1. GLOBAL WARMINGGlobal warming is the rise in the average temperature of Earths atmosphere and oceans since thelate 19th century and its projected continuation. Since the early 20th century, Earths meansurface temperature has increased by about 0.8 °C (1.4 °F), with about two-thirds of the increaseoccurring since 1980. Warming of the climate system is unequivocal, and scientists are morethan 90% certain that it is primarily caused by increasing concentrations of greenhouse gasesproduced by human activities such as the burning of fossil fuels and deforestation.These findings are recognized by the national science academies of all major industrializednations.[A]Climate model projections were summarized in the 2007 Fourth Assessment Report (AR4) by theIntergovernmental Panel on Climate Change (IPCC). They indicated that during the 21st centurythe global surface temperature is likely to rise a further 1.1 to 2.9 °C (2 to 5.2 °F) for their lowestemissions scenario and 2.4 to 6.4 °C (4.3 to 11.5 °F) for their highest. The ranges of theseestimates arise from the use of models with differing sensitivity to greenhouse gasconcentrations.According to AR4, warming and related changes will vary from region to region around theglobe. The effects of an increase in global temperature include a rise in sea levels and a changein the amount and pattern of precipitation, as well a probable expansion of subtropicaldeserts. Warming is expected to be strongest in the Arctic and would be associated with thecontinuing retreat of glaciers, permafrost and sea ice. Other likely effects of the warming include amore frequent occurrence of extreme-weather events including heat waves, droughts and heavyrainfall, ocean acidification and species extinctions due to shifting temperature regimes. Effectssignificant to humans include the threat to food security from decreasing crop yields and the lossof habitat from inundation.Proposed policy responses to global warming include mitigation by emissions reduction,adaptation to its effects, and possible future geoengineering. Most countries are parties to theUnited Nations Framework Convention on Climate Change (UNFCCC), whose ultimateobjective is to prevent dangerous anthropogenic (i.e., human-induced) climate change. Partiesto the UNFCCC have adopted a range of policies designed to reduce greenhouse gasemissions:10:9 and to assist in adaptation to globalwarming.:13:10 Parties to the UNFCCC have agreed that deep cuts in emissionsare required, and that future global warming should be limited to below 2.0 °C (3.6 °F) relativeto the pre-industrial level.[B] Reports published in 2011 by the United Nations EnvironmentProgramme and the International Energy Agency suggest that efforts as of the early 21stcentury to reduce emissions may be inadequate to meet the UNFCCCs 2 °C target.
2. OBSERVED TEMPERATURE CHANGEDThe Earths average surface temperature rose by 0.74±0.18 °C over the period 1906–2005. Therate 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 verysmall, estimated to account for less than 0.002 °C of warming per decade since 1900. Temperatures in the lower troposphere have increased between 0.13 and 0.22 °C (0.22and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Climateproxies show the temperature to have been relatively stable over the one or two thousandyears before 1850, with regionally varying fluctuations such as the Medieval Warm Period and theLittle Ice Age.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. Examples include sea  level rise (water expands as it warms), widespread melting of snow and ice, increasedheat   content of the oceans, increased humidity, and the earlier timing of spring events, e.g., the flowering of plants. The probability that these changes could have occurred by chance is virtually zero.Recent estimates by NASAs Goddard Institute for Space Studies (GISS) and the National ClimaticData Center show that 2005 and 2010 tied for the planets warmest year since reliable, widespreadinstrumental measurements became available in the late 19th century, exceeding 1998 by a few hundredths of a degree. Estimates by the Climatic Research Unit (CRU) show 2005 as thesecond warmest year, behind 1998 with 2003 and 2010 tied for third warmest year, however, "theerror estimate for individual years ... is at least ten times larger than the differences between these three years." The World Meteorological Organization (WMO) statement on the status of theglobal climate in 2010 explains that, "The 2010 nominal value of +0.53 °C ranks just ahead of thoseof 2005 (+0.52 °C) and 1998 (+0.51 °C), although the differences between the three years are not statistically significant..."NOAA graph of Global Annual Temperature Anomalies 1950–2011, showing the El Niño-Southern OscillationTemperatures in 1998 were unusually warm because global temperatures are affected by the ElNiño-Southern Oscillation (ENSO), and the strongest El Niño in the past century occurred during that year. Global temperature is subject to short-term fluctuations that overlay long term trendsand can temporarily mask them. The relative stability in temperature from 2002 to 2009 is consistent with such an episode. 2010 was also an El Niño year. On the low swing of theoscillation, 2011 as an La Niña year was cooler but it was still the 11th warmest year since records
