Amplification of global warming through pH-dependence of DMS-production simulated with a fully coupled Earth system model
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We estimate the additional transient surface warming Δ<i>T</i><sub><i>s</i></sub> caused by a potential reduction of marine dimethyl sulfide (DMS) production due to ocean-acidification under the high emission scenario RCP8.5 until the year 2200. Since we use a fully coupled Earth system model, our results include a range of feedbacks, such as the response of marine DMS-production to the additional changes in temperature and sea-ice cover. Our results are broadly consistent with the findings of a previous study that employed an off-line model set-up. Assuming a medium (strong) sensitivity of DMS-production to pH, we find an additional transient global warming of 0.30 K (0.47 K) towards the end of the 22nd century when DMS-emission are reduced by 7.3 Tg S yr<sup>−1</sup> or 31 % (11.5 Tg S yr<sup>−1</sup> or 48 %). The main mechanism behind the additional warming is a reduction of cloud albedo, but a change in short-wave radiative fluxes under clear-sky conditions due to reduced sulfate aerosol load also contributes significantly. We find an approximately linear relationship between reduction of DMS-emissions and changes in top of the atmosphere radiative fluxes as well as changes in surface temperature for the range of DMS-emissions considered here. For example, global average <i>T</i><sub><i>s</i></sub> changes by −0.041 K per 1 Tg S yr<sup>−1</sup> change in sea-air DMS-fluxes. The additional warming in our model has a pronounced asymmetry between northern and southern high latitudes. It is largest over the Antarctic continent, where the additional temperature increase of 0.56 K (0.89 K) is almost twice the global average. We find that feedbacks are small on the global scale due to opposing regional contributions. The most pronounced feedback is found for the Southern Ocean, where we estimate that the additional climate
change enhances sea-air DMS-fluxes by about 9 % (15 %), which counteracts the reduction due to ocean acidification.