What Is the Evidence for Human-Caused Climate Change?

Multiple studies published in peer-reviewed scientific journals show that 97 percent or more of actively publishing climate scientists agree: Climate-warming trends over the past century are extremely likely due to human activities.

Rapid and large changes in warming

Earth's climate has changed throughout history. In the past 650,000 years, there have been seven cycles of glacial advance and retreat, with the abrupt end of the last ice age about 11,700 years ago marking the beginning of the modern climate era—and of human civilization. Most of these climate changes are attributed to very small variations in Earth's orbit that alter the amount of energy our planet receives from the sun. But the warming we've seen over the past few decades is too rapid to be linked to changes in Earth's orbit and too large to be caused by solar activity.

Ice cores drawn from Greenland, Antarctica, and tropical mountain glaciers show that Earth's climate responds to changes in greenhouse gas levels. Ancient evidence can also be found in tree rings, ocean sediments, coral reefs, and layers of sedimentary rocks. This ancient, or paleoclimate, evidence reveals that current warming is occurring roughly 10 times faster than the average rate of ice-age-recovery warming. Carbon dioxide from human activity is increasing more than 250 times faster than it did from natural sources after the last ice age.

Observable evidence of rapid climate change includes:

• Global temperature rise
• Warming ocean
• Shrinking ice sheets
• Retreating glaciers
• Decreased snow cover
• Sea level rise
• Declining arctic sea ice
• Extreme weather events
• Ocean acidification

Explore the evidence for rapid climate change in detail at climate.nasa.gov.

Greenhouse effect

Scientists attribute the global warming trend observed since the mid-20th century to the human expansion of the "greenhouse effect"—warming that results when the atmosphere traps heat radiating from Earth toward space. Certain gases in the atmosphere block heat from escaping. The heat-trapping nature of carbon dioxide and other gases was demonstrated in the mid-19th century.

Over the past century the burning of fossil fuels like coal and oil has increased the concentration of atmospheric carbon dioxide (CO₂). This happens because the coal- or oil-burning process combines carbon with oxygen in the air to make CO₂.

The role of human activity

CO₂ levels in Earth's atmosphere have increased from 280 parts per million (ppm) to 414 ppm in the past 150 years. (Scientists use ppm to measure what fraction of the air is made up of a certain molecule, in this case CO₂. One ppm of CO₂ would mean that for every 1 million air molecules you breathe in, one would be CO₂.)

The Intergovernmental Panel on Climate Change (IPCC), a United Nations organization that includes 195 member countries and thousands of independent scientific experts, published a Synthesis Report in March 2023. It concluded that human activities, principally through greenhouse gas emissions, have unequivocally caused global warming. The main drivers of these emissions are energy use, land use, and the consumption and production of goods.