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Global Warming Global warming This page is about the current warming of the Earth’s IEA CO2 Emissions per Year vs. IPCC Scenarios climate system. "Climate change" can also refer to 32 IEA data climate trends at any point in Earth’s history. For other Emitted 2 B2 uses see Global warming (disambiguation) 30 A1 28 A1T Global Land–Ocean Temperature Index A2 26 A1F1 0.6 Annual Mean 24 0.4 5‐year Running Mean 22 0.2 1990 1995 2000 2005 2010 Billions of Tonnes Fossil Fuel CO 0 Year Fossil fuel related carbon dioxide (CO2) emissions com- −Temperature Anomaly (°C) 0.2 pared to five of the IPCC’s "SRES" emissions scenarios. The dips are related to global recessions. Image source: Skeptical Science. −0.4 1880 1900 1920 1940 1960 1980 2000 Global mean surface temperature change from 1880 to Global warming and climate change are terms for the 2014, relative to the 1951–1980 mean. The black line is observed century-scale rise in the average temperature of the annual mean and the red line is the 5-year running the Earth’s climate system and its related effects. mean. The green bars show uncertainty estimates. Multiple lines of scientific evidence show that the cli- Source: NASA GISS. mate system is warming.[2][3] Although the increase of near-surface atmospheric temperature is the mea- sure of global warming often reported in the popular press, most of the additional energy stored in the cli- mate system since 1970 has gone into ocean warming. The remainder has melted ice, and warmed the con- tinents and atmosphere.[4][lower-alpha 1] Many of the ob- served changes since the 1950s are unprecedented over decades to millennia.[5] Scientific understanding of global warming is increasing. In its 2014 report the Intergovernmental Panel on Cli- mate Change (IPCC) reported that scientists were more than 95% certain that most of global warming is caused World map showing surface temperature trends (°C per by increasing concentrations of greenhouse gases and decade) between 1950 and 2014. Source: NASA other human (anthropogenic) activities.[6][7][8] Climate GISS.[1] model projections summarized in the report indicated that during the 21st century the global surface temper- ature 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.[9] These findings have been recognized by the national science academies of the major industrialized nations.[10][lower-alpha 2] Future climate change and associated impacts will dif- fer from region to region around the globe.[12][13] An- 1 2 1 OBSERVED TEMPERATURE CHANGES ticipated effects include warming global temperature, rising sea levels, changing precipitation, and expansion of deserts in the subtropics.[14] Warming is expected to be greatest in the Arctic, with the continuing retreat of glaciers, permafrost and sea ice. Other likely changes include more frequent extreme weather events includ- ing heat waves, droughts, heavy rainfall, and heavy snowfall;[15] ocean acidification; and species extinctions due to shifting temperature regimes. Effects significant to humans include the threat to food security from de- creasing crop yields and the abandonment of populated areas due to flooding.[16][17] Possible societal responses to global warming include mitigation by emissions reduction, adaptation to its ef- Two millennia of mean surface temperatures according to dif- fects, building systems resilient to its effects, and possible ferent reconstructions from climate proxies, each smoothed on a future climate engineering. Most countries are parties to decadal scale, with the instrumental temperature record overlaid the United Nations Framework Convention on Climate in black. Change (UNFCCC),[18] whose ultimate objective is to prevent dangerous anthropogenic climate change.[19] The UNFCCC have adopted a range of policies designed to reduce greenhouse gas emissions[20][21][22][23] and to as- sist in adaptation to global warming.[20][23][24][25] Parties to the UNFCCC have agreed that deep cuts in emissions are required,[26] and that future global warming should be limited to below 2.0 °C (3.6 °F) relative to the pre- industrial level.[26][lower-alpha 3] 1 Observed temperature changes Main article: Instrumental temperature record The global average (land and ocean) surface tempera- Energy change inventory, 1971-2010 NOAA graph of Global Annual Temperature Anomalies 1950– 2012, showing the El Niño Southern Oscillation Ice 3% Oceans Continents °C per decade).