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.