Warming

Fig 1 Average global temperatures from 2014 to 2018 compared to a baseline average from 1951 to 1980, according to NASA's Goddard Institute for Space Studies.

Fig 2 The average annual temperature at the earth's surface has risen since the late 1800s, with year-to-year variations (shown in black) being smoothed out (shown in red) to show the general warming trend.

Global warming is the long-term rise in the average temperature of the Earth's climate system. It is a major aspect of current climate change, and has been demonstrated by direct temperature measurements and by measurements of various effects of the warming.^[1][2]

The terms global warming and climate change are often used interchangeably.^[3] However, speaking more properly, global warming denotes the mainly human-caused increase in global surface temperatures and its projected continuation,^[4][5] but climate change includes both global warming and its effects, such as changes in precipitation and impacts that differ by region.^[6] There have been prehistoric periods of global warming,^[7] but many observed changes since the mid-20th century have been unprecedented over decades to millennia.^[1][8]

Climate proxy records show that natural variations offset the early effects of the Industrial Revolution, so there was little net warming between the 18th century and the mid-19th century,^[27][28] when thermometer records began to provide global coverage.^[29] The IPCC has adopted the baseline reference period 1850–1900 as an approximation of pre-industrial global mean surface temperature.^[27]

Multiple independently produced instrumental datasets confirm that the 2009–2018 decade was 0.93 ± 0.07 °C warmer than the pre-industrial baseline (1850–1900).^[30] Currently, surface temperatures are rising by about 0.2 °C per decade.^[31] Since 1950, the number of cold days and nights have decreased, and the number of warm days and night have increased.^[32] Historical patterns of warming and cooling, like the Medieval Climate Anomaly and the Little Ice Age, were not as synchronous as current warming, but may have reached temperatures as high as those of the late-20th century in a limited set of regions.^[33]

Although the most common measure of global warming is the increase in the near-surface atmospheric temperature, over 90% of the additional energy stored in the climate system over the last 50 years has warmed ocean water.^[34] The remainder of the additional energy has melted ice and warmed the continents and the atmosphere.^[35]

The warming evident in the instrumental temperature record is consistent with a wide range of observations, documented by many independent scientific groups;^[36] for example, in most continental regions the frequency and intensity of heavy precipitation has increased.^[37] Further examples include sea level rise,^[38] widespread melting of snow and land ice,^[39] increased heat content of the oceans,^[40] increased humidity,^[41] and the earlier timing of spring events,^[42] such as the flowering of plants.^[43]

The Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report concluded, "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century."[9] The largest human influence has been the emission of greenhouse gases such as carbon dioxide, methane, and nitrous oxide. Climate model projections summarized in the report indicated that during the 21st century the global surface temperature is likely to rise a further 0.3 to 1.7 °C (0.5 to 3.1 °F) in a moderate scenario, or as much as 2.6 to 4.8 °C (4.7 to 8.6 °F) in an extreme scenario, depending on the rate of future greenhouse gas emissions and on climate feedback effects.[10] These findings have been recognized by the national science academies of the major industrialized nations[11] and are not disputed by any scientific body of national or international standing.[12][13]

The effects of global warming include rising sea levels, regional changes in precipitation, more frequent extreme weather events such as heat waves, and expansion of deserts.[14] Surface temperature increases are greatest in the Arctic, which has contributed to the retreat of glaciers, permafrost, and sea ice. Overall, higher temperatures bring more rain and snowfall, but for some regions droughts and wildfires increase instead.[15] Climate change threatens to diminish crop yields, harming food security, and rising sea levels may flood coastal infrastructure and force the abandonment of many coastal cities.[16][17] Environmental impacts include the extinction or relocation of many species as their ecosystems change, most immediately the environments of coral reefs,[18] mountains, and the Arctic.[19] Due to the persistence of CO2 in the atmosphere and the inertia of the climate system, climatic changes and their effects will continue for millennia even if carbon emissions are stopped.[20]

Possible societal responses to global warming include mitigation by emissions reduction, adaptation to its effects, and maybe climate engineering. Countries work together on climate change under the umbrella of the United Nations Framework Convention on Climate Change (UNFCCC), which has near-universal membership. The ultimate goal of the convention is to "prevent dangerous anthropogenic interference with the climate system".[21] Although the parties to the UNFCCC have agreed that deep cuts in emissions are required[22] and that global warming should be limited to well below 2 °C (3.6 °F) in the Paris Agreement,[23] the Earth's average surface temperature has already increased by about half this threshold[24] and current pledges by countries to cut emissions are inadequate to limit future warming.[25] Some scientists question the feasibility, in higher emissions scenarios, of climate adaptation.[26]