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Very interesting piece Ray. I need to read it a few more times to fully understand it so maybe my question is a bit foolish. You say Europe has warmed 3 times as fast as the NH average and it is due to increased water vapour. Increased water vapour is the principle feedback mechanism which drives CO2 induced AGW. So are European CO2 levels increasing at 3 times the NH average or do we have an enhanced "loop gain" in Europe? Or could there be an entirely different cause for the water vapour increase? It does seem a tad unfair that having done such a splendid job in cutting our CO2 emissions we should be warming at faster than the rest of the NH many of whom are making little or no effort.
[Response: First to you and all the others reading my piece, I wish to express my appreciation for your being willing to wade through such a long and detailed article. It's a little long on technical detail, but in this case I felt it necessary in order to understand the implications of the papers. Your very apt query about European warming vs the NH average cuts to the heart of the matter. Europe can warm more than the average, and it can still be water vapor that is the aggravating factor, precisely because the water vapor feedback being discussed in the GRL paper is a different water vapor feedback than the one that provides the general amplification of the CO2 effect. The latter water vapor feedback works through its effect on the top of atmosphere budget. The one discussed by Philipona et al works through the surface budget. Quite different things. As for your comment about it being unfair that Europe gets the worst of NH climate change, it's a sad fact that physics is unfair. Global warming has global effects, unevenly distributed, and the worst polluters do not pay the worst price. That's precisely why global warming is a harder problem to tackle politically than more local things, like the old London smog. –raypierre]
Comment by David H — 21 Nov 2005 @ 5:21 AM
Thanks for this nice article. I read the Soden et al. paper and found it quite convincing. However, I was wondering whether you could comment on how this relates to a paper published last year by Minschwaner and Desler [J. Clim. 17, 1272 (2004)], who found that in the tropics the relative humidity in the upper troposphere was decreasing, and that the GCMs were overestimating the moistening. Is this just a matter of regional changes differing from global ones, or is it due to different ways of measuring the humidity, or something else?
[Response: I hope somebody else will chime in on this, but my reading of the situation is that Minschwaner and Dessler were dealing with water vapor very high up in the tropics. The OLR is sensitive to water vapor over a very deep layer, and the water vapor diagnostic used by Soden et al is more sensitive to WV in this layer than to the (less important) water vapor that M&D focused on. –raypierre ]
Comment by Brian Jackson — 21 Nov 2005 @ 7:51 AM
Like David H, I've not read this yet, I'm at work so this'll be for tonight. But I'd just like to link to the transcript of a BBC Horizon programme on Global Dimming. The document is here http://www.bbc.co.uk/sn/tvradio/programmes/horizon/dimming_trans.shtml and if you search (ctrl F, or find) using "it may already be happening. In Western" it'll get you to the salient part.
In essence the suggestion is that anti-polution efforts in the EU have reduced 'global dimming' and increased solar radiation at the ground (and throughout the troposphere). Thus increasing the regional global warming trend.
[Response: We discussed this documentary in a number of posts already:
Global Dimming?, Global Dimming II and Global dimming may have a brighter future?. – gavin]
[Response: I'm glad you picked up on the connection to global dimming, because Philipona et al essentially show that, whatever global dimming there has been, the water vapor effect and cloud greenhouse effect together overwhelm it over the period they study. They regard the primary forcing as due to the clear sky surface water vapor feedback, because the net cloud effects offset each other. However, an equally valid way to look at the situation is to say that the combined surface IR effect of clouds and water vapor more than offsets any dimming due to cloud changes and other things that reduce solar radiation –raypierre]
Comment by Chris Reed — 21 Nov 2005 @ 8:32 AM
Thanks Gavin,
I was merely making this apropos of William's post 15/5/05 "Global Dimming (GD) may have a brighter future": "The "dimming" may have lead to a slight negative radiative forcing, somewhat masking the global warming signal; the reversal, Wild et al. suggest, may have removed this masking effect and lead to the signal being more obvious in the 1990s. Aerosol emissions have decreased, particularly in Europe and the US over the 1990s, largely due to clean air legislation. " i.e. one major point of the Horizon prog (when you take away their usual apocalyptic hyperbole).
As with Soden et al's earlier paper re Pinatubo, GD seems to me to be likely to be one of the factors in Europe that is then amplified by Water Vapour (WV) Feedback, just as Pinatubo's volcanic plume material's effect (although -ve) is shown by Soden et al to be amplified, by WV Feedback. Previously I'd said I'd not read this posting – sorry, should have stated not "fully" read. It's just that upon my initial reading I couldn't figure out whether the results re Europe had a bearing the GD research of Wild et al and the anti-pollution measures in Europe.
Comment by Chris Reed — 21 Nov 2005 @ 10:16 AM
Soden et al, is only focussing on Pinatubo but fails to compare it with the other two registered major aerosol producing eruptions, El Chinon 1982 and Agung 1963 which do show up clearly in MSU4 / radio-sonde (UKMO4), but not in MSU2LT. Furthermore there are several similar dips in MSU2LT without either a volcanic trigger or a clear strong El Nino (which is arguable in case of El Chinon).
Consequently, one could say that Soden fails on the reproducibility requirement. I also keep wondering why his MSU 2LT graph start just prior Pinatubo instead of showing the whole range, to prove that there was only one dip and that was Pinotuba. Because it wasn't
Philipona et al shows a compelling case for water vapor being a strong greenhouse gas but he keeps it calling "feedback" consistently without any substantiation whatsoever. It's clear that it's more moist in that area but it's also clear that it's more arid in other areas. Why is the one (positive) feedback and the other is not?
As to cause and effect, why is there a strong increase in the last dozen years over the Alps and why not gradually? Is there any other thing that changed in the Alps in the last decennium? Let me tell you that it is currently Europe's main and foremost aviation highway junction and perhaps also the busiest one of the world. Where-ever you fly, you have to cross the Alps. In no other place you can see more than 5-10 airlines at any time of the day, each producing some kg of water vapor (greenhouse gas) per second, every second, every year. And it's my job to monitor that. So any idea why exactly there is so much water vapor GHG forcing in the higher levels above the normal weather? And would it have anything to do with CO2 feedback? One thing is for sure, it's anthropogenic, but that's all.
This post is shadow posted in UKww.
[Response: Because there's no shortage of source of water in the troposphere, the input of water vapor by commercial airlines is unlikely to be much of a factor in water vapor feedback. (It's a different story in the stratosphere, though). As I said in my post,
Philipona et al do not treat the attribution problem. There's not much controversy about low level water vapor being a feedback responding to other
climate forcings, but the paper doesn't say which forcing this feedback is amplifying.]
Comment by Andre Bijkerk — 21 Nov 2005 @ 12:04 PM
The remarks on Soden et al (2002)? is re #4, not re the recent publication.
Comment by Andre Bijkerk — 21 Nov 2005 @ 12:10 PM
Is this right? So the converse would be, no extra WV if there were not that triggering CO2 to get the positive feedback mechanism working. Or another way of putting it, just as people (not guns) kill people, so too the entire extra warming is both directly & indirectly (through the intervening variable of WV) attributable to CO2 (plus other non-WV GHGs).
Reminds me of the el ninos some 10 years ago (not sure of the exact years). Some people were attributing all the warming & weather issues to el ninos — but the el ninos themselves were increasing in frequency — I figured most likely due to GW (but no one was connecting those dots to come up with that pausible explanation). So we cause the warming; the warming increases the frequency & intensity of el ninos, which cause various weather-related happenings. Or does someone have proof that GW has not been impacting el ninos?
BTW, there is an excellent poem about GW on http://www.MarkLynas.org
Comment by Lynn Vincentnathan — 21 Nov 2005 @ 5:33 PM
***Or does someone have proof that GW has not been impacting el ninos?***
It's often impossible by nature to "prove a negative."
There are some suggested links to increasing El Nino/decreasing La Nina frequency with the 1976-1977 Pacific Decadal Oscillation shift. That could be an entirely plausible "natural" explanation for the observed change in El Nino vs La Nina balance since 1976 (according to this site, prior to the shift, we had 3 straight years of La Nina, two classified as "strong," and 4 La Nina years out of 5). And the warmer Pacific temps brought about by the PDO shift are in-line theoretically with increased El Nino frequency and magnitude along with decrease La Nina frequency and magnitude.
Or does someone have proof the 1976-1977 PDO shift has not been impacting El Ninos?
Comment by Michael Jankowski — 21 Nov 2005 @ 7:42 PM
Ray, I think that you are being far too kind in your description of the Philipona et al paper. I don't think that they appreciate the crucial distinction between the effects of changes in top of atmosphere and changes in surface fluxes. The misleading media attention is in part a consequence of a poorly thought out argument. I doubt that their basic conclusion is correct, that the enhanced downward infrared flux has something to do with the warming in this region. I suspect that you could take a climate model, fix the lower tropospheric water vapor being input into the radiation code over Europe only, and hardly affect temperatures locally. I might be wrong, but I do not see anything in the paper to convince me otherwise.
[Response: Isaac, you know me. I'm just an old bunny rabbit. I also had the impression that Philipona et al are confused about T.O.A. vs. surface water vapor feedback, but although their paper is unclear on this and their comments to reporters are even more unclear, I didn't feel comfortable saying what I thought they were really thinking. You have a point though. The numerical experiment you describe would be interesting to carry out. Also, though their conclusion that the low level WV increase has caused the European warming may be somewhat dubious, I feel pretty confident, based on some simple radiative-convective models I ran, that an effect like this could give you an extra degree or so of warming in cases when the boundary layer is not already optically thick in the infrared. –raypierre]
Comment by Isaac Held — 21 Nov 2005 @ 10:28 PM
#9, I don't think the cited WV effect is unique to Europe, the Arctic has had a huge increase in temperature likely rivaling or exceeding Western Europe's trend. Other regions in the world have similar or higher temperature increases with respect to GT.
