A new post by James Hansen and Makiko Sato looks at why global warming has been accelerating, pushing 2020 to record highs despite La Nina. They present intriguing evidence that aerosols may be playing a role.
http://www.columbia.edu/~jeh1/mailings/2020/20201214_GlobalWarmingAcceleration.pdf

One troubling feature of the recent acceleration in global warming is that it's coming despite a low phase in the solar cycle. (2/)

Greenhouse gases are all increasing, but not enough to explain the recent acceleration in warming. (3/)

Hansen & Sato: "The real-world acceleration (in warming) tells us that there must be another forcing, which is unmeasured. There is only one good candidate: aerosols." In other words, aerosols are no longer masking as much ghg warming as PM declines. http://www.columbia.edu/~jeh1/mailings/2020/20201214_GlobalWarmingAcceleration.pdf
(4/)

I hypothesized in this 2016 paper that cuts in sulfur from coal and ships could accelerate warming. Ship fuel sulfur rules went global in 2020, and coal use has declined with the pandemic. Contrails and ship-track clouds are diminished too. (5/) https://earthzine.org/could-cuts-in-sulfur-from-coal-and-ships-help-explain-the-2015-spurt-in-northern-hemisphere-temperatures/

Aerosols and their impacts on clouds are the biggest unknown in climate modeling. If aerosol cooling has been stronger than we thought historically, then there's more of a warming bump yet to come as we cut PM to protect health, & greater climate sensitivity than we thought. (6/)