Ocean warming is induced by rising levels of greenhouse gases preventing heat radiated from the Earth's surface from escaping as freely as it once did. The majority of the excess heat in the atmosphere is returned to the ocean. Since the 1970s, the oceans have absorbed over 93 per cent of the excess heat dispersed from greenhouse gas emissions. Because the oceans are vast in size, and because water takes much more energy to heat up than air, that equates to a temperature increase of just a little over 1 degree Fahrenheit, on average over the past century. But the ocean warming has rapidly increased and as a result, the upper ocean heat content has risen significantly in recent decades. This is mainly due to fossil fuel consumption.
The topmost part of the ocean, down to about 700 meters, has absorbed the bulk of the extra heat. The bottom of the ocean is also not immune to warming; they have sucked up another third of that excess warmth.
Ocean warming leads to deoxygenation a reduction in the amount of oxygen dissolved in the ocean.
What Causes Deoxygenation?
Excess nutrient inputs from land-based sources can drive hypoxia. Large watersheds deliver vast amounts of nutrients to the oceans and other coastal ecosystems. These nutrients fuel blooms of algae that eventually die off and decay. Microbes that utilize aerobic (oxygen-dependent) respiration and break down the algae consume oxygen during that decomposition. Hypoxia occurs when the rate of oxygen consumption exceeds its replenishment through photosynthesis and mixing of the water column.
Climate change is magnifying the problem of deoxygenation as a result of the effects on the physical properties of water and on the respiration rates of microbes and animals. Oxygen is not easily dissolved in water, which holds less oxygen as it warms. Warmer water is also less dense than colder water and tends to stratify above colder water layers. Warmer waters carry less dissolved oxygen, thus temperature can affect the growth and survivorship of fish due to low oxygen availability (PÃ¶rtner and Knust 2007). Warming can also reduce oxygen solubility and increase the strength and duration of stratification between the water layers (Townhill et al. 2016). This leads to a reduction in oxygen flow from the upper to lower water columns, and can exacerbate eutrophication-driven low-oxygen (hypoxic) dead zone on coastal waters( Rabalais et all.2009).
In areas such as parts of the Arctic and Antarctic, glacial meltwater from non-saline snow and ice is less dense than seawater and contributes to surface water layers as well. The net effect of stratification is to reduce the mixing of bottom and surface waters such that bottom waters are not re-aerated sufficiently
Effect on marine life
Most ocean life, from plankton to fish to sharks to whales, live in the upper part of the ocean, right in the middle of where the temperatures are rising quicker. Many of these animals are susceptible to small changes in temperature. Corals are highly sensitive to water temperature in their surroundings. A one degree Celsius rise in sea surface temperature can stress them and cause the phenomenon called âbleachingâ.
Marine fishes, seabirds and marine mammals all face very high risks from increasing temperatures, including high levels of mortalities, loss of breeding grounds and mass movements as species search for favourable environmental conditions. Coral reefs are also affected by increasing temperatures which cause coral bleaching and increase their risk of mortality.
Physiological processes of plants and animals are highly dependent on temperature, and shifts in environmental temperatures strongly affect the performance of species (PÃ¶rtner and Farrell 2008). The nature of the performance response depends on the thermal tolerance of the individual, which is dependent on adaptation (over evolutionary time) and acclimation (over the lifespan of the individual) (Somero 2010).
Abundance of species can be strongly linked to warming through multiple ecological mechanisms. In established local populations, warming can directly facilitate enhanced reproduction, settlement or growth by providing more optimal physiological conditions (Parmesan, 2006). However excessive warming can also reduce species abundance by directly negatively affecting these physiological processes, and thus reducing survivorship and growth (e.g. Neuheimer et al. 2011). Warming can facilitate immigration and the survivorship of immigrants, by providing new opportunities to colonise a habitat that was previously too cold to operate within (Parmesan and Yohe 2003; Wernberg et al. 2016). It can also lead to loss of species from waters that become too warm, either due to death or migration of adults, or failed recruitment of individuals (Wernberg et al. 2016).
Warming can increase abundance indirectly, by improving prey abundance, or reducing the abundance of natural enemies, such as predators, prey or pathogens (Hughes 2000). However it can also lead to population declines, by the loss of important prey or symbiont species. Corals in particular are highly dependent on their algal symbionts (zooxanthellae) that provide key nutrients, but thermal stress results in the expulsion of the symbionts, resulting in coral bleaching, and gradual coral starvation (Hoegh-Guldberg 1999).
