Global warming occurs when the levels of greenhouse gases in the atmosphere increase, causing more solar energy to be trapped in the climate system. Human activity results in the emission of carbon dioxide and methane, which are two of the most important greenhouse gases. As more greenhouse gases enter the atmosphere, the average temperature of the Earth goes up.
The energy from the Sun is absorbed by the Earth's surface and then radiated back into space. However, some of this energy is then absorbed by gases in the atmosphere, such as carbon dioxide, methane and water vapor. These gases then radiate some of this absorbed energy back towards the surface of the Earth. This greenhouse effect is what warms the Earth above the freezing point.
Various human activities, such as burning fossil fuels to produce electricity and cutting down forests, result in the emission of carbon dioxide. The gas enters the atmosphere and remains there for decades. Before the Industrial Revolution, levels of carbon dioxide were around 280 parts per million. In 2010, the level was measured at 380 parts per million. As this level continues to rise, more solar energy is trapped and radiated back into the climate system, causing global temperatures to increase.