Volcanoes Damped Recent Warming

We've been putting ever more CO2 into the atmosphere, which should be causing ever faster warming.  But warming has not been as fast as expected (globally) since around the turn of the century.  What's going on?

We recently went through the deepest solar minimum in a long time, but solar cycles have too small an effect to explain it.  Turns out it's mostly been aerosols.

Earth can be cooled by a kind of atmospheric sunscreen: sulfur gasses make tiny droplets - aerosols - that are reflective and have a cooling effect.  In fact this idea is being advanced as an emergency cooling measure: aerosol-forming compounds can be sprayed by aircraft.

Aerosols don't stay in the air very long (months to a few years), so aerosol spraying would have to be kept up.  CO2, in contrast, stays in the air for thousands of years.  As soon as the aerosol rains out, wham!  Warming comes back.

You can see the effect of volcanic cooling (Mt. Pinatubo in 1991), and the recent slowdown of warming, in the most recent NASA data - note the dip to the right of the word "Global":

Direct measurements showed that there were aerosols coming from somewhere.  It was assumed, since there haven't been enough really big volcanic eruptions, that the aerosols must have been coming from dirty new coal plants in Asia.

It presents the same problem: when you clean up coal plant aerosols, the warming that was masked will come to bear.  That's more or less what happened when post-WWII emissions were cleaned up to fix acid rain and other problems.  Warming started in earnest in the mid-1970s.

But it turns out that a lot of smaller volcanoes can do it too.  Close study reveals that's what's been happening.

The implication is that we have a lot of warming that's been hidden.  Time will tell how volcanoes behave in the future.  What they do, and how long aerosol shielding lasts, will have a big effect on how much warming happens in the coming years.