BodyTwo degrees Celsius: the point of no return. Once average global temperatures exceed preindustrial levels by this amount, scientists warn, a climate catastrophe could become inevitable. Current projections indicate that it would be too late to prevent sea ice from disappearing, ice sheets from collapsing and rising seas from swallowing heavily populated coastlines. A whole new climate would emerge. Lasting millennia, this hotter world would be humankind’s most enduring legacy, perhaps outlasting our species. Fears over this new climate order spurred more than 100 nations to sign the Copenhagen Accord, drafted in 2009. Although nonbinding, the signatories agree that actions are needed to hold global warming to no more than 2 degrees Celsius (3.6 degrees Fahrenheit). But CO2 isn’t the only culprit in global warming. Over the last several decades, researchers have identified a substantial supporting cast of co-conspirators. Four pollutants that make a relatively brief but powerful appearance in the atmosphere are known as the short-lived climate warmers: methane, near-surface ozone, a class of industrial chemicals called hydrofluorocarbons and the black-carbon component of soot. These typically stick around the atmosphere for no more than a week to roughly a decade. A growing number of climate scientists say the quickest way to stall Earth’s slowly rising fever in the short term is to reduce the output of these more ephemeral warmers. In fact, some studies indicate, clearing them from the atmosphere might drastically shrink the rate of climate change in a mere 10 to 20 years and help the world avoid as much as 0.5 degrees Celsius of warming by 2050. Such cuts could buy nations a little time to tackle the more challenging problem of weaning themselves from CO2-producing fossil fuels, the only way to prevent catastrophic warming in the long run. Cleaning up methane, black carbon and other short-lived warmers requires no new technology and would improve air quality, an added benefit that could save millions of lives annually while boosting agricultural productivity. As a result, countries are joining the short-lived-warmers bandwagon. In 2011, the European Parliament passed a resolution urging its 28 member nations to adopt climate policies that target these pollutants. Last year, the United States partnered with several other countries to found the Climate and Clean Air Coalition, which promotes initiatives to cut emissions of short-lived warmers. In September, the United States, China and dozens of other countries took a more concrete step and agreed to reduce the use of hydrofluorocarbons, also known as HFCs. But some critics argue that these fleeting climate warmers may offer more hype than hope. Scientific uncertainty still surrounds some of them, especially black carbon. Depending on the conditions, it can either warm or cool the climate. It also enters the atmosphere along with a mix of climate-cooling pollutants. So targeting soot may not control near-term warming as reliably as researchers have claimed. Worse, some scientists worry, eliminating it might actually aggravate global warming. The power of soot View larger image | Produced by diesel trucks, forest fires, wood-burning stoves and some industrial activities, black carbon warms the atmosphere by absorbing sunlight and by capturing heat radiated from the Earth. It also enhances warming by breaking up clouds, but in other conditions counteracts warming by seeding clouds. After settling on the ground, black carbon also encourages the melting of snow and ice. Credit: Source: T. Bond et al/J. of Geophys. Res.: Atmospheres 2013; credit: M. AtarodWith the climate doomsday clock ticking away, viable options that offer even a smidgen of relief can’t be ignored. “Dealing with these [warmers] doesn’t get you off the hook of dealing with carbon dioxide,” says climate scientist Drew Shindell of the NASA Goddard Institute for Space Studies in New York City. But they “at least offer a way to make some progress.” A motley crew Without a doubt, CO2 is the chief driver of human-caused climate change. People have flooded the atmosphere with so much CO2 that it’s responsible for more than half of the extra energy humans have trapped around the planet. So focusing on ways to chop its emissions is “a sensible thing,” Shindell says. But it’s not the only greenhouse gas humans unleash into the air. Like CO2, methane absorbs heat emitted from the Earth. Fossil fuel production releases methane to the atmosphere. So do landfills and wastewater treatment plants, where microbes generate the gas. Cows and other livestock also belch out a substantial amount. Although atmospheric CO2 is more than 200 times as plentiful, methane is actually the more potent heat-trapper: Over 100 years, a kilogram of methane emitted into the atmosphere will absorb 25 times as much heat as a kilogram of CO2. Methane is one of the main gases that contribute to the formation of another short-term warmer: ground-level ozone. Way up in the stratosphere, ozone filters out much of the sun’s biologically harmful ultraviolet light. Closer to the ground, ozone isn’t helpful. In the troposphere, it absorbs heat, creates smog and interferes with plant growth. Over the last century, near-ground ozone concentrations in the Northern Hemisphere have more than doubled. Enlarge Weighing the options View larger image | Reducing different greenhouse pollutants will produce warming reductions that vary in both magnitude and timing (graph, left). These projections are based on the fact that some gases are more potent warmers, and some are relatively short-lived (table, right). That means reducing their emissions will bear fruit sooner compared with cuts in pollutants like CO2, which persists in the atmosphere