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Global warming

Global warming

Overview
Global warming refers to the rising average temperature
Instrumental temperature record
The instrumental temperature record shows fluctuations of the temperature of the global land surface and oceans. This data is collected from several thousand meteorological stations, Antarctic research stations and satellite observations of sea-surface temperature. Currently, the longest-running...

 of Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about 0.8 C-change with about two thirds of the increase occurring over just the last three decades. Warming of the climate system is unequivocal, and scientists are more than 90% certain most of it is caused by increasing concentrations of greenhouse gases
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

 produced by human activities such as deforestation
Deforestation
Deforestation is the removal of a forest or stand of trees where the land is thereafter converted to a nonforest use. Examples of deforestation include conversion of forestland to farms, ranches, or urban use....

 and burning fossil fuel
Fossil fuel
Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...

.
Discussion
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Quotations

Global warming causing climate change may be the ultimate issue that unites us all.

Louise Burfitt-Dons,2008

"Why try to change the future when the present is so bleak"

Santa Bush 11/09/2007

"There's a better scientific consensus on this than on any issue I know -- except maybe Newton's second law of dynamics," said D. James Baker, administrator of the U.S. National Oceanic and Atmospheric Administration. "Man has reached the point where his impact on the climate can be as significant as nature's."

Warrick, Joby. “Consensus Emerges Earth Is Warming – Now What?” Washington Post 12 Nov. 1997: A01.

"The answer to global warming is in the abolition of private property and production for human need. A socialist world would place an enormous priority on alternative energy sources. This is what ecologically-minded socialists have been exploring for quite some time now."

Louis Proyect, Columbia University

‘Since global warming Eskimos now have twenty different words for water.’

John O'Farrell - This Is Your Life 2001

“If you asked me to name the three scariest threats facing the human race, I would give the same answer that most people would: nuclear war, global warming and Windows.”

Dave Barry

“While human-induced global warming is not going to turn present-day Earth into present-day Mars, global warming is dire enough that our most distinguished scientists recently concluded that as many as 1 million species on the planet could be extinct by 2050 if affairs do not change.”

Jay Inslee
Encyclopedia
Global warming refers to the rising average temperature
Instrumental temperature record
The instrumental temperature record shows fluctuations of the temperature of the global land surface and oceans. This data is collected from several thousand meteorological stations, Antarctic research stations and satellite observations of sea-surface temperature. Currently, the longest-running...

 of Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about 0.8 C-change with about two thirds of the increase occurring over just the last three decades. Warming of the climate system is unequivocal, and scientists are more than 90% certain most of it is caused by increasing concentrations of greenhouse gases
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

 produced by human activities such as deforestation
Deforestation
Deforestation is the removal of a forest or stand of trees where the land is thereafter converted to a nonforest use. Examples of deforestation include conversion of forestland to farms, ranches, or urban use....

 and burning fossil fuel
Fossil fuel
Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...

. These findings are recognized by the national science academies of all the major industrialized countries.

Climate model
Climate model
Climate models use quantitative methods to simulate the interactions of the atmosphere, oceans, land surface, and ice. They are used for a variety of purposes from study of the dynamics of the climate system to projections of future climate...

 projections are summarized in the 2007 Fourth Assessment Report
IPCC Fourth Assessment Report
Climate Change 2007, the Fourth Assessment Report of the United Nations Intergovernmental Panel on Climate Change , is the fourth in a series of reports intended to assess scientific, technical and socio-economic information concerning climate change, its potential effects, and options for...

 (AR4) by the Intergovernmental Panel on Climate Change
Intergovernmental Panel on Climate Change
The Intergovernmental Panel on Climate Change is a scientific intergovernmental body which provides comprehensive assessments of current scientific, technical and socio-economic information worldwide about the risk of climate change caused by human activity, its potential environmental and...

 (IPCC). They indicate that during the 21st century the global surface temperature is likely to rise a further 1.1 C-change for their lowest emissions scenario
Special Report on Emissions Scenarios
The Special Report on Emissions Scenarios was prepared by the Intergovernmental Panel on Climate Change in 2000, based on data developed at the Earth Institute at Columbia University. The emissions scenarios described in the Report have been used to make projections of possible future climate...

 and 2.4 C-change for their highest. The ranges of these estimates arise from the use of models with differing sensitivity to greenhouse gas concentrations
Climate sensitivity
Climate sensitivity is a measure of how responsive the temperature of the climate system is to a change in the radiative forcing. It is usually expressed as the temperature change associated with a doubling of the concentration of carbon dioxide in Earth's atmosphere.The equilibrium climate...

