Multiple lines of evidence, including measurements of a variety of planetary indicators, show that climate change is happening and is caused primarily by human activity. As the concentration of carbon dioxide and other heat-trapping gases in the atmosphere has increased — much of it from burning fossil fuels — land and ocean temperatures have risen, along with sea levels, while sea and land ice have declined.
The changes are consistent with the basic physics of the greenhouse effect that have been understood since the mid-1800s and map to what is expected from human activity.