Comment by wayne davidson — 21 Nov 2005 @ 11:17 PM
Pierre, could you comment on what, exactly, is new in the recent Philipona paper, compared with the two similar papers they published last year ("Greenhouse forcing outweighs decreasing solar radiation driving rapid temperature rise over land", "Radiative forcing – measured at Earth's surface – corroborate the increasing greenhouse effect")? Both of these papers suggested a strong water vapour feedback.
From what I gather from the AGU press release, the idea is that water vapour feedback explains the difference in temperature trends in north-eastern and south-western Europe (Above all, their measurements demonstrate strong water vapor feedback that rapidly warms Central and Northeastern Europe, where sufficient water is available from plants and the surface, known as evapotranspiration. However, in disucssions over at UKWW, it's been suggested that the difference can be explained by trends in the NAO, without needing to invoke differences in moisture availability for evapo-transpiration
Comment by Tom Rees — 22 Nov 2005 @ 5:25 AM
In your figure1 you suggest that the top of atmosphere cooling results from unbalance ("increased CO2, out of equilibrium ,236W/m2″).
Is the stratospheric cooling also the consequence of this unbalance?
[Response: Not really. The stratospheric cooling is partly a consequence of the increased in situ infrared emission from increased CO2, and also due to ozone changes.]
Comment by Pascal — 22 Nov 2005 @ 5:36 AM
Sorry, it was not figure 1 but figure 2.
Comment by Pascal — 22 Nov 2005 @ 5:38 AM
Ray, now I have read your essay again there are a couple more questions. Am I right to assume that the near surface WV feedback is ignorant of the source of the heating? That is that it will also amplify the effects of any change in solar input, urbanisation or land use change.
[Response: That's right. There could be some minor differences in the response, depending on the nature of the forcing, but in a broad brush picture what you say is true. –raypierre]
The second of your figures reminded me that we are talking about a dynamic system and I wondered what time constants we are looking at. I found James Hansen's editorial essay at http://www.columbia.edu/~jeh1/hansen_slippery.pdf interesting in this respect. He cites ocean time constants as 50-100 years. He estimates ice sheet time constants as hundreds of years for their disintegration because it is a wet process but millennia for their growth which is a dry process. I am a bit surprised that they do not appear to be known with any precision. Do you not need them to build a realistic climate model?
[Response: Depends on the time scale of the process you are looking at. Certainly, you can't treat the melting of Greenland without a dynamic ice-sheet model that has the right time-constants built in, through the physics of ice.–raypierre]
Hansen also makes the point that we already have 0.5 deg C warming in the pipe line. This is about what we saw in the last 60 years. Does this imply that if world had been generally cooling for very long time that the change in forcing that caused it to start warming must have occurred well before we detected any change in surface temperature?
[Response: You have to be careful here: though we speak of "thermal inertia," there is no inertia in the climate system that is like the inertia of Newtonian mechanics. A system that starts cooling does not have a tendency to continue cooling. What thermal inertia does is slow the approach to equilibrium.–raypierre]
Comment by David H — 22 Nov 2005 @ 10:31 AM
I read with interest your excellent article on recent papers dealing with water vapour.
I am certainly not convinced by the final statement in Rolf Philipona's article: "radiation measurements in central Europe and ERA-40 reanalysis data for all Europe demonstrate greenhouse warming and manifest a two to three times larger positive water vapor feedback than predicted by GCM calculations in zones where sufficient surface water is available for evapo-transpiration." As evidence for this, the authors cite one of their other GRL articles by Philipona et al. (2004) which in turn cites 2xCO2 experiments performed by Wild et al. using a coupled GCM which is free-running and unlikely to reproduce regional variability relating to changes in the large-scale atmospheric circulation.
Aside from the valid arguments about top of atmosphere verses surface radiation, where is the evidence that GCMs do not reproduce the relationship between low-level water vapour and temperature? Perhaps I missed this? Studying their Figure 4 shows integrated water vapour to increase approximately at the rate of 7 percent per oC increase in temperature. This is about the rate expected from the Clausius Clapeyron equations if relative humidity is constant (e.g. Wentz and Schabel 2000, Nature 403, p.414; Trenberth et al. 2005, Climate Dynamics, 24, p.741) and, I would expect, is likely to be reproduced by GCMs that are forced with the observed changes in temperature, even over continental Europe.
[Response: I'm really glad you brought up this point. I had noticed the statement regarding GCM's underestimating the (surface) water vapor feedback, but hadn't tracked back the justification through the earlier papers. The statement in the 2005 paper could easily being misread as saying that the GCM's underestimate the increase of water vapor with temperature, and that this is the root cause of the under-prediction of temperature increase. The experiments cited do not support this, however. As you note, the Wild et al GCM experiments only tell us that their particular simulation underestimates the European temperature increase. This simulation was not designed with an accurate reproduction of the decadal regional changes in mind. In particular, the "cold start" of the run at 1985 may affect the earlier parts of the simulation. It's not entirely clear to me how to design a clean simulation to test the relative role of low level water vapor vs. circulation changes in accounting for the mismatch, but there is certainly no indication in Wild et al that there is any problem in the GCM handling of low level water vapor. If the simulations underestimate the regional temperature increase for reasons independent of the low level water vapor feedback (e.g. large scale circulation patterns misrepresented in the simulation), then the low level water vapor and the surface fluxes will disagree with observations for reasons that have nothing to do with the way the models are handling the water vapor. In brief, I completely agree that there is nothing in the Philipona group oeuvre to support the contention that models underestimate the degree to which low level water vapor increases with temperature. That said, Figure 2 of the 2005 paper does suggest that the low level water vapor increases somewhat more rapidly than one would expect on the basis of constant relative humidity, at least in some places and months. For example, the March Alpine data shows a 40% increase in column water vapor for a 2.9 degree warming. Clausius-Clapeyron suggests a change on the order of only 20%. Other months and regions show less of a mismatch with fixed relative humidity. Philipona et al essentially have nothing to say about whether this kind of behavior is captured by GCMs. The essential problem is to disentangle the water vapor errors that are due to errors in the modeled regional temperature change, from water vapor errors due to problems in the water vapor dynamics itself. What one needs to answer is the question of whether the model would have gotten the water vapor right if it had gotten the termperature right. Perhaps there is some way to do an experiment like this using a "nudging" protocol, where observed data is used to force the model to track the real temperature, while nevertheless allowing the model to compute its own moisture field. –raypierre]
Nonetheless, the work of Atsumu Ohmura, Martin Wild and Rolf Philipona has provided the community with a well calibrated surface radiation budget dataset and their efforts should be commended and their careful analysis of the data and conclusions are extremely welcome.
Comment by Richard Allan — 22 Nov 2005 @ 12:26 PM
Raypierre, thanks for the technical explanation. It will be a pleasure to read a second time. Here's something tying your post to one on "balance". Too bad someone with better guesses wasn't asked.
Quoting the article: Other scientists say humans will be able to adapt, arguing that IPCC models may be wrong. They point, for instance, to a lingering dispute about whether temperatures are rising more slowly higher in the atmosphere than at the surface.
"I don't see the catastrophic effects from warming that others predict," said John Christy, a professor at the University of Alabama in Huntsville who says satellite data since 1979 shows temperatures rising fastest at the surface.
"All I can offer is guesses," he said of the temperature discrepancy. "Perhaps as the surface warms the atmosphere has a capacity to release warmth to space in a way the climate models don't take into account."
[Response: Hmmm. That is only a popular article, so maybe one shouldn't be too hard, but if that is all Christy said, he is being disingenuous. He knows full well that other records show more warming, and that another guess is that his dataset is simply wrong. Indeed, thats the mostly likely – William]
[Response: Indeed, the way this article quotes Christy is "balance" journalism at its worst. In this case, Christy doesn't even pretend to offer any scientific support for his gut feeling that all manner of things will be well and we shouldn't worry. By the way, I found the article's discussion of the effect of winter warming on Sami reindeer herders very interesting. I spend a lot of time in Northern Sweden and Finland myself, and have picked this up anecdotally from Sami. Also, my family an I like to go to Akaslompolo, somewhat north of the Arctic Circle in Finland, for cross country skiing in the spring. Usually it's around -3C in the daytime but last time I went it was +15 in week 16 and everything was melting like crazy. Locals say the past few years were the same. It's just anecdotal, but as I was scraping the klister off my skis it planted the idea in my mind to figure out a way to see if the European arctic warming trend could be attributed to sea ice changes and other anthropogenic effects. The general problems in reproducing regional warming, noted in Philipona et al, would be an impediment, though. –raypierre]
Comment by Steve Latham — 22 Nov 2005 @ 1:11 PM
Ray, in reference to your last comment I was thinking more in terms of electrical analogues with the sun as the input signal summed with various positive and negative feed backs to drive an amplifier (in reality of course many amplifiers, non linear and chaotic function blocks). If as Hansen suggests we can have a half degree in the pipe line which looks like oceans time constant, I am asking whether, to first approximation, we are looking at a 50 -100 year delay in seeing the consequence of any change in GHG forcing in the surface trend.
[Response: In principle, the delay effect is incorporated in transient ocean-atmosphere climate runs. These runs aren't perfect, but if the main problem in European regional temperature increase were due to just the delay, the models would probably be doing OK. Regional trends are notoriously problematic for models, and seems more likely to me that the underprediction of European warming has to do with either the modeled ocean temperature pattern, the modelled atmospheric response to this pattern, or some problem related to the local hydrological cycle and boundary layer moisture dynamics. Philipona et al clearly favor the last of these, but it's far from an open-and-shut case. –raypierre]
Comment by David H — 22 Nov 2005 @ 1:38 PM
Re #16 (SL): Sounds as if Christy is now pushing Lindzen's discredited "iris effect." To start doing that now, however vaguely, seems kind of strange, as Lindzen himself appears to have given up trying to defend the idea in the journals.
