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Reducing Soot Could Help Cool the Arctic, New Study Says

By Alyson Kenward

The increasing concentration of carbon dioxide (CO2) in the atmosphere may be the primary culprit for warming the global climate, but new research indicates that reducing another atmospheric pollutant — soot — could offer some short-term help for the Arctic, which is one of the most rapidly warming regions on the planet.

By tracking the worldwide sources of soot, its transport through the atmosphere, and its overall warming potential, Stanford University researcher Mark Jacobson has found that eliminating man-made soot emissions during the next 15 years could reduce global warming by as much as one degree Fahrenheit from what it might otherwise be. That’s about how much the Earth has warmed since 1990. And because soot particles only remain in the atmosphere for a week or two, the climate would respond quickly to stopping soot emissions, as residual soot in the atmosphere settles out to the ground. However, targeting CO2 emissions, which can stay in the atmosphere for up to a century, will still be necessary to lessen long-term climate change, but may not offer a quick enough solution to prevent complete summer melting of Arctic sea ice.

“If you stop emitting soot, you can get a quick climate feedback,” Jacobson, an environmental engineer, said in an interview with Climate Central. “So, controlling it is the fastest way of controlling global warming.” These new results are based on a climate model he has developed during the past two decades.

In particular, Jacobson says limiting soot emissions may be the only way to prevent the complete loss of sea ice in the Arctic during summertime, which many projections show is likely to occur by the middle of this century. According to his calculations, published in the Journal of Geophysical Research on July 29, and previous results from other research groups, the majority of soot-induced warming occurs in the Northern Hemisphere, and soot from northern areas such as Europe can make its way into the Arctic.

Consequently, he says, controlling emissions could reduce warming in the Arctic by nearly 3 degrees Fahrenheit and at least curb the pace of significant changes to the ice and snow landscape while cuts to long-lived greenhouse gases such as CO2 are made. In recent decades, the Arctic has warmed at nearly twice the rate of the global average, and this accelerated warming trend is expected to continue if greenhouse gas emissions are not curtailed.

Monthly July Arctic sea ice extent for the period 1979 to 2010
Monthly July Arctic sea ice extent for the period 1979 to 2010 shows a 6.4% decline per decade.
Credit: National Snow and Ice Data Center (NSIDC)

“We cannot afford to lose the Arctic,” said Durwood Zaelke, president of the Institute for Governance and Sustainable Development, in a press release on Jacobson’s findings. “Targeting [soot] with aggressive, fast action today is the most important strategy for saving the Arctic.”

Soot in the Arctic

Considered by some scientists to be the one of the strongest contributors to the planet’s current rising temperature behind CO2, soot is released from the burning of certain fuels and biomass. According to an analysis of worldwide fuel usage, fossil fuels, such as diesel oil, contribute about 40 percent of the world’s soot, whereas soot from biofuels — especially wood and dung-burning stoves — are responsible for approximately 20 percent of soot emissions (open burning of biomass, including wildfires, makes up the other 40 percent).

Soot contains a number of individual pollutants, some of which can have a slight cooling impact. The overall effect of soot emissions, however, is to warm the climate, due largely to a particular component called black carbon, a solid black residue that forms during incomplete fuel combustion.

Soot’s role in climate change is far from straightforward. In the atmosphere, for example, black carbon in the soot can warm the air by absorbing sunlight and emitting heat. Soot can also alter the composition of clouds and change how much sunlight they absorb or reflect, switching some clouds from net reflectors of solar radiation to net absorbers. The magnitudes of these feedbacks vary in different regions of the world.

A key concern in the Arctic is that when the black carbon in soot deposits on bright white snow, it can darken the surface, leading to greater absorption of solar radiation and melting. This complicated atmospheric behavior of soot poses a challenge for scientists hoping to predict exactly how it will influence the climate system.

“Black carbon is very difficult to treat in [computer] models,” said Veerabhadran Ramanathan, an atmospheric scientist from Scripps Institution of Oceanography at the University of California, San Diego and a prominent expert on atmospheric soot. The result is that few models have accurately dealt with all of black carbon’s nuances, which can lead to overly simplified conclusions. “Jacobson’s model is one of the few that deals with this complexity in a reasonable way,” he said in an email interview.

There are a number of reasons black carbon is such a potent warming agent. For one, black carbon is actually comprised of tiny black particles (unlike CO2 and methane, which are released into the atmosphere as gases) that, when present in large enough quantities, are visible to the naked eye. When this fine black residue absorbs solar radiation it warms the surrounding air.

