Global warming

Global warming is the long-term rise in Earth’s average surface temperature caused primarily by the accumulation of human-emitted greenhouse gases. The strongest attribution studies find that human influence explains essentially all of the observed warming since the mid-20th century: for 1951–2010, the estimated human contribution was 0.6–0.8°C, compared with about 0.65°C of observed warming, while natural forcing and internal variability contributed little on average. [citation_1w8v46] A separate attribution analysis estimated 2020 warming at 1.22 ± 0.15°C above 1850–1900, with 1.15 ± 0.15°C human-induced; greenhouse-gas warming was partly offset by other human influences, especially aerosols. [citation_1wfjmj] The main drivers are fossil-fuel combustion, cement production, deforestation and other land-use change, alongside methane and nitrous-oxide emissions from agriculture and energy systems. [citation_1x321u]

The warming is already altering physical and biological systems. Observed and expected consequences include glacier, snow, sea-ice and permafrost loss; sea-level rise; changing precipitation; more frequent or intense heat extremes; ecosystem disruption; biodiversity loss; and risks to food, water and human health. [citation_1x321u] The effects are uneven: Arctic regions warm especially rapidly, while societies that depend heavily on climate-sensitive agriculture or have fewer resources to adapt face greater exposure. [citation_1xlod8] Short-term fluctuations, including the apparent 1998–2012 “hiatus,” do not contradict the long-term trend; changes in ocean heat uptake, natural variability and observational coverage help explain that interval, and newer datasets show stronger recent warming. [citation_1xlor4]

Limiting further warming requires rapid, large emissions reductions across the energy system rather than a single intervention. Evidence from mitigation modeling identifies low-carbon electricity as a high-impact priority, combined with energy efficiency, electrification of transport and buildings, industrial decarbonization, methane reductions, and protection and restoration of forests and other carbon sinks. [citation_1xlkr2] Buildings offer substantial cost-effective potential through efficiency improvements, particularly lighting and other measures that deliver the same energy services with less consumption. [citation_1xlpjz] Policies that have shown progress include renewable-energy deployment, vehicle fuel-efficiency standards, passenger-vehicle electrification and forestry measures, but replication of existing sectoral progress alone was estimated to cut emissions only about 20% below a current-policies pathway by 2030—insufficient for a cost-effective pathway below 2°C. [citation_106wys] The central implication is that mitigation and adaptation must proceed together: emissions cuts reduce the magnitude of future warming, while adaptation addresses impacts that are already unavoidable.