Climate change is a long term alteration in the statistical characteristics of Earth's climate, including changes in average temperature, precipitation, atmospheric circulation, sea level, snow and ice, and the frequency or intensity of some climate extremes. Climate refers to the long term pattern of weather over a region or across the planet, while weather describes short term atmospheric conditions. Climate change therefore cannot be identified from one unusually hot day or one unusually cold winter.
Scientists examine changes over sufficiently long periods, traditionally using averages and variability over approximately thirty years or longer, while also studying longer historical and geological records. Climate can change naturally through variations in solar activity, volcanic eruptions, orbital changes, ocean circulation and internal variability. However, the climate change observed since the industrial era is dominated by human caused increases in greenhouse gases, particularly carbon dioxide, methane and nitrous oxide, with additional influences from aerosols and land use. Modern climate change is consequently both a scientific description of changing climate conditions and a subject involving interactions among the atmosphere, ocean, land, ice, ecosystems and human activities.
The climate system consists of several interconnected components, including the atmosphere, oceans, land surfaces, cryosphere, biosphere and frozen water reservoirs. Energy enters the Earth system mainly as solar radiation and leaves primarily as outgoing infrared radiation. Climate remains relatively stable over a period when incoming and outgoing energy are balanced over appropriate timescales, but changes in energy flows can alter temperature and other climatic properties.
Greenhouse gases absorb and re emit some outgoing infrared radiation, influencing the amount of energy retained in the lower atmosphere and surface system. Oceans absorb a large fraction of the excess heat associated with modern global warming and can transport heat around the planet through currents. Ice and snow reflect sunlight and can provide feedbacks through changes in surface reflectivity. Vegetation and soils exchange carbon and water with the atmosphere. These interactions mean that climate change is not simply a change in air temperature but a reorganisation of the physical state and energy balance of a complex planetary system.
Human activities are the principal cause of the current long term warming trend. Burning coal, oil and natural gas releases carbon dioxide that had previously been stored underground as fossil carbon. Cement production also releases carbon dioxide, while deforestation and land use change can release stored carbon and reduce the capacity of ecosystems to absorb atmospheric carbon. Agriculture, livestock production, waste management and fossil fuel extraction contribute methane and nitrous oxide through different processes.
Industrial activities can emit additional greenhouse gases such as fluorinated gases. These gases alter atmospheric radiative properties and produce a positive radiative forcing that warms the climate system. Human activities can also emit aerosols that reflect sunlight or influence clouds and therefore exert a cooling effect that partially offsets some greenhouse gas warming. The observed climate is therefore the result of multiple influences, but the persistent global warming trend is primarily explained by human greenhouse gas emissions.
Climate change affects many parts of the Earth system. Rising global mean surface temperature is accompanied by widespread changes in land and ocean temperatures, loss of glaciers, reductions in many forms of snow and ice, rising sea levels and changes in ocean heat content. A warmer atmosphere can hold more water vapour, influencing the hydrological cycle and potentially increasing heavy precipitation where atmospheric conditions support rainfall.
At the same time, increased evaporation and changing circulation can contribute to drying in some regions. Climate change can shift the timing of seasons, affect ecosystems and species distributions and alter conditions for agriculture. Extreme events are also changing, although the direction and magnitude vary by event and location. Heat extremes have become more frequent in many regions, while heavy precipitation has increased in many areas. The relationship between climate change and individual events must be assessed using physical understanding and event attribution rather than assuming every weather event has the same cause.
Sea level rise is an important component of climate change and results primarily from two processes associated with warming. First, seawater expands as it warms, a process called thermal expansion. Second, melting glaciers and ice sheets transfer additional water from land to the ocean. Changes in terrestrial water storage can also affect sea level. Ice sheets respond over long timescales and involve complex interactions among snowfall, surface melting, ice flow, ocean temperatures and atmospheric conditions.
Sea level rise increases the baseline from which storm surges and coastal flooding occur, meaning that an event producing the same temporary surge can cause greater inundation in a future with higher mean sea level. Regional sea level changes can differ from the global average because of ocean circulation, gravitational effects, land movement and changes in water storage. Climate change therefore has consequences for coastlines even where local air temperature changes are relatively modest.
Climate change can be described through the concepts of mitigation and adaptation. Mitigation means reducing the causes of climate change, particularly by lowering greenhouse gas emissions or increasing the removal and storage of carbon dioxide. Examples include replacing fossil fuel energy with low carbon energy sources, improving energy efficiency, electrifying transport and heating where electricity is low carbon, reducing methane emissions and protecting or restoring carbon rich ecosystems.
Adaptation means reducing harm from climate impacts that are occurring or expected, through measures such as heat action plans, flood protection, drought resilient water management, infrastructure changes and agricultural adaptation. Some changes cannot be avoided immediately because the climate system responds over time to accumulated greenhouse gases and because some physical processes have long response times. Mitigation and adaptation therefore address different aspects of the problem and are complementary rather than interchangeable.
Scientists reconstruct past climates using tree rings, ice cores, ocean sediments, corals, pollen, fossils, historical observations and other natural archives. Instrumental observations provide direct measurements of temperature, precipitation, sea level, ocean heat, atmospheric composition and other variables over the modern period. Satellite observations have expanded global coverage since the late twentieth century. Climate models combine physical equations describing atmospheric, oceanic, land surface, cryospheric and other processes to simulate climate and explore how different forcing scenarios affect future conditions.
Confidence in major aspects of modern climate change comes from the convergence of independent evidence rather than from one dataset or one model. Scientists also distinguish between projections, which describe possible future outcomes under specified assumptions, and predictions, which attempt to forecast a particular future state. Uncertainty remains about some regional details, feedback strengths and future human emissions, but uncertainty does not mean that all explanations of observed warming are equally supported.
Climate change is sometimes incorrectly treated as a synonym for global warming, but the terms have different scopes. Global warming specifically describes the long term increase in Earth's average surface temperature, whereas climate change includes warming as well as changes in precipitation, circulation, sea level, ice, ecosystems and climate variability. Another misconception is that natural climate variability has stopped.
Natural variability continues and can temporarily accelerate or slow the observed warming trend over shorter periods, even while the long term human driven trend persists. Similarly, a cold day or snowy winter does not contradict global warming because local weather varies around a changing climatic average. Understanding climate change requires separating short term weather from long term climate statistics, distinguishing natural variability from external forcing and recognising that regional impacts can differ substantially from global averages.
