Global warming

Global mean land-ocean temperature change from 1880–2012, relative to the 1951–1980 mean. The black line is the annual mean and the red line is the 5-year running mean. The green bars show uncertainty estimates. Source: NASA GISS. (click for larger image)

The map shows the 10-year average (2000–2009) global mean temperature anomaly relative to the 1951–1980 mean. The largest temperature increases are in the Arctic and the Antarctic Peninsula. Source: NASA Earth Observatory^[1]

Fossil fuel related CO₂ emissions compared to five of the IPCC's "SRES" emissions scenarios. The dips are related to global recessions. Image source: Skeptical Science.

Global warming is the rise in the average temperature of Earth's atmosphere and oceans since the late 19th century and its projected continuation. Since the early 20th century, Earth's mean surface temperature has increased by about 0.8 °C (1.4 °F), with about two-thirds of the increase occurring since 1980.^[2] Warming of the climate system is unequivocal, and scientists are 95-100% certain that it is primarily caused by increasing concentrations of greenhouse gases produced by human activities such as the burning of fossil fuels and deforestation.^[3][4][5] These findings are recognized by the national science academies of all major industrialized nations.^[6][A]
Climate model projections were summarized in the 2007 Fourth Assessment Report (AR4) by the Intergovernmental Panel on Climate Change (IPCC). They indicated that during the 21st century the global surface temperature is likely to rise a further 1.1 to 2.9 °C (2 to 5.2 °F change) for their lowest emissions scenario and 2.4 to 6.4 °C (4.3 to 11.5 °F change) for their highest.^[7] The ranges of these estimates arise from the use of models with differing sensitivity to greenhouse gas concentrations.^[8][9]
Future climate change and associated impacts^[10] will vary from region to region around the globe.^[11] The effects of an increase in global temperature include a rise in sea levels and a change in the amount and pattern of precipitation, as well as a probable expansion of subtropical deserts.^[12] Warming is expected to be strongest in the Arctic, with the continuing retreat of glaciers, permafrost and sea ice. Other likely effects of the warming include more frequent extreme weather events including heat waves, droughts and heavy rainfall; 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 loss of habitat from inundation.^[13][14]
Proposed policy responses to global warming include mitigation by emissions reduction, adaptation to its effects, and possible future geoengineering. Most countries are parties to the United Nations Framework Convention on Climate Change (UNFCCC),^[15] whose ultimate objective is to prevent dangerous anthropogenic (i.e., human-induced) climate change.^[16] Parties to the UNFCCC have adopted a range of policies designed to reduce greenhouse gas emissions^{[17]:10[18][19][20]:9} and to assist in adaptation to global warming.^{[17]:13[20]:10[21][22]} Parties to the UNFCCC have agreed that deep cuts in emissions are required,^[23] and that future global warming should be limited to below 2.0 °C (3.6 °F) relative to the pre-industrial level.^[23][B] Reports published in 2011 by the United Nations Environment Programme^[24] and the International Energy Agency^[25] suggest that efforts as of the early 21st century to reduce emissions may be inadequate to meet the UNFCCC's 2 °C target.

Observed temperature changes

Main article: Instrumental temperature record

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

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.^[26] It is very likely that human activities substantially contributed to this increase in ocean heat content.^[27]

