Climate change is a persistent alteration in the average conditions or variability of climate, occurring over decades or longer. It includes changes in temperature, precipitation, atmospheric circulation, and the frequency or intensity of extremes. Unlike weather, which describes short-term atmospheric conditions, climate change concerns sustained patterns. The term encompasses natural changes throughout Earth’s history, but contemporary usage commonly refers to human-driven warming and its wider consequences. Global warming specifically describes increasing global average surface temperature; climate change includes the accompanying transformations of the climate system. (science.nasa.gov)
Physical mechanisms and causes
Climate depends on Earth’s energy budget: the balance between incoming solar radiation and energy returned to space. Greenhouse gases absorb and emit infrared radiation, producing the natural greenhouse effect. Increasing their concentrations alters this balance, warming the surface and lower atmosphere. Carbon dioxide, methane, and nitrous oxide are important contributors to the human-enhanced greenhouse effect. Some atmospheric particles, or aerosols, counteract part of this warming by reflecting sunlight or modifying clouds. (science.nasa.gov)
Burning fossil fuels, industrial processes, agriculture, and land-use change increase greenhouse gas concentrations. Deforestation releases stored carbon and can reduce uptake by vegetation. Human activities therefore alter the carbon cycle as well as atmospheric composition. The Intergovernmental Panel on Climate Change concluded in its 2023 synthesis that human activities, principally greenhouse gas emissions, had unequivocally caused global warming. (ipcc.ch)
Natural causes include changes in Earth’s orbit, solar output, and volcanic activity. Internal variability, including the El Niño–Southern Oscillation, redistributes heat and affects temperatures and rainfall over years or decades. These influences help explain historical changes and short-term fluctuations, but do not explain the sustained modern warming trend without human influence. (science.nasa.gov)
Climate feedbacks modify the response to an initial disturbance. Warming increases atmospheric water vapour, which strengthens greenhouse warming. Loss of reflective snow and ice increases solar absorption. Clouds affect both incoming sunlight and outgoing infrared radiation; differences in their responses remain an important source of uncertainty in the magnitude of future warming. (ipcc.ch)
Observations and attribution
Evidence comes from surface thermometers, satellites, ocean measurements, and records of snow and ice. Paleoclimatology reconstructs earlier climates using ice cores, tree rings, sediments, and other natural archives. Agreement among independent indicators establishes that warming is not simply an artefact of one measurement system. (science.nasa.gov)
The Intergovernmental Panel on Climate Change assessed global surface temperature during 2011–2020 as approximately 1.1°C above the 1850–1900 average. Global mean sea level rose approximately 0.20 metres between 1901 and 2018, with an increasing rate over that period. These figures describe specified historical intervals rather than present-day conditions. Widespread changes also include ocean warming, retreating glaciers, and shrinking Arctic sea ice. (ipcc.ch)
Attribution research combines observations, physical understanding, and climate models to distinguish human influence from natural variability. Comparisons of simulations with and without human influences show that natural factors alone cannot reproduce the observed warming. Attribution of individual extreme events is more specific: it examines how climate change altered an event’s likelihood or intensity, rather than treating every event as exclusively human-caused. (ipcc.ch)
Environmental and human consequences
Warming affects the water cycle, increasing atmospheric moisture and changing evaporation, precipitation, and drought conditions. Heavy precipitation has intensified in many regions, while changes in drought depend on location and the type of drought measured. Hot extremes have become more frequent and intense across most land regions. Effects differ geographically because circulation, terrain, and local conditions shape regional responses. (ipcc.ch)
The ocean absorbs much of the excess heat accumulating in the climate system. Thermal expansion and melting land ice contribute to sea-level rise. Changes in the cryosphere, including retreating glaciers, affect freshwater supplies and coastal risks. Separately, ocean uptake of carbon dioxide causes ocean acidification, a chemical change related to emissions rather than a direct consequence of warming. (ipcc.ch)
Climate change alters ecosystems, species distributions, and seasonal biological activity, placing additional pressure on biodiversity. Human consequences include disruption of food production, water availability, livelihoods, and settlements. Risk depends not only on climatic hazards but also on exposure and vulnerability. Poverty, ecosystem degradation, and limited institutional capacity can increase vulnerability; impacts therefore differ substantially among and within regions. (ipcc.ch)
Projections and responses
Future climate projections are conditional on emissions, land use, and socioeconomic assumptions, rather than unconditional forecasts. Additional warming increases many risks, and some changes, especially sea-level rise, persist for centuries or longer. Limiting human-caused warming requires net-zero carbon dioxide emissions alongside substantial reductions in other greenhouse gases. Net zero means that anthropogenic emissions are balanced by anthropogenic removals over a specified period. (ipcc.ch)
Mitigation addresses causes through emissions reductions and increased carbon uptake. Measures assessed in scientific reports include renewable energy, energy efficiency, electrification, and changes in land management. Carbon dioxide removal can balance residual emissions but is distinct from preventing emissions and cannot substitute for deep reductions. Adaptation addresses consequences through measures such as flood protection, improved water management, and adjustments to farming. Its effectiveness depends on resources and institutions, and some systems face limits to adaptation. (ipcc.ch)
The Paris Agreement, adopted on 12 December 2015, established an international framework for climate action. Its temperature objective is to keep warming well below 2°C above pre-industrial levels while pursuing efforts to limit it to 1.5°C. Participating countries communicate nationally determined contributions and periodically strengthen their climate commitments within the agreement’s framework. (unfccc.int)