Biomass is any living organic matter that’s used for fuel. There’s something comforting about a wood fire, but that comfort fades when you see the carbon emissions behind the flames—and that’s especially true when you move beyond an individual fireplace to a massive incinerator.

Forty-four percent of bioenergy in the United States comes from wood, cut from forests and burnt in biomass incinerators that emit higher levels of carbon dioxide than most coal-fired plants. Some wood pellets are created from sawdust or unusable waste from industries, but increasingly, they come from clear-cut forests. Deforestation and forest degradation together are a huge source of carbon emissions, second only to the burning of fossil fuels.

What about other forms of biomass? The net effect of biomass depends on its source, growing conditions, what would otherwise be grown, and the process by which it’s converted to energy.

Historically, biofuels have been produced from food crops such as corn and soy. Ethanol, for example, is a gasoline substitute derived from corn that is politically popular in the early-primary state of Iowa. The theory is that because corn pulls carbon dioxide out of the air as it grows, the carbon dioxide released when we burn it is the same carbon it absorbed, resulting in net-zero emissions. But when you consider fertilizer, farm equipment, transportation, energy used in the conversion to liquid fuels, and potential deforestation, it’s not such a sweet deal.

Some biofuels may play a useful role, however. Clean biofuels come from crops that don’t require much fertilizer, water, or other care. Switchgrass has potential, provided it’s grown on farmland unsuitable for most food crops, especially if grown near a refinery.