Our Most Abundant Fossil Fuel

Coal is the most abundant fossil fuel on Earth, and the United States holds some of the largest coal reserves in the world: enough to last a few hundred years at today's rate of use. But coal is not a perfect fuel. Trapped inside it are traces of impurities such as sulfur and nitrogen. When coal burns, these are released into the air, where they can combine with water vapor in clouds to form droplets of weak sulfuric and nitric acid that fall to the ground as "acid rain."

Coal also contains tiny specks of minerals that don't burn. Most of them become the ash left behind in a power plant's boiler, but some are carried up the smokestack with the hot gases. Some of these particles are so small that 30 of them laid side by side would barely equal the width of a human hair.

Like all fossil fuels, coal is made mostly of carbon; it formed from plants that lived hundreds of millions of years ago. When coal burns, its carbon combines with oxygen in the air to form carbon dioxide, a colorless, odorless gas that traps heat in the atmosphere. Coal releases more carbon dioxide for each unit of energy than any other fossil fuel, and the burning of fossil fuels is the main cause of the warming of Earth's climate since the 1800s.

Engineers have developed ways to make coal cleaner. Modern power plants can capture more than 99 percent of the fine particles and remove most of the sulfur dioxide that causes acid rain. Removing carbon dioxide is much harder: systems that capture it and store it deep underground exist, but only a few are in operation. Partly for these reasons, coal's share of U.S. electricity has fallen sharply since the 2000s as power companies have switched to natural gas, wind and solar power.