for a century or more. Credit: Chart, source: Y. Xu et al/Atmos. Chem. Phys. 2013; table, source: EPA, IPCC, T. Bond et al/Journal of Geophysical Research: Atmospheres 2013Hydrofluorocarbons are a more recent concern. Companies are increasingly turning to these organic compounds as greener replacements for stratospheric-ozone-destroying chlorofluorocarbons in refrigeration, air conditioning and foam production. Although HFC emissions currently are very low, one molecule of these chemicals can soak up hundreds of times more heat than a molecule of CO2. And their releases are expected to double within seven years. Many recent climate simulations have largely overlooked the advantages of cutting HFCs because their emissions are still so small. “The benefit you gain is by preventing them from growing,” Shindell explains. Given the pace of growth in HFC emissions, researchers have started giving them a closer look. Unchecked, HFC emissions could contribute nearly as much to global warming by 2100 as black carbon does today, concludes Veerabhadran Ramanathan, an atmospheric scientist at the Scripps Institution of Oceanography in La Jolla, Calif. Replacing HFCs with any of several available substitutes could shave a half-degree from global warming by century’s end, he and several colleagues report June 26 in Atmospheric Chemistry and Physics. Black sheep of quick warmers Among short-term warmers, black carbon is the most concerning and complicated. Produced by incomplete combustion, it is one among many tiny solid particles called aerosols that enter the atmosphere in the exhaust of diesel-fueled trucks, the smoke from charcoal-burning cookstoves and the haze from forest fires. In general, aerosol particles produced by combustion cool the planet by scattering sunlight. Not black carbon. In the early 2000s, atmospheric scientist Mark Jacobson of Stanford University demonstrated that black carbon delivers a warming double-whammy. Unlike greenhouse gases, these particles both absorb incoming sunlight and trap outgoing heat. As if that weren’t bad enough, these particles also can boost temperatures by breaking up light-reflecting clouds. And even after it settles back to Earth, black carbon can continue to foster warming. Particles that land in the Arctic or the Himalayas darken ice and snow, boosting heat absorption and melting. Enlarge Added benefits View larger image | Many countries would see boosts in crop yields (values in millions of metric tons) from the reduction of ozone that would result from cutting black carbon and methane emissions. Cuts using existing technologies could thus have benefits beyond climate. Credit: Source: D. Shindell et al/Science 2012In the past, many climate simulations have failed to include all of these varied climate effects, leading some scientists to underestimate soot’s warming potential, notes Tami Bond, an engineer and atmospheric scientist at the University of Illinois at Urbana-Champaign. In the June 16 Journal of Geophysical Research: Atmospheres, she and her colleagues offer the most comprehensive soot assessment to date. It pegs black carbon’s direct effect on warming — not counting cloud- or ice-related impacts — at double some estimates. It’s the second-largest contributor to human-driven warming, causing almost two-thirds as much over the past two centuries as CO2. But in contrast to CO2’s century-long life span, black-carbon particles typically settle out of the atmosphere within a few days to weeks. The potency and short life span associated with non-CO2 greenhouse pollutants is giving climate-mitigation analysts some much-needed optimism. Last year a team led by NASA’s Shindell reported in Science that adopting 14 tactics to curb emissions of black carbon and methane (and by extension tropospheric ozone) could reduce global warming by roughly half a degree Celsius within the next 35 years or so. These reductions would come through activities such as capturing the methane that currently escapes from oil and natural-gas wells and pipelines, adding soot filters to diesel vehicles and switching to cleaner-burning cookstoves in developing countries. For this simulation, the researchers assumed that cuts began in 2010 and only gradually reached full implementation by 2030. These reductions would do more than just fight global warming, Shindell’s group adds. By trimming black carbon and other aerosols that make up particulate matter, as well as ground-level ozone, the pollution cuts would prevent up to 4.7 million deaths annually. And at the same time, global crop yields would grow by as much as 135 million metric tons a year. Cuts in short-lived warmers would have their biggest impacts on the Arctic. Temperatures there are rising at about twice the global average. At that rate, summer sea ice could disappear within the next 10 to 30 years, says Stanford’s Jacobson. “If you control all the CO2 in the world, it’s not going to save the Arctic sea ice because of the long lifetime of CO2.” Controlling the shorter-lifetime pollutants is the only way to do it, he argues. In 2010, Jacobson demonstrated exactly how much influence black carbon alone has on the Arctic. Over just a 15-year span, controlling soot emissions by installing filters and replacing dirty cookstoves would slash Arctic warming by 1.7 degrees, he reported in the Journal of Geophysical Research: Atmospheres. Removing sooty emissions from fossil-fuel burning alone dropped average air temperatures in the Arctic by 1.2 degrees over that period. A cool idea, but . . . Cutting emissions of ephemeral warmers could also dampen sea level rise. Without cuts to these pollutants, the ocean could swell an average of 112 centimeters during this century, Ramanathan and colleagues conclude in the August Nature Climate