Scientists generally agree that to limit global warming to less than 2.4 °C--and avoid the worst effects of climate change--greenhouse-gas emissions must be reduced 50 percent by 2050. But humanity is a long way from being weaned from the petroleum, natural gas, and coal whose use causes much of this pollution.

In fact, global energy demand is expected to increase about 40 percent over the next two decades. By 2030, the use of petroleum, coal, and natural gas is expected to jump by 23 percent, 44 percent, and 37 percent, respectively. "You look at the world of renewables and you see a lot of progress, but they are not going to outpace the growing demand for energy," says Peter Jackson, a senior director at IHS Cambridge Energy Research Associates, an energy consultancy and think tank.

The near-term challenge, therefore, is to find ways to extract fossil fuels safely and use them in ways that emit as little carbon dioxide as possible. But dwindling supplies in conventional oil fields are forcing the petroleum industry to drill in deeper waters and pursue hard-to-extract deposits such as the tar sands of Alberta, Canada.

In the next decade, some oil can be replaced by ethanol: U.S. producers will make about 12 billion gallons of it from corn this year. But that's a small fraction of the roughly 170 billion gallons of gasoline and diesel that will be consumed in 2010. Making biofuels at a significantly larger scale will mean using cellulosic biomass as a feedstock for ethanol and other fuels, and deriving fuels from custom-engineered microbes.

Coal, the dirtiest fossil fuel, accounts for 42 percent of electricity production worldwide and 45 percent in the United States. The good news is that natural gas is far more abundant than was thought only a few years ago. And replacing coal plants with natural-gas plants could greatly reduce carbon dioxide emissions.