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. Importantly, the warming of the planet, which is far more rapid than anything experienced in many millennia, cannot be explained by natural variations in the climate, despite claims to the contrary.
Scientists also know that fossil fuels are the main source of the carbon dioxide because of the chemical signature of the molecules in the atmosphere.
The evidence for climate change is overwhelming. As NASA says, there is “unequivocal evidence that Earth is warming at an unprecedented rate” and that human activity “is the principal cause.” The 2023 National Climate Assessment similarly states, “Human activities—primarily emissions of greenhouse gases from fossil fuel use—have unequivocally caused the global warming observed over the industrial era.”