Black Carbon: The Secret Climate Threat

New study shows that soot is almost as bad for the planet as carbon dioxide

Black Smoke, Soot, Environment, Climate
Forrest Anderson/Liaison
Black smoke emerges from a coal factory's chimney.
By |

Climate scientists have shed new light on a serious threat to our way of life: black carbon. (No, it's not a metal band, though there is one that's pretty close.) New research suggests that this substance – more commonly known as soot – is one of the biggest causes of global warming, second only to carbon dioxide in slowly making parts of our planet uninhabitable.

Earlier this month, a group of climate scientists made waves in the environmental community when they published a study arguing that black carbon is fully twice as bad as previously thought. In fact, they estimate it causes two-thirds as much warming as carbon dioxide. That's the bad news. The good news – and "good" is relative when it comes to climate – is that this may be one of the few areas where nations rich and poor can come together and actually do something about climate change.

Obama's Climate Challenge

A brief explanation of the problem: When almost anything burns, it releases tiny particles of soot into the air. That black stuff in the plume of diesel smoke you see pouring out of a garbage truck's exhaust pipe? That's black carbon. We've long known that those particles contribute to global warming in several ways, as well as leading to millions of premature deaths worldwide. Most simply, when struck by light, soot heats up like a black leather car seat on a sunny day. When it floats down onto ice or snow, it turns what was once a bright, shiny, heat-reflective surface into a filthy heat-sink. Picture what happens when you toss a rock onto a frozen pond. By the next day, the rock has sunken in and created a little melt pool around it. Black carbon essentially does this to the entire Arctic.

The biggest sources of black carbon are grass and forest fires, but the runners-up are diesel engines and household stoves burning wood, coal or other material for heating and cooking, mostly in Africa and Asia. This has led to an interesting irony in the United States' approach to black carbon. We have long resisted global efforts to reduce greenhouse gas emissions; Congress infamously failed to ratify the Kyoto Protocol, the now-outmoded treaty to cut pollution in developed countries, and Republicans have blocked Democratic proposals for a carbon dioxide cap at home. But pretty much everyone, including GOP members, has long agreed that we ought to reduce black carbon emissions.

Much of this agreement is likely because cutting carbon dioxide emissions would hurt powerful industries such as oil and gas or manufacturing, but cutting black carbon won't. While the U.S. accounts for about 19 percent of the world's CO2 emissions, it's responsible for only 8 percent of black carbon. Reining in soot pollution would focus on eliminating dirty cook stoves in places like Kenya, not dirty U.S. power plants. Perhaps this is why the U.S. is already a founding member of a United Nations effort to reduce black carbon and other ancillary climate-warming emissions. Or why the State Department launched an effort last year to replace 100 million cook stoves with clean-burning models by 2020. But whatever the debatably fair reasons, the U.S. government is already behind the initiative.

Don't Ignore the Drought

There's another argument for targeting black carbon: instant gratification. While carbon dioxide lingers in the atmosphere for decades, slowly warming the climate, black carbon sticks around for only days or weeks before its effects disappear. Reduce emissions now, and we'll see results right away.

There are some complications. Not all soot is created equal – at least when it comes to climate change. The particles react with other gases emitted by whatever fire created them, so some sources are effectively harder on the climate than others (diesel engines appear to be the worst). Soot also interacts with clouds in complex ways. In some circumstances, black carbon emissions can actually have a cooling effect, in part by leading to thicker cloud cover, for example.

But the bottom line is that while there are plenty of unknowns about exactly how bad black carbon is, and which sources are the worst, there are some, particularly diesel engines, that are unambiguously bad. If we cleaned up those engines everywhere from U.S. oil rigs to Indian truck fleets, we'd go a long way to nudging ourselves towards a future where New York remains above sea level.

"If we did everything we could to reduce these emissions, we could buy ourselves up to half a degree less warming – or a couple of decades of respite," said Piers Forster, one of the authors of the study and a professor at the University of Leeds's School of Earth and Environment, in a statement earlier this month. So this is far from a permanent solution. We'll still have to drastically cut carbon dioxide emissions to slow or reverse climate change. Still, in a field as dismal as climate science, any good news – even if it’s shrouded in bad news ­– is something to embrace.

x