Cooling from atmospheric particles may mask greater warming

There’s troubling new research that suggests the scientific community may not fully understand the impacts of aerosols – tiny particles in the air – when it comes to their role in climate change.

The paper published in Science by Dr. Daniel Rosenfeld of Hebrew University, and a team of scientists working in China, Germany and the United States, details why they believe that the global cooling caused by aerosols is dramatically underestimated. That may sound like good news at first, but here’s the thing: If the cooling effect is far greater than previously thought, but we’re still seeing evidence of an alarming rate of global warming, that may mean it’s masking the rate of warming that’s been underestimated too.

At least that’s one of the possibilities in a complex scenario. When we think about global warming, we understand the relationship between greenhouse gas (GHG) emissions and rising temperatures, ice melt, sea level rise and other consequences. But the aerosols – some of which occur naturally, like dust, but much of which comes from the pollution of cars or industrial smokestacks – primarily have a cooling effect.

They do this because of a key role in how clouds are formed and behave. Winds create the clouds, but the more particles they contain, the more area the clouds will cover. They’ll also hold more water and take longer to dissipate. That means they’re deflecting more solar energy away from the earth, although dark particles can absorb heat too. Rosenfeld’s team doesn’t directly address this, but this same science is the premise for some geoengineering strategies that propose “seeding” the atmosphere with aerosols to artificially manufacture more global cooling.

Scientists haven’t been sure, though, how much of the cooling effect comes from the winds that promote cloud formation in the first place rather than the aerosols themselves. Rosenfeld and a colleague in China – Yannian Zhu from the Meteorological Institute of Shaanxi Province – found a new way to use satellite images to separately measure the influence of the winds and the aerosol in the clouds. They used the technique on low cloud cover over wide swaths of the world’s oceans, from the equator at the tropics to latitude 40 South, which is about even with Wellington, New Zealand.

What they discovered is that the cooling impact of the aerosol particles, much of which is pollution that humans have been pumping into the atmosphere, is nearly twice what scientists previously thought. If that’s the case, the discrepancy points to the likelihood that our warming is already overriding more cooling than we thought was there. It also sets up another question, because much of the aerosol particles cooling the planet are derived from our dirty fuels. Ironically, clean-energy sources could reduce that cooling.

Scientists already know that loss of this aerosol cooling is a potential problem: A January 2018 study found that “cleaning up” the pollution particles could mean an average global surface temperature rise of 0.5 to 1.1°C, and precipitation increases of 2.0 to 4.6 percent, along with an uptick in extreme weather. It’s almost as if we can’t live with the pollution – it’s literally killing people – but now we can’t live without its protective effects either.

There’s another possibility that Rosenfeld plans to investigate, and that’s whether or not the aerosols warm rather than cool once they reach a height of 10 kilometers or more above the earth. He’ll be working with Israel’s Space Agency and France’s National Center for Space Studies (CNES) to find out. Either way, Rosenfeld says, the current climate prediction models are getting the aerosol particle impacts wrong, and that means reworking the numbers on the 1.5 to 4.5°C window for temperature rise in this century.