[Response: Lindzen has given up trying to defend it in the journals, but he still trots around the idea at meetings. He was still pushing the same idea at the Blois climate conference a year and a half ago, and the word I got from Stefan is that he trotted out the Iris idea at a public debate on global warming held recently at Yale. –raypierre ]
Comment by Steve Bloom — 22 Nov 2005 @ 2:45 PM
Response on respomse #5
Response: Because there's no shortage of source of water in the troposphere, the input of water vapor by commercial airlines is unlikely to be much of a factor in water vapor feedback. (It's a different story in the stratosphere, though).
The usual cruising altitudes are about 33-40,000 feet, tropopause, not exactly the lower troposphere. Here, in virtually absolute humitidy free area, they create a thick additional layer of water vapor. Exactly the area where small changes have large effects in the semi-logaritmic relationship between concentration and forcing.
Furthermore Philipona et al wonder about the accelleration in the last decade and the local effect on mid Europe and start talking about regional variation. However both the timing and spatial variation is consistent with the increase in aviation.
Comment by Andre Bijkerk — 22 Nov 2005 @ 3:41 PM
Thanks for your reply, and thanks very much for the link to your paper "On the relative humidity.." (I've been after something like that for ages).
The point I was (clumsily) trying to make in my posts 3 & 4 was that I didn't understand why the processes Philipona outlines would be specific to Europe. I've not got access to the original paper. But when I read comments like Isaac Held's, It strngthens my concern that the paper you refer to may not provide a credible explanation for variation in warming rates across a region as large and geographically variable as Europe. I have read your essay several times and still don't draw an explanation of regional variation specific to Europe from it. But as I've only been learning about climate science for less than a year, I'm hardly in a position to start an argument!
Andre, I've just seen your comments re Soden et al 2002. (Science vol 296). Sorry but I don't want to get into a re-run of our discussion on that, this is not the forum. In your post 19 you may have a point, I'd like to see some peer reviewed research on it though.
Chris (aka Cobblyworlds at BBC Science DG)
[Response: There isn't anything in Philipona et al that says that the surface water vapor feedback they note should be specific to Europe. One could think of reasons it could be, having to do with wind or soil moisture effects, but they do not make a case for it. The case that surface water vapor feedback is in some way the root cause of European warming is weak to say the least. What I found interesting in the paper was the analysis showing how much water vapor increases contribute to the increase in downward infrared flux. This is an effect that can amplify temperature trends caused by any mechanism. It's not a new effect, but it is nice to see it come out so clearly in the observations. It would have been nicer if Philipona et al had done a better job of distinguishing surface water vapor feedback from the top-of-atmosphere feedback. –raypierre]
Comment by Chris Reed — 23 Nov 2005 @ 8:09 AM
In this neck of the woods (Minnesota and Wisconsin), meteorologists like to refer to dewpoints – on those hot and humid days in July and August.
Record high dewpoints were measured last August at a few stations in southern Minnesota, with dewpoints measured in the low-mid 80s F).
Links to work I've done on trends in dewpoint averages, temperatures and the timing of spring snowmelt runoff over the last 50-100 years are listed below.
Special Report: Air Temperatures & Dew Points Great Lakes States (2003)
Earlier in the Year Snowmelt Runoff and Increasing Dewpoints for Rivers in Minnesota, Wisconsin and North Dakota (2003)
Annual Air Temperature, Minneapolis weather station (1820-2004)
Annual dewpoint averages: Minneapolis 1918-2002, (r.h. used 1918-1944)
Comment by Pat Neuman — 23 Nov 2005 @ 6:54 PM
Thanks for this impressive explanation! It is quite clear that there is a lot of confusion of what causes the extra water feedback in Europe.
Here follows a few remarks I have on water vapour and radiation budgets.
First, as the Swiss researchers said, about one fourth of a W/m2 is directly attributable to the increase in CO2 (CH4 plays no role here, as there is no increase in recent years) but the Modtran radiation calculation only gives some 0.063 W/m2 for the increase in CO2 in the period of interest. I suppose that the 0.25 W/m2 mentioned includes the CO2 induced water vapour feedback? Even then, the influence of CO2 changes is only some 6% of the total radiation increase… Thus it is important to know what the source of the extra heat/water vapour is. The NAO seems to be the first cause, as there is a direct correlation between NAO index and temperatures and precipitation in Europe. The positive NAO index of the last decades induced higher temperatures, less near the ocean, more towards inland regions for most of Europe. For the Mediterranean, this is reversed in winter: the precipitation is much lower and the temperatures are lower too (see the graphs at UKww discussion)
As the influence of more water inflow to East Europe probably is higher in a relative dry region, compared to Western Europe, this may trigger higher temperatures and thus more evaporation… The inflow of cold(er), dryer air in the Mediterranean winter season with a positive NAO results in lower precipitation and a decreasing temperature (see Fig. 13&14 of Marriotti ea.). Again the influence of the NAO is clear…
Next, current models seems to get the global water vapour feedback right, according to the research by Soden e.a, but there are very large regional to hemispheric differences. This makes an equal global treatment of the water vapour feedback rather questionable. If you look at the (sub)tropics, despite the increase in sea surface temperature and CO2, the upper troposphere humidity reduced in the period 1985-1994 (see Chen ea.). The large change in TOA radiation (~3 W/m2 extra loss to space) in the full (sub)tropics (30N-30S) is almost entirely from a reduction in (low) cloud cover.
If there is less humidity in the warmer tropics, and a global increase in humidity, due to higher temperatures, then the extra-tropics must be much wetter (which is anyway the case for Europe)â?¦ In the more moderate climate zone, it seems that changes in cloud cover have cancelling effects for the overall radiation budget and temperature changes are largely influenced by water vapour. In the Arctic, again the difference in cloud cover in summer (increasing) and winter (decreasing) both temper the warming one can expect from the decrease in albedo and inflow of warmer/wetter air from lower altitudes. I havenâ??s seen figures for the (relative) humidity of that region, but as that probably is directly correlated to cloud cover, that means that even the change of humidity in different seasons is an important point that is not covered by current climate models. The change in cloud cover in the Arctic winter is large enough to (near) cancelling out the extra heat input and change in albedo of the other seasons…
[Response: I was also wondering how the Swiss got .25 W/m**2 out of Modtran. When I ran it myself for the increase in downward IR due to a 12ppm increase of CO2, I got about .09 W/m**2 for midlatitude summer conditions. I don't think their figure is supposed to include the water vapor effect, since they're accounting for that separately in their budget. It's a puzzle. I also tried using the NCAR radiation model and also get an expected increase of only about a tenth of a W/m**2. I didn't mention this since there are a lot of ways the number could change if one did a proper calculation based on the observed humidity and temperature profiles, but still it seems like a long way to go to get the calculation up to a quarter of a Watt per square meter. –raypierre]
Comment by Ferdinand Engelbeen — 25 Nov 2005 @ 12:56 PM
[Response: Rolf, it's nice to meet you. I'm glad you're reading this. Welcome to RealClimate. You're among friends here — we're all just trying to understand how climate works as best we can. Pull up a chair… (raypierre here, and in the following)]
Answer from Rolf Philipona to Raypierre,
Changing greenhouse effect:
I do not agree with your explanation of measuring "changes" of the greenhouse effect looking from space or from the surface. I agree with you that if we measure the total flux we get different results because as you mention the bulk of the signal measured from the satellite is from a colder region than the signal measured from the ground, hence you show correctly that the flux measured from above is smaller than that from the surface. This is because part of the radiation is fully absorbed by greenhouse gases, or in other words some spectral regions are saturated, the atmosphere is opaque for these wavelengths.
But then your statement that "increasing" greenhouse gases would affect the two radiations in different ways, I do not agree. Let's assume that we are adding ozone in the stratosphere. If an instrument looks from below it sees the additional ozone because ozone absorbs and emits at around 10 microns in the atmospheric window. In the window the atmosphere is not opaque and the instrument sees through the entire atmosphere into space, and hence, it receives the signal from the additional ozone molecules. If an instrument looks from a satellite it sees all the way down to the surface and receives the same signal from the added ozone as the instrument from the surface. The only difference is that it gets a decrease while the one from the surface an increase. The value is the same. Also it is not important where the additional ozone is in the stratosphere or in the troposphere, it can be observed from space or the surface at any location.
Now if we add water vapor to the atmosphere it increases the greenhouse effect in the spectral regions that are not saturated not opaque, which means in the atmospheric window. It contributes to the water vapor continum in the window and this is similar to the ozone, producing a flux change that can be seen from above as from the surface. And this is the same thing for other greenhouse gases too. The warming depends on how much we additionally close the window with more greenhouse gases and this must be observable from both sides.
[Response: But what I have said just stems from radiation physics of the most basic sort. Since (as you agree) radiation emitting to space comes from a different altitude and passes through a different medium from radiation emitted to the surface, it stands to reason that it will be affected by addition of greenhouse gases in different ways. This can be verified an any spectrally resolved radiation model. To use the Paris heat-wave sounding as an example, if I keep everything else the same and increase CO2 by 12ppm, then (using the NCAR radiation model) the OLR goes down by .17 W/m**2, while the surface radiation goes up by .037 W/m**2. One goes down, the other goes up, and the amount one goes down is different from the amount the other goes up. I'd call that "greenhouse gas increases affecting the two radiations in different ways." CO2 is well- mixed, but for a gas like water vapor which has strong vertical gradients, there is the additional effect that changing upper level water vapor affects the emissivity at a level closer to that from which radiation is emitted to space. For example, using my Paris sounding again, if I increase the relative humidity above the boundary layer from 50% to 60%, keeping CO2 fixed, then OLR goes down by 3.8 W/m**2 while the downward IR into the surface goes up by 1.96 W/m**2. Your example would only work if the entire atmosphere were transparent in the unsaturated bands. ]
What is warming what?
