by Carolyn Gramling Thursday, January 5, 2012
Black carbon — fine particles of soot in the atmosphere produced from the burning of fossil fuels or biomass — has been known to be a health hazard for decades, a major contributor to the thick hazes of pollution hovering over cities around the world. But over the last decade, scientists have been examining in increasing detail the various ways in which these particles contribute to another hazard: heating up the planet.
In 2007, the Intergovernmental Panel on Climate Change (IPCC) reflected this increasing interest in black carbon by including the particles' impact on climate in an assessment report for the first time. The IPCC estimated that the particles could heat up the planet by 0.34 Watts per square meter — about equivalent to the warming potential of ozone, and somewhat less than that of methane (at 0.48 Watts per square meter). Other estimates over the last decade have suggested a similar or slightly higher climate impact.
Then, a 2007 study raised black carbon’s profile even higher, suggesting that it is a climate change threat second only to carbon dioxide. Atmospheric scientist Veerabhadran Ramanathan of the Scripps Institution of Oceanography and chemical engineer Greg Carmichael of the University of Iowa suggested in Nature Geoscience that black carbon could be responsible for as much as 18 percent of the planet’s warming (carbon dioxide is responsible for about 40 percent) — three to four times higher than the IPCC estimated. That would make it a more powerful warming agent than methane, CFCs, nitrous oxide or tropospheric ozone.
But the picture scientists have of black carbon’s impact is still hazy. Adding to the uncertainty is that the particles also influence the climate and the planet’s heat budget in indirect — and often conflicting — ways. For example, they can reduce the albedo of snow-covered regions such as the Arctic (reducing how much solar radiation the white snow reflects back into space and thus further warming the planet). Conversely, they can increase cloud cover, which can help cool the planet. Much of this is only now being observed, studied and included in climate models, and the overall impact of these particles is still far from clear.
"We are just beginning to unravel the impact of black carbon on human health, indoor and outdoor air quality, temperature, cloudiness, precipitation, mountain glaciers, sea ice and snowpacks," Ramanathan told scientists at the American Geophysical Union’s (AGU) annual meeting in San Francisco, Calif., last December.
And the more we learn about how black carbon affects climate, the hazier the picture becomes. Given these uncertainties, scientists are wrestling with a nagging question: When it comes to climate change mitigation, what should we do about black carbon?
“Black carbon” is a catchall term for particles from a variety of sources: diesel exhaust, coal combustion, biofuel burning, the burning of crops and grasses, forest fires, even primitive cook stoves in rural villages, says Monika Kopacz, a climate scientist and program manager at NOAA’s Climate Program Office in Silver Spring, Md.
“We — modelers — tend to call all of these things black carbon,” Kopacz says. “But everything is a mixture; it’s not all just graphite, elemental carbon.” Scientists use words such as “carbonaceous aerosols” and “soot,” often interchangeably, to refer to the same uncertain mixture of particles — and this uncertain terminology is a manifestation of the uncertainty of the field, she adds. For example, there’s a growing interest in a related topic: so-called “brown carbon,” which may also be a mixture of black carbon, organic carbon and other particles.
Scientists studying black carbon do agree on some points. Black carbon’s negative impact on health is well-established, Kopacz says. And the overall, direct atmospheric impact of soot particles suspended in the atmosphere is to warm it up by absorbing incoming solar radiation, says Dorothy Koch, a climate scientist and program manager in the Climate and Environmental Sciences Division at the U.S. Department of Energy in Germantown, Md.
Still, Koch notes, there is disagreement over how powerfully that atmospheric impact affects climate. “There’s still quite an array of estimates,” she says.
In 2001, atmospheric scientist Mark Jacobson of Stanford University presented one of the earliest arguments for the importance of black carbon in heating up the climate. Black carbon, he reported in Nature, appeared to be unique among aerosols. Unlike aerosols such as sulfates, which have a cooling effect because they reflect sunlight back into space, black carbon is a climate warmer: It absorbs the solar radiation, heating up the atmosphere.
Even more significantly, Jacobson found that when heated by the sun, the soot in the atmosphere warmed at a rate of 0.55 Watts per square meter — making it more absorbing by mass than methane and carbon dioxide, which warms at a rate of 1.56 Watts per square meter. Ramanathan and Carmichael’s 2007 estimate was even higher, suggesting that black carbon’s direct warming effect is about 0.9 Watts per square meter.
But other global models of black carbon’s direct effect on climate, while still showing that it is a warming agent, suggest a considerably smaller impact — still important, but not second to carbon dioxide, Koch says. “We all came out with the same sign,” she says. But “we’re all struggling to understand why we’re getting different answers.”
The challenge of figuring out the exact effect of black carbon on the atmosphere gets considerably more complicated because the impact of black carbon on climate and warming varies depending on where it is around the planet: not only regionally, but also its altitude within the atmosphere — and whether it is still suspended or has settled out, particularly in snow-covered regions.
