Climate is changing. The clearest evidence comes from widespread thermometer records, which show that Earth’s average surface air temperature has increased by about 1°C (1.8°F) since 1900. Questions about the accuracy of these temperature records have been addressed and resolved, and the analyses have been replicated by multiple independent groups. A wide range of other observations—of Arctic summer sea ice, Northern Hemisphere snow cover, Greenland and West Antarctic ice sheets, and sea level rise—provides a comprehensive picture of warming throughout the climate system.
Humans are changing Earth’s climate. CO2 is one of the main greenhouse gases of importance to Earth’s energy balance. The present level of atmospheric CO2 is almost certainly unprecedented in the past million years, during which time modern humans evolved and societies developed. Human activities—including extracting long-buried fossil fuels and burning them for energy, deforestation, and other land use changes—are responsible for the increase in atmospheric CO2.
Climate change is happening much faster than most previous natural climate variations. All major climate changes, including natural ones, are disruptive. Past climate changes led to extinction of many species, population migrations, and pronounced changes in the land surface and ocean circulation. At the current pace, Earth is warming about ten times faster than it did at the end of the last ice age, making it more difficult for human societies and the natural world to adapt.
Earth’s climate will continue to warm in the future. The amount of warming that will be experienced in the future depends on the amount of CO2 and other greenhouse gases that accumulate in the atmosphere. Even if emissions of greenhouse gases were to suddenly stop, Earth’s surface temperature would not cool and return to the level in the pre-industrial era for thousands of years. If CO2 emissions continue their current trajectory of increases, without either technological or regulatory abatement, then global average surface temperature will increase an additional 2.6 to 4.8 °C (4.7 to 8.6 °F) by 2100. If steps are taken to curtail emissions or remove carbon from the atmosphere, we would expect to see less warming.
Computer models of the climate system help us understand why climate changed in the past and how it may change in the future. Climate models are based on mathematical equations that represent the best understanding of the basic laws of physics, chemistry, and biology that govern the behavior of the atmosphere, ocean, land surface, ice, and other parts of the climate system, as well as the interactions among them. The models are tested by comparing their simulations to a broad range of observed climate and weather variations. Although they are imperfect representations of the natural world, these models accurately simulate many features of the climate system, and can usefully inform decisions related to future climate conditions.
An Introduction to Climate Change in 60 seconds:
Additional Resources on Understanding the Climate System