You say: "Remember that the whole troposphere warms more or less as a unit. That means that the air near the ground must warm also with the rest. In this way, we see that the warming of the entire troposphere can mostly be inferred just by thinking about the top of atmosphere budget, without bringing the surface budget into the picture in any detail."
From papers in the IPCC report we learn that the lower troposphere is warming and the upper troposphere is cooling.
[Response: No, not at all. It is the stratosphere that is cooling, not the troposphere. The troposphere warms with a vertical profile that is given approximately by the constraint that things stay on the moist adiabat. See Figure 12.8 of the IPCC Third Assessment Report. The stratosphere can be somewhat decoupled from the rest of the atmosphere precisely because convection doesn't reach into the stratosphere. However, the stratosphere accounts for a relatively small portion of the mass of the atmosphere,hence a relatively small portion of the greenhouse effect. The effects of stratospheric cooling are not completely insignificant with regard to the energy budget, but they are a decidedly secondary effect. ]
Also, the surface absorbs about 50% of the solar radiation. Further, you mentioned yourself that a large part of the energy available at the surface is brought into the atmosphere by sensible and latend heat fluxes. So how can you claim that all depends on the top of the atmosphere radiation budget without bringing the surface radiation budget into the picture?
[Response:Because the surface energy budget must close; that means all the energy absorbed at the surface makes its way into the atmosphere. For many purposes one doesn't need to know exactly which term is doing the transfer. Note that I didn't say that the surface budget was irrelevant to everything. The surface budget is important in determining the temperature difference between the free troposphere and the solid surface, and in some cases changes in this temperature jump can add to or take away from the general tropospheric warming. To determine how much this happens, though, it is necessary to look in detail at how the evaporation and sensible heat transfers respond. Where there's a sufficient moisture supply, it's hard for the surface to air temperature jump to get large enough to make much difference. ]
Let us look at the measurements shown in our paper. In figure 4 we show that over the Iberian peninsula temperature decreased while water vapor decreased also. In central Europe however we see a strong increase of temperature over the same time period and a strong water vapor increase. We show the total integrated water vapor increase in the atmosphere but we can show that this is very well related to the increasing moisture at the 2 meter level over ground. In figure 3b we show that in central Europe the annual mean shortwave net radiation (that what gets absorbed) decreased by 1.1 Watt m-2 over the period. Hence the increased temperature is not due to solar radiation. But if we add to the shortwave net radiation the longwave downward radiation then we see in figure 3c that we have now good correlation between total incoming radiation and the temperature. Figure 3 shows always the changes for the different months (trend over the monthly means from 1995 to 2002). Hence, even though this is trivial we prove experimentally that the surface temperature is driven by the radiation fluxes at the surface.
[Response: Not exactly. To determine the change in surface temperature,you need to complete the argument by saying how evaporative and sensible heat transfers respond. Note that I didn't say myself that the increased downward IR flux COULDN"T lead to the temperature increase. It's just that the argument is incomplete. Also, there's nothing in the description you've given here that's incompatible with my description of your paper as dealing with a different water vapor feedback than the top-of-atmosphere feedback discussed in,e.g. Held and Soden.]
Now since shortwave decreased and longwave increased it looks like the warming is due to increased longwave radiation. The longwave radiation from the cloud-free atmosphere can only increase due to increasing surface temperature or due to increasing greenhouse gases, who additionally close the window.
Increased longwave radiation due to increased surface temperature:
The calculations which you present do not match our calculations shown in the paper because you did not take into account the apparent sky emittance. You only calculated the longwave radiation emitted from the surface with a respective temperature change. But the atmosphere absorbs and emits only part of the upward radiation. The apparent sky emittance is about 0.7 . If you use this you will find for the 2.7 Wm-2 the temperature difference of 0.8°C as we show in figure 3a).
[Response: I don't understand which of my arguments you are referring to here. In Figure 1, I use the NCAR radiation model, and don't make any assumption about the apparent sky emittance. It is computed from the model. In Figure 2, I do assume a unit emittance, but that was just to make the point that greenhouse warming does not rely on greehouse gas increases being able to directly increase the downward IR. In cases when the lower atmosphere is transparent enough to allow the greenhouse gas increase to affect downward IR, it can give a bit of additional warming, but the amount depends on evaporation and sensible heat fluxes as well. ]
What else than anthropogenic greenhouse gases and water vapor is increasing the temperature?
After we had made the correction on longwave downward radiation for the surface temperature increase we are left with an increase on the annual means of 1.18 Wm-2 as shown in figure 3e. For individual months this can be up to 5 Wm-2. We show that this shows good correlation with the moisture change at the surface. A radiative transfer model and a sensitivity study helped us to subtract the part which is due to the absolute humidity increase. After that we are left with a longwave radiation increase of 0.35 Wm-2 and this is close to what is expected from anthropogenic greenhouse gases.
[Response: I agree that the most likely root cause of the European warming is anthropogenic greenhouse gas increases, but I don't see anything in your paper that actually supports the attribution. Process of elimination is not a very convincing argument. The only way I see to do the attribution is to have a model that reproduces the European warming, then re-run it without CO2 increases to see how things change. Has anybody actually done this?]
From measurements we know that aerosols rather decreased over Europe over the last ten years and this should have increased the shortwave radiation, but our measurements show a decrease. Hence, increasing water vapor in the atmosphere apparently absorbed more solar radiation and overcompensated the aerosol effect.
What else than greenhouse gases is left as a forcing if solar-, aerosol- and cloud- are not the culpit?
[Response: Well, something like a circulation changed forced by the NAO pattern (which may in turn be affected by greenhouse gases) might cause an increase in European air temperatures, which in turn would allow low level moisture to increase if there is enough moisture supply,which would then constitute an amplification of a signal driven remotely. Again, without looking at the response of evaporation and sensible heat transfer, one can't determine whether this amplification shows up as additional surface warming, or additional evaporation.]
Radiative transfer models:
We have been comparing radiative transfer models to surface longwave radiations in many occasions. As an experimentalist I have learned to appreciate these models, who very well agree with longwave radiation instruments that are traced to absolute standards. These models allow us to demonstrate what I explained above.
[Response: Is the way you do the radiative calculation explained in more detail in some other publication? As I mentioned in my reply to Ferdinand above, there isn't enough detail in your GRL paper to allow me to reproduce the result.]
Answer from Rolf Philipona to Isaac Held
We are not asking Raypierre nor you nor anybody else to be kind with us. But everybody should be fair and first thoroughly study our work before critisising it. Whether our arguments are poorly thought out, future will show. Well, for the time being I wait and see whether you can take a climate model and fix it as you suspect, such that it provides better arguments to explain why temperature decreases in the southwest and increases in central and northeastern Europe, than what we have shown with measurements in our paper.
Answer from Rolf Philipona to Tom Rees
In the first paper the investigation is based only on measurements in the central Alps. This did not allow us to show where the additional water vapor came from. We thought that it might be related to a positive NAO index.
In the second paper we show the temperature in central Europe and compare it to the temperature increase in the northern hemisphere. We show what solar radiation is decreasing since 1980 and that only with the very sunny summer 2003 the slope gets slightly positive. We show the radiation budget for the period 1995-2002 and 1995-2003 in order to show the impact of the hot summer 2003.
In the last paper we extend our analysis over all Europe. Figure 1 and figure 4 shows with data from CRU and ERA-40 how the temperature increases respectively decreases in Europe over the last two decades. Figure 2 shows how temperature and water vapor evolves from 1995-2002 for the individual month. With this picture we show that the water vapor and temperature increase, which show a strong gradient from southwest to northeast can not be due to the NAO but is most likely due to water vapor feedback.
Answer from Rolf Philipona to Ferdinant Engelbeen
Let me quote what is in our paper. "Stand-alone MODTRAN radiative transfer model calculations show a +0.26 Wm-2 annual mean longwave downward forcing for the 12 ppm CO2 and other greenhouse gas increases in Europe from 1995 to 2002, apart from water vapor." We did not say that we have 0.26 Wm-2 just from the 12 ppm C02 increase! There are other greenhouse gases that did increase also from 1995-2002 and these are included, but again without the water vapor. The water vapor increase alone, which we measured makes 0.83 Wm-2 longwave radiation increase as shown in the paper. This is backed up by a sensitivity study between longwave radiation measurements, radiosonde profiles and MODTRAN.
I do not know what you did with your MODTRAN in order to get only 0.063 for 12 ppm. Since I do not have MODTRAN ad hand right now I did another simple calculation.
From 1750 to now CO2 increased from 280ppm to 375ppm. IPCC calculates for that increase a forcing of 1.4 Wm-2. Proportional to that numbers we get for 12ppm increase a forcing of 0.18 Wm-2. For all the greenhouse gases IPCC gave a forcing of 2.4. Again with the numbers above we get 0.3 Wm-2 forcing from 1995-2002. Hence our 0.26 is not far off.
[Response: You can do Modtran calculations using Dave Archer's web version of the model. Just follow the link in Ferdinand's comment. When I did the calculation using the standard Modtran midlatitude summer profile for clear sky conditions, I find that increasing CO2 by 12ppm only increases the downward IR flux by about .09 W/m**2. (In the model, set the detector to be at the ground, looking upward). I also tried using the NCAR radiation model, with various European soundings. For the warm Paris sounding, the optical thickness of the boundary layer cuts the change in surface IR to well below .09 W/m**2. If I use a colder sounding, I still only get a surface effect of .06 W/m**2. Low clouds would reduce the CO2 effect even more. Maybe methane and other GHG changes could bump the number up somewhat, but I would need to understand the details of what you did in order to understand how you get the number all the way up to .26. By the way, the IPCC radiative forcing numbers you are quoting are for top-of-atmosphere radiative changes, not surface radiative forcing. These are not the same thing, and I am wondering if the Modtran results you mention also used top-of-atmosphere results by mistake. ]
If you can show a direct correlation between the NAO index and temperature in central Europe for the years 1995 to 2002, I would be very interested. We did not find this correlation. In the early 1990 the NAO index was high and the temperature in Europe increased but since the late 1990 the index dropped but the temperature increased even more.
