Thousands of studies conducted by researchers around the world have documented increases in temperature at Earth’s surface, as well as in the atmosphere and oceans. Many other aspects of global climate are changing as well. High temperature extremes and heavy precipitation events are increasing, glaciers and snow cover are shrinking, and sea ice is retreating. Seas are warming, rising, and becoming more acidic, and flooding is become more frequent along the U.S. coastline. Growing seasons are longer, and large wildfires occur more frequently. Many species are moving to new locations, and changes in the seasonal timing of important biological events are occurring in response to climate change.

These trends are all consistent with a warming world and are expected to continue.

Many lines of evidence demonstrate that human activities, especially emissions of heat-trapping greenhouse gases from fossil fuel combustion, deforestation, and land-use change, are primarily responsible for the climate changes observed in the industrial era, especially over the last six decades. The atmospheric concentration of carbon dioxide, the largest contributor to human-caused warming, has increased by about 40% over the industrial era. This change has intensified the natural greenhouse effect, driving an increase in global surface temperatures and other widespread changes in Earth’s climate that are unprecedented in the history of modern civilization.

Greenhouse gas emissions from human activities will continue to affect Earth’s climate for decades and even centuries. Humans are adding carbon dioxide to the atmosphere at a rate far greater than it is removed by natural processes, creating a long-lived reservoir of the gas in the atmosphere and oceans that is driving the climate to a warmer and warmer state.

Beyond the next few decades, how much the climate changes will depend primarily on the amount of greenhouse gases emitted into the atmosphere; how much of those greenhouse gases are absorbed by the ocean, the biosphere, and other sinks; and how sensitive Earth’s climate is to those emissions.

To learn more about the science of climate change, explore:

• NCA4 Volume I, Climate Science Special Report
• NCA4 Volume II, Ch.2: Our Changing Climate
• USGCRP Indicators