Change. This estimate reflects both the expected melting of glaciers and ice sheets and the thermal expansion of seawater. If reductions in short-lived pollutants began within the next two years, however, their impact on temperature could help shave 25 centimeters — 22 percent — from the sea level rise expected by 2100, the new simulations project. Climate benefits might extend to rainfall as well. Soot doesn’t disperse evenly across the globe, so its warming effect isn’t evenly distributed. This patchy warming creates temperature gradients in the atmosphere that can alter wind and rain patterns, Shindell says. In his team’s 2012 study, black carbon reductions were projected to reduce recent shifts in South Asia’s monsoon patterns and to lower drought risk in parts of Europe and Africa. To some climate scientists, the potential benefits from targeting ephemeral warmers sound too good to be true. “Black carbon has gotten a bit too much emphasis lately,” argues Gunnar Myhre, an atmospheric scientist at the Center for International Climate and Environmental Research in Oslo. “The uncertainties are really large and much larger than for the greenhouse gases.” For instance, although technologies are readily available to address many sources of the pollution, that doesn’t mean implementing them will be a cinch. Replacing the cookstoves used by 3 billion people in India, China and other developing countries, for example, is a monumental task. In reality, change may not come soon enough to reap all of the near-term benefits promised by climate simulations, says climate scientist Steven Smith of the Joint Global Change Research Institute in College Park, Md. Part of the problem, notes Smith, is that black carbon isn’t emitted on its own. It enters the atmosphere with an assortment of other aerosols — and “aerosols are the big wildcard.” Taken as a whole, aerosols should cool the planet, but by how much remains unclear. Since there aren’t ways to separate black carbon from other particles in soot, cutting black carbon could actually lead to warming if the other lost aerosols would have had a cooling effect. That’s why targeted cuts are key, Bond says. “You shouldn’t go blindly reducing emissions from a source because it happens to emit black carbon.” The smartest move would be to focus on sooty sources that cough out more black carbon relative to other particles. Removing diesel engines from the road or adding particle traps to diesel vehicles is a better bet than, say, trying to limit agricultural burning or control forest fires. Of even greater uncertainty is what black carbon does to clouds. In general, aerosols, including soot particles, help seed clouds, serving as surfaces on which cloud droplets can form. With more cloud droplets, clouds can grow bigger and brighter — and reflect more sunlight away from Earth. Yet if too much black carbon builds within clouds, its heating could end up burning the clouds away. This is why black carbon’s relationship with clouds is so complicated. Depending on where soot is released, its altitude and the type of cloud it encounters, a black-carbon particle can either help build or destroy clouds, says Dorothy Koch, who until recently studied black carbon’s interactions with clouds at Columbia University. It’s difficult to sort out these cloud interactions because it’s hard to trace where black-carbon particles go in the atmosphere and how they change over time as they bump into other aerosols. The June assessment of black carbon by Bond’s team tried to tally all of its influences on clouds. In the end, the researchers found a slight net warming of the atmosphere. But that’s not a given; Bond acknowledges that uncertainties surrounding this result are so big that soot’s interactions with clouds might actually end up being a net source of climate cooling. Still, any cooling from cloud effects is probably insignificant, Shindell says, and unlikely to completely offset black carbon’s warming influence. “There’s very little chance you’d have a negative consequence in terms of climate change” if you cut black carbon. Moreover, he adds, even if the climate benefits prove smaller than anticipated, the health benefits alone would be reason enough to curb black carbon. Smith agrees: “It’s definitely a win-win.” But he doubts the climate benefit from cutting back on short-lived warming pollutants will prove as robust as some proponents tout. In a new climate simulation, he and Andrew Mizrahi, also at the Joint Global Change Research Institute, assumed that between 2015 and 2035, nations begin phasing in all “maximally feasible” cuts to short-lived warmers. The result: Global temperatures will drop only 0.16 degree by 2050 relative to current projections. Their findings appeared August 12 in the Proceedings of the National Academy of Sciences. Smith says there are several reasons that his calculations predict a much smaller benefit than previous studies. In addition to differences in computing emission reductions, he and Mizrahi used a more complex climate simulation that integrates a larger number of factors than typical models. In their scenario, the climate didn’t respond as quickly to declines in short-lived warmers, resulting in a smaller temperature change. Other researchers may quibble with the pair’s calculations, but all agree that any major climate policy must address all warmers. “There’s no need for them to compete with each other,” Shindell contends. In fact, the 2012 simulation by his team suggests the world will surpass the 2-degree warming benchmark by the end of the century if CO2 isn’t also regulated. Ultimately, Shindell says, limiting short-lived warmers can slow the rate of climate change — but not stop it.
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