Effect on Aquaculture
The effects of warming oceans on marine aquaculture are largely unclear, but there are concerns that the temperature rises will affect the suitability of areas for growing particular species and reduce feed intake and food conversion efficiency (Gubbins 2006). Warming seas can also influence the prevalence of disease (Bell et al. 2013), by increasing parasite growth rates, promoting the geographic spread of novel pathogens (Harvell et al. 2002; Brander 2007), or by reducing the immunocompetence of the farmed species (Callaway et al. 2012).
There is evidence of thermal effects on species within farmed systems, which could potentially affect production. Behavioural research shows Atlantic salmon actively prefer to occupy a 16â18Â°C temperature zone within aquaculture cages, and they display an active avoidance of water warmer than 18Â°C (Oppedal et al. 2011). This matches evidence that the optimal temperature range for growth of Atlantic salmon in seawater is 14â18Â°C (Jobling 1981; Johansson et al. 2009), and that there are reductions in performance by 20â25 per cent when temperatures reach 16â20Â°C (Oppedal et al. 2011). It is estimated that summer sea temperatures consistently exceeding 18Â°C in Scotland by 2050 under the RCP8.5 emissions scenario, and may be avoided under the less extreme RCP2.6 scenario. Rising sea temperatures could also drive increases in hypoxia within sea cages, due to reduced dissolved oxygen in warmer water, further impairing performance (Oppedal et al. 2011).
In addition to direct evidence of temperature on marine aquaculture species, there is a need for further information on effects of temperature on parasites, pathogens and organisms that foul cages.
Key parasites of Atlantic salmon include the sea lice (Lepeophtheirus salmonis and Caligus elongatus). In 2006 they were estimated to cost the industry Â£28.6 million per year (Euro 33.6 million) in lost production and parasiticide use (Costello 2009). Their early life history is known to be temperate dependent, with shorter generation times in warmer waters, thus future sea-lice infestations are potentially greater than currently observed and treated (Costello 2006).
Warming seas may allow the introduction, establishment and spread of new pathogenic parasites. For example, in North America warmer winters have encouraged the spread of protozoan parasites that caused mass mortality in Eastern oysters (Hofmann et al. 2001). They may also facilitate the dispersal and spread of non-native species that foul cages and block water flow, such as the tunicate Styela clava (Cook et al. 2013).
There are concerns that warming seas will increase risks to aquaculture from harmful algal blooms, jellyfish poisoning and the incidence of bacterial diseases that infect shellfish, for example Vibrio species, that are also harmful to human health when ingested (Callaway et al. 2012).
Impact on humans
A 2012 report by the Food and Agriculture Organization of the United Nations estimates that marine and freshwater capture fisheries and aquaculture provide 4.3 billion people with about 15% of their animal protein. Fisheries and aquaculture are also a source of income for millions of people worldwide. By altering distributions of fish stocks and increasing the vulnerability of fish species to diseases, ocean warming is a serious risk to food security and people's livelihoods globally. Economic losses related to ocean warming are likely to run from tens to hundreds of millions of dollars.
Warming ocean temperatures are linked to the increase and spread of diseases in marine species. Humans risk direct transmission of these diseases when consuming marine species, or from infections of wounds exposed in marine environments.
This ocean warming affects primary productivity and nutrient cycles, the global distribution and survival of marine organisms, and the amount and type of fish that are caught in fisheries. Increasing temperature can result to loss of breeding grounds for fish and subsequently, higher mortality levels for some species. With rising temperature, marine life will be forced to find suitable alternative environmental conditions in order for them to survive.
What can be done?
Limit greenhouse gas emissions
This will help prevent the massive and irreversible impacts of growing temperatures on ocean ecosystems and their services.
Protect marine and coastal ecosystems
Well-managed protected areas can help conserve and protect ecologically and biologically significant marine habitats. This will regulate human activities in these habitats and prevent environmental degradation.
Restore marine and coastal ecosystems
This can include building artificial structures such as rock pools that act as surrogate habitats for organisms, or boosting the resilience of species to warmer temperatures through assisted breeding techniques.
Improve human adaptation
Governments can introduce policies to keep fisheries production within sustainable limits to prevent overfishing. Coastal setback zones which prohibit all or certain types of development along the shoreline can minimise the damage from coastal flooding and erosion. New monitoring tools can be developed to forecast and control marine disease outbreaks.
Strengthen scientific research
Governments can increase investments in scientific research to measure and monitor ocean warming and its effects. This will provide more precise data on the scale, nature and impacts of ocean warming, making it possible to design and implement adequate and appropriate mitigation and adaptation strategies.