Climate Catch-22: If We Eradicate Particle Smog, Earth Will Get Hotter

And if we don’t, we’ll all get sicker and Earth will warm anyway, says Israeli-Chinese study that warns the particle cooling effect has been grossly underestimated

D. Rosenfeld et al., Science Jan

Whoever thought coal dust and other crud in our air would have a silver lining? We know that air pollution is giving us everything from asthma to cancer. But if that smog is broken down into components, the greenhouse gas component is warming the climate, but the particulate component is cooling it.

Thing is, we have grossly underestimated that cooling effect of the tiny particles that float in the air, called aerosols, says a new paper by Israeli and Chinese scientists published Thursday in Science Advances.

So, if Earth is being cooled much more than we realized by airborne particles, that means that without them, global warming would have been much worse than we have grasped, they say.

Daniel Rosenfeld of the Hebrew University of Jerusalem and Yannian Zhu of the Meteorological Institute of Shaanxi Province in China developed a new method of measuring the cooling effect of aerosols, based on satellite imagery, which they applied to low-lying clouds above the oceans. They discovered that aerosols’ cooling effect was nearly twice as great as previously thought. That’s quite the stunning error.

“The same source has two opposing effects,” Rosenfeld, the lead author of the study, told Haaretz. “Cars, industry, agriculture — everything we humans do has the well-known warming effect due to greenhouse gas emission, which is offset, to a poorly known extent, by the particulate air pollution.” Which is also hazardous to our health, he points out.

Damned if you do, cooked if you don’t. The entirety of modern civilization relies on fossil fuels in a thousand ways, and their use releases methane and carbon dioxide (making the world hotter) and aerosol (the small particles — making the world cooler). What to do?

It’s a climatic Catch-22. If we make the combustion cleaner, we’ll get the same greenhouse gases with fewer aerosols, so the warming will be less countered. It will get even hotter on average. That is bad.

If we don’t make our combustion cleaner, then in the short run we keep the Earth cooler but we pay for it by coughing our lungs out and it won’t help a whit in the long run. That’s because in the long run, the greenhouse gasses are accumulating in the atmosphere but the particles are short-lived. They get washed out of the sky with the rain. That is bad. So, what should we do?

Happily, Rosenfeld thinks the answer is a no-brainer.

In the long run, we need to make combustion more efficient and therefore cleaner because in any case, if we don’t, we will have more and more and more greenhouse gases but the particles won’t stay in the sky anyway.

But the true answer is to decarbonize: to replace fossil fuels, from gasoline to coal, with renewable energy sources, which at this stage means wind and sun.

Renewable energy is not zero-carbon throughout its lifespan but it’s enormously better than fossil fuels. We can only hope that one day a new technology will come along and save us from ourselves before large swathes of the planet, including the Middle East, become unsurvivable in the midday heat.

Technically, the team discovered that the cooling due to the impact of the air pollution on low marine clouds is much greater than we previously thought.

“This is a big deal because such clouds cover much of the ocean, and therefore, much of the Earth’s surface, so it has a large impact on the climate,” Rosenberg says.

How, by the way, do the particles cause cooling? Whatever else they do, they cause clouds to form, that reflect sunlight radiation back into outer space. Satellite imagery shows, quite starkly, cloud formation over the exact lines where ships passed, emitting pollution from their smokestacks.

DARRIN ZAMMIT LUPI/REUTERS

The image above shows how pollution influences cloud formation over the sea — in this case, the Pacific Ocean.

White and blue is zero to little cloud cover; from green to yellow through to the red, we see increasing cloud cover. Strikingly, we see red stripes “outlined” in yellow. These are clouds formed in the wake of pollution trails from liners and other ships belching out smoke. “We see clouds that wouldn’t have existed. They exist because of the pollution,” Rosenfeld explains. And by existing, they return more radiation back to space.

Clouds form when wind rises and gets colder, but the composition of the cloud depends on the aerosols.  The more particles a low-lying cloud contains, the more small water droplets it will hold.  Rain happens when these droplets bind together, get heavy and fall. 

Since it takes longer for small droplets to bind together than it does for large droplets, aerosol-filled or “polluted” clouds contain more water, remain in the sky longer (while they wait for droplets to bind and rain to fall, after which the clouds will dissipate) and cover a greater area. 

All the while, the aerosol-laden clouds reflect more solar energy back into space, thereby cooling the Earth’s overall temperature. Q.E.D.

Or not. Rosenfeld adds that another hypothesis to explain why Earth is warming despite the cooling by aerosols, is that when these particles get lodged in “deep clouds” 10 kilometers or more above the surface, they may actually behave in reverse and exacerbate the warming. It’s being checked.