[31] 3% 93% Atmosphere The average temperature of the lower troposphere has in- 1% creased between 0.13 and 0.22 °C (0.23 and 0.40 °F) per decade since 1979, according to satellite tempera- ture measurements. Climate proxies show the temper- ature to have been relatively stable over the one or two thousand years before 1850, with regionally varying fluc- Earth has been in radiative imbalance since at least the 1970s, tuations such as the Medieval Warm Period and the Little where less energy leaves the atmosphere than enters it. Most of Ice Age.[32] this extra energy has been absorbed by the oceans.[28] It is very likely that human activities substantially contributed to this in- The warming that is evident in the instrumental temper- crease in ocean heat content.[29] ature record is consistent with a wide range of obser- vations, as documented by many independent scientific ture shows a warming of 0.85 [0.65 to 1.06] °C in the groups.[33] Examples include sea level rise,[34] widespread period 1880 to 2012, based on multiple independently melting of snow and land ice,[35] increased heat con- produced datasets.[30] Earth’s average surface tempera- tent of the oceans,[33] increased humidity,[33] and the ture rose by 0.74±0.18 °C over the period 1906–2005. earlier timing of spring events,[36] e.g., the flowering of The rate of warming almost doubled for the last half of plants.[37] The probability that these changes could have that period (0.13±0.03 °C per decade, versus 0.07±0.02 occurred by chance is virtually zero.[33] 3 1.1 Trends 2 Initial causes of temperature changes (external forcings) Temperature changes vary over the globe. Since 1979, land temperatures have increased about twice as fast as Main article: Attribution of recent climate change ocean temperatures (0.25 °C per decade against 0.13 Thermal radiation into space: 195 Directly radiated [38] Solar Radiation °C per decade). Ocean temperatures increase more from surface: 40 absorbed by Earth slowly than land temperatures because of the larger ef- 235 W/m² Greenhouse gas fective heat capacity of the oceans and because the ocean absorption: 350 67 [39] Heat and energy loses more heat by evaporation. The northern hemi- 452 in the atmosphere sphere is also naturally warmer than the southern hemi- sphere mainly because of meridional heat transport in the The Greenhouse oceans, which has a differential of about 0.9 petawatts 168 324 Effect 492 northwards,[40] with an additional contribution from the Earth's land and ocean surface albedo differences between the polar regions. Since the warmed to an average of 14℃ beginning of industrialisation the temperature difference Greenhouse effect schematic showing energy flows between the hemispheres has increased due to melting of between space, the atmosphere, and Earth’s surface. sea ice and snow in the North.[41] Average arctic temper- Energy exchanges are expressed in watts per square atures have been increasing at almost twice the rate of meter (W/m2). the rest of the world in the past 100 years; however arctic temperatures are also highly variable.[42] Although more greenhouse gases are emitted in the Northern than South- ern Hemisphere this does not contribute to the difference in warming because the major greenhouse gases persist long enough to mix between hemispheres.[43] 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 com- mitment studies indicate that even if greenhouse gases were stabilized at year 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.[44] Global temperature is subject to short-term fluctuations This graph, known as the Keeling Curve, documents that overlay long-term trends and can temporarily mask the increase of atmospheric carbon dioxide (CO2) them. The relative stability in surface temperature from concentrations from 1958–2015. Monthly CO2 mea- 2002 to 2009, which has been dubbed the global warming surements display seasonal oscillations in an upward hiatus by the media and some scientists,[45] is consistent trend; each year’s maximum occurs during the Northern with such an episode.[46][47] Recent updates to account for Hemisphere's late spring, and declines during its growing differing methods of measuring ocean surface tempera- season as plants remove some atmospheric CO2. ture measurements show a significant positive trend over the recent decade.[48][49] The climate system can respond to changes in exter- nal forcings.[56][57] External forcings can “push” the cli- mate in the direction of warming or cooling.[58] Ex- amples of external forcings include changes in atmo- spheric composition (e.g., increased concentrations of greenhouse gases), solar luminosity, volcanic eruptions, 1.2 Warmest years and variations in Earth’s orbit around the Sun.[59] Orbital cycles vary slowly over tens of thousands of years, and at present are in a cooling trend.[60] The variations in orbital Nine of the 10 warmest years in the instrumental
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