Black Carbon May Contribute Almost as Much as Carbon Dioxide to Global Warming

Black carbon’s role in driving warming is much higher than previously thought

Coal-fired stoves are a major source of black carbon.
Coal-fired stoves are a major source of black carbon. Michael Davis-Burchat

Black carbon—an atmospheric pollutant “formed by the incomplete combustion of fossil fuels, biofuels, and biomass“—holds an even tighter grip on the Earth’s climate than we thought. Based on new research by scientists with the International Global Atmospheric Chemistry project, black carbon may in fact be the second most important factor driving modern anthropogenic climate change.

According to a news release by the American Geophysical Union, which published the study, cutting back on black carbon emissions would have “an immediate cooling impact.” One of the study’s authors, Tami Bond, says:

“This study shows that this is a viable option for some black carbon sources and since black carbon is short-lived, the impacts would be noticed immediately. Mitigating black carbon is good for curbing short-term climate change, but to really solve the long-term climate problem, carbon dioxide emissions must also be reduced.”

In Asia and Africa, coal and biomass burning are the main culprits of black carbon emissions. In North America, Latin America and Europe, it’s diesel engines.

But wherever it comes from, black carbon messes with the Earth’s climate in a number of ways. Black carbon absorbs sunlight, trapping heat. It seeds clouds, which both trap heat and reflect sunlight. And by reacting with other chemicals in the atmosphere, it creates a range of downstream effects. Unlike carbon dioxide, the effects of which are felt world-wide owing to its long stay in the atmosphere, the effects of black carbon are often much more local.

All in all, says the study, black carbon is accountable for trapping around 1.1 watts of energy per square meter of the Earth’s surface every year. This value, 1.1 W/m^2, compares with the 1.56 W/m^2 of energy trapped by carbon dioxide and the 0.86 W/m^2 trapped by methane, another greenhouse gas. However, the uncertainty wrapped up in the measure of black carbon’s potential is huge: the 1.1 W/m^2 comes with an uncertainty of 90%, meaning that the real energy-trapping potential could realistically fall anywhere from 0.17 to 2.1 watts per square meter.

The uncertainty in the measure of black carbon’s effect on the Earth’s energy budget comes from a few places, the authors say. No one has pinned down exactly how black carbon’s interactions with clouds affect energy trapping. Nor is there a solid number for the total amount of black carbon being emitted each year. Sorting these values out to a higher degree of precision would cut down some of the uncertainty in understanding black carbon’s role as a heat trapper.

Even still, realizing the warming potential of black carbon also points to an opportunity to mitigate on-going warming. The BBC:

“Reducing emissions from diesel engines and domestic wood and coal fires is a no-brainer as there are tandem health and climate benefits,” said Professor Piers Forster from the University of Leeds.

“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,” he added.

Half a degree of warming is much less than the total expected warming we are set to face, but with modern warming already starting to affect people’s daily lives, it is likely worth trying anything that could bring temperatures down.

The results come on the heels of reports by both NASA and the National Oceanic and Atmospheric Administration that, globally, 2012 was one of the warmest years on record, with the top 10 years all occurring within the past 14 years.

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