The Earth's average surface temperature rose by 0.74±0.18 °C over the period 1906–2005. The rate of warming over the last half of that period was almost double that for the period as a whole (0.13±0.03 °C per decade, versus 0.07±0.02 °C per decade). The urban heat island effect is very small, estimated to account for less than 0.002 °C of warming per decade since 1900.^[28] Temperatures in the lower troposphere have increased between 0.13 and 0.22 °C (0.22 and 0.4 °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.^[29]
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.^[30] Examples include sea level rise (water expands as it warms),^[31] widespread melting of snow and ice,^[32] increased heat content of the oceans,^[30] increased humidity,^[30] and the earlier timing of spring events,^[33] e.g., the flowering of plants.^[34] The probability that these changes could have occurred by chance is virtually zero.^[30]
Recent estimates by NASA's Goddard Institute for Space Studies (GISS) and the National Climatic Data Center show that 2005 and 2010 tied for the planet's warmest year since reliable, widespread instrumental measurements became available in the late 19th century, exceeding 1998 by a few hundredths of a degree.^[35][36][37] Estimates by the Climatic Research Unit (CRU) show 2005 as the second warmest year, behind 1998 with 2003 and 2010 tied for third warmest year, however, "the error estimate for individual years ... is at least ten times larger than the differences between these three years."^[38] The World Meteorological Organization (WMO) statement on the status of the global climate in 2010 explains that, "The 2010 nominal value of +0.53 °C ranks just ahead of those of 2005 (+0.52 °C) and 1998 (+0.51 °C), although the differences between the three years are not statistically significant..."^[39] Every year from 1986 to 2012 has seen world annual mean temperatures above the 1961-1990 average.^{[40][41][dated info]}
Temperatures in 1998 were unusually warm because global temperatures are affected by the El Niño-Southern Oscillation (ENSO), and the strongest El Niño in the past century occurred during that year.^[42] Global temperature is subject to short-term fluctuations that overlay long term trends and can temporarily mask them. The relative stability in temperature from 2002 to 2009 is consistent with such an episode.^[43][44] 2010 was also an El Niño year. On the low swing of the oscillation, 2011 as a La Niña year was cooler but it was still the 11th warmest year since records began in 1880. Of the 13 warmest years since 1880, 11 were the years from 2001 to 2011. Over the more recent record, 2011 was the warmest La Niña year in the period from 1950 to 2011, and was close to 1997 which was not at the lowest point of the cycle.^[45]
Temperature changes vary over the globe. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).^[46] Ocean temperatures increase more slowly than land temperatures because of the larger effective heat capacity of the oceans and because the ocean loses more heat by evaporation.^[47] The northern hemisphere is also naturally warmer than the southern hemisphere mainly because of meridional heat transport in the oceans which has a differential of about 0.9 petawatts northwards,^[48] with an additional contribution from the albedo differences between the polar regions. Since the beginning of industrialisation the interhemispheric temperature difference has increased due to melting of sea ice and snow in the North.^[49] Average arctic temperatures have been increasing at almost twice the rate of the rest of the world in the past 100 years, however arctic temperatures are also highly variable.^[50] Although more greenhouse gases are emitted in the Northern than Southern Hemisphere this does not contribute to the difference in warming because the major greenhouse gases persist long enough to mix between hemispheres.^[51]
The thermal inertia of the oceans and slow responses of other indirect effects mean that climate can take centuries or longer to adjust to changes in forcing. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.^[52]

Initial causes of temperature changes (external forcings)

Main article: Attribution of recent climate change

Greenhouse effect schematic showing energy flows between space, the atmosphere, and earth's surface. Energy exchanges are expressed in watts per square meter (W/m²).

This graph, known as the Keeling Curve, shows the increase of atmospheric carbon dioxide (CO₂) concentrations from 1958–2008. Monthly CO₂ measurements display seasonal oscillations in an upward trend; each year's maximum occurs during the Northern Hemisphere's late spring, and declines during its growing season as plants remove some atmospheric CO₂.

The climate system can respond to changes in external forcings.^[53][54] External forcings can "push" the climate in the direction of warming or cooling.^[55] Examples of external forcings include changes in atmospheric composition (e.g., increased concentrations of greenhouse gases), solar luminosity, volcanic eruptions, and variations in Earth's orbit around the Sun.^[56] Orbital cycles vary slowly over tens of thousands of years and at present are in an overall cooling trend which would be expected to lead towards an ice age, but the 20th century instrumental temperature record shows a sudden rise in global temperatures.^[57]