Why can't we burn CO₂ to stop global warming?

If it could work it might be a good idea. Unfortunately it's a no-go, and here's why:

All fuels have carbon in them. Coal is almost pure carbon, gasoline and oil are mostly carbon and hydrogen, alcohol is carbon, hydrogen, and oxygen. When you burn any of these fuels, carbon atoms in the fuel combine with oxygen from the air to make carbon dioxide, and energy is released.

Once carbon has been combined with oxygen you can't add any more oxygen to the carbon -- in other words, carbon dioxide doesn't burn. In fact, carbon dioxide is often used in fire extinguishers precisely because it does not burn and can smother a fire.

It is possible to turn carbon dioxide back into carbon and oxygen, but this always costs energy. You get energy by burning carbon and making carbon dioxide, but to reverse this you need to add in energy.

Suppose that it were somehow possible (which it isn't) to burn carbon dioxide and somehow get back carbon and oxygen. If this got you energy, or even if it could be done with no cost, it would mean that you could start off with some carbon, burn it, get energy, then get the carbon back again, and start all over. This would mean getting endless energy for nothing -- which is obviously impossible.

The best way to turn carbon dioxide back into carbon-containing fuels and oxygen is to let plants handle the job. They combine carbon dioxide and water to make carbon-based fuels (like wood) using the energy from the sun.

Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.