Earth has a natural “greenhouse effect” that results from gases like water vapor, carbon dioxide (CO2), and methane absorbing heat radiated from the Earth’s surface and lower atmosphere and radiating that heat back towards the surface. Without this natural effect, Earth’s climate would be much colder than that which gave rise to civilization.
The problem of global warming has resulted from human activities, primarily the burning of fossil fuels and clearing of forests. These activities increase the amount of heat-trapping gases, thereby intensifying the natural greenhouse effect. The amount of CO2 (as well as methane and nitrous oxide) in the atmosphere has increased sharply since the Industrial Revolution. The concentration of CO2 is now more than 40 percent higher than at any time during at least the past 800,000 years. If current emission trends continue, by 2100, CO2 concentration would be higher than the Earth has seen in more than 10 million years.