You can see it as you're flying into New Delhi or rather, you can't see a thing. As the plane descends to the Indian capital on an ordinary November day, it is immersed in air so polluted as to be opaque, a brownish sludge that scatters any sunlight. The air clears a bit once you've deplaned, but the horizon still contracts, pollution closing off the New Delhi sky like a dome.
That soupy brown air is the result of so-called black carbon expelled into the atmosphere in and around the Indian capital, from the burning of biomass for cookstoves and of black coal for electricity, and the incomplete combustion in the old diesel engines that propel most of the cars and trucks in the city. Breathing here isn't all that good for you there's a reason the city is home to the "Delhi cough" and now scientists are discovering that the sooty air isn't good for the climate either. According to some estimates, black carbon may be responsible for as much as 18% of the planet's warming, making it the No. 2 contributor to climate change after carbon dioxide, which accounts for 40%. "The world could think that we just cut CO2 and the problem is solved and we all go home, but it's not," says Veerabhadran Ramanathan, a climatologist from the Scripps Institution of Oceanography and an expert on black carbon. "That's my nightmare."
Black carbon and CO2 each contribute to global warming in different ways. CO2 intensifies the greenhouse effect, allowing sunlight from space to enter through the atmosphere, then trapping the sun's energy as it bounces off the surface of the planet like a greenhouse. (Of course, without some greenhouse warming, the earth would be a cold, dead place, but too much CO2 accelerates the effect and could make the earth too hot to be habitable. The temperature on Venus, for instance, where the atmosphere is 96% CO2, is over 400°C, or 750°F.) By contrast, black carbon in the air actually absorbs sunlight as it comes from space, directly heating up the atmosphere. "The soot particles are like the parts of a blanket, and it's getting thicker," says Ramanathan. "The smoke absorbs sunlight and heats the blanket directly."
Unlike CO2, which can hang around in the atmosphere for centuries CO2 that was emitted by the first coal-powered train is probably still in the air, warming the planet black carbon has a relatively brief life span. It remains just a few weeks in the air before it falls to earth. That's key, because if the world could reduce black carbon emissions soon, it could help blunt warming almost instantly. "You can wait a week or a month and the totals in the atmosphere can be significantly different," says Eric Wilcox, an atmospheric scientist with NASA. Meanwhile, if we were to vastly reduce new CO2 emissions immediately, the billions of tons that already exist in the atmosphere would keep warming the planet for decades.
All of this has particular importance for developing Asian countries, especially India, where a mix of development means that biomass-burning and diesel combustion remains prominent. (In developed countries like the U.S., there's much less burning of biomass and any diesel combustion tends to be much cleaner, as the clearing skies over major U.S. cities demonstrate.) Though India is responsible for less than 3% of global greenhouse-gas emissions, according to Ramanathan it is responsible for about 6% of global black-carbon emissions, give or take a significant margin of error. India and other developing countries rightly argue that rich nations are responsible for the majority of carbon already in the atmosphere, and should therefore take the lead on cutting emissions, but if black carbon is definitively proven to play a large role in warming, poor nations will still be on the hook.
The science is evolving it's so new that black carbon wasn't even listed as a warming agent in the most recent report from the Intergovernmental Panel on Climate Change but it cannot be ignored. Black carbon is already having an impact on the ice atop the Himalayas, the massive glaciers that feed the major rivers of Asia when they melt each spring. Thanks to global warming, these glaciers are receding, threatening the long-term water supplies for the region. Ramanathan, Wilcox and an Indian glaciologist Syed Iqbal Hasnain are working to figure out the impact of black carbon on glacial loss. Beyond warming the atmosphere, black carbon can also speed the melting of glaciers by literally turning them black soot on snow makes the ice heat up faster. "When black carbon falls on the snow, it darkens it," says Ramanathan. "If the snow is white, it reflects 80% of the sunshine, but with black carbon it absorbs the sunlight."
The good news is that while taking CO2 out of our energy cycle has proven very difficult especially in poorer developing nations black-carbon emissions should be easier to curb. Reducing deforestation will help the burning of tropical rain forests is a big contributor to the black-carbon load. Next, diesel filters in cars can be upgraded, and biomass-burning stoves can be exchanged for technology that uses solar power or natural gas. These changes will cost money, but they should be cheaper than decarbonization. And cutting back on black carbon will also pay immediate health dividends, with less air pollution and fewer deaths from respiratory diseases. We might even be able to see the sky in New Delhi again.