You can’t see them, but you are surrounded by billions of tiny particles. When bits of dust or droplets hang in the air, scientists call them “aerosols“. In the atmosphere, aerosols protect us a little from being too hot. They hide some of the warming we have already created. As smog, they kill us by the millions every year. What happens to aerosols as the world heats up even more? We need to know.
Our guest Dr. Robert Allen wades into what may be one of the most complex questions of science. After getting his Pd.D. in Atmosphere, Ocean and Climate Dynamics at Yale, Robert was a Postdoctoral Scholar at various places, including at the Scripps Institution of Oceanography. I called Dr. Allen about his new paper in Nature Climate Change: “Enhanced landsea warming contrast elevates aerosol pollution in a warmer world“.
Show by Radio Ecoshock, reposted under CC License. Episode details at https://www.ecoshock.org/2019/02/out-of-the-smog-into-the-sea.html
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The composition of these little drops or flecks isn't simple. Many things are up there. Scientists talk about aerosol “species” and “populations”. I’ve seen a greatly-magnified photo of a tiny solid piece which looked like an asteroid. It had other things smeared on it, like maybe pesticides or materials from car exhausts.
When sunlight arrives on Earth, some of it bounces and radiates back out into space. But some of the Sun’s energy is held within the Earth’s atmosphere by greenhouse gases like carbon dioxide. A significant portion of the Sun’s light never reaches the surface, because it strikes aerosol particles first.
There is a stunning statement in the introduction, which says “aerosols cause a net cooling effect, which has probably offset ~40% of [Greenhouse Gas] GHG warming.” That is almost half the real warming potential getting shielded by pollution, both man-made and natural. Does that mean the warming we have already created is really 40% WORSE than we are experiencing so far, or does it imply a change in time, putting off the arrival of the heat?
Robert Allen says the amount of heating hidden by aerosols could be up to half a degree C. If we are currently approaching 1.5 degrees C of warming, as suggested by several scientists on this show, doesn’t that mean we really risk 2 degrees of warming as soon as pollution is greatly reduced? Because the citizens of Asian cities like New Delhi, Mumbai, and dozens of giant cities in China are complaining, and dying from smog, cleaning it up is a high political priority. The United States started lowering pollution in the 1970’s with the Clean Air Act, and it did reduce both smog and acid rain. It seems almost certain that other countries will cut aerosol production, especially as coal power plants are phased out in favor or renewable energy. So we will get that extra heat. Meanwhile, it drives me crazy that other scientists, the IPCC and governments don’t mention the extra load of heat just waiting for us, as though aerosol cooling isn’t a real factor the public needs to know.
The studies by Robert Allen, and our previous Radio Ecoshock guest Yangyang Xu, show that expected meteorological changes will enhance the effect of aerosols. Yes a warmer atmosphere will hold more moisture, but that is expected to show up in more extreme rainfall events. In between those extreme rains, in some parts of the world there will be more dry days in between, and more stagnant weather patterns. This leads to a worsening pollution (with all the attendant health affects, and a slight cooling when those conditions exist).
The paper led by Robert Allen deals mainly with the effects of climate change on aerosols in the Northern Hemisphere. I have not forgotten my listeners in Australia and New Zealand. But most of the aerosol pollution comes out of the much more heavily populated northern hemisphere. Unlike carbon dioxide, which distributes itself evenly around the world’s atmosphere over time, aerosol pollution mainly stays in the hemisphere where it was created. A lot of the excess unfortunately collects in the Arctic, where glaciers and snows are darkening. That leads to earlier spring melts, and more exposed darker land for longer periods, another spin-off effect that adds to the heating of the planet.
THE TROPICS ARE EXPANDING TOWARD YOU
We break off for a minute to discuss something I learned from Allen’s previous paper in 2012 “Recent Northern Hemisphere tropical expansion primarily driven by black carbon and tropospheric ozone“. I did not know the Tropics are expanding north and south as the world warms. Although emissions are a factor, that is not directly causing tropical expansion, but something else. Dr. Allen tells us the drivers of tropical expansion are different for the North and Southern Hemispheres.