Global black carbon emissions double previous estimates

Black carbon is one of the most potent air pollutants that contributes to global climate change, and is produced by the incomplete combustion of fossil fuels and forest fires. While scientists have known about the role of black carbon for decades, there’s been limited research to calculate global emissions. Now, MIT and National University of Singapore (NUS) researchers have developed a new method to calculate global black carbon emissions.

In a study published in last week’s Journal of Geophysical Research, the researchers calculated the global total of black carbon emissions at 17 teragrams a year between 2000 and 2005. This result is significantly larger than the majority of global air pollution modeling studies, which employ a bottom-up approach — adding together the emissions estimates from different economic sectors to find a global total. One such study found humans emitted only 7.5 teragrams of black carbon per year during the same five-year period — less than half the new estimate.

“Our results are the first to produce a global top-down estimation of the emissions of black carbon,” says Chien Wang, a senior research scientist with the MIT Joint Program on the Science and Policy of Global Change and co-author of the study.

The top-down method used by Wang and his co-author Jason Cohen of NUS relies on gathering data from air measurement stations and satellites to sufficiently cover every region of the globe. In this study, data was collected from 238 different stations. They then compiled this data and used inverse modeling to determine the emissions from each of the major pollution regions.

“Current emissions inventories are mainly obtained by adding up estimates of emissions from ever sector of the economy and the environment to obtain a global estimate. This method creates uncertainty in the projections,”  Wang says. “Our method eliminates some of these uncertainties by more accurately factoring in population and economic changes around the globe.”

The differences in the emissions estimates are most apparent in China and Southeast Asia. Wang predicts this is the result of the bottom-up method not capturing the rapid socioeconomic growth that has occurred in this region over the past 15 years.

Black carbon enters the atmosphere as small particles and warms the planet by absorbing heat and reducing the ability of the Earth to reflect light back out to space. Because black carbon plays a key role in air pollution and global climate change, it is essential for policymakers to have an accurate picture of the severity of the problem.

“This top-down method isn’t perfect and still creates some unknowns in the estimates produced,” Wang says, but he suggests that combining both approaches would potentially improve estimates and further eliminate uncertainty. “We hope this work will open the door to further efforts to better quantify and reduce uncertainty in black carbon emissions estimates.”