If you don't understand aerosols, you don't understand

The missing half of the climate story

A heat shield, up close.

The story in the papers about climate change is simple. Too simple. The climate used to be stable, then we added huge volumes of greenhouse gasses to the atmosphere. It started warming. And that made the weather go haywire. Nice, neat, simple, unidirectional change.

The story as scientists understand it is much more complex. The climate used to be stable, then we added huge volumes of greenhouse gasses, but also quite a lot of aerosol pollution to the atmosphere: sulfur dioxide, soot, lots of other stuff. The two pollutants pull in opposite directions. Greenhouse gasses tend to warm the atmosphere, but the aerosols offset much of that effect by reflecting some sunlight back out to space. When we cut back on aerosol emissions, temperatures jumped. 

None of this makes sense without a firm grasp of aerosol effects. But the public doesn’t have that, because climate journalism is narrowly fixated on the greenhouse gas side of the equation. 

And yet the impact of aerosol pollution isn’t small. Aerosols are mostly emitted in the northern hemisphere, where most people live and most industrial activity concentrates. Shielding just one hemisphere from so much solar radiation creates a serious temperature imbalance between the hemispheres that has far-reaching effects on the climate.

Aerosol pollution shifted the Intertropical Convergence Zone south, disrupting rain patterns throughout the developing world. It cooled the Sahara desert enough to dry out the Sahel, setting off a multi-decade drought that displaced millions of farmers, and contributed to famines that took the lives of 100,000 people. Aerosol pollution accounts for a lull in Atlantic hurricane activity in the Atlantic lasting from the late 1960s to the 1990s that coincided with a huge building boom in the U.S. Eastern Seaboard, leaving much more infrastructure exposed to hurricanes once aerosol cooling was withdrawn, and hurricane activity resumed. 

Aerosol pollution turned out to be much easier to clean up than carbon dioxide. Aerosols are an accidental byproduct of combustion, not core to its functioning. They get emitted when you burn low-quality fuels — coal, bitumen, low-grade fuel oil, stuff with lots of impurities, ranging from sulfur and nitrogen compounds, to trace metals, dirt, all kinds of gunk.

You can get rid of that stuff and still obtain all the energy you were after.

Carbon dioxide isn’t like that. It’s inescapable; built deep into the logic of combustion. Aerosols are what you get when you burn stuff you probably shouldn’t be trying to burn. Carbon dioxide is what’s left when your burn is as “clean” as it can be.

First wave environmentalism, dating back to the 1960s, was all about cracking down on aerosol pollution — that’s what the “Clean Air” in “Clean Air Act” referred to. And for good reason. Doctors had pieced together that aerosols made people sick: particulate matter in your lungs gives you emphysema, or cancer. Sulfur dioxide turns the rain acid, and destroys nearby ecosystems.

Luckily, the technology to clean all that up was at hand: you could scrub emissions at the smokestack and the tailpipe, as well as by cleaning up the fuels themselves. Catalytic converters became required in new cars. Industry was forced to adopt new standards. 

Particularly from the 1990s, we began to emit far fewer aerosols. This was absolutely needed, don’t let anyone tell you otherwise: aerosol pollution really was killing millions of people. It still is — though far fewer than it once did.

But it came at a cost. Because aerosols don’t just make you sick. They also reflect sunlight out to space. Clean them up and you withdraw what had amounted to a heat shield. And since the 1990s, we’ve been withdrawing that aerosol heat shield quickly.

The Climate Industrial Complex is heavily invested in portraying it as purely a result of increasing greenhouse gas emissions. 

It’s not that simple.

The rapid warming we’ve seen is the result of simultaneously continuing to add greenhouse gasses to the atmosphere and withdrawing the aerosol heat shield that had been offsetting a large portion of it. 

This is a reality that simply hasn’t broken through to the public yet, despite the occasional, meek article describing climate demasking. For a climate movement committed to maximizing alarm at the climate path we’re on, fast temperature rises are almost good news, or at least, useful in the quest to raise consciousness about climate change. 

A message that foregrounds aerosol effects is one they will never want to foreground, because it muddies the advocacy waters. Acknowledging the tradeoffs involved in cleaning up aerosol pollution runs counter to their whole mission, which portrays pollution as simply and unproblematically bad. Treating fast rising temperatures as, in part, an outcome of cleaning up pollution is too uncomfortable. This is one inconvenient truth they’re more than happy to sweep under the rug.

And it’s not just that: the message about aerosol demasking gets downplayed for fear that folks will draw the obvious inference: if aerosols were so useful for keeping temperatures down, maybe we ought to think about going back to that strategy. Not with toxic chemicals like we ignorantly used to do, back when we were doing this by accident, but with benign stuff like sea salt, or with sulfur compounds spread high up in the atmosphere where nobody has to breathe it. 

If our climate conversation was frank about aerosols, we’d gravitate naturally enough to the conclusion that altering earth albedo to keep temperatures down isn’t actually unprecedented. It’s extremely precedented! It’s what we spent most of the last hundred years doing…though, admittedly, we didn’t realize that was what we were doing at the time. 

These obvious inferences aren’t made, because the climate conversation is in deep denial about aerosol effects, profoundly uncomfortable about the complexities of a world where deadly pollution turns out to have some positive effects, and desperate for the simplicity of an “emissions=bad” framing that just doesn’t capture our atmospheric reality.

Scientists, who know better, don’t have a way to make their voices heard. The advocacy organizations that speak for them obscure the results of their research. And so we’re sleepwalking towards a climate disaster we would be able to forestall, if only we dared.