The circulation of the earth's atmosphere and oceans, and most life on earth, is powered by energy from the sun. That energy largely arrives in the form of visible light. About 30% of it is immediately reflected back into space. This reflected fraction is called the "planetary albedo."
About 30% of the energy that comes in to the earth from the sun is directly reflected back into space. This fraction is called the "planetary albedo." The balance is absorbed by the earth. It is trapped by various "greenhouse gasses," such as water vapor and carbon dioxide. As a result, the planet warms until the same amount of energy is radiated back to space.
The other 70% of the energy is absorbed by clouds, land and the oceans. If that energy just continued to be absorbed, the earth would rapidly heat up, and we—and all life—would quickly fry. To keep the earth at a constant temperature, somehow, the 70% of solar input that is absorbed must be radiated back into space as heat (infrared). However, while the atmosphere is transparent to visible light, it is opaque to infrared. This is because of absorption by water vapor, CO2 and other trace gasses that occur naturally in the atmosphere. As a consequence, the earth heats up until it reaches a temperature at which enough heat energy is radiated off the top of the atmosphere to bring the outward flow of energy into balance with that being absorbed.
As a result of this "greenhouse warming" the average temperature of the earth is about 33°C (about 60°F) warmer than it would be if there were no atmosphere (making earth a hospitable place for life).
As human activity (burning coal, oil and natural gas and clearing land) releases more CO2 (and other greenhouse gases) into the atmosphere, the amount of warming increases.
When fossil fuels such as coal, oil or natural gas, are burned, CO2 is released into the atmosphere where much of it remains for over 100 years. As a consequence, the atmospheric concentration of CO2 has been steadily rising, and with it the average temperature of the earth. This in turn is inducing a variety of other changes in climate. Despite national and international efforts to reduce the emissions of CO2and other greenhouse gasses, growing concentrations are leading to global warming and climate change.
Details on climate change and its impact can be found in the reports of the Intergovernmental Panel on Climate Change (IPCC). Additional details on impacts of climate change on the United States can be found in the US National Assessment.