It rises from the chimneys of mansions and from simple hut stoves. It rises from forest fires and the tail pipes of diesel-fueled trucks rolling down the highway, and from brick kilns and ocean liners and gas flares. Every day, from every occupied continent, a curtain of soot rises into the sky.
What soot does once it reaches the atmosphere has long been a hard question to answer. It’s not that scientists don’t know anything about the physics and chemistry of atmospheric soot. Just the opposite: it does so many things that it’s hard to know what they add up to.
To get a clear sense of soot — which is known to scientists as black carbon — an international team of 31 atmospheric scientists has worked for the past four years to analyze all the data they could. This week, they published a 232-page report in the Journal of Geophysical Research. “It’s an important assessment of where we stand now,” says Veerabhadran Ramanathan of the Scripps Institution for Oceanography, an expert on atmospheric chemistry who was not involved in the study.
The new estimate of black carbon’s heat-trapping power is twice that made by the IPCC.
The big result that jumps off the page is that black carbon plays a much bigger role in global warming than many scientists previously thought. According to the new analysis, it is second only to carbon dioxide in the amount of heat it traps in the atmosphere. The new estimate of black carbon’s heat-trapping power is about twice that made by the Intergovernmental Panel on Climate Change in 2007.
This result suggests that cutting black carbon emissions could go a long way to slowing climate change. But the authors of the new study warn that we’ll need to be careful about the sort of black carbon we choose to cut. “There’s a significant potential, but you have to be very targeted,” said co-author Sarah Doherty of the University of Washington.
Soot is made up of tiny dark particles. When it rises from fires, it mixes with dust, sulphates, and other material rising from the ground. As it ascends through the atmosphere, it can drift into clouds, mixing with the water droplets. Rain and snow then wash out the black carbon particles and bring them back to Earth.
Along the way, black carbon exerts all sorts of influences, some of which help warm the atmosphere and some of which cool it. When sunlight strikes black carbon, its dark hue causes it to heat up, something like the way a black tar roof gets hot on a sunny day. When black carbon falls on ice and snow, it smudges their bright white reflective surfaces. As a result, less sunlight bounces back out to space, leading to more warming.
In clouds, black carbon has a dazzling number of effects. “The more we study it, the more mechanisms people find,” says Doherty.
If black carbon heats up the layer of the atmosphere where clouds are forming, for example, they will evaporate. They can no longer reflect sunlight back into space, and so the soot-laced clouds end up warming the atmosphere. But black carbon that hangs above low-lying stratocumulus clouds has a different effect. It stabilizes the layer of air on top of the clouds, promoting their growth. It just so happens that thick stratocumulus clouds are like shields, blocking incoming sunlight. As a result, black carbon also ends up cooling the planet.
Black carbon has a dazzling number of effects on clouds.
All these effects depend, ultimately, on how much soot is in the air, which, in turn, depends on the many different kinds of sources of soot all over the world. Estimating that flux is a major challenge, and so it’s not too surprising that different teams of scientists have ended up with markedly different estimates for the net effect of soot on the climate.
In 2009, Doherty and her colleagues set out to make careful estimates of all sources of black carbon, using data from monitoring stations around the world. They then ran computer models of the atmosphere to measure the effects of the black carbon, based on what scientists have learned about chemical reactions in clouds from experiments and observations. Along with the effect that soot had on clouds, the scientists also estimated the total amount of warming that occurred as the soot directly absorbed sunlight, and as it darkened snow and ice.
After the scientists had taken into account all of these effects, they tallied them up to calculate how much extra energy was being stored in the atmosphere thanks to black carbon. Climate scientists typically express that energy as watts per square meter of the Earth’s surface. The number they got — 1.1 watts — was enormous. Carbon dioxide, the biggest heat-trapper in the atmosphere, is responsible for an estimated 1.56 watts per square meter. Black carbon takes second place. “It took a while to convince ourselves it was correct,” says Doherty.
“It took a while to convince ourselves it was correct,” a co-author of the study says.
If black carbon is responsible for trapping so much heat, then reducing soot may be an effective way to slow down the planet’s warming. It’s even more attractive because black carbon washes quickly out of the atmosphere, and so reducing soot emissions would lead to a fast fall in the concentration of black carbon in the atmosphere. Carbon dioxide, by contrast, lingers for centuries in the atmosphere.
James Hansen of the Goddard Institute for Space Studies has been arguing for such a strategy for over a decade. But the new study reveals a paradox in reducing soot to fight global warming. If tomorrow we could shut down every brick kiln, every burning farm field, and every other source of soot, we would, on balance, have no effect on global warming whatsoever.
How can this be? Because when things burn, black carbon is not the only thing they produce. A forest fire produces black carbon as well as organic carbon molecules. The forest fire black carbon helps to warm the planet, but the organic carbon creates a haze that blocks sunlight, cooling the atmosphere. The two emissions cancel each other out. “In the real world you can’t just get rid of black carbon emissions,” says Doherty. “You get rid of other things as well.”
But Doherty and her colleagues found that some sources of soot — including coal and diesel fuel — produce a lot of warming with very little compensating cooling. They suggest that these sources should be the top priority for efforts to fight global warming.
The study suggests some sources of soot should be a top priority in fighting global warming.
Diesel fuel looks to be an especially ripe target. “That message is loud and clear,” says Ramanathan. Making diesel an even more attractive candidate for attack is the fact that reducing much of its black carbon emissions might simply be a matter of upgrading old, soot-spewing engines with newer technology. Developing countries, in particular, could put in place regulations about burning diesel to upgrade their rapidly growing auto fleets.
Coal is another potent source of warming from soot, the scientists found, whether burned industrially or at home. So are the small stoves that billions of people use to cook. Fueled by wood or coal, they spew billows of sooty smoke. Engineers in recent years have designed efficient, cheap stoves that release much less black carbon. Getting those stoves into people’s homes would take a lot of warming soot out of the atmosphere.
Doherty does not see her new study as the end of the story. While she and her colleagues conclude that soot most likely produces 1.1 watts per square meter, they still put a margin of error on their results. They calculate that there’s a 90 percent chance the actual figure falls between .17 and 2.1 watts. To tighten that range, they still need to better understand the many ways that soot alters clouds, and also get a better fix on the amount of soot each source produces. “We need to dig deeper on that,” she says.
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Nevertheless, Doherty and her colleagues see many good reasons not to wait for a more precise understanding of soot before taking steps to reduce it. Along with its effect on the global climate, a number of studies also indicate it has powerful influences on some regions of the planet. A lot of soot falls onto the glaciers of Himalayas, for example, speeding up their melting. Millions of people depend on that ice for their water supply. Soot also has a particularly large effect on the circulation of the atmosphere around India, which ultimately reduces the amount of rainfall produced by monsoons.
Even before soot gets far into the air, it has a particularly harmful effect: it makes people sick. In recent days, news reports from China have provided startling images of Beijing swaddled in a blanket of sooty smog. That air pollution, from cars and coal-fired plants, takes a terrible toll on the country’s health. Far from the world’s urban centers, poor people suffer from air pollution in their own homes when they cook with smoky stoves and breathe in black carbon and other pollutants.
These benefits of cutting black carbon were already apparent before Doherty and her colleagues published their new study; now it’s clear that cutting soot could help not just personal health, but planetary health as well.