Ocean acidification may speed up total warming this century as much as 0.9°F, a new study finds. This amplifying feedback is not to be found in the forthcoming climate report from the Intergovernmental Panel on Climate Change (IPCC) — one more reason it provides an instantly out of date lowball estimate of future warming.
But unrestricted carbon pollution doesn’t merely threaten to wipe out coral reefs, oysters, salmon, and other ocean species we rely on. Researchers find “Global warming amplified by reduced sulphur fluxes as a result of ocean acidification,” in a new Nature Climate Change study (subs. req’d).
As an accompanying news article in Nature explains:
Atmospheric sulphur, most of which comes from the sea, is a check against global warming. Phytoplankton — photosynthetic microbes that drift in sunlit water — produces a compound called dimethylsulphide (DMS). Some of this enters the atmosphere and reacts to make sulphuric acid, which clumps into aerosols, or microscopic airborne particles. Aerosols seed the formation of clouds, which help cool the Earth by reflecting sunlight….
More recently, thinking has shifted towards predicting a feedback in the opposite direction, because of acidification. As more CO2 enters the atmosphere, some dissolves in seawater, forming carbonic acid. This is decreasing the pH of the oceans, which is already down by 0.1 pH units on pre-industrial times, and could be down by another 0.5 in some places by 2100. And studies using ‘mesocosms’ — enclosed volumes of seawater — show that seawater with a lower pH produces less DMS. On a global scale, a fall in DMS emissions due to acidification could have a major effect on climate, creating a positive-feedback loop and enhancing warming.
How much extra warming may occur because of ocean acidification? The study, led by scientists at the Max Planck Institute for Meteorology, concludes:
The reduced DMS emissions induce a significant additional radiative forcing, of which 83% is attributed to the impact of ocean acidification, tantamount to an equilibrium temperature response between 0.23 and 0.48 K. Our results indicate that ocean acidification has the potential to exacerbate anthropogenic warming through a mechanism that is not considered at present in projections of future climate change.
So we have up to 0.9°F warming from acidification this century that isn’t in the IPCC’s Fifth Assessment Report models. You can add that to the carbon feedback from the thawing permafrost — also unmodeled by the new IPCC report — which is projected to add up to 1.5°F to total global warming by 2100.
That means actual warming this century might well be 2°F higher than the IPCC projects. In the case where humanity keeps taking little or no action to restrict carbon pollution, that means actual warming by 2100 from preindustrial levels could exceed 10°F.