Global warming

This page is about the current warming of the Earth's climate system. "Climate change" can also refer to climate trends at any point in Earth's history. For other uses see Global warming (disambiguation)

Global mean surface temperature change from 1880 to 2015, relative to the 1951–1980 mean. The black line is the annual mean and the red line is the 5-year running mean. Source: NASA GISS.

World map showing surface temperature trends (°C per decade) between 1950 and 2014. Source: NASA GISS.^[1]

Fossil fuel related carbon dioxide (CO₂) emissions compared to five of the IPCC's "SRES" emissions scenarios, published in 2000. The dips are related to global recessions. Image source: Skeptical Science.

Fossil fuel related carbon dioxide emissions over the 20th century. Image source: EPA.

Global warming and climate change are terms for the observed century-scale rise in the average temperature of the Earth's climate system and its related effects.^[2] Multiple lines of scientific evidence show that the climate system is warming.^[3][4][5] Although the increase of near-surface atmospheric temperature is the measure of global warming often reported in the popular press, most of the additional energy stored in the climate system since 1970 has gone into ocean warming. The remainder has melted ice and warmed the continents and atmosphere.^[6][a] Many of the observed changes since the 1950s are unprecedented over tens to thousands of years.^[7] On 12 November 2015, NASA scientists reported that human-made carbon dioxide (CO₂) continues to increase above levels not seen in hundreds of thousands of years: currently, about half of the carbon dioxide released from the burning of fossil fuels is not absorbed by vegetation and the oceans and remains in the atmosphere.^{[8][9][10][11]}
Scientific understanding of global warming is increasing. The Intergovernmental Panel on Climate Change (IPCC) reported in 2014 that scientists were more than 95% certain that global warming is mostly being caused by increasing concentrations of greenhouse gases (GHG) and other human (anthropogenic) activities.^[12][13][14] 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) for their lowest emissions scenario using stringent mitigation and 2.6 to 4.8 °C (4.7 to 8.6 °F) for their highest.^[15] These findings have been recognized by the national science academies of the major industrialized nations^[16][b] and are not disputed by any scientific body of national or international standing.^[18]
Future climate change and associated impacts will differ from region to region around the globe.^[19][20] Anticipated effects include warming global temperature, rising sea levels, changing precipitation, and expansion of deserts in the subtropics.^[21] Warming is expected to be greater over land than over the oceans and greatest in the Arctic, with the continuing retreat of glaciers, permafrost and sea ice. Other likely changes include more frequent extreme weather events including heat waves, droughts, heavy rainfall with floods and heavy snowfall;^[22] ocean acidification; and species extinctions due to shifting temperature regimes. Effects significant to humans include the threat to food security from decreasing crop yields and the abandonment of populated areas due to rising sea levels.^[23][24] Because the climate system has a large inertia and CO₂ will stay in the athmosphere for a long time, many of these effects will not only exist for decades or centuries, but will persist for tens of thousands of years.^[25]
Possible societal responses to global warming include mitigation by emissions reduction, adaptation to its effects, building systems resilient to its effects, and possible future climate engineering. Most countries are parties to the United Nations Framework Convention on Climate Change (UNFCCC),^[26] whose ultimate objective is to prevent dangerous anthropogenic climate change.^[27] The UNFCCC have adopted a range of policies designed to reduce greenhouse gas emissions^{[28][29][30][31]} and to assist in adaptation to global warming.^{[28][31][32][33]} Parties to the UNFCCC have agreed that deep cuts in emissions are required,^[34] and that future global warming should be limited to below 2.0 °C (3.6 °F) relative to the pre-industrial level.^[34][c]
Public reactions to global warming and general fears of its effects are also steadily on the rise, with a global 2015 Pew Research Center report showing a median of 54% who consider it "a very serious problem". There are, however, significant regional differences. Notably, Americans and Chinese, whose economies are responsible for the greatest annual CO2 emissions, are among the least concerned.^[36]

Observed temperature changes

Main article: Instrumental temperature record

2015 – Warmest Global Year on Record (since 1880) – Colors indicate temperature anomalies (NASA/NOAA; 20 January 2016).^[37]

Earth has been in radiative imbalance since at least the 1970s, where less energy leaves the atmosphere than enters it. Most of this extra energy has been absorbed by the oceans.^[38] It is very likely that human activities substantially contributed to this increase in ocean heat content.^[39]

Two millennia of mean surface temperatures according to different reconstructions from climate proxies, each smoothed on a decadal scale, with the instrumental temperature record overlaid in black.

NOAA graph of Global Annual Temperature Anomalies 1950–2012, showing the El Niño Southern Oscillation

The global average (land and ocean) surface temperature shows a warming of 0.85 [0.65 to 1.06] °C in the period 1880 to 2012, based on multiple independently produced datasets.^[40] Earth's average surface temperature rose by 0.74±0.18 °C over the period 1906–2005. The rate of warming almost doubled for the last half of that period (0.13±0.03 °C per decade, versus 0.07±0.02 °C per decade).^[41]
The average temperature of the lower troposphere has increased between 0.13 and 0.22 °C (0.23 and 0.40 °F) per decade since 1979, according to satellite temperature measurements. Climate proxies show the temperature to have been relatively stable over the one or two thousand years before 1850, with regionally varying fluctuations such as the Medieval Warm Period and the Little Ice Age.^[42]
The warming that is evident in the instrumental temperature record is consistent with a wide range of observations, as documented by many independent scientific groups.^[43] Examples include sea level rise,^[44] widespread melting of snow and land ice,^[45] increased heat content of the oceans,^[43] increased humidity,^[43] and the earlier timing of spring events,^[46] e.g., the flowering of plants.^[47] The probability that these changes could have occurred by chance is virtually zero.^[43]