Every winter, a heavy haze of pollution envelops many Asian cities.
The smog is a local hazard: Reduced visibility grounded planes in New Delhi in December, while toxic amounts of particulate matter kept Beijing residents indoors several days last month.
New research showing that a component of this pollution also plays a big role in global warming could boost efforts to make air-pollution control in Asia part of the international climate agenda.
A study last month by the International Global Atmospheric Chemistry Project found black carbon or soot – the smoky particles released by inefficient burning of fuel – to be the second biggest contributor to warming after carbon dioxide.
The findings are especially relevant for India and China, which are among the biggest emitters of black carbon, largely from the use of coal and wood for cooking and heating, and from the rising number of vehicles on the road.
But with both countries wary of a focus that would push more of the climate burden onto them, and considerable scientific uncertainty remaining over soot's effects, experts here say that domestic concern about public health is still the most potent reason to cut emissions in the region.
Ambient air pollution is the sixth biggest risk factor for death in South Asia, according to the World Health Organization's Global Burden of Disease Report released last month. Indoor air pollution – from burning wood and coal for cooking food – is the third biggest risk factor.
"The Indian government already has a good reason to act on pollution – the health of the people," says Anumita Roychowdhury, director of the air pollution and transportation program at New Delhi's Centre for Science and Environment (CSE). "If climate benefits can help do that too, good. But the approaches should be separate and they should not divert attention from carbon dioxide."
A Way to 'Buy Time'
Last month's soot study bolsters a recent push by the US government and the United Nations Environment Program (UNEP) to cut emissions of "short-lived climate forcers" – pollutants like soot and methane that vanish from the air relatively quickly – as a way to "buy time" in the battle to hold warming down to two degrees Celsius in the next two decades.
At Doha last month, where climate talks on reducing greenhouse gases made little headway, UNEP's Climate and Clean Air Coalition including Europe, Japan and some Latin American countries agreed to cuts in methane, soot, and ozone. The cuts, they said, could represent a 0.5 C reduction in warming by 2050 – and an estimated 2 million lives saved in air-pollution related deaths.
So far, Bangladesh is the only developing country in Asia to join the UNEP coalition – a sign of the challenges ahead.
"A voluntary global initiative on short-term climate forcers is welcome," said R. R. Rashmi, joint secretary of the Indian ministry of environment and forests and lead climate negotiator, in an e-mail interview, "if it has the potential to address issues relating to technological interventions, availability of appropriate technologies and financial resources at the global level."
"The science and impact of black carbon on global climate change is yet to be fully established or understood," Mr. Rashmi emphasized, and has a bearing on "the availability of energy sources and energy access for large rural populations."
(Read More: India's Air the World's Unhealthiest, Study Says)
Some of the caution has to do with the paradoxical effects of black carbon.
Dark soot particles absorb heat and warm the air. But they also promote cloud formation – which shades and cools the earth. Soot is emitted as part of a family of particles known as aerosols. Some aerosols like soot warm the atmosphere, but some like organic carbon and sulfate have a more cooling effect.
Soot is the second biggest global warming agent only when you don't take into account the influence of these cooling companions.
When the effects of all these co-travelers are added, "the net effect of all aerosols is estimated to be near zero, with large uncertainty," said Tami Bond, a coauthor of the new soot study and associate professor at the University of Illinois in Urbana-Champaign, in an e-mail.
Whether cutting soot emissions will reduce warming thus depends on what other aerosol the source is emitting. Curbing certain industrial emissions or open field and savannah burning – the latter emits 40 percent of global soot – could lead to more warming, since those sources also emit a lot of offsetting cooling sulfates.
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Even burning wood for cooking produces organic carbon that offsets warming, pointing to the challenges of syncing health and climate solutions. The climate gains of clean cookstoves would depend on the choice of replacement fuel or technology.
"This is why we say you can reduce a fraction of the global warming from black carbon, but not nearly all of it, by selecting the right sources," says Ms. Bond.
The caveats are what make Jayaraman Srinivasan, head of the Divecha Centre for Climate Change at the Indian Institute of Science in Bangalore, skeptical that controlling soot will slow global warming.
"Unless we find a way to control black carbon without controlling its cousins that come along with it, cooling is not guaranteed," he says.
The scientific uncertainty is still too large to guide policy, Mr. Srinivasan says. "What is certain is that all aerosols are bad for health and that should be our guide for action."
UNEP advisers say that local health concerns are what will help sell climate soot control measures. That dual benefit also drives the US-funded Global Alliance on Clean Cook stoves campaign, which aims to help 100 million poor households convert to cleaner fuels and stoves by 2020.
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The other advantage is that the solutions for short-lived forcers are already at hand. "It's easier than shifting our economies from fossil fuels to renewable energy," agrees Indrajit Bose, a climate policy researcher at CSE. "But remember the benefits are also short-term.
Tackling the three major sources of soot in India – diesel engines, wood and biofuel stoves and brick kilns – requires no technological innovation. What is needed is equally challenging: capital investments, complex logistics, and enormous political will.
Replacing India's 100,000 outdated brick kilns would mean outlays of at least $25,000, according to one study. Upgrading cook stoves is not expensive, but it requires converting the behavior of millions of families across the country.
India has done it before to curb vehicular pollution – but with some judicial prodding. Court orders mandating natural gas-fueled buses and taxis helped clean up Delhi air a decade ago. The government's 2003 autofuel policy forced vehicle manufacturers and oil companies to meet Euro IV-equivalent emission standards in 20 Indian cities and Euro III standards in the rest of the country by 2012.
Those air quality gains have since been cancelled out by a jump in vehicle numbers, especially more-polluting diesel cars.
The rising pollution – including this winter's smog – may be spurring action again: Environmental authorities recently recommended a ban on diesel cars in Delhi. Long-standing diesel subsidies, which were originally meant to help farmers and transporters but have skewed the passenger car market, were drawn down for the first time in January.
"Where we are slipping is we are still waiting for the next auto roadmap," says CSE's Ms. Roychowdhury, to convert vehicles to the latest Euro VI emission standards.
That upgrade won't be cheap: $1,330 dollars per diesel car, $4,810 per diesel truck, estimates the ICCT, and almost $2 billion on refinery investments.
But the ensuing air quality improvements could save half a million lives in cities alone. "Do we really need a climate benefit to act on this front?" asks Roychowdhury.