Amplification of global warming through pH dependence of DMS production simulated with a fully coupled Earth system model

Abstract

Abstract. We estimate the additional transient surface warming T s 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 offline 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 emissions are reduced by 7.3 Tg S yr −1 or 31 % (11.5 Tg S yr −1 or 48 %). The main mechanism behind the additional warming is a reduction of cloud albedo, but a change in shortwave 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 T s changes by −0.041 K per 1 Tg S yr −1 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.

Impacts of Shifts in Phytoplankton Community on Clouds and Climate via the Sulfur Cycle

“…Culture experiments have confirmed that DMSP and DMS cell contents (normalized to cell volume) can be reduced by about 3 and 10 times in a dominant coccolithophorid species, Emiliania huxleyi, when exposed to doubled atmospheric CO 2 concentrations (Avgoustidi et al, 2012). In CALC, we reduced coccolithophore cellular organosulfur content in the DMS module by 90% for the high CO 2 scenario to provide a potential upper bound of the impact of ocean acidification on DMS production and compare with previous simulations by Six et al (2013) and Schwinger et al (2017). Simulated differences in DMS concentrations and other fields represent the consequences of an acidification-induced decline in ocean DMS production by coccolithophores.…”

“…This is mostly attributable to different configurations of our simulations from Six et al (2013) as discussed previously, as well as missing feedback in the uncoupled simulations in Six et al (2013). Similarly, our simulated changes in radiative effect and temperature in CALC are smaller than results from Schwinger et al (2017), which suggested 0.19-0.30 W/m 2 changes of…”

“…This reduction in DMS emissions is much smaller than estimates in Six et al (2013) and Schwinger et al (2017), which suggested an additional reduction of 5%-17% and 17%-27%, respectively, depending on the prescribed sensitivity relationships. This is because Six et al (2013) and Schwinger et al (2017) applied empirical relationships between pH and the total DMS production, while we assume that only the DMS production attributable to calcifiers changes due to acidification. Our CALC simulation likely underestimates the total impact of ocean acidification on DMS production because of we limited our scope to the impacts of calcifying organisms.…”

“…Mesocosm and laboratory experiments suggested that increasing acidity might lead to decreasing DMS concentrations Avgoustidi et al, 2012;Hopkins et al, 2010). Six et al (2013) and Schwinger et al (2017) suggested that decreases in DMS emissions resulting from ocean acidification might strongly amplify global warming. Given various potential changes in marine ecosystems, the DMS production will likely be affected.…”

The Norwegian Earth System Model, NorESM2 – Evaluation of theCMIP6 DECK and historical simulations

“…2.4), hence allowing for a direct biogeochemical climate feedback in coupled simulations. The DMS air-sea flux is simulated as a function of upper-ocean biological production following the formulation of Six and Maier-Reimer (1996) and 135 was first tested in the NorESM model framework by Schwinger et al (2017).…”