How high the particles are in the atmosphere, such as whether they’re suspended above or below clouds, matters: Warmed-up soot near the land or ocean surface can alter updrafts and dissipate clouds; soot higher in the atmosphere has a longer lifespan, and can travel for long distances and more efficiently absorb heat than particles lower in the atmosphere or obscured by clouds, Koch says.
And black carbon’s warming influence isn’t limited to the atmosphere. When transported to snow-covered regions, the particles can settle on the snowpack and darken it — reducing its ability to reflect solar radiation back into space, which then further warms the planet and melts more snow.
The soot that settles onto glaciers and mountain snowpack comes from a variety of sources. Although some sources, such as diesel burning, are more likely to contribute a warming mix of aerosols than others, such as cookstoves, there’s little dispute that any atmospheric black carbon that lands on snow-covered areas promotes warming, Kopacz says.
Still, the relative importance of different sources of black carbon settling in snow-covered regions may be very different from what dominates the atmosphere, says atmospheric chemist Dean Hegg of the University of Washington. And although they are all warming agents, understanding the sources to these regions can have significant implications for mitigation strategies, he says.
Variations in the relative importance of these sources are due in part to regional differences in source outputs, Hegg says. For example, biomass burning from cookstoves in India is considerably more important as an input source to the Himalayas and the Tibetan Plateau, while crop and grass burning — such as the slash-and-burn method to fertilize soil and develop farmlands — is more prevalent in other parts of Eurasia.
But there are also significant differences in atmospheric transport patterns. The primary global sources of black carbon are industrial and biofuel emissions from South Asia, as well as biomass burning, reported Koch and James Hansen of NASA Goddard Institute for Space Studies in the Journal of Geophysical Research in 2005. That black carbon, they noted, rises quickly to high altitudes — higher than 3 kilometers — and thus readily travels poleward to the Arctic.
But, Hegg notes, black carbon traveling at that altitude is much too high to be incorporated in precipitation. So, as it turns out, South Asian black carbon doesn’t have much of an impact on the snow. Furthermore, as Koch and Hansen noted, South Asia contributes only about 25 percent of the low-altitude springtime “Arctic haze,” the visible pollution that hovers over the dry Arctic for as much as a month at a time. The rest derives from biomass burning in Russia, North America and Europe.
Sorting out these relative contributions matters, not only because different types of light-absorbing aerosols may have a different impact on warming, but also because it is crucial to developing short-term mitigation plans. To that end, Hegg has been working to distinguish the relative importance of black carbon sources to different locations in the Arctic: Siberia, the Canadian lower Arctic and Greenland. Because the black carbon will likely have the strongest impact on the region’s albedo in springtime — when light begins to return to the Arctic but before the snow melts — Hegg’s research has focused on black carbon concentrations measured in the spring over three different years: 2007, 2008 and 2009. For all three years, Hegg reported at the AGU meeting last December, more than 90 percent of the black carbon in the Arctic sites was derived from biomass burning. Hegg and his co-authors also discovered that in these sites, the biomass was primarily crops and grasses, rather than forests or wood.
That suggests some ideas for mitigation, Hegg says. “If crop and grass burning is the more important source, the strategy for mitigating black carbon in the snow is quite different than for industrial sources,” he says. “The way to mitigate may be to change the time at which you burn biomass — say you delayed burning residue from crops and grass to fertilize soil by a few weeks.” Although that would attenuate the growing season for some farmers, he adds, it would also delay a large source of pollution until much of the Arctic snow was already melted. “This gives you an idea of why it’s important to try to determine the sources.”
The components of black carbon also turn out to matter quite a bit in the atmosphere as well. One often-overlooked issue, Koch says, is that in addition to soot, different black carbon sources emit a variety of other chemicals — and “depending on what’s mixed in, you get a different impact on the climate.”
Indeed, resolving the overall impact of such co-emitted pollutants is one of the major uncertainties in quantifying the link between black carbon emissions and climate effects, says atmospheric scientist Tami Bond of the University of Illinois at Urbana-Champaign.
For example, Bond says, coal burning emits black carbon, but it also emits sulfur dioxide that is transformed into sulfate particles in the atmosphere — which ultimately have a cooling effect on climate. A 2010 study led by Carmichael and published in Nature Geoscience confirmed that the two aerosols have a conflicting impact. Carmichael and his colleagues examined ground-level air samples, atmospheric samples at 30 meters and atmospheric samples at 5,000 meters altitude, and found that the higher the ratio of black carbon to sulfate in the atmospheric samples at all altitudes, the more solar radiation was absorbed.
Wood burning, meanwhile, produces both black carbon and organic carbon — which, when mixed together, are more absorbing than when they stay separated, Koch says. Different sources can also emit differently sized soot particles, which affects their absorbance, she adds. “The biomass burning particles tend to be a little bit bigger, and tend to be mixed with organic carbon, which is a little more hygroscopic than diesel.” As a result, those particles were more likely to become the seeds of clouds.
