This is the latest in the story of black carbon.
A new study spotlights the role of black carbon in global warming. Hey, hey good news: maybe we can allay the global warming blues by turning off black carbon emissions. But no, no, my, my, not so fast; it's not quite so easy as all that.
Let's start with a review of the drivers of global warming. There are essentially two categories of global warming agents that drive climate change: carbon dioxide (CO2) and everything else. On an individual basis, CO2 is the biggie, but collectively the other shorter-lived atmospheric constituents warm about as much as or more than CO2. These shorter-lived constituents include methane, hydrofluorocarbons (HFCs), nitrous oxide, and, the topic for today, black carbon.
Black Carbon Primer
Also known as soot, black carbon is produced from incomplete combustion of fossil fuels and biomass. Because it is black, it absorbs sunlight and warms the atmosphere. On the positive side, it only stays in the atmosphere for several days to weeks.
Most U.S. black carbon emissions come from transportation (which represents about 52 percent of U.S. emissions, with a large portion coming from diesel and some 35 percent coming from the burning of biomass, which includes wildfires). In less developed countries coal and biomass burning tend to dominate.
Black carbon can lead to warming in a number of ways, including by:
- direct effect: A bit like getting overheated by wearing a black T-shirt on a sunny summer day, black carbon absorbs sunlight, thus heating the atmosphere.
- indirect effect related to clouds: When black carbon bounces around the atmosphere, it heats it up and causes clouds to evaporate -- fewer clouds mean even less radiation reflected to space and more warming.
- indirect effect related to changes in reflectivity of snow and ice: If black carbon is deposited on snow or ice, it lowers the reflectivity of the ice, causing more heat from the sun to be absorbed.
Previous assessments have concluded that warming from black carbon is significant. For example, the last assessment report by the Intergovernmental Panel on Climate Change (IPCC) estimated that warming from both the direct effect of black carbon from fossil fuels and the indirect effect from albedo changes on snow was about 0.3 watts per meter squared compared to 1.66 watts per meter squared for CO2. Maybe not a dominant player but something to be reckoned with.
New Data on Black Carbon
Now a team of 31 scientists led by Tami Bond of the University of Illinois at Urbana-Champaign have completed a new assessment aimed at "Bounding the role of black carbon in the climate system." Expected to appear shortly in the Journal of Geophysical Research, their work is available now in pre-publication form online.
The study’s broad conclusion -- which has received a good deal of play in the media (see for example here) -- is that black carbon warming is significantly larger than previously estimated. At about 1.1 watts per meter squared of warming (when all three effects listed above are considered), black carbon moves into the No. 2 spot for most powerful warming agents, ahead of methane and behind CO2.
Why, you might wonder, is the new estimate so much larger than previous assessments? Because earlier studies underestimated the global emissions of black carbon and failed to fully account for the indirect effects.
Bad News and Good News
OK, so the first reaction, and the one that's swirling around the media, is one of bad news: sort of a Holy mackerel, that's a lot of warming response.
But such a conclusion also implies a good news retort: We can solve this problem presto chango by simply dialing back on black carbon emissions. Think about it. Addressing CO2 emissions is really, really challenging. But if black carbon warming is so large, we can forget about CO2 for a while and focus on black carbon instead and make some solid progress toward tackling the problem.
And, since black carbon comes from plain old air pollution and we have a lot of experience eliminating air pollution, it should be a relatively easy task.
Indeed this is largely the thinking behind the State Department's Climate and Clean Air Coalition to Reduce Short-Lived Climate Pollutants. With backing from the United Nations, this voluntary group of six nations, formed last year, plans to target black carbon along with emissions of methane and HFCs -- pollutants that have "the potential to reduce the warming expected by 2050 by as much as 0.5 Celsius degrees."
With no foreseeable plan to address CO2 emissions, addressing shorter-lived warmers has gained considerable momentum.
Not So Fast on the Good News
Alas, when it comes to black carbon, it's not quite that simple, and such nuance is the part that some media outlets have kinda swept under rug. (See here.)
It turns out that burning stuff like coal and oil doesn't produce only black carbon, and some of that other stuff produced by burning fossil fuels cools instead of warms the atmosphere. (See related post.) In other words, while air pollution is generally bad, bad, bad, some of it has actually prevented some warming that would have occurred were those cooling agents not helping block it.
For example, burning coal emits sulfur oxides into the atmosphere where they are converted into sulfate particles; these particles, in contrast to black carbon, tend to reflect sunlight and cool the atmosphere. Depending upon the relative amounts of black carbon and sulfur oxides emitted, dialing down on emissions from coal burning may reduce warming but it also may exacerbate warming.
So which is it? Bond and co-authors were not able to give a definitive answer but were of a mind that it would be kind of a wash: "Uniform elimination of all emissions from black-carbon-rich sources could lead to no change in climate warming." They recommend that a much more selective approach be taken that focuses on those black carbon source that are definitively and strongly tied to black carbon emissions, like that coming from diesel engines.
In short, burning stuff may produce black smoke but that's not all, and what that is is best not forgotten. As in so many other things, when it comes to black carbon, there's more to the picture than meets the eye.