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 (CO 2 ) 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 CO 2 than you started with.

Consider the proposal as a chemical reaction: CO 2 plus energy yields carbon and oxygen. This formula essentially reverses coal combustion (carbon plus oxygen yields CO 2 and energy). If energy from coal were applied to drive the decomposition reaction, more CO 2 would be released than consumed, because no process is perfectly efficient.