As the sun's rays reach the Earth's surface, some are absorbed and re-emitted as heat. Greenhouse gases such as water vapour and carbon dioxide absorb and re-radiate some of this heat. Increased amounts of greenhouse gases in the atmosphere mean more heat is trapped, warming the Earth. Human activities, especially the burning of fossil fuels, have increased concentrations of atmospheric carbon dioxide by 40%, mainly since 1900. Global average surface temperature has increased by 0.8℃ over that time. Other changes to the climate in recent decades can be seen in the warming of the oceans. A rise in sea level. Decreasing snow and ice cover in the northern hemisphere. And a decline in sea ice in the Arctic. If emissions continue unchecked then further warming of 2.6℃ to 4.
8℃ would be expected by the end of this century. Even at the low end, this would have serious implications for human societies and the natural world. For more information about climate change from leading science academies, please visit royalsociety.org/climatechange or nas-sites.org/americasclimatechoices.