Increase in average global temperature
Increase in average global temperature since the 1850s due to CO2 and short-lived species including black carbon. In the Arctic, black carbon is a more significant contributor to warming. Chart reproduced from an AMAP technical report (2008). Cooling from short-lived pollutants is not included in this depiction.

In contrast, CO2 absorbs lower energy long wave infrared radiation that is naturally emitted by the planet, so each molecule of CO2 doesn’t heat the air up as much — there are just so many more molecules of CO2 that they have had a huge cumulative warming impact on the planet in the past 50 years.

Black carbon packs some additional punches when it comes to warming, however. After soot is spewed out from tailpipes, cook stoves, or forest fires, it will last in the air for only a week or two before falling out of the sky or being washed out by precipitation.

While white snow and snow-covered ice reflect nearly all sunlight, the buildup of soot darkens the snowy surface and increases the amount of solar radiation that is absorbed. As this soot-covered surface begins to warm up, snow and ice can melt, which leads to further surface darkening. Of course, this darker watery surface will absorb even more radiation, which warms the surrounding surface and air; this cycle of warming, melting and further melting is known as the “ice-albedo feedback loop.”

Jacobson isn’t the only scientist who says reducing black carbon emissions is an attractive approach to slowing down global warming and Arctic sea ice loss in the near-term. In fact, a growing collection of research articles and public policy papers indicate that removing soot from the atmosphere is an attractive short-term goal because of the immediacy of its cooling effects.

These researchers consistently emphasize, though, that cuts in CO2 emissions are still needed to prevent long-term global warming. Tackling soot is not a substitute for reducing CO2, they say, but rather can contribute to a more holistic approach.

“The good news is that we can shut off black carbon’s warming today, like flipping a switch,” said Tami Bond, an engineering professor at the University of Illinois in a recent press release. “We can’t ignore our long-term problem — greenhouse gases, which will stay around for decades. But black carbon is washed out of the atmosphere within a couple of weeks, and its snow warming lasts about a season. If we stop emitting, this warming will stop too.”

Significant Uncertainties Remain

Though a plethora of models point to black carbon as a major contributor to global and Arctic warming, there is a hiccup with that idea. Though the Arctic has been warming rapidly over the past few decades, the observed levels of soot in parts of that region have actually decreased by about 55 percent since the early 1990’s.

There simply doesn’t seem to be a clear correlation in the Arctic between the amount of soot and warming, says Sangeeta Sharma, an atmospheric chemist from Environment Canada who has been looking at the surface level soot measurements from three different locations in the Arctic.

Then there is the issue of co-pollutants. Because soot is emitted from diesel vehicles and cook stoves using coal or biomass, along with many other pollutants that can either increase and decrease atmospheric warming (including sulfates), she says that soot isn’t the only chemical that matters when it comes to short-term Arctic warming. "Just looking at one [pollutant] doesn’t give a complete answer," she explains. "Sulfate levels in the high Arctic have also decreased by about the same amount as the soot over the same period of time, which could result in increased warming.”

Although decreasing levels of soot have been observed across the Arctic, Sharma cautions that because those measurements are only taking place at the surface, it is difficult to say how much soot is actually in the column of air above. It could be that at the surface, black carbon has decreased but that at higher altitudes it has actually increased, she says, but for now it is hard to measure that difference.

Considering all the unanswered questions of how much soot is actually making its way into the Arctic, Sharma says that modeling studies like Jacobson’s are essential for better understanding how changing emissions of soot and other co-pollutants will impact the region’s climate, but cautions that they can't predict exactly what changes will be observed in the future.

Curtailing Soot Emissions

Despite the uncertainty that still exists between the role soot has played in the Arctic warming trend, the increasing evidence connecting soot to short-term climate change has prompted scientists like Jacobson and Ramanathan to push for reductions of the pollutant. The technologies, they say, already exist to eliminate most soot emissions from fossil fuels and biofuels. For example, diesel vehicles can be retrofitted with particulate filters that catch soot, and cleaner-burning cook stoves already exist, although relatively few have been deployed in many developing countries. Ramanathan is currently working to bring clean burning cook stoves to villages in India.

“I think that’s pretty low-hanging fruit,” said Jacobson. “There just hasn’t been an incentive to make a change, even though the technology is there.” He also points out that biofuel soot released from wood or dung-burning cook stoves is responsible for nearly a million and half deaths worldwide each year, and that any attempt to eliminate this form of air pollution would be positive in terms of global health.