Anthropogenic ocean acidification
While everybody has heard about climate change by now, it is less well known that fossil fuel emissions have more consequences than increasing atmospheric temperatures, rising sea-levels and changes in climatic regimes. Another direct, but often overlooked, effect is the acidification of the world’s oceans. The atmosphere and ocean constantly exchange gasses at the water surface and are in a natural equilibrium. With the increase of CO2 in the atmosphere, the amount of CO2 that dissolves into the surface water increases as well. Because of this, a large part of the CO2 emitted into the atmosphere actually ends up in the ocean. Currently about one-third of the anthropogenic CO2 has been absorbed by the oceans and in the long run (millennia) about 90% will be absorbed by the ocean. It is well known by chemists that CO2 is a weak acid as it changes the chemistry of the surface water in such a way that the amount of H+ ions increases and the pH (a measure for the acidity) decreases. CO2 emissions in the atmosphere thus make the ocean surface water more acidic (or more correctly: less alkaline, as the pH of the water is still above 7).
When realizing that many organisms in the oceans secrete calcite (or aragonite) shells and that calcite dissolves in acids, it becomes clear that the acidification of oceanic water might pose problems for these organisms. Now the acidification will not go as far that calcite shells will actually dissolve in the surface water any time soon (though aragonite undersaturation may already occur in the Southern Ocean by 2100), but marine calcifyers may find it increasingly difficult to secrete their shells in more acidic waters. Laboratory experiments show that calcification rates of many marine calcifyers, like certain corals, foraminifera, coccolithophores and shellfish, reduce under more acidic (high CO2) conditions. Geological studies show that over the last glacial-interglacial transition, during which there was an increase of atmospheric CO2 from ~180 to ~280 ppm, shell weights of foraminifera shells decreased significantly (by about one-third).
The rise in atmospheric CO2 since the industrial revolution is of a similar magnitude as the change in CO2 over glacial-interglacial transitions. Hence, we would expect that anthropogenic ocean acidification has already affected marine calcification over the last century. This is however difficult to detect and only very recently (2009) two studies have shown that modern foraminifera shells have indeed lower weights and thinner shells compared to older ones. How this affects the organisms itself (will they survive?) or the global carbon cycle (will it moderate or exaggerate atmospheric CO2 concentrations?) is not well known. Furthermore, there could be possible impacts on physiological processes such as photosynthesis, growth and reproduction, but there is also little known about effects on these processes either.
There is, however, plenty of reason to be worried nonetheless. Studies on natural CO2-enriched places and the geological past have shown, however, that in low pH conditions biodiversity is highly reduced, regime shifts of ecosystems take place and there seems to be a link between elevated CO2 concentrations in the air and the extinction of marine species. Besides that, some species are already under a lot of pressure because of global warming and pollution, like coral reefs, and many of the potentially affected species are at the base of the marine food chain (in the plankton) and any changes there would have significant knock-on effects. Furthermore, while the exact impacts of ocean acidification are unclear at the moment, it is relatively straightforward to predict the severity of future ocean acidification itself as the chemical process is well known. Currently, the average pH of the world’s oceans is about 8.1, which is already about 0.1 units lower compared to pre-industrial times and corresponds to an increase in acidity of 30%. Model calculations show that by the end of the 21st century pH levels will have dropped another 0.4 units under a business as usual scenario and possibly even 0.7 units lower than pre-industrial values by the year 2300. Due to the logarithmic nature of the pH scale this corresponds to three (0.5 units) and five (0.7 units) fold increase of H+ ions compared to the pre-industrial situation. These changes are probably larger than any pH change in the past 300 million years because of the extreme speed at which it takes place. The ability of species to adapt to their changing environment might therefore be limited.
EPOCA (EU FP7 program on acidification)
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