What is global warming? Global warming is the result of heat-trapping pollution in the atmosphere. These pollutants act like a blanket around the Earth, causing the planet's temperature to rise. This ability of certain gases to trap heat is sometimes called the greenhouse effect.

What causes global warming? Fossil fuels—the coal, oil, and natural gas that we burn for energy and transportation—are the largest source of heat-trapping pollution. The second largest source is deforestation and land clearing. Other important sources include: cattle ranching, conventional agriculture, landfills, and various industrial processes. (For an overview of emissions in the United States, click here.)

What's wrong with a little warming? In the 19th century, it was thought that global warming would simply make the planet a bit cozier. We now understand that by forcing the planet to heat up at an unnatural rate, we are throwing the Earth out of balance. It is not just a question of being a bit warmer—the planet is responding in ways we hardly understand: we're having record storms, heat waves, and wildfires; ice caps are melting; the ocean is becoming more acidic; pest infestations are wiping out forests; species are going extinct; long-term drought is affecting entire regions.

What can we do about it? The simple answer is: stop polluting and stop clearing forests. By applying technologies that exist today, we can move to a super-efficient, clean energy economy and eliminate much of the global warming pollution that we're producing. But because we have delayed action so long, the warming is now well underway. To avoid dangerously disrupting the climate, we must act at an unprecedented pace. That will require national and global policies that are science-based, not politically driven.