BERKELEY, Calif. — Think, for a moment, of carbon dioxide as garbage, a waste product from burning fossil fuels. Like other garbage, almost all of that CO 2 is thrown away — into the atmosphere, where it contributes to climate change. A small amount is captured and stored underground to keep it out of the air.

But increasingly, scientists are asking, rather than throwing away or storing CO 2 , how about recycling some of it?

At laboratories around the world, researchers are working on ways to do just that. The X Prize Foundation has created an incentive, a $20 million prize for teams that by 2020 come up with technologies to turn CO 2 captured from smokestacks of coal- or gas-fired power plants into useful products.

But perhaps the ultimate goal of researchers in this field is to turn the waste product of fuel-burning into new fuel. In theory, if this could be done on a large scale using renewable energy or even sunlight, there would be no net gain of emissions — the same carbon dioxide molecules would be emitted, captured, made into new fuels and emitted again, over and over.