Dragoset, B., A historical reflection on reflections, The Leading Edge, 2005, v. 24, p. S46-S71, Box 1.

A good singlebeam acquisition program will allow you to choose the method of bottom tracking. Often these methods are not associated with the first return but rather the "strongest" part of the first return within some specified distance from the last known bottom position. This allows the minimization of error created by, for example, fish swimming in the path of the beam (see image below), and also the weaker first returns of soft vegetation.

  Fish in the water column as seen on a singlebeam survey. A red band indicates the strongest return amplitude. An acquisition program would likely classify the bottom at or near the top of the red band. Image reproduced from the University of New Hampshire Ongoing Education website.

So, if we assume that the acquisition program's bottom tracking capability is acceptable, the errors that Posidonia's leaves introduce will be minuscule, because the leaves do not present a large change in impedance and thus will not reflect much of the beam. You're left with deciding whether or not you want to classify the "bottom" of the water column as Posidonia's root mat, which may be several inches thick in a large colony, or the top of the sediment below. This is where your uncertainty will likely lie. What I don't know, and what may be difficult to figure out without some rather clinical testing, is whether or not the root mat provides a strong-enough first return to trigger the bottom tracking. I suspect it would, but I can't say that with any confidence without looking at a controlled test survey. I imagine companies like Odom and Kongsberg do this regularly and might be able to give you a more specific answer.

Tobbe also correctly notes that lower frequencies won't be affected as much by vegetation as higher frequencies. If you were going to try to run a bathymetric survey over Posidonia—or any area with high concentrations of seafloor plant life—you would want to consider using a device capable of lower frequency surveys...at the cost of diminishing your resolution.


...so how does it get from 1000 to 10,000 feet - what lifts CO2 up?

Hmm... how does it get to the mid-troposphere and higher?
Well, first, to understand that one may find some helpful information in comments below the question. Second, it always helps to review our own assumptions.
In the question, there's an implicit assumption that CO2 does not mix with the rest of the atmosphere at the molecular level. As it turns out, it does!
Also, the primary mixing mechanism in the troposphere takes place through eddy diffusion, not molecular diffusion, and that's an important distinction. Storms and jet streams, convection, and turbulence (from storms and clouds down to the nearly molecular size) all contribute to mixing.
As one goes higher in the atmosphere, the balance between the magnitude of eddy diffusion and molecular diffusion shifts. This happens above the tropopause.


Aquifers are relatively permeable zones of material that transmit water. Common aquifer materials include layers of unconsolidated sedimentary rock, like sands and gravels; and poorly cemented “bedrock” units like sandstone. Interconnected solution cavities in limestone, called karst, are common in some areas. Calling something an “aquifer” generally infers that the unit is capable of producing enough water via a well to be of interest to humans.

Aquifers are typically delineated using geologic and geophysical logs of boreholes from wells and test holes. These kinds of logs are typically required to be provided to a specified public agency for future use by others. Using these logs, a geologist can create cross sections of an area to evaluate the extent and thickness of these units. This is done keeping in mind the geometry of how these were originally deposited, such as linear streams, sheet-like sands, etc.

Surface geophysical techniques can also be used to assess subsurface conditions, but typically require “ground truth.” I remember asking a geophysicist what his data were telling him about an area, and he said, “I won’t know until you put some drill holes in.” It wasn’t as bad/funny as it seem. He could help color in between the data points, but first he needed to know what the specific geophysical response he was seeing meant as far as real rock present in the subsurface.

It should be rare to hit an aquifer zone unknowingly, as you described. It’s possible they hit a permeable fault zone, which can be tricky to know about beforehand. But I need to mention that engineers are notorious for keeping geologists out of the loop until a problem arises. They often don’t want to pay for upfront studies, the kind that geologists use to assess such things.


Why does the shoreline sometimes recede prior to a Tsunami?

It has nothing to do with the geological cause of the tsunami. Instead, it's a result of the way waves propagate. You can see the same effect on ordinary wind-generated ocean waves — the waterline draws back before each wave peak arrives and washes up the beach. Tsunamis are much bigger waves, in terms of both amplitude and wavelength, so the effect is more dramatic.

The particles in some surface waves, including wind waves and Rayleigh waves (a component of what is often called ground roll), have in a circular or elliptical motion — in the case of a wind wave the motion is clockwise if the wave is traveling from left to right (see this animated comparison for Rayleigh waves). The 'backwards' motion in the trough results in the drawback you are asking about.


Ore reserves represent the economic portion of a mineral deposit. They are reported as tonnes and grade above a minimum value (cut-off grade).

