At last, world leaders have recognized that climate change is a threat. And to slow or reverse it, they are launching initiatives to reduce greenhouse gases, especially carbon dioxide, the gas responsible for about half of global warming to date. Significantly reducing emissions of carbon dioxide is essential, as they will likely become an even greater cause of global warming by the end of this century. But it is a daunting task: carbon dioxide remains in the atmosphere for centuries, and it is difficult to get governments to agree on reducing emissions because whereas the benefits of doing so are shared globally, the costs are borne by individual countries. As a result, no government is moving fast enough to offset the impact of past and present emissions. Even if current emissions were cut in half by 2050 -- one of the targets discussed at the 2008 UN Climate Change Conference -- by then, humans' total contribution to the level of carbon dioxide in the atmosphere would still have increased by a third since the beginning of this century.
Meanwhile, little attention has been given to a low-risk, cost-effective, and high-reward option: reducing emissions of light-absorbing carbon particles (known as "black carbon") and of the gases that form ozone. Together, these pollutants' warming effect is around 40-70 percent of that of carbon dioxide. Limiting their presence in the atmosphere is an easier, cheaper, and more politically feasible proposition than the most popular proposals for slowing climate change -- and it would have a more immediate effect.
Time is running out. Humans have already warmed the planet by more than 0.5 degrees Celsius since the nineteenth century and produced enough greenhouse gases to make it a total of 2.4 degrees Celsius warmer by the end of this century. If the levels of carbon dioxide and nitrous oxide in the atmosphere continue to increase at current rates and if the climate proves more sensitive to greenhouse gases than predicted, the earth's temperature could rise by as much as