3. began in 1880. Of the 13 warmest years since 1880, 11 were the years from 2001 to 2011. Overthe 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.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). Oceantemperatures 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. The northernhemisphere warms faster than the southern hemispherebecause it has more land and because ithas extensive areas of seasonal snow and sea-ice cover subject to ice-albedo feedback. Althoughmore greenhouse gases are emitted in the Northern than Southern Hemisphere this does notcontribute to the difference in warming because the major greenhouse gases persist long enough to mix between hemispheres.The thermal inertia of the oceans and slow responses of other indirect effects mean that climate cantake centuries or longer to adjust to changes in forcing. Climate commitment studies indicate thateven if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.Initial causes of temperaturechanges (externalforcings)Greenhouse effect schematic showing energy flows between space, the atmosphere, and earths surface. Energyexchanges are expressed in watts per square meter (W/m 2).This graph, known as the Keeling Curve, shows the increase of atmosphericcarbon dioxide (CO2) concentrationsfrom 1958–2008. Monthly CO2measurements display seasonal oscillations in an upward trend; each years maximumoccurs during theNorthern Hemispheres late spring, and declines during its growing season as plants remove someatmospheric CO2. The climate system can respond to changes in external forcings. External forcings can "push" the climate in the direction of warming or cooling. Examples of external forcings include changesin atmospheric composition (e.g., increased concentrations of greenhouse gases), solar
4. luminosity, volcanic eruptions, and variations in Earths orbit around the Sun. Attribution of recentclimate change focuses on the first three types of forcing. Orbital cycles vary slowly over tens ofthousands of years and at present are in an overall cooling trend which would be expected to leadtowards an ice age, but the 20th century instrumental temperature record shows a sudden rise in global temperatures.GREENHOUSE GASESThe greenhouse effect is the process by which absorption and emission of infrared radiation bygases in the atmosphere warm a planets lower atmosphere and surface. It was proposed by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896.Naturally occurring amounts of greenhouse gases have a mean warming effect of about 33 [C]°C (59 °F). The major greenhouse gases are water vapor, which causes about 36–70% of thegreenhouse effect; carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4– 9%; and ozone (O3), which causes 3–7%. Clouds also affect the radiation balancethrough cloud forcings similar to greenhouse gases.Human activity since the Industrial Revolution has increased the amount of greenhouse gases inthe atmosphere, leading to increased radiative forcing from CO 2, methane, troposphericozone, CFCs and nitrous oxide. The concentrations of CO2 and methane have increased by 36% and 148% respectively since 1750. These levels are much higher than at any time during the last 800,000 years, the period for which reliable data has been extracted from ice cores. Lessdirect geological evidence indicates that CO2 values higher than this were last seen about 20 million years ago. Fossil fuelburning has produced about three-quarters of the increase in CO2 fromhuman activity over the past 20 years. The rest of this increase is caused mostly by changes in land-use, particularly deforestation.Per capita greenhouse gas emissions in 2005, including land-use change.Total greenhouse gas emissions in 2005, including land-use change.Over the last three decades of the 20th century, gross domestic product per capita and population growth were the main drivers of increases in greenhouse gas emissions. CO2 emissions are :71continuing to rise due to the burning of fossil fuels and land-use change. Emissions canbe attributed to different regions. The two figures opposite show annual greenhouse gas emissionsfor the year 2005, including land-use change. Attribution of emissions due to land-use change is a :289controversial issue.
5. Emissions scenarios, estimates of changes in future emission levels of greenhouse gases, havebeen projected that depend upon uncertain economic, sociological, technological, and natural developments. In most scenarios, emissions continue to rise over the century, while in a few, emissions are reduced. Fossil fuel reserves are abundant, and will not limit carbon emissions in the 21st century. Emission scenarios, combined with modelling of the carbon cycle, have beenused to produce estimates of how atmospheric concentrations of greenhouse gases might changein the future. Using the six IPCC SRES"marker" scenarios, models suggest that by the year 2100, the atmospheric concentration of CO2 could range between 541 and 970 ppm. This is anincrease of 90–250% above the concentration in the year 1750.The popular media and the public often confuse global warming with ozone depletion, i.e., the destruction ofstratospheric ozone by chlorofluorocarbons. Although there are a few areas oflinkage, the relationship between the two is not strong. Reduced stratospheric ozone has had aslight cooling influence on surface temperatures, while increased tropospheric ozone has had a somewhat larger warming effect.