The massive global economic shutdown in response to COVID-19 is clearing the skies as less pollution is being emitted from the burning of fossil fuels, including emissions of fewer colorless greenhouse gases. But emitting fewer greenhouse gases alone will not cool Earth, only reduce extra warming that would have otherwise taken place. And counterintuitively, as fossil fuel pollutants are reduced, global average temperatures warm even more due to the loss of global cooling aerosols (air pollution) that are emitted alongside greenhouse gases when fossil fuels are burned.
These aerosols are short-lived, and consist of things like smoke or soot and sulfate particles. They can cool by reflecting light back into space if they are bright aerosols (sulfate) or warm by absorbing light and turning it into heat if they are dark aerosols (soot). So many cooling aerosols are involved that they have been masking nearly a third of the planetary warming that should have occurred to date. Halting their emissions is like turning off the air conditioner on a hot summer day. But this hidden warming is only part of the story. Warming feedbacks creating especially worrisome heat waves and droughts are more likely today on a warmer Earth than they have been in the past. It’s these extremes that matter most: Averages don’t kill that often, but extremes do.
Reports of greenhouse gas emissions reductions of up to 25 percent from January to February in China, if extended for six months and applied globally, would reduce future emissions of carbon dioxide in the sky 0.3 percent total, slowing the overall rate of planetary warming. Like after 9/11 and the Great Recession in 2008, economic rebound eventually erases the effects of reduced emissions. In the meantime, however, we benefit from the clean air and fewer people die from respiratory challenges.
An article in Forbes even speculates that the decreased death toll from fossil fuel pollution might rival or exceed the death toll from COVID-19. The World Health Organization says total global deaths from outdoor air pollution are 4.2 million annually. Worst-case-scenario COVID-19 mortality rates in just the U.S. are 2.2 million, according to the Imperial College COVID-19 Response Team in the U.K., excluding possible deaths from a collapsed health care system.
The likelihood that global fossil fuel use will remain substantially below historic levels is pretty high for at least a few weeks to a few months. This creates a significant risk that the reductions of global cooling aerosol emissions will reveal abrupt warming during the Northern hemisphere’s summer, plausibly creating heat wave and drought conditions beyond anything we have experienced. The risks are two-fold: from reduction of global cooling pollution and from drought/heat wave feedback loops.
Aerosols mask 31.8 percent of the planetary warming we should have experienced already with the concentrations of greenhouse gases currently in our atmosphere, according to the latest Intergovernmental Panel on Climate Change report. This cooling works in two ways: by blocking sunlight, creating global dimming; and by enhancing cloud formation that does the same.
The aerosols mostly disappear after a few weeks to a few months. When they disappear, it’s like pulling down a giant canopy in mid-summer at the park. The sun’s rays bear down in all their ferocity and strike an object, whether that be one’s skin or plants, built objects or water, and that light energy is transformed into infrared energy or heat, where it is then trapped on Earth by greenhouse gases instead of being harmlessly reflected back into space as cold light energy.
The heat wave feedback is the other factor that needs to be considered. A little more warming may not sound like much, but the vast majority of how our weather responds to planetary warming is due to such feedback loops. The drought/heat feedback is one of the biggest because it is directly tied to the biggest greenhouse gas of all: water vapor. Heat dries the atmosphere. The drier it gets, the hotter it becomes because less-humid air heats up more than humid air. Then, the increased heat makes it drier still.
An article in the Bulletin of the American Meteorological Society by David Rup and Philip Mote, for instance, reported that the 2011 drought in Texas was made 20 times more likely because of global warming. Similarly, Texas State Climatologist John Nielsen-Gammon said that 74 percent of that drought was caused by the drought/heat feedback loops.
The bottom line is that climate extremes are far from well-understood, and it’s these extremes that are the most important. The way they work is full of such feedback loops. Some are known, but others haven’t even been discovered yet. If they were, climate scientists would likely be able to model them to better predict when they will occur and just exactly how extreme they will be.A little bit of extra warming can create impacts that are far greater than the original warming. This is one of those laws of physics climate scientists have been warning us about for 30 years now. Physics doesn’t generally happen linearly, especially where heat is concerned. With heat, physics happens like the spread of COVID-19, nonlinearly and exponentially.
The short-term projections of weather and extremes are still too difficult to predict with total accuracy, in contrast to longer-term climate patterns that have so far been unnervingly accurate. It’s the short-term events — the extreme events — that remain the challenge for climate scientists. Short-term extreme weather events can change the direction of the future on a dime.
An example would be a tropical storm event striking during the summer as a major drought is unfolding. Because it is much more difficult for nature to heat up humid air after a hurricane, drought and heat waves are much more likely to be stopped, or at least reduced, in severity by a tropical rain event. Because hurricane intensity, formation and landfall remain difficult to predict, some of us will be spared extra heat because of COVID-19 … maybe.
The rest of us, whoever we may be, should likely add another thing to our lists to keep us awake at night.