.

An increase in global temperature will cause sea levels to rise
Current sea level rise
Current sea level rise potentially impacts human populations and the wider natural environment . Global average sea level rose at an average rate of around 1.8 mm per year over 1961 to 2003 and at an average rate of about 3.1 mm per year from 1993 to 2003...

 and will change the amount and pattern of precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

, and a probable expansion of subtropical
Subtropics
The subtropics are the geographical and climatical zone of the Earth immediately north and south of the tropical zone, which is bounded by the Tropic of Cancer and the Tropic of Capricorn, at latitudes 23.5°N and 23.5°S...

 desert
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...

s. Warming is expected to be strongest in the Arctic
Arctic shrinkage
Ongoing changes in the climate of the Arctic include rising temperatures, loss of sea ice, and melting of the Greenland ice sheet. Projections of sea ice loss suggest that the Arctic ocean will likely be free of summer sea ice sometime between 2060 and 2080, while another estimate puts this date at...

 and would be associated with continuing retreat of glaciers
Retreat of glaciers since 1850
The retreat of glaciers since 1850 affects the availability of fresh water for irrigation and domestic use, mountain recreation, animals and plants that depend on glacier-melt, and in the longer term, the level of the oceans...

, permafrost
Permafrost
In geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of...

 and sea ice
Sea ice
Sea ice is largely formed from seawater that freezes. Because the oceans consist of saltwater, this occurs below the freezing point of pure water, at about -1.8 °C ....

. Other likely effects of the warming include more frequent occurrence of extreme weather
Extreme weather
Extreme weather includes weather phenomena that are at the extremes of the historical distribution, especially severe or unseasonal weather. The most commonly used definition of extreme weather is based on an event's climatological distribution. Extreme weather occurs only 5% or less of the time...

 events including heatwaves, droughts and heavy rainfall events, species extinctions
Extinction risk from climate change
The extinction risk of global warming is the risk species have of becoming extinct due to the effects of global warming.-Current projections:The scientific consensus in the IPCC Fourth Assessment Report is that...

 due to shifting temperature regimes, and changes in agricultural yields. Warming and related changes will vary from region to region around the globe, with projections being more robust in some areas than others. In a 4 °C world, the limits for human adaptation are likely to be exceeded in many parts of the world, while the limits for adaptation for natural systems would largely be exceeded throughout the world. Hence, the ecosystem services upon which human livelihoods depend would not be preserved.

Proposed responses to global warming include mitigation
Mitigation of global warming
Climate change mitigation is action to decrease the intensity of radiative forcing in order to reduce the potential effects of global warming. Mitigation is distinguished from adaptation to global warming, which involves acting to tolerate the effects of global warming...

 to reduce emissions, adaptation
Adaptation to global warming
Adaptation to global warming and climate change is a response to climate change that seeks to reduce the vulnerability of natural and human systems to climate change effects. Even if emissions are stabilized relatively soon, climate change and its effects will last many years, and adaptation will...

 to the effects of global warming, and geoengineering
Geoengineering
The concept of Geoengineering refers to the deliberate large-scale engineering and manipulation of the planetary environment to combat or counteract anthropogenic changes in atmospheric chemistry The Intergovernmental Panel on Climate Change concluded in 2007 that geoengineering options, such...

 to remove greenhouse gases from the atmosphere or reflect incoming solar radiation back to space. The primary international effort to prevent dangerous anthropogenic climate change
Avoiding Dangerous Climate Change
The related terms "avoiding dangerous climate change" and "preventing dangerous anthropogenic interference with the climate system" date to 1995 and earlier, in the Second Assesment Report of the International Panel on Climate Change and previous science it cites.In 2002, the United Nations...

 ("mitigation") is coordinated by the 194-nation UNFCCC. The Kyoto Protocol
Kyoto Protocol
The Kyoto Protocol is a protocol to the United Nations Framework Convention on Climate Change , aimed at fighting global warming...

 is their only legally binding emissions agreement and only limits emissions through the year 2012. Afghanistan and the USA are the only nations in the UNFCCC that have not ratified the original protocol, and as of October 2011 several others have refused to extend the emissions limits beyond 2012. Nonetheless, in the 2010 Cancun Agreements
2010 United Nations Climate Change Conference
The 2010 United Nations Climate Change Conference was held in Cancún, Mexico, from 29 November to 10 December 2010. The conference is officially referred to as the 16th session of the Conference of the Parties to the United Nations Framework Convention on Climate Change and the 6th session of the...

