Climate change refers to long term shifts in climate driven mainly by human activities, especially the emission of greenhouse gases like carbon dioxide and methane. These gases trap heat in the atmosphere and enhance the natural greenhouse effect. Since the late nineteenth century, global average temperature has risen a little more than 1 degree Celsius, with most of the warming occurring in recent decades.

Evidence for climate change comes from multiple independent observations. Instrumental records show increasing temperatures on land and at sea, while satellite measurements confirm changes in the energy balance of the planet. Glaciers and ice sheets are shrinking, Arctic sea ice is declining in extent and thickness, and sea level is rising due to both thermal expansion of seawater and melting land ice. Shifts in the timing of seasons, changes in the distribution of plants and animals, and increases in the frequency or intensity of some extreme weather events further support ongoing change.

Climate models that include human emissions reproduce the observed warming patterns, while those that consider only natural factors fail to do so. This strengthens the conclusion that human activities are the dominant cause of recent climate change. Projections indicate that future warming will depend on how quickly emissions are reduced. Higher emission paths lead to larger temperature increases, more severe impacts on water resources, agriculture, ecosystems, and human health, and a higher risk of crossing climate tipping points. Mitigation through rapid emission reductions and adaptation to unavoidable changes are both required to limit long term risks.