In our new paper in AGU Advances, we are able to attribute increases in cloud brightness to pollution from the shipping industry in the southeast Atlantic Ocean. In that region, we calculate ~2 W/m2 of cooling due to more sunlight being reflected back to space by the clouds rather than being absorbed at the ocean surface. This is a pretty large number — in comparison, the warming (in energy units) that would result from a doubling of atmospheric carbon dioxide concentrations is ~4 W/m2.
But that’s just for the southeast Atlantic. What does this mean for the rest of the world?
By using estimates of the increase in all industrial sulfate pollution from “historical” (1850 to 2015) simulations produced by the latest generation of global climate models, we took the relationships between sulfate pollution and cloud properties from the southeast Atlantic shipping corridor and scaled them worldwide. Our “observationally-informed” global value for cooling due to pollution-induced cloud brightening is ~1 W/m2. This is approximately twice as large as the “best guess” value reported in the most recent Intergovernmental Panel on Climate Change (IPCC) assessment report.
The IPCC estimates that the warming due to increasing greenhouse gases (like carbon dioxide, methane, and nitrous oxide) is ~3 W/m2 today. If our global estimate is correct, then, it means that almost one-third of the warming we could have experienced due to greenhouse gas emissions has been “masked” by the cooling effect of pollution-induced cloud changes.
So far, the world has already warmed ~1 °C (1.8 °F) since the late 1800s. Without the cooling effect of pollution-cloud interactions, it is possible we would have already warmed by 1.5 °C (2.7 °F). The IPCC has found that there would be significant negative effects on society and natural ecosystems if the world were to warm that much, with even more harmful effects being felt at 2 °C (3.6 °F).
Diamond group @ Florida State 
Very interesting research. Thank you for your great work. A few questions:
-The preprint (see link below) mentions a best estimate effect of -0.62 W/m², while the final article estimates -1.0 W/m². Can you elaborate on what causes this difference?
-Did you estimate the ERF effect in low clouds from shipping emissions alone? Earlier research differs greatly (with for example a (indirect aerosol effect toa) low estimate of -0.086 W/m² (Sofiev et al. 2018) compared to a high estimate of −0.60 W/m² (Lauer et al. 2007).
-The final article notes the ERF effect of anthropogenic aerosols on low clouds-1.0 W/m², how would this compare to all clouds?
Looking forward to hear from you, best regards,
Leon Simons
Link to preprint:
https://www.researchgate.net/publication/337677504_Substantial_Cloud_Brightening_from_Shipping_in_Subtropical_Low_Clouds
Hi Leon,
Thanks for the interest! Some quick answers below:
1. During revisions, I realized what I was calling the “cloud albedo” is better thought of as the “overcast scene albedo” because it had effects from the overlying atmospheric column mixed in (see a fuller discussion in the new Methods section). “Cloud albedo” is quite a bit higher than the “overcast scene albedo.” When using the scene albedo, the proper downwelling shortwave flux to use is at TOA, not the attenuated flux at 850 hPa or the (clear-sky) surface. Either way you redo the calculations, the ERF goes up by ~30%.
2. In the southeast Atlantic, the estimate is from shipping (in the corridor only, not background transport) alone. For the global estimates, I use the rise in SO4 from all industrial sources as estimated via historical simulations from the CMIP6 archive. It’s not in the paper, but a back-of-the-envelope calculation to get the shipping effect alone winds up closer to the Sofiev+2018 end of the spectrum than Lauer+2007 or the Peters+ papers.
3. Great question! The short answer is I don’t know. My best guess is small negative forcing from mixed-phase clouds and some positive forcing in deep convective clouds and cirrus just about cancel each other, but there’s a lot of uncertainty. Some scholars like Danny Rosenfeld have argued for a large positive forcing in deep convective clouds, which I think deserves more attention but I am not overall convinced. The enhancement of lightning over shipping corridors in the Indian Ocean and South China Sea suggests there is definitely something to look at here.
Cheers,
Michael