, member nations agreed that urgent action is needed to limit global warming to no more than 2 C-change above pre-industrial levels. Current scientific evidence, however, suggests that 2°C is the "threshold between ‘dangerous’ and ‘extremely dangerous’ climate change", that this much warming is possible during the lifetimes of people living today, and that steep reductions in global emissions must be made by 2020 in order to have a 2-out-of-3 chance of avoiding global warming in excess of 2°C.

Observed temperature changes


Evidence for warming of the climate system includes observed increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level. The Earth's average surface temperature, expressed as a linear trend, 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 °C ± 0.02 °C per decade). The urban heat island
Urban heat island
An urban heat island is a metropolitan area which is significantly warmer than its surrounding rural areas. The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon. The temperature difference usually is larger at night...

 effect is estimated to account for about 0.002 °C of warming per decade since 1900. Temperatures in the lower troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

 have increased between 0.13 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements
Satellite temperature measurements
The temperature of the atmosphere at various altitudes as well as sea and land surface temperatures can be inferred from satellite measurements. Weather satellites do not measure temperature directly but measure radiances in various wavelength bands...

. Climate proxies
Proxy (climate)
In the study of past climates is known as paleoclimatology, climate proxies are preserved physical characteristics of the past that stand in for direct measurements , to enable scientists to reconstruct the climatic conditions that prevailed during much of the Earth's history...

 show the temperature to have been relatively stable over the one or two thousand years
Temperature record of the past 1000 years
The temperature record of the 2nd millennium describes the reconstruction of temperatures since 1000 CE on the Northern Hemisphere, later extended back to 1 CE and also to cover the southern hemisphere. A reconstruction is needed because a reliable surface temperature record exists only since about...

 before 1850, with regionally varying fluctuations such as the Medieval Warm Period
Medieval Warm Period
The Medieval Warm Period , Medieval Climate Optimum, or Medieval Climatic Anomaly was a time of warm climate in the North Atlantic region, that may also have been related to other climate events around the world during that time, including in China, New Zealand, and other countries lasting from...

 and the Little Ice Age
Little Ice Age
The Little Ice Age was a period of cooling that occurred after the Medieval Warm Period . While not a true ice age, the term was introduced into the scientific literature by François E. Matthes in 1939...

.

Recent estimates by NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

's Goddard Institute for Space Studies
Goddard Institute for Space Studies
The NASA Goddard Institute for Space Studies , at Columbia University in New York City, is a component laboratory of NASA's Goddard Space Flight Center Earth-Sun Exploration Division and a unit of The Earth Institute at Columbia University...

 (GISS) and the National Climatic Data Center
National Climatic Data Center
The United States National Climatic Data Center in Asheville, North Carolina is the world's largest active archive of weather data. The center became established in late 1951, with the move into the new facility occurring in early 1952....

 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. Current estimates by the Climatic Research Unit
Climatic Research Unit
The Climatic Research Unit is a component of the University of East Anglia and is one of the leading institutions concerned with the study of natural and anthropogenic climate change....

 (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.” The World Meteorological Organization
World Meteorological Organization
The World Meteorological Organization is an intergovernmental organization with a membership of 189 Member States and Territories. It originated from the International Meteorological Organization , which was founded in 1873...

 (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...”

Temperatures in 1998 were unusually warm because the strongest El Niño
El Niño-Southern Oscillation
El Niño/La Niña-Southern Oscillation, or ENSO, is a quasiperiodic climate pattern that occurs across the tropical Pacific Ocean roughly every five years...

 in the past century occurred during that year. 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.

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). 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. The Northern Hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...

 warms faster than the Southern Hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...

 because it has more land and because it has extensive areas of seasonal snow and sea-ice cover subject to ice-albedo feedback
Ice-albedo feedback
Ice-albedo feedback is a positive feedback climate process where a change in the area of snow-covered land, ice caps, glaciers or sea ice alters the albedo. This change in albedo acts to reinforce the initial alteration in ice area...

. 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.

The thermal inertia
Volumetric heat capacity
Volumetric heat capacity , also termed volume-specific heat capacity, describes the ability of a given volume of a substance to store internal energy while undergoing a given temperature change, but without undergoing a phase change...

 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 would still occur.