With figure 2 in our paper we show with the monthly changes that there are two different phenomenas which influence temperature in Europe. One phenomena influences temperature and water vapor similarly all over Europe. We have strong evidence that this is due to changes of large scale weather pattern (circulations). But then on top of that we observe a gradient of temperature and humidity changes from southwest towards northeast. There is no way to explain both phenomenas with large scale circulations. Hence we assume that the gradient is because water vapor increases due to water vapor feedback in regions where water is available for additional evapotranspiration.
[Response: Again, to complete this argument, you need to turn the radiative forcing into a surface temperature change using all of the surface fluxes, not just the radiative ones. Further, if the root cause of the European warming is low level water vapor feedback, then why don't GCM's reproduce it? They have all the physics necessary to moisten the boundary layer. It's possible that there is some problem with the parameterizatio of evapotranspiration, but it would have to be a pretty big problem to miss the effect by so much. As I said in my response to Ferdinand, the paper you cite in support of the claim that GCM's underestimate the feedback in fact only says that the particular GCM in question underestimates the European warming, for one reason or another.
Certainly, your results will be valuable to modellers seeking to check whether GCM's are doing low level water vapor correctly. I do hope we can agree on two things though:
(1) The water vapor feedback you are talking about is a completely different thing from the top-of-atmosphere water vapor feedback discussed by Held and Soden, and most other users of that term,
(2) The observed increase in downward infrared due to low level water vapor feedback means it is possible that this effect plays a role in the enhanced European warming, but the attribution of the warming to this cause would require additional analyses that have not yet been done. (–raypierre)]
Comment by Rolf Philipona — 26 Nov 2005 @ 3:13 PM
Years with warmer Pacific ocean waters (El Ninos) are correlated with higher annual dewpoints, in the Midwest.
Comment by Pat Neuman — 26 Nov 2005 @ 6:54 PM
Dear Ray,
Andre Bijkerk wrote (22 Nov 2005 @ 3:41 pm ) :
19. Response on respomse #5
Response to: 'Because there's no shortage of source of water in the troposphere, the input of water vapor by commercial airlines is unlikely to be much of a factor in water vapor feedback. (It's a different story in the stratosphere, though). '
The usual cruising altitudes are about 33-40,000 feet, tropopause, not exactly the lower troposphere. Here, in virtually absolute humitidy free area, they create a thick additional layer of water vapor. Exactly the area where small changes have large effects in the semi-logaritmic relationship between concentration and forcing.
Furthermore Philipona et al wonder about the accelleration in the last decade and the local effect on mid Europe and start talking about regional variation. However both the timing and spatial variation is consistent with the increase in aviation."
Unquote (Comment was made by Andre Bijkerk â?? 22 Nov 2005 @ 3:41 pm )
May I suggest for you all to have a look at my website http://www.contrails.nl for a selection of pictures of this 'thick additional layer of water vapor'. These pictures were taken over the past ten years, mainly in The Netherlands.
I am just a concerned citizen and almost a complete ignoramus on this subject. Still, I cannot imagine that this extra layer of man-made cirrus in the tropoapsue should have no consequences at all on weather, climate and the quality and quantity of the Sun's radiation that reaches the surface of Earth.
Comment by Robert C. VanWaning — 27 Nov 2005 @ 10:00 AM
Re #23,
Rolf and Ray,
This discussion is very interesting and may lead to a better understanding of what influences local/regional/global climate.
About the NAO and European temperature: average European temperature has a good correlation with the NAO, with some exceptions, over the whole period 1900-2000, see the graph at app. one third of the page in the UKww discussion. After 2000, temperature remains high, while the NAO is decreasing. The increase in CO2 for 2000-2002 is too low to explain the higher temperatures than expected. Maybe there is/was some latent heat (less permafrost?) in the colder regions after the long period of high NAO/temperature? Or other oscillations (AO, AMO) which weigh in?
About the period of interest: 1995 is a rather unfortunate start of the measurements, as that coincides with a very strong negative spike in NAO and European temperatures. The strong positive NAO started in 1990 and induced a considerable jump in temperatures. But the 1995+ period allows for a detailed investigation of a changing climate. Of more interest than the full 1995-2002 period is a split of 1995-2000 and the later period, as the first period shows a strong increase in NAO and T, while from 2000 on, the weakening NAO is not followed by a strong temperature decrease.
Of interest is to see the change of water vapour in absolute figures, not only in %, as the base moisture content and temperature near the oceans (West Europe) is higher than more inland (East Europe). This is important as for the same (or even smaller) absolute change, the influence in % and radiation/temperature is larger at low basic levels.
About radiation absorptions/emissions: it would be interesting to know more accurate what the real source is of increasing/decreasing budgets. Several individual wavelengths are measured in Switzerland. Maybe a few can be used (or are used or should be added) on unsaturated absorption/emission bands directly related to the main greenhouse gases… Or alternatively some accurate full spectrum analyses of the different wavelengths…
Comment by Ferdinand Engelbeen — 27 Nov 2005 @ 5:42 PM
Essenhigh's letter in which he proves (to his own satisfaction, at least) that H2O accounts for 90+% of Earth's greenhouse effect is back on the web:
Can anyone tell me what mistakes, if any, he's making?
[Response: Mainly he is not doing a proper intergral across the whole spectrum. When you do you get numbers as seen in the table here. – gavin]
[Response: The calculation Essenhigh describes is done in any standard radiative transfer model, and done very accurately based on well validated spectroscopic data. As the Chem and Eng. News respondent said in his comment on Essenhigh's calculation, and as is pretty much confirmed in Gavin's water vapor post, the 95% figure for water vapor vs. CO2 is bogus. The real figure is more like 2/3 water to 1/3 CO2, and even that gives a misleading impression of the extent to which CO2 controls climate, given that the water vapor acts as a feedback and is more or less controlled by CO2. As a complement to Gavin's post, you can also take a look at my water vapor article from the Caltech general circulation volume. It has a graph showing the relative importance of CO2 and water vapor in determining the Earth's radiation budget. You can find the paper here . From the brief description of the work, I can't tell exactly how Essenhigh did his calculation, but in addition to the error Gavin noted, I believe that Essenhigh did not solve the radiative transfer equation properly, if at all. In essence, he seems to be looking at the amount of radiation from the surface which is absorbed by the whole column of the atmosphere. That is a different thing from computing the effect of water vapor and CO2 on the amount of radiation that escapes to space, since that radiation has contributions from a wide range of different altitudes. –raypierre]
Comment by Barton Paul Levenson — 28 Nov 2005 @ 2:49 PM
Note that Essenhigh's anti-CO2 op-ed appeared in the rather obscure and transient journal "Chemical Innovations." From the web site of this publication, I found the following:
"Written by practitioners, Chemical Innovation is an authoritative, readable magazine that brings you practical insights (and maybe a smile or two). Feature articles and regular departments explore a wide range of chemical-related technologies, commercial development ideas, environmental concerns, research and development strategies, and legal issues."
"Chemical Innovation ceased publication after the December 2001 issue."
If they published nonsense like Essenhigh's piece in the guise of science, its no wonder they folded. Or, notice the comment about providing "a smile or two." Maybe Essenhigh's op-ed was supposed to be a joke.
Comment by R. T. Pierrehumbert (raypierre) — 1 Dec 2005 @ 1:36 AM
Here is simple calculation of the relative effect of water vapor, treating water vapor as a well mixed gas. Assuming water vapor is 10,000 ppm, there is 32 time more of it than carbon dioxide, or five doublings. Water vapor is 1.5 times as effective as carbon dioxide at the same concentration (from this Pierrehumbert paper), so water vapor has 7.5 times the effect as carbon dioxide, or about 88% of the greenhouse effect, compared to the standard 65%.
Of course, water vapor is not well mixed. I think the difference must be in the vertical distribution of water vapor. As its concentration depends on temperature, it may be less concentrated higher up where greenhouse gases have the most effect. Does this make sense? Are there any figures for relative concentration of water vapor by altitiude?
[Response: I'm pleased you read my paper. Either I was unclear, or you misinterpreted, what I said about the radiative effects of water vapor vs. CO2. Regarding the sensitivity, what I said (or at least meant) was that if you take the present water vapor concentration in "typical" midlatitude sounding, and doubled it or halved it at each level, you change the OLR by about 6W/m**2 as opposed to 4W/m**2.You can't use that figure to extrapolate back to zero water vapor and thus get the total effect of the present amount of water vapor in the atmosphere. That result is given, based on direct radiative calculations, in my Figure 1. I re-did that calculation myself, but the numbers are not particularly novel. They're essentially the same as what Gavin reported in his RC piece, based on a somewhat different calculation with a somewhat different radiation code. Let's not mince words, though — there is no question that water vapor is extremely important to the radiation budget. As I showed in my paper, if you take out half the water in the atmosphere, you precipitate an ice age. If you take it all out, you through Earth into a globally frozen "snowball earth" state. –raypierre]
Comment by Blair Dowden — 1 Dec 2005 @ 8:26 AM
I think that cloud cover cannot be ignored. Water vapor condenses into clouds. Clouds will generally cause cooling during the day. Inceased water vapor = increased clouds which moderates the earth's temperature.
