Climate change

Climate describes patterns and statistics of weather over long periods, rather than the conditions on a particular day. Climate has changed throughout Earth’s history through natural processes, including changes in solar energy and the planet’s orbit. The present period of rapid warming is different: assessments find that human activity is the principal cause of the recent trend.

The natural greenhouse effect keeps Earth warm enough for life. Greenhouse gases absorb and re-emit some of the infrared energy leaving the surface, slowing heat loss to space. Human activities have increased concentrations of these gases, especially carbon dioxide from burning coal, oil and gas and from land-use change; methane and nitrous oxide also contribute. The resulting extra energy warms the atmosphere, ocean and land. Natural influences still affect climate, but they do not account for the observed modern warming trend.

Evidence comes from multiple independent measurements, including surface records, ocean observations, satellites, and natural records such as ice cores and tree rings. The IPCC assessed global surface temperature in 2011–2020 as about 1.1°C above 1850–1900 and concluded that human activities, principally greenhouse-gas emissions, have unequivocally caused global warming. Observed changes include warming oceans and air, shrinking ice, rising sea levels and shifts in some climate extremes.

Climate change affects natural systems and human societies through interacting hazards, including heat, heavy precipitation, drought, coastal flooding and ocean warming. The consequences vary by region and depend on exposure and vulnerability as well as the physical hazard. Impacts can affect health, water supplies, food production, ecosystems, infrastructure and livelihoods. Risks generally grow as warming increases, although the exact effects depend on local conditions and future emissions.

Mitigation limits climate change by reducing greenhouse-gas emissions or increasing removals, for example through cleaner energy, energy efficiency and protection or restoration of carbon-storing ecosystems. Adaptation reduces harm by preparing for present and expected effects, such as through resilient infrastructure, risk planning and changes in water or agricultural management. The two approaches complement one another: mitigation limits future warming, while adaptation addresses impacts that are already occurring or cannot be avoided. The Paris Agreement provides an international framework for countries to strengthen climate action, pursue emissions reductions and adapt to climate effects; implementation and support remain important challenges.