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. To a lesser extent, the clearing of land for agriculture, industry, and other human activities also has increased concentrations of greenhouse gases.

The role of human activity

In its Fifth Assessment Report, the Intergovernmental Panel on Climate Change (IPCC), a group of 1,300 independent scientific experts from countries all over the world under the auspices of the United Nations, concluded there's a more than 95 percent probability that human activities over the past 50 years have warmed our planet.

The industrial activities of modern civilization have raised atmospheric carbon dioxide levels from 280 parts per million to 414 parts per million in the past 150 years. The panel also concluded there's a better than 95 percent probability that human-produced greenhouse gases such as carbon dioxide, methane, and nitrous oxide have caused much of the observed increase in Earth's temperatures over the past 50 years.