Climate is the long-term statistical description of weather, including average conditions, seasonal patterns, variability, and extremes. It is commonly described through temperature, precipitation, humidity, wind, and other environmental measurements. Whereas weather concerns conditions at particular times and places, climate describes their distribution over longer periods, typically decades or more. In a broader sense, climate encompasses the state of the interacting components of the Earth system, not merely conditions in the air. (ipcc.ch)
Description and measurement
Climate descriptions use statistics to characterize both typical conditions and departures from them. Annual averages alone are insufficient: two places with similar annual temperatures may have very different winters, summers, or rainfall seasons. Measures of variability and extremes help describe conditions that averages conceal. Climate therefore includes the range and frequency of events as well as their mean values. (ipcc.ch)
The World Meteorological Organization defines climatological standard normals using consecutive 30-year periods, updated each decade. The 1991–2020 period is one such reference interval. Normals provide a consistent basis for comparing observations, but they do not imply that climate is unchanging. A climate anomaly is a departure from a specified reference value; its interpretation therefore depends on the baseline used. Fixed historical baselines can support comparisons across successive decades. (wmo.int)
The climate system
The climate system comprises the atmosphere, hydrosphere, cryosphere, land, and living organisms, together with their interactions. These components exchange heat, moisture, and other substances. The ocean is especially important because its large heat capacity slows the surface-temperature response to changes in the planet’s energy balance. Consequently, different parts of the system respond over different timescales. (ipcc.ch)
The Sun supplies most of the energy driving climate. Earth’s energy budget depends on incoming solar radiation, reflected sunlight, and outgoing thermal radiation. Uneven heating between low and high latitudes drives atmospheric and oceanic circulation, which redistributes energy. Evaporation, convection, precipitation, and currents connect this redistribution with the water cycle. (science.nasa.gov)
The greenhouse effect arises because certain atmospheric gases absorb and emit thermal infrared radiation. Naturally occurring greenhouse gases help maintain surface temperatures above those of an otherwise comparable planet without this effect. Clouds and reflective surfaces also influence the energy budget: snow and ice reflect sunlight, while changes in their extent can reinforce an initial warming or cooling. Such responses are examples of climate feedbacks. (science.nasa.gov)
Geographic controls
Latitude strongly influences climate because solar illumination varies with position on Earth and with the seasons. Elevation, prevailing winds, proximity to large bodies of water, and ocean currents modify this broad pattern. Coastal areas generally experience smaller temperature ranges than continental interiors because water heats and cools more slowly than land. The precise influence depends on circulation and local geography. (oceantoday.noaa.gov)
Mountains affect temperature and precipitation. Rising moist air can cool and produce precipitation on windward slopes, leaving drier conditions downwind—a rain shadow. Higher elevations generally have lower temperatures, although local atmospheric conditions can produce exceptions. These controls allow sharply contrasting climates to occur within relatively short distances. (oceantoday.noaa.gov)
Climate classification
The Köppen climate classification organizes climates principally by temperature and precipitation, including their seasonal distribution. Its five major groups are tropical, dry, temperate, continental, and polar, with subdivisions distinguishing more specific regimes. Classification provides a compact way to compare regions without reproducing their full observational records. (nesdis.noaa.gov)
Tropical climates remain warm throughout the year; dry climates have limited precipitation; temperate climates have relatively mild conditions; continental climates have more pronounced seasonal temperature contrasts; and polar climates remain cold. These are broad categories rather than complete descriptions. Local elevation and geographic setting can create conditions that differ from the surrounding regional pattern. (nesdis.noaa.gov)
Variability and long-term change
Climate variability includes fluctuations beyond individual weather events, arising from internal interactions or external influences. The El Niño–Southern Oscillation is an important example: changes in tropical Pacific ocean temperatures and atmospheric circulation affect seasonal and interannual conditions across many regions. Its warm, cool, and neutral phases alter patterns of rainfall and temperature without accounting for every regional weather anomaly. (ipcc.ch)
Climate change concerns persistent shifts in climate characteristics. Natural influences include volcanic eruptions, solar variations, and changes in Earth’s orbit. Human activities alter atmospheric composition and land surfaces. The Intergovernmental Panel on Climate Change’s 2023 synthesis report concluded that human activities, principally greenhouse-gas emissions, had unequivocally caused global warming; it assessed global surface temperature in 2011–2020 as approximately 1.1°C above the 1850–1900 average. (ipcc.ch)
Reconstruction and modeling
Paleoclimatology investigates climates preceding widespread instrumental observations. Researchers use indirect evidence preserved in tree rings, ice cores, corals, pollen, and sediments. These climate proxies record different environmental influences: tree growth can reflect moisture or temperature, while ice preserves information about precipitation, atmospheric composition, and volcanic activity. Combining records extends climate knowledge beyond the instrumental period. (ncei.noaa.gov)
Climate models represent physical processes mathematically and simulate interactions among atmosphere, ocean, and land. They are tested against observations and used to investigate past conditions and possible future changes. Unlike forecasts of particular weather events, long-term projections describe statistical outcomes under specified assumptions about future influences. Their uncertainties reflect internal variability, imperfect representation of processes, and differing future emissions pathways. (climate.gov)