[Response: Why don't you think for yourself instead of just parrotting whatever sound-bite you've picked up from the Cato Institute or its equivalent. The answers to your questions have been amply dealt with here, and in any number of elementary and popularized books on climate. I just can hardly begin to count how many ways your are wrong. Nobody is ignoring clouds or cloud cover, not me, not climate models, not Philipona's paper. Clouds don't invariably cause cooling during the day because clouds can have a strong greenhouse effect — and you've got no cause to ignore the night-time half of the equation. Increased water vapor does not lead to increased clouds, since increased temperature can dissipate clouds. Clouds are not simply related either to water vapor content or to temperature. Hence the idea of a simple cloud thermostat like you are describing is just snake oil. Ferdinand E. has a bit of a skeptical streak, I think, but I enjoy responding to his posts because he makes an effort to educate himself about what is already known, and as a result comes up with interesting arguments. Remarks like yours are just meaningless noise. –raypierre]
Comment by Gord Checknita — 2 Dec 2005 @ 6:52 AM
Raypierre, I am still seeking the "missing" carbon dioxide greenhouse effect, the difference between my simplistic 12% and the actual 35%. If the logarithmic relationship does not hold at low levels, it seems carbon dioxide must have relatively more effect than water vapor when levels are low. Is this due to the wider absorption band of water vapor?
[Response: Loosely speaking, its due to the different concentration values at which water vapor and co2 absorption bands saturate. Yes, the band width and distribution comes into this, but so does the fact that wv has a big vertical gradient whereas CO2 doesn't. –raypierre]
The other source of the difference may be the upper troposphere, or Free Tropospheric Humidity. Humidity depends on temperature, which falls with altitude. Would it be correct to think that water vapor declines relative to carbon dioxide as altitude increases, thus reducing its effect where it is most important? Your paper demonstrates that there is room for variations in water vapor in the upper troposphere, and these variations would have significant effect. I could not find a profile of humidity vs. altitude. Does this account for a difference of a factor of three times compared to my simple calculation?
[Response: Actually, my thumbnail number of 6 W/m**2 per doubling for wv already incorporated the vertical structure of water vapor, since I got that by holding the vertical structure fixed at a "typical" profile, and just scaling it down by equal factors at each altitude. Since you used my number, the effects due to vertical distribution are already factored in. Water vapor goes down approximately exponentially with height, but there can be a lot of structure. Take a look at the subtropical soundings in my earlier water vapor review, from the Chapman book on millennial variablility. –raypierre]
When I examine your Figure 1, I notice that the average outgoing longwave radiation seems to exceed the 342 W/m2 incoming radiation. Also striking is the huge difference in outgoing radiation between the north and south polar regions. Is this because of the high albedo of the Antarctic ice cap? The other obvious observation is that water vapor has more relative effect in the tropics, where it is moister.
[Response:You're forgetting the cosine weighting of area vs latitude, or maybe looking at the wrong curve. For the OLR computed with observed water vapor,the net emission is 270 W/m**2. Allowing for an albedo of .3, the absorbed solar radiation is 240 W/m**2, not 342. The computed emission is about 30 W/m**2 greater than this because the OLR I show in the figure is the clear-sky value. Clouds bring down the OLR by the required amount. Note that when we say the albedo is .3, that also includes cloud effects. It turns out that clouds have a net cooling effect on the present climate, though that doesn't mean that changes in clouds will have a net cooling effect when you double CO2. As for North Pole vs. South Pole, that's mainly because Antarctica gets colder than the NP, which is because Antarctica is a continent and has a whacking great tall ice sheet on it. It's not primarily an albedo effect. All that, by the way, would apply to the annual mean. However, note that the figure in my paper is for January, when the OLR is lower at the NP because it's winter there, hence colder. Nice comments all 'round. I'm pleased you have gotten so much out of my paper.–raypierre]
Comment by Blair Dowden — 2 Dec 2005 @ 9:02 AM
For the past few years I've been trying to teach myself how to write radiative-convective models of planetary atmospheres. I can reproduce the surface temperatures of Earth and Mars very closely, but I always come out with too low a figure for Venus, even when assuming the atmosphere layers are blackbodies in the thermal IR. There's a lot I don't understand about this stuff, and despite having such useful sources as Houghton's "Introduction to Atmosphere Physics" and Goody and Yung's book, I seem to be having a great deal of trouble assimilating the concepts. I was about to send a paper to Icarus about how there had to be another source of Venus's high temperature, but, thank God, I caught myself in time — I must be doing something incredibly stupid that a first-year student could correct me on. Would someone be willing to look at my model, and/or my abortive article, and tell me what I'm doing wrong? I'm a middle-aged computer programmer with no prospect of being able to afford going back to school.
[Response: Fortunately there's a lot of educational material available on the web for free. My guess is that you're trying to use some kind of a one-layer slab atmosphere model, which doesn't begin to work for an atmosphere as thick as that of Venus. Actually, I'd say such models give a bad picture of the greenhouse effect for just about any atmosphere. A first shot at the explanation of Venus can be gotten using a grey gas atmosphere, as Carl Sagan did. There are much more sophisticated treatments now, but the greygas tells the main story. You can take a look at the discussion of Venus in the old version of my lecture notes here , or even better check back there in a few weeks after I've posted the first radiation chapter from my new climate book.–raypierre]
Comment by Barton Paul Levenson — 6 Dec 2005 @ 3:34 PM
The link doesn't work.
[Fixed – gavin]
I'm actually using a 20-layer atmosphere (and 1 layer of ground, 1 meter water for Earth and 1 meter granite for Venus and Mars) with absorptivity set by a (very crude) band model for H2O, CO2, O3 and clouds. Reflectivity is simply assigned, constrained by the planetary bolometric Bond albedo and the known level of illumination at the surface. I put clouds in layer 1 for Venus, layer 20 for Mars, and for Earth I used the cloud scheme of Kiehl and Trenberth (1997). The fact that I'm not dealing with scattering explicitly is probably a big source of error.
[Response: Sorry, I was assuming you made the most common "mistake" (though I prefer to call it a learning experience!). You're tackling this at a more sophisticated level than I thought. If you replace your band model with a grey-gas model with the same nunber of vertical levels, you'll find that you can get Venus plenty hot, if you assume that convection sets the temperature profile to the CO2 dry adiabat. I am pretty sure that the problem with the band model you are using is that it doesn't include enough bands to represent the optical thickness of a 90 bar atmosphere. When you have an atmosphere that dense, all sorts of dinky CO2 bands we don't usually think of come out of the woodwork. If you do a Google Scholar search on "David Crisp" you'll find out more about CO2 spectroscopy in the Venus regime than you'd ever want to know. Work by Bullock and Grinspoon is a good reference, but I'm not sure their papers give enough details to reproduce their spectroscopy model. My understanding of the situation is that you can't get all the way to Venus temperature with CO2 alone, but you can get close. The rest involves clouds and a few minor constituents, and I think Grinspoon is the latest word on that. I should emphasize that the grey-gas model is not a good quantitative model for Venus, but it's arguably closer to the real thing than a few-band model. –raypierre ]
Comment by Barton Paul Levenson — 7 Dec 2005 @ 8:19 AM
If you go to page 22, section 3.4 of Ray's notes, you will find that he also has problems with Venus, and in fact with the slab model, the basis of all RC models. The answer to this conundrum is that there is not just one error in the way the slab model is used, there are in fact two. In the case of Mars and Earth, the errors tend to cancel each other out, but for Venus this is not true.
[Response: When I say the "slab model" I'm referring to the single layer slab model. That's the only one that has trouble with Venus. The continuously stratified grey gas model works fine, if you include the effects of convection. I'm not sure what you mean when you say the "slab model" is the basis of all radiative-convective models. If you just mean that all RC models subdivide the atmosphere into slabs, then your remark about the "error" due to slab models is incorrect. –raypierre]>
The first error is to calculate the radiation re-emitted by greenhouse gases based on Planck's function, which uses the temperature of the layer. The natural linewidth of a spontaneous emission from greenhouse gases "is an intrinsic property of the transition and cannot be changed by modifying the conditions." P.W. Atkins.
[Response: This isn't true, except for an isolated molecule in a vacuum. The line width depends on local pressure (collisional broadening) and local temperature (doppler broadening). –raypierre]>
Here is a couple of spectra from Mars and you can see that the emission from CO2 at 667 cm^-1 is independent of the surface temperature, and is fixed at a Planckian temperature of about 220K. http://mars.spherix.com/spie2/Reprint62.htm Spectra of the Earth also show CO2 with a Planckian temperature of 220K.
[Response: It's not enough to look at emission at the center of the band. It's important to look at the effect of the wings of the band as well. –raypierre]>
The second error is that the models use bands, where the lines, which escape to space, are averaged with the lines, which are totally absorbed near the surface. This makes it seem as though the atmosphere is warmed at all altitudes, whereas in fact all the warming is in the boundary layer. The greenhouse gases are transparent to the lines of radiation in the greenhouse bands, which reach the higher altitudes.
[Response: Also wrong in part. The bands used in models represent statistical average effects of many, many lines. They aren't perfect models, but they've been extensively checked against line-by-line calculations and there's no major problem. As for the vertical distribution of the warming, remember that in the troposphere convection and other dynamic heat transport really do communicate the warming to all altitudes. It is absolutely incorrect to say it's all at the surface. It is true that the asymmetry between what comes out the top and what goes into the boundary layer is the basis of the flaw of the single-layer slab model. –raypierre]>
Aerosols, including clouds, do radiate blackbody radiation and the Asian Brown Cloud explains why the free troposphere is warming in the Northern Hemisphere. However radiosondes show little warming of the troposphere in the SH, where there is less industrial activity and large deserts like the Sahara to provide aerosols.
Cheers, Alastair.
