Climate change is leading to more extreme weather of certain types—especially those types of weather events related to temperature. Warming due to climate change is leading to more heat waves, and extremely hot days and nights have become more likely. Extreme cold events are less frequent and less severe than in previous decades. Climate change is also leading to more heavy rainfall and snowfall events as Earth’s atmosphere is becoming warmer and moister. Climate change can exacerbate droughts, as increased surface temperature increases evaporation, which leaves the land drier than it would be at a cooler temperature. Hurricanes (when they occur) will be more intense, produce more rainfall, and possibly be larger. In addition, rising sea level contributes to stronger storm surges.

In order to mitigate the power of climate change and survive it’s devastating effects already in motion, we must understand our climate system and how it works.


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.

Key resources: Climate Change: Evidence and Causes; Advancing the Science of Climate Change; Surface Temperature Reconstructions for the Last 2,000 Years


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.

Key resources: Climate Change: Evidence and Causes; Advancing the Science of Climate Change


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.

Key resources: Climate Change: Evidence and Causes; Abrupt Impacts of Climate Change: Anticipating Surprises; VIDEO: Abrupt Impacts of Climate Change


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.

Key resources: Climate Change: Evidence and Causes; Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia; Warming World: Impacts by Degree


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.

Key resources: A National Strategy for Advancing Climate Modeling; Climate Modeling 101