Animation depicting the greenhouse effect, the process by which atmospheric gases warm the Earth. Heat energy from the Sun (yellow) reaches the Earth, where some is reflected back into space, but most passes through the atmosphere and heats the surface. The Earth warms up, and emits some of this heat as longer-wavelength thermal infrared energy (red). Some of this also escapes into space. In the atmosphere, molecules of the three main greenhouse gases are shown: water vapour (H20), methane (CH4) and and carbon dioxide (CO2). These molecules are able to absorb the thermal infrared emitted by the Earth, energising the molecules. The molecules re-emit this energy in all directions, warming the lower atmosphere and surface. Emissions of carbon dioxide by power stations and other processes increases the amount of this gas in the atmosphere. This rise in CO2, from around 280 parts per million in the pre-industrial age to some 400ppm now, causes more thermal infrared to be absorbed and re-emitted, leading to the observed global warming.