[Response: In summary, I would argue that it's not that the slab model is used incorrectly, but that it's fatally flawed — for reasons related to, but not precisely the same as, those you note. The only case in which the slab model makes sense is for an optically thin atmosphere, and even then it has to be interpreted properly. A way of re-stating what you're saying is that, in some loose average sense, Earth and Mars act more like the optically thin case, even though they aren't actually optically thin near the peak of the main CO2 (and H2O) absorption bands. Because the slab model is very misleading, I've eliminated it from the text of my book (remember to check back in a few weeks for the new radiation chapter!) and relegated it to the problems section. It's still important for people to understand it, since it's so widespread, but it's not a good way to understand the greenhouse effect. Note also that Barton's problem turns out not to be the use of the slab model (see my previous comment) –raypierre]
Comment by Alastair McDonald — 7 Dec 2005 @ 5:43 PM
Thanks very much for the info, I greatly appreciate it. I have been using the Planck function (an approximation I wrote for it, since the integral can't be integrated directly) for the radiation from a layer of atmosphere. The expression was Ai * sigma * T^4, where Ai is the absorptivity (I assume emissivity = absorptivity, Kirchhoff's Law under LTE), sigma is of course the Stefan-Boltzmann constant and T is absolute temperature. Is there some other function I can use? Where can I find the relevant equations? I know people have written successfuly RCMs of Venus — Sagan in 1961, Pollack in 1980, Bullock in 1997 to name just three — so there must be some way to do it.
Again, thanks for your help.
Comment by Barton Paul Levenson — 8 Dec 2005 @ 10:18 AM
Ray, thanks very much for the further comments, you clarified things quite a bit for me. The mention of the adiabat makes me wonder if my problem is with use of the ideal gas equation. My atmospheres all come out very shallow, e.g. 28 km for Earth. Partly this is unavoidable because I've quantized the atmosphere, so it has a "top." But maybe I'm doing something else wrong as well. If my Venus atmosphere was *taller,* the lapse rate from ground to radiating level would allow for a higher temp. at the surface.
[Response: Yes, the Venusian atmosphere is a headache on many fronts. At 10 bars or more, the deviations of CO2 from the ideal gas law are very significant. The thermodynamic regime on Venus is accessible to experiment, though, and Venus modellers use various empirical equations of state that are reasonably well validated. Doing the spectroscopy right is another matter, and there continue to be developments around the edges of the subject. Venus atmospheric studies should get a real boost soon, with the ESA Venus Express mission. Because of the expense of doing the Venus radiative transfer calculation, there hasn't been much modelling of the Venus general circulation. That's about to change, owing to the diligent efforts of my friends at LMD/Jussieu in Paris. –raypierre]
Comment by Barton Paul Levenson — 8 Dec 2005 @ 1:21 PM
Kirchhoff's law doesn't apply to gases, because the greenhouse molecules can absorb more energy that they emit. Eventually they lose that excess energy in collisions, where it is thermalised and so the atmosphere just heats up. On Mars the atmosphere gets so hot, global dust storms result. The air then heats the dust which loses the heat to space through the blackbody radiation from the dust grains. Note that the air is hotter than the dust because it has been heated by the radiation in the CO2 band, acting in a similar way to a microwave oven, or the pumping of a laser.
On Venus, the sulphur dioxide clouds serve the same purpose as the dust storms on Mars, radiating outgoing blackbody radiation to space (at a height where the carbon dioxide is no longer optically thick.) Heat is transferred to the clouds, partly by IR radiation from the surface and partly by convection of the hot atmosphere.
The greenhouse effect on the surface of Venus is not only from carbon dioxide. It is also caused by the blackbody radiation from the underneath of the clouds. But I will now try to write my own model of the Venusian atmosphere. BTW, Venus may not be stable. The surface may be slowly heating up until the a 'resurfacing' happens. Then a vast quantity of SO2 will be released, adding to the cloud and cooling the planet down.
Of course, this conflicts with the way Ray (and everyone else) sees these things.
Comment by Alastair McDonald — 8 Dec 2005 @ 8:11 PM
If I can return to my much simpler atmospheric model on Earth, I note two facts: In a vertical section of atmosphere a doubling of water vapor has 1.5 times the effect of a doubling of carbon dioxide. And most of the greenhouse effect is at high altitudes where water vapor levels are much lower than at the surface. This suggests a little water vapor goes a long way. My question is at the same altitude and concentration, how does the greenhouse sensitivity of carbon dioxide compare to water vapor?
Comment by Blair Dowden — 8 Dec 2005 @ 11:41 PM
On H2O versus CO2, the constraint I've been using is that H2O is about 10/3 as effective an absorber, on average (it varies with wavelength or frequency) based on Houghton's (2002) tables. I define optical thickness (dimensionless) with an absorption coefficient of so many square meters per kilogram, times the "specific mass" of the absorber in so many kilograms per square meter (the usual treatment is absorber density times path length, which you can see works out to the same units when multiplied together). From
tau = k SM
where k is absorption coefficient and SM specific mass, you can then do
T = exp(-tau)
where T is transmissivity (which must be between 0 and 1, thus the negative sign on the optical thickness).
One big, big problem for me has been finding absorption coefficients on the web. Except for one British source which gave figures of 130 m2/kg for low and middle clouds and 65 m2/kg for high clouds everywhere from 4 microns up, I haven't been able to find figures anywhere… and I've been looking for years. What I should really do is get my butt to the library some day and look up the journal articles people cite. Articles in peer-reviewed journals still beat any source on the web (except, of course, when the articles are available on the web!).
Comment by Barton Paul Levenson — 9 Dec 2005 @ 11:15 AM
This thread continues to be interesting, but I will soon be moving on to other things. Please forgive me if I make responses only sporadically, if at all.
One thing I will note, globally: RealClimate has a pretty open posting policy, and most comments are vetted only for appropriate subject matter, not for correctness. Thus, readers shouldn't assume that something posted is true just because a RC blogger hasn't bothered to correct it. In the present thread, and across RealClimate in general, this remark applies to many or most of Alastair MacDonald's posts. His view of radiative transfer and the greenhouse effect has enough technospeak in it to sound authoritative, but in many other regards borders on science fiction. It is not a productive use of time to correct him in every instance.
Kirchoff's law in particular is well established both from statistical phyics and experiments, for the thermodynamic regime of most interest to planetary climates. Alastair's remark that it doesn't apply to gases is just nonsense. That said, I would like to point out that Alastair's discomfort with Kirchoff's law is not entirely off the mark. The usual equilibrium argument for Kirchoff's law applied to individual wavelengths involves a lot of hidden assumptions and these were thought about by such greats as David Hilbert. It does involve second-law questions that are difficult to express precisely, and it is easy to come up with exotic examples (fluourescence, e.g.) in which the law is invalid, These are all far from thermodynamic equilibrium, though. The essence of Kirchoff's law comes from an assumption that if electromagnetic radiation is used to bring matter into equilibrium, then the Second Law still works even if we consider radiative exchange one wavelength at a time. This is extremely well established for the reasonably dense portions of planetary atmospheres.
Comment by R. T. Pierrehumbert (raypierre) — 9 Dec 2005 @ 2:23 PM
Thank you for posting. I appreciate your effort to clarify these things.
I understand that you are almost certainly far too busy to look at my program, but could you point to someone who would be willing to, perhaps a grad student? I'm still stuck as to why I'm getting wrong results for Venus. Thanks for any help.
Comment by Barton Paul Levenson — 9 Dec 2005 @ 3:09 PM
Re #41, I am willing to have a look at your work. In fact I am keen to see if I can adapt it to give the correct results. That would show that my ideas could be right :-) My e-mail adress is firstname.lastname@example.com . I will acknowledge receipt and give you my honest opinion, but that may not be what you want!
Cheers, Alastair.
Comment by Alastair McDonald — 9 Dec 2005 @ 7:25 PM
Okay, thanks very much. I'll attach the source code to an e-mail. It's in the Turbo Pascal 5.5 language, which is considered obsolete, but is blindingly fast on a modern PC.
Comment by Barton Paul Levenson — 10 Dec 2005 @ 8:16 AM
Oops — I forgot the two input data files, one for the band scheme and one for assorted gas data. Coming right up.
Comment by Barton Paul Levenson — 10 Dec 2005 @ 8:20 AM
Perhaps I should have explained what I meant when I said Kirchhoff's Law did not apply to gases. As you will be aware there are, in fact, two sets of laws named after Kirchhoff: his electrical laws and his thermodynamic laws. Confusion arises because the second of his electrical laws; that the sum of the currents flowing in and out of a node is zero, gets confused with the first of his laws of thermodynamics; that a hot solid, liquid or gas under high pressure (i.e. blackbodies) gives off a continuous spectrum; See http://www.pa.msu.edu/courses/2000spring/PHY232/lectures/kirchhoff/kirchhoff.html and http://astrosun2.astro.cornell.edu/academics/courses//astro201/kirchhoff.htm
His second thermodynamic law, that a hot gas under low pressure produces a bright-line or emission line spectrum, applies to all gases in terrestrial conditions. Thus what I should have written was that Kirchhoff's first thermodynamic law does not apply to terrestrial conditions, but that was not what I meant!
When the second electrical law is combined with the first thermodynamic law, you get what is generally known as Kirchhoff's law; that radiation emitted equals radiation absorbed. See: http://scienceworld.wolfram.com/physics/KirchhoffsLaw.html However, note that is only true in equilibrium conditions. In the case of a blackbody, if it absorbs more radiation then it will heat up and emit more radiation thus automatically reaching equilibrium conditions. In the case of a (low pressure) gas, its line emission does not change as the absorption increases. Kirchhoff was the first to discover that. Therefore, for line radiation it is not true that absorbed equals emitted. In fact, what is a line but a case of absorption being unequal to emission. Greenhouse gases emit and absorb line radiation and so do not obey Kirchhoff's Law! That is what I meant.