Irrespective of whether the ore reserve will be mined by surface or underground methods the deposit is blocked out and the mass of each block and the mass of the valuable commodity (zinc, copper, gold, etc.) is calculated for each block. From this the grade of each block is also determined. The reserve is determined by summing the masses of all the blocks and determining the overall grade of the total.

In your example, a deposit of 100,000 tonnes grading 1% zinc means there is 100,000 tonnes of rock that can be mining and treated by a processing plant (mill). The amount of zinc metal contained within that 100,000 tonnes of rock is 1000 tonnes.

Similarly, a gold deposit that contains 2 Mt @ 8 g/t, contains 2 million tonnes of rock grading 8 grams per tonne, or 8 parts per million. The amount of gold contained in that 2 million tonnes is 16,000,000 g (16,000 kg or 16 t).


How did this hole / hollow form within this rock?

It looks to me that there were voids in this rock when it was formed, caused by gas bubbles These voids might have become filled with sand or other friable material. Weathering eroded the side of the hollow, enlarging it and allowing any finely divided material inside it to trickle out. You should have included something to show scale, but my impression is that your rocks are about six feet tall and not very solid, therefore easily eroded. The round stones you refer to are geodes, which are hollow and usually encrusted in the inside with quartz crystals. In some parts of England they are referred to as 'potato stones', which gives you an idea of what they look like. Some recent posts on this site give you photos of them.


A geochemical model, like phreeqc calculates the saturation index (SI) for minerals based on the composition of the solution, temperature, and possibly pressure. These are used to calculate the activities of the ions, based on the activity coefficients. These are then compared to the activities that would be present at saturation as defined by the solubility product (Ksp). The saturation index is < 0 if the solution is undersaturated with respect the mineral, = 0 at saturation, and > 0 if supersaturated.

So when you say the model shows you are saturated with respect to gypsum, it indicates that by definition you are in equilibrium with solid phase gypsum. It does not indicate if you got there by dissolving gypsum or by precipitating gypsum. It is highly unlikely (but theoretically possible) that your SI is exactly 0 without one of those things happening.


what is stopping someone from building a high-pressure, high-temperature system and dumping graphite in and making millions of diamonds?

  What is stopping someone from making these HPHT machines and making tons of money?

No one, and it is done. First of all, synthetic diamonds have been out on the market for quite some time now and it's a well established business. You've heard of diamond saws, diamond drills, diamond polishing paste and all kinds of other diamond products. So people make them, sell them, and make money.

It's not even that hard: In our own lab we got a machine that can make diamonds (among other HPHT) stuff. Someone once even put some peanut butter and made diamonds out of it (micrometre sized, though). Here's a picture of the machine (called a piston cylinder):

Here's a picture of how this might look:

Note that this one is not from our lab, but it's a really nice picture of how this stuff looks when you make it in such machines. This one's from this paper. The diamond crystals are about 30 μm across. Not something you'd put on a ring.

The big question is how do you make gemstone quality diamonds. Chemical vapour deposition seems like the most successful method. There are other methods, but it's pretty easy to distinguish them from natural diamonds (example). It is still quite hard to do it: it's not something anyone can just do in their backyard. With technology it becomes easier though. Here comes the question of regulation. Gemesis, one of the companies that makes them, engraves a serial number using laser on each synthetic gem-quality diamond they make. But what about some evil diamond makers who want to simulate natural stones? As with any other synthetic material, there are imperfections that are different from imperfections found in natural stones.  Moreover, each mine or region where diamonds come from has a different fingerprint (they use that to trace blood diamonds). There are methods to analyse the diamonds to detect this. UV and IR spectroscopy, laser ablation, isotope analysis, cathodoluminescence and others. Some are easier and cheaper than others.

So you can't just make up impurities to say "look it's natural", they have to be consistent with a specific diamond source you're trying to imitate. And that's - not very easy. Likely impossible with today's technology.


What color is the Sahara desert, as seen from orbit?

Neither. The Sahara Desert is heterogenous. It’s yellow, red, orange, brown, black, and even green in certain places and at certain times of year.

Slightly less cheeky answer: neither. Both examples are both approximations of human vision, displayed on computer screens that can be highly variable. The 2002 version of the Blue Marble is a composite of 4 months of atmospherically-corrected data in narrow wavelength bands. The composite data is scaled to approximate the nonlinear response of human vision. In particularly bright areas the image is clipped in some bands, leading to incorrect hue and loss of contrast.

For all the gory details, here’s the user’s guide for the MODIS land surface reflectance product (which has always looked slightly greenish to me): http://modis-sr.ltdri.org/guide/MOD09_UserGuide_v1.4.pdf

The Apollo 17 Blue Marble is a color-corrected scan of 40-year-old film (stored in a freezer at NASA’s Johnson Space Center) and is also not a particularly good representation of what an astronaut would see (as Pont commented earlier). The best description of the appearance of the Earth from space is from astronaut Piers Sellers, who said it’s “brilliant blue, and too bright to look at directly—like sunlight reflected off snow while skiing at high altitude”.

Brutally honest answer: neither. To paraphrase Mapbox’s Charlie Loyd, there’s an irreducible subjectivity to color. How we perceive color is affected by ambient light, adjacent colors, overall amount of light, individual physiology, our age, and probably mood and what we had to eat for breakfast. Viewed from space there are even more variables: particularly atmospheric conditions (which influence both incident and reflected light) and viewing angle.

Current state-of-the-art is probably NOAA View, http://www.nnvl.noaa.gov/view/#TRUE which uses a multi-band algorithm to simulate true color with Visible Infrared Imaging Radiometer Suite (VIIRS) data. According to that algorithm, the Sahara is more red than yellow.

For the record, I’ve worked on color-correcting MODIS imagery, the Blue Marble datasets, and astronaut photography on the NASA Earth Observatory & Visible Earth sites (including the referenced version of AS17-148-22727—unfortunately I can’t find the original TIFF.) Perhaps that means I think the Sahara is both ochre and yellow. I also wrote the guide to color-correcting Landsat imagery linked by Jezibelle (thank you).


How different is the proportion of heavy water ($\ce{^2H2O}$) in the different oceans?

For detailed information, see Isotopes of the Earth's Hydrosphere, especially chapter 3 which is "Isotopic Composition of Ocean Water".

At the surface, deuterium fraction is maximized in regions of high evaporation, and minimized in areas with ice.

In the deep ocean, there is about 2% above average deuterium in arctic, and 2% below average near Antarctica.  There is a general increasing trend from south to north.


What are the differences between down-looking and side looking radar?

A radar measures the time of arrival of the backscattered signal to infer the distance between the target and the platform (satellite, aircraft, whatever). If two targets are at the same distance from the platform, one on the left and one on the right because the platform is looking down, then their signals will arrive at the same time, making them impossible to distinguish. Looking sideways removes this ambiguity. Look at this figure from the Center for Space Research of the University of Texas (there is also a text explanation in the link).


Is there evidence to delineate where a southern extension of the East African Rift 'plate boundary' is developing?

There is considerable evidence that as the East African rift system is still developing, that it is not been possible to delineate southern rifting structures, which would extend into the Indian Ocean south of Madagascar. Looking at the evidence found in recent studies confirm this:

First to set the overall tectonic scene by Saemundsson (1), the East African Rift is a series of parallel rifts, particularly the east and west rift systems.  Rifting is propagating southwards, with rifting rates decreasing from 2.6 cm/year in the Red Sea area to less than 0.5 mm/year south of Kenya/Tanzania.

The entire rifting region is seismically active, with seismicity extending south into Mozambique, as can be seen in the earthquake map below (2):

There are no recorded earthquakes extending from the African continent to the plate boundaries to the south or east.

Peter Bird, of UCLA, published observations of the stress and strain features of southern Africa, particularly as the southern extension of the East African Rift provides a unique opportunity to observe new plate boundary formation.  Observational evidence provided by Bird et al (3) have found that there may be some very slow rotation of the developing 'Somali Plate' (East African plate) with respect to the 'Nubian plate' (rest of Africa) which is meeting resistance through an unbroken lithosphere - which may also partly explain the different spreading rate in the rift itself from north to south (1) (mentioned above).

The stress direction model developed by Bird et al. is shown below (3), what can be see is that the normal faulting from the East African Rift continues someway off the coast of Mozambique, becoming transversal (strike-slip), leading to a compressive feature (thrust faulting) near the plate boundary.

More studies would be required in the Indian Ocean off the coast of Mozambique would be required to definitively determine the extent and dynamics of this possible southern extent of the East African rift.

References

(1) Saemundsson, 2010, East African Rift: An Overview, Presented at Short Course V on Exploration for Geothermal Resources

(2) Midzi et al. Seismic Hazard Assessment in Eastern and Southern Africa

(3) Bird et al. 2006, Patterns of stress and strain rate in southern Africa, Journal of Geophysical Research