Instead of sequestering carbon dioxide to reduce its effects on global climate, why don’t we split it into harmless carbon and oxygen?
—J. Henderson, Devon, Pa.

James E. Miller, a chemical engineer at Sandia National Laboratories, breaks it down:

Splitting carbon dioxide (CO₂) into carbon and oxygen can in fact be accomplished, but there is a catch: doing so requires energy. If hydrocarbon fuels, which produce the greenhouse gas in the first place, supply that energy, thermodynamics tells us that the net result will be more CO₂ than you started with.

Consider the proposal as a chemical reaction: CO₂ plus energy yields carbon and oxygen. This formula essentially reverses coal combustion (carbon plus oxygen yields CO₂ and energy). If energy from coal were applied to drive the decomposition reaction, more CO₂ would be released than consumed, because no process is perfectly efficient.

Another option would be to harness a carbon-free energy source to drive a reaction that does not merely undo the combustion process but instead uses carbon dioxide as an input to generate useful, energy-rich products. At Sandia National Laboratories, we are working to apply concentrated sunlight to drive high-temperature thermal reactions that yield carbon monoxide, hydrogen and oxygen from CO₂ and water. Carbon monoxide and hydrogen are basic chemical building blocks that find use in producing synthetic fuels, so we call this process “sunshine to petrol.”