Warming effects of reduced sulfur emissions from shipping

Abstract. The regulation introduced in 2020 that limits the sulfur content in shipping fuel has reduced sulfur emissions over global open oceans by about 80 %. This is expected to have reduced aerosols that both reflect solar radiation directly and affect cloud properties, with the latter also changing the solar radiation balance. Here we investigate the impacts of this regulation on aerosols and climate in the HadGEM3-GC3.1-LL climate model. The global aerosol effective radiative forcing caused by reduced shipping emissions is estimated to be 0.13 W m−2, which is equivalent to an additional ∼50 % to the net positive forcing resulting from the reduction in all anthropogenic aerosols from the late-20th century to the pre-2020 era. Ensembles of global coupled simulations from 2020–2049 predict a global mean warming of 0.04 K averaged over this period. Our simulations are not clear on whether the global impact is yet to emerge or has already emerged because the present-day impact is masked by variability. Nevertheless, the impact of shipping emission reductions either will have already committed us to warming above the 1.5 K Paris target or will represent an important contribution that may help explain part of the rapid jump in global temperatures over the last 12 months. Consistent with previous aerosol perturbation simulations, the warming is greatest in the Arctic, reaching a mean of 0.15 K Arctic-wide and 0.3 K in the Atlantic sector of the Arctic (which represents a greater than 10 % increase in the total anthropogenic warming since pre-industrial times).