Initial causes of temperature changes (External forcings)


External forcing refers to processes external to the climate system (though not necessarily external to Earth) that influence climate. Climate responds to several types of external forcing, such as radiative forcing
Radiative forcing
In climate science, radiative forcing is generally defined as the change in net irradiance between different layers of the atmosphere. Typically, radiative forcing is quantified at the tropopause in units of watts per square meter. A positive forcing tends to warm the system, while a negative...

 due to changes in atmospheric composition (mainly greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

 concentrations), changes in solar luminosity, volcanic
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

 eruptions, and variations in Earth's orbit
Orbital forcing
Orbital forcing is the effect on climate of slow changes in the tilt of the Earth's axis and shape of the orbit . These orbital changes change the total amount of sunlight reaching the Earth by up to 25% at mid-latitudes...

 around the Sun.} Attribution of recent climate change
Attribution of recent climate change
Attribution of recent climate change is the effort to scientifically ascertain mechanisms responsible for recent changes observed in the Earth's climate...

 focuses on the first three types of forcing. Orbital cycles
Milankovitch cycles
Milankovitch theory describes the collective effects of changes in the Earth's movements upon its climate, named after Serbian civil engineer and mathematician Milutin Milanković, who worked on it during First World War internment...

 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
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...

, but the 20th century instrumental temperature record
Instrumental temperature record
The instrumental temperature record shows fluctuations of the temperature of the global land surface and oceans. This data is collected from several thousand meteorological stations, Antarctic research stations and satellite observations of sea-surface temperature. Currently, the longest-running...

 shows a sudden rise in global temperatures.

Greenhouse gases



The greenhouse effect is the process by which absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...

 and emission
Emission spectrum
The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted by the element's atoms or the compound's molecules when they are returned to a lower energy state....

 of infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 radiation by gases in the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 warm a planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

's lower atmosphere and surface. It was proposed by Joseph Fourier
Joseph Fourier
Jean Baptiste Joseph Fourier was a French mathematician and physicist best known for initiating the investigation of Fourier series and their applications to problems of heat transfer and vibrations. The Fourier transform and Fourier's Law are also named in his honour...

 in 1824 and was first investigated quantitatively by Svante Arrhenius
Svante Arrhenius
Svante August Arrhenius was a Swedish scientist, originally a physicist, but often referred to as a chemist, and one of the founders of the science of physical chemistry...

 in 1896.

Naturally occurring amounts of greenhouse gases have a mean warming effect of about 33 C-change. The major greenhouse gases are water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

, which causes about 36–70 percent of the greenhouse effect; carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 (CO2), which causes 9–26 percent; methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 (CH4), which causes 4–9 percent; and ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...

 (O3), which causes 3–7 percent. Clouds also affect the radiation balance through cloud forcing
Cloud forcing
Cloud forcing is, in meteorology, the difference between the radiation budget components for average cloud conditions and cloud-free conditions...

s similar to greenhouse gases.

Human activity since the Industrial Revolution
Industrial Revolution
The Industrial Revolution was a period from the 18th to the 19th century where major changes in agriculture, manufacturing, mining, transportation, and technology had a profound effect on the social, economic and cultural conditions of the times...

 has increased the amount of greenhouse gases in the atmosphere, leading to increased radiative forcing from CO2, methane, tropospheric ozone, CFCs
Chlorofluorocarbon
A chlorofluorocarbon is an organic compound that contains carbon, chlorine, and fluorine, produced as a volatile derivative of methane and ethane. A common subclass are the hydrochlorofluorocarbons , which contain hydrogen, as well. They are also commonly known by the DuPont trade name Freon...

 and nitrous oxide
Nitrous oxide
Nitrous oxide, commonly known as laughing gas or sweet air, is a chemical compound with the formula . It is an oxide of nitrogen. At room temperature, it is a colorless non-flammable gas, with a slightly sweet odor and taste. It is used in surgery and dentistry for its anesthetic and analgesic...

. 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 core
Ice core
An ice core is a core sample that is typically removed from an ice sheet, most commonly from the polar ice caps of Antarctica, Greenland or from high mountain glaciers elsewhere. As the ice forms from the incremental build up of annual layers of snow, lower layers are older than upper, and an ice...

s. Less direct geological evidence indicates that CO2 values higher than this were last seen about 20 million years ago. Fossil fuel
Fossil fuel
Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...

 burning has produced about three-quarters of the increase in CO2 from human activity over the past 20 years. The rest of this increase is caused mostly by changes in land-use, particularly deforestation
Deforestation
Deforestation is the removal of a forest or stand of trees where the land is thereafter converted to a nonforest use. Examples of deforestation include conversion of forestland to farms, ranches, or urban use....