So where does that leave Schwarzschild's equation? What Chandrasekhar described as the Schuster-Schwarzschild method, which is the basis of all climate modelling? Well all three of them were astrophysicists, and stars are made of high-pressure gases, hence Kirchhoff's law applies in their field of interest. But here on the surface of the Earth, not only do the gases not emit blackbody radiation, the other condition for Kirchhoff's Law, equilibrium, does not apply either. Every day the sun rises heating the surface of the Earth, which emits constantly changing blackbody radiation. Equilibrium is never reached, with the air being warmed during the day and being cooled by radiating greenhouse emission back to the surface at night.
At first sight, it might seem that Kirchhoff's Law must apply to line radiation because one assumes that the system must eventually reach equilibrium, but that is not true if it is a dynamical system.
Of course, I should construct a model and write a paper, but what editor would pass on a paper for review that began by stating that Kirchhoff's Law was wrong? Moreover, how long would it take for me to produce the model? I suspect that the predictions that it could produce would have happened long before the model was completed.
Comment by Alastair McDonald — 10 Dec 2005 @ 10:18 AM
Answer from Rolf Philipona to Raypierre,
(I would appreciate if you would respond only at the end of my comments).
In your response to my comment #23 you are asking whether we can agree on the two following things:
(1) The water vapor feedback you are talking about is a completely different thing from the top-of-atmosphere water vapor feedback discussed by Held and Soden, and most other users of that term.
(2) The observed increase in downward infrared due to low level water vapor feedback means it is possible that this effect plays a role in the enhanced European warming, but the attribution of the warming to this cause would require additional analyses that have not yet been done.
To point (2) I agree that this requires additional analysis:
Let us look again at your second figure of your initial overview on measuring an increasing greenhouse effect from space or from the ground. You show that in an unperturbed situation (left graph) the outgoing longwave radiation (OLR) is emitted from the troposphere at a temperature of 255K with an irradiance of 240Wm-2. This irradiance is equal to the solar radiation absorbed by the planet. You also show that from low altitude at temperature of 280K longwave downward radiation (LDR) is emitted to the ground with an irradiance of 349Wm-2.
Then you show the out of equilibrium situation with increased CO2 (middle graph). With more CO2 OLR is now emitted from a higher colder region of 254K and hence irradiates 4Wm-2 less or 236Wm-2. From the lower troposphere you still show LDR being emitted from the same level and by the same amount. However, in your response to my answer #23 you agree that with increased CO2 LDR emittance increases even though the increase may not be as large as the decrease of OLR. I hope we agree, that with increasing greenhouse gases LDR is from a lower level with higher temperature and is further increased because the atmospheric window is additionally closed. Your graph should therefore show LDR from a lower level and 1 or 2Wm-2 increased and therefore show about 351Wm-2.
Then you show the equilibrium restored situation (right graph). Here the temperature of the troposphere increased and with it OLR by 4 Wm-2 and is now again equal to the unperturbed situation. Since the temperature increased also at the lower troposphere, LDR increased by about 3Wm-2 and now shows a 5Wm-2 higher value or 354Wm-2 than during the initial unperturbed situation.
What can be observed from space and what from the ground?
If we assume now that the temperature of the entire troposphere increased more or less equally (as shown in IPCC Fig 12.8) then we must conclude that at the top of the atmosphere no change of OLR between 1995 to 2002 could have been observed. The idea of putting 2 times CO2 in the atmosphere is an interesting Gedankenexperiment. However, in reality CO2 increases by about 2 ppm per year and the observations show that the atmosphere with its rather low heat capacity responds to that quite rapidly. With the 90 ppm CO2 increase since 1750 atmospheric temperature already considerably increased. Of course the oceans and the earth itself have a large heat capacity and will therefore respond slowly and with a delay. However, it is the atmosphere with increased greenhouse gases which makes the additional insulation and this is what effects the changing radiative fluxes that we are talking about. The observed fact that temperatures increases slower over the oceans than over land demonstrates that the large heat capacity of the ocean tries to hold back the warming of the air over the ocean and produces a delay at the surface but nevertheless the atmosphere responds quit rapidly to increasing greenhouse gases.
With all this we must conclude that as long as the Earthâ??s albedo (shortwave) does not change, the OLR hardly shows any changes at the top of the atmosphere even if the greenhouse gases increase (the shift of the emission to higher levels is always compensated by increasing temperature in the troposphere). From outside planet Earth is therefore more or less in equilibrium because the atmosphere has responded to the changes. I say more or less because the slow uptake of heat in the ocean and the Earth mantle produces a long delay and therefore a very small decrease of OLR.
But, can this be measured from space?
If we observe from the ground LDR increases if greenhouse gases increase since LDR is now emitted from lower warmer levels. Over land atmospheric temperature increases faster since the heat uptake over land is lower than over the oceans and with increasing temperature LDR increases even more. This is what we have measured and reported in our paper. Of course in our case not only CO2 increased but also other greenhouse gases and the water vapor, which made the LDR rises large enough that they can be observed even in the short time period.
To point (1): different effect of water vapor feedback at higher versus lower troposphere
Water vapor in the atmosphere increases with increasing temperature (if there is water available on the ground) and is therefore a feedback and with regard to the atmospheric greenhouse an additional insulator. If the temperature increases similarly throughout the entire troposphere then water vapor will increase at any altitude and will increase temperature at the surface. In the saturated spectral part increasing water vapor will shift OLR emission upward in the upper troposphere and LDR emission downward in the lower troposphere. For spectral parts in the atmospheric window additional longwave radiation will be emitted from the water vapor continuum, and this is observable from space and from the ground.
In our experiment we observed that the relative humidity more or less stayed constant. We also show that specific humidity as well as the integrated water vapor increased with temperature such that it more or less follows the Clausius-Clapeyron relation.
Under such circumstances it is not clear why increasing water vapor should be more important in the upper versus the lower troposphere. Water vapor and other greenhouse gases have an insulating effect that must be measurable on both sides. However, as previously shown, if measured from space it is masked by effects of the increasing temperature whereas if measured from the ground it is not masked. I have studied again the paper of Held and Soden (2000) to understand their arguments about different water vapors but I am not convinced. I would however refer to their final remarks where they state that their test of models are limited to observations of natural climate variability. Our paper shows increases of radiative fluxes measured at the surface while greenhouse gases did increase and water vapor strongly increased, most probably not due to natural climate variability.
[Response: Dear Rolf– Since the comment form is closed and there may not be many readers with us any more, I'll just try to make a few final points by way of clarification and rounding out the discussion, without usurping the "last word." Naturally, I look forward to continuing this discussion in other venues.
With regard to the discussion of my Figure 2, please note that this was put in just to make the point that surface temperature can increase even if the lower atmosphere is so optically thick that increasing CO2 has no direct effect on the downward IR at the surface. I'm not saying that the midlatitudes conform to this idealization, nor am I saying that the sequence of large imbalance followed by large adjustment is a realistic description of what is seen in the atmosphere, which has a more gradual CO2 increase. It was just a convenient way to explain how the greenhouse effect works. One can also explain it via a change of radiating level for a system that stays near equilibrium (that's how I do it in my climate book) but in my experience most people find that explanation a bit harder to grasp. My reason for emphasizing a distinction between the role of the top-of-atmosphere vs. surface effects is that somebody who does a study like yours elsewhere in the world (e.g. in a cloudy part of the Tropics) could well find that the direct effect of greenhouse gases on downward radiation (i.e. the effect apart from temperature change) was insignificant. If they didn't keep the top-of-atmosphere effects in mind, they and their readers would conclude erroneously that the greenhouse effect wasn't operating, and something else was causing any observed temeprature change. I hope there is nothing in my article that would seem to devalue your work, which I think is interesting, even though my interpretation of what is significant about it is different from what the press seems to have picked up on it.
On the matter of the radiative effects of upper level vs. lower level water vapor, I only want to emphasize that the effects discussed by Held and Soden and by many others do not derive from the parts of models that are debatable, but from radiation physics such as Kirchoff's Law and from band-models of radiation — all of which are well confirmed by laboratory experiments, observations, and comparison with detailed line-by-line radiative calculations. The essence of the argument is simply that, in a part of the spectrum where water vapor is a good absorber (hence, by Kirchoff, a good emitter) when one puts some water vapor at a high,cold level one blocks the infrared coming from below, and replaces it with infrared emitted at a lower temperature by the high-cold layer. Putting more water vapor near the ground, where the air temperature is nearly the same as the ground temperature, does not do this because you are replacing one radiating surface with another with nearly the same temperature. In fact, if the air is warmer than the underlying surface, as often happens, putting more water vapor in the boundary layer will actually increase the OLR, until adjustment occurs to bring the system into balance. The other thing to keep in mind is that the radiative effects of water vapor are approximately logarithmic in concentration, like most greenhouse gases. That is why a doubling or halving of the relatively small amounts of water in the mid to upper troposphere is still radiatively significant.
What your paper made me think of, though, is that the midlatitudes are midway between the optically thin and optically thick limit. That means that the radiating temperature to space and the radiating temperature to the ground, are different, but not all that different. The heat wave example I chose in Fig. 1 somewhat exaggerates the difference as compared to cooler, drier profiles. That all means that the increase in infrared opacity due to low level water vapor causes an additional surface forcing that needs to be thought about. The degree of warming this surface IR forcing causes depends on the response of the other terms in the surface energy budget.
I want to conclude by thanking you for taking the time to contribute your comments to RealClimate. They add greatly to the educational value of our site. –raypierre]
Comment by Rolf Philipona — 19 Dec 2005 @ 5:49 AM
Sorry, the comment form is closed at this time.
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