Impact of SO2 reduction on global warming

We've focused a lot on the effects of low sulfur fuel standards on recent warming, but in some ways whats happened in China is even a bigger story. They have reduced their SO2 emissions nearly 75% since 2005, resulting in ~0.06C (0.02C to 0.11C) warming. To put this in perspective, this represents around 17% of the estimated anthropogenic warming the world has experienced since 2005. Thats a pretty big factor, drive by some pretty impressive emissions declines, the magnitude of which has been a bit under-appreciated.

We've talked a lot in recent years, both here and in our client reports and presentations, about the hypothesis that global warming has accelerated recently. One of the potential reasons for this acceleration is the mandate for reduced sulfur emissions from shipping via the IMO 2020 regulations: https://lnkd.in/et8xCNrH Sulfur effectively acts as a brake on the warming driven by increased greenhouse gases, as it is an effective blocker of sunlight. Decreasing sulfur emissions in the name of cleaner air, a worthy cause, then inadvertently accelerates warming via an increase in net solar radiation at the surface. The data below support this hypothesis, as we have seen an acceleration in sea surface temperatures since the IMO 2020 regulations went into effect on January 1, 2020. The plots in the attached graphic show the annual SST anomalies across the major shipping lanes of the mid-latitudes in the North Pacific and the North Atlantic, and there is clearly "something different" going on over the last five years. Further, there is an emerging body of scientific literature that also suggests IMO 2020 has inadvertently accelerated global warming (two papers cited in attached graphic). So, while we have been successful in reducing greenhouse gas emissions in the western world in recent decades, there may be other factors at play that are considerably more important with regards to modulating the rate of global warming.

Amid the global heat wave, the theoretical warming effect of the IMO 2020 shipping regulation is starting to come up again (not as the main factor, but as some kind of factor). This regulation, implemented Jan. 1, 2020, dramatically and rapidly reduced global shipping’s sulfur emissions (estimated to account for 13% of all global SOx emissions prior to the regulation) – a big health benefit to the people who would otherwise breathe in those emissions, but with a climate side effect. Prior to 2020, there were numerous studies conducted on how this regulation could affect global temperature. Because sulfate aerosols in the atmosphere have a cooling effect, the studies claimed that removing such a significant portion of the sulfates in the atmosphere via IMO 2020 – without concurrently cutting shipping’s carbon emissions (which won’t happen for decades even in an optimistic scenario) – would have a net positive effect on temperature over time. https://lnkd.in/e4UGsXhJ