.
Over the last three decades of the 20th century, gross domestic product per capita and population growth
Population growth
Population growth is the change in a population over time, and can be quantified as the change in the number of individuals of any species in a population using "per unit time" for measurement....

 were the main drivers of increases in greenhouse gas emissions. CO2 emissions are continuing to rise due to the burning of fossil fuels and land-use change.
Emissions can be attributed to different regions. The two figures opposite show annual greenhouse gas emissions for the year 2005, including land-use change. Attribution of emissions due to land-use change is a controversial issue.
Emissions scenarios, estimates of changes in future emission levels of greenhouse gases, have been projected that depend upon uncertain economic, sociological
Sociology
Sociology is the study of society. It is a social science—a term with which it is sometimes synonymous—which uses various methods of empirical investigation and critical analysis to develop a body of knowledge about human social activity...

, technological
Technology
Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures. The word technology comes ;...

, 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
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...

, have been used to produce estimates of how atmospheric concentrations of greenhouse gases might change in 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 an increase of 90–250% above the concentration in the year 1750.

The popular media and the public often confuse global warming with the ozone hole, i.e., the destruction of stratospheric
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

 ozone by chlorofluorocarbons. Although there are a few areas of linkage, the relationship between the two is not strong. Reduced stratospheric ozone has had a slight cooling influence on surface temperatures, while increased tropospheric ozone
Tropospheric ozone
Ozone is a constituent of the troposphere . Photochemical and chemical reactions involving it drive many of the chemical processes that occur in the atmosphere by day and by night...

 has had a somewhat larger warming effect.

Particulates and soot


Global dimming
Global dimming
Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth's surface that was observed for several decades after the start of systematic measurements in the 1950s. The effect varies by location, but worldwide it has been estimated to be of the order of a 4%...

, a gradual reduction in the amount of global direct irradiance
Irradiance
Irradiance is the power of electromagnetic radiation per unit area incident on a surface. Radiant emittance or radiant exitance is the power per unit area radiated by a surface. The SI units for all of these quantities are watts per square meter , while the cgs units are ergs per square centimeter...

 at the Earth's surface, has partially counteracted global warming from 1960 to the present. The main cause of this dimming is particulates produced by volcanoes and human made pollutant
Pollutant
A pollutant is a waste material that pollutes air, water or soil, and is the cause of pollution.Three factors determine the severity of a pollutant: its chemical nature, its concentration and its persistence. Some pollutants are biodegradable and therefore will not persist in the environment in the...

s, which exerts a cooling effect by increasing the reflection of incoming sunlight. The effects of the products of fossil fuel combustion—CO2 and aerosols—have largely offset one another in recent decades, so that net warming has been due to the increase in non-CO2 greenhouse gases such as methane. Radiative forcing due to particulates is temporally limited due to wet deposition which causes them to have an atmospheric lifetime of one week. Carbon dioxide has a lifetime of a century or more, and as such, changes in particulate concentrations will only delay climate changes due to carbon dioxide.

In addition to their direct effect by scattering and absorbing solar radiation, particulates have indirect effects on the radiation budget. Sulfates act as cloud condensation nuclei
Cloud condensation nuclei
Cloud condensation nuclei or CCNs are small particles typically 0.2 µm, or 1/100 th the size of a cloud droplet ) about which cloud droplets coalesce. Water requires a non-gaseous surface to make the transition from a vapour to a liquid. In the atmosphere, this surface presents itself as tiny...

 and thus lead to clouds that have more and smaller cloud droplets. These clouds reflect solar radiation more efficiently than clouds with fewer and larger droplets, known as the Twomey effect
Twomey effect
Twomey effect — describes how cloud condensation nuclei , possibly from anthropogenic pollution, may increase the amount of solar radiation reflected by clouds. This is an indirect effect....

. This effect also causes droplets to be of more uniform size, which reduces growth of raindrops and makes the cloud more reflective to incoming sunlight, known as the Albrecht effect
Albrecht effect
The Albrecht effect describes how cloud condensation nuclei , possibly from anthropogenic pollution, may increase cloud lifetime and hence increase the amount of solar radiation reflected from clouds. Because it does not directly interact with incoming or outgoing radiation, it has an indirect...

. Indirect effects are most noticeable in marine stratiform clouds, and have very little radiative effect on convective clouds. Indirect effects of particulates represent the largest uncertainty in radiative forcing.

Soot
Soot
Soot is a general term that refers to impure carbon particles resulting from the incomplete combustion of a hydrocarbon. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolyzed fuel particles such as cenospheres,...

 may cool or warm the surface, depending on whether it is airborne or deposited. Atmospheric soot
Soot
Soot is a general term that refers to impure carbon particles resulting from the incomplete combustion of a hydrocarbon. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolyzed fuel particles such as cenospheres,...

 directly absorb solar radiation, which heats the atