No discussion of climate change can get very far without focusing on greenhouse gases — pollutants including carbon dioxide, methane, nitrous oxides and more, which are trapping heat and driving the planet’s temperature upward.
But according to a report published Tuesday in the Journal of Geophysical Research: Atmospheres, the second most important heat-trapping pollutant isn’t a gas at all: it’s black carbon, more commonly known as plain old soot, generated mostly from the burning of diesel fuel, coal and woody plant material. “There’s a relatively small amount in the atmosphere,” said the study’s lead author, Tami Bond, of the University of Illinois at Urbana-Champaign, in an interview. “But it’s very powerful.”
This isn’t the first hint that black carbon might have an outsize effect on climate. Research published in 2008, for example, concluded that black carbon’s global-warming effect was about twice as great as the Intergovernmental Panel on Climate Change’s major report in 2007 had estimated — the same conclusion reached in the new study — but that conclusion wasn’t considered definitive.
Even so, scientists and policymakers understood that reducing emissions of black carbon could be a relatively quick and easy way to slow the pace of global warming, and a number of initiatives including one created by the U.S. and another by the United Nations have been created to try and deal with the problem. The particles can also cause or aggravate lung disease, so there’s a public-health benefit to reducing emissions as well.
But without a true sense of how big an issue black carbon really was, scientists and policymakers couldn’t make truly informed decisions about how to attack it. So Bond and 30 other scientists launched a truly comprehensive assessment, coordinated by the International Global Atmospheric Chemistry Project, of how much black carbon there really is in the atmosphere, where it comes from and how much it warms the planet.
In the end, the study took four years. “It wasn’t supposed to take that long, but the science isn’t entirely straightforward,” Bond said. On the most basic level, black carbon contributes to global warming simply because it’s black. “The particles absorb just about every ray of light that hits them,” she said, heating up like dark clothing on a sunny summer day and warming the air around them. In the Arctic, black carbon particles eventually settle onto ice and snow, where they help speed melting.
Particles of black carbon come in different sizes, however, and in some cases they’re small enough that they can affect cloud formation — in some cases, at least, helping to create clouds that reflect sunlight. Some sources of black carbon, moreover, such as the open burning of plant waste, generate other kinds of particles that tend to cool the planet. “If we shut those sources off, you could actually make the warming worse,” Bond said.
The new report tries to take all of these complexities into account. “It’s clear that reducing emissions from diesel engines is the biggest priority, followed by some kinds of wood and coal burning in homes — for cooking, and maybe for heating,” Bond said.
Emissions from some kinds of small industry might also be a target, but not from all. “We tried to estimate the net effect of each source, along with the uncertainties in our estimates,” Bond said. “In some cases, the uncertainty is so large that it’s clear we need more understanding.”
Nevertheless, Bond said reducing the most clearly harmful sources of emissions is something we already know how to do. “Black carbon was really high in the U.S. in the early 1900s,” she said. “Since then, we’ve increased our use of fossil fuels dramatically without increasing those emissions. This is not rocket science.”
The black carbon study may not have all the details nailed down, and it doesn’t do away with the thornier, longer-term issue of carbon dioxide emissions. But it does point to a way to slow the pace global warming significantly and quickly. Given that the dire effects of warming are showing everywhere — as a new report makes clearer than ever — this could prove invaluable as a guide to effective policy.