Black carbon has only a short lifetime in the atmosphere — about a week, compared to carbon dioxide’s 100-year lifespan. But in that short time, black carbon can “age” considerably, undergoing multiple chemical transformations that further complicate the picture. And, to make models of black carbon’s impact even more complicated, that aging process can convert it from a climate-warmer to a climate-cooler.
The interaction of black carbon and clouds is one of the biggest question marks in aerosol modeling — and in climate modeling, as clouds are generally thought to have a cooling effect. In a 2010 review paper in Atmospheric Chemistry and Physics, Koch and physicist Anthony Del Genio of NASA Goddard Institute for Space Studies outlined 10 aerosol-related processes that could either increase or decrease cloud cover — factors such as the relative altitudes of a plume of smoke and a cloud, the type of cloud and other meteorological conditions. For example, they noted, soot embedded in clouds promotes cloud evaporation, but soot blown from land up and over stratocumulus clouds over the oceans has been observed to stabilize clouds. Overall, they found, more strongly absorbing aerosols — like black carbon — have the strongest impact on clouds.
One effect of black carbon’s “aging,” in fact, can be to make the particles gradually more reactive with water until they become the seeds of future clouds, called cloud condensation nuclei. The environmental and climate factors that moderate the transformation of water-hating (hydrophobic) black carbon into water-loving (hydrophilic) cloud seeds are the subject of considerable research.
In one such study, atmospheric chemist Torsten Tritscher and a team of scientists at the Paul Scherrer Institut in Switzerland examined how diesel soot ages in the lab. Although freshly emitted, “pure” soot is hydrophobic, over time, photochemical reactions produce a more hydrophilic coating around the particles, transforming them into the seeds of clouds, Tritscher’s team reported at the AGU meeting in December. Other studies presented at the same meeting examined different ways in which black carbon could transform to become more hydrophilic. For example, in laboratory studies that simulated the aging of carbon, graduate student Beth Friedman of the University of Washington and her colleagues found that as the particles became coated with different atmospheric contaminants, they became more likely to nucleate ice.
These kinds of studies, including dozens presented at AGU examining how black carbon transforms into clouds, help to indicate the complexity of the problem, Koch says: Many of these environmental and atmospheric factors tend to be regional, so it’s difficult to model the impact of black carbon on cloud formation on a global scale.
Indeed, the study of cloud formation, as with the albedo impact, is a field “where a more regional analysis is useful,” Kopacz says. Globally, she says, “there are so many sources of uncertainty. It’s a really complicated picture.”
The growing interest in the black carbon question has been driven in part by Jacobson and Ramanathan’s work and in part by the IPCC’s 2007 report, Kopacz says. “In general, there has been a push toward studying short-lived climate forcers,” she says. “There’s been a recognition that it’s not just carbon dioxide or long-lived greenhouse gases that warm the atmosphere, but a whole set of things that can really contribute to climate change” — and in still very uncertain ways. In the IPCC’s 2007 report, she notes, the overall contribution from all aerosols toward warming was basically zero — with huge error bars in both directions. “That’s prompted a lot of people to see what’s going on with aerosols.”
Indeed, as policy battles over carbon dioxide rage on, many scientists have suggested that reducing soot emissions might be the quickest way to mitigate global warming in the short term because of black carbon’s short atmospheric lifespan and large potential impact on climate. Jacobson, speaking before the U.S. House of Representatives Committee on Oversight and Government Reform in 2007, suggested that curtailing the emissions of all black carbon sources might seem to be the safest bet.
Other scientists have also urged Congress to take action. In March 2010, Bond testified before the House Select Committee on Energy Independence and Global Warming, stating that the question of what to do about black carbon is more urgent than it appeared even a few years ago in the 2007 IPCC report. That report, she said, may have underestimated the direct heating impact of black carbon, because its models did not include the transformations that black carbon undergoes in the atmosphere. “Black carbon collides and interacts with other particles, so that each particle contains many chemicals, not just black carbon,” she told the committee. “This mixing increases the absorption of black carbon by about 50 percent.” That in turn makes the forcing “much higher than most models predict.”
But what action to take is not entirely clear. The strong impact of aerosols on clouds, and the generally cooling impact of clouds, “suggests the need for caution when pursuing mitigation of soot in order to cool climate,” Koch and Del Genio noted in their 2010 study. At the same time, they noted, “relatively few global model studies have been conducted, and the global model cloud responses should be better tested against cloud scale models and field studies.”
Still, many scientists insist that curtailing black carbon emissions may be the safest bet. In her testimony before the House Committee last year, Bond said, “Reducing black carbon and ozone in the atmosphere is like applying an emergency brake in a car out of control. It will slow the vehicle quickly and give you a little time to think.” But, she added, it’s only a partial, quick fix. “The problem will continue if you don’t take your foot off the gas pedal — that is, if carbon dioxide emissions are maintained.”
And Ramanathan, testifying before the same committee, made his case even more plainly: “Black carbon is an important, fast-action tool in mitigating long-term warming.”
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