Greenhouse gases absorb heat energy from the sun and re radiate it back to Earths surface and keep the earths temperature at a constant 15 degrees Celsius, which is 30 degrees warmer than if they were absent. This is a warm enough mean temperature to sustain life, known as the greenhouse effect. However, the enhanced greenhouse effect has been caused by human activity, such as coal burning for fuel, and has lead to a rise in the amount of these greenhouse gases in the atmosphere. This absorbs more of this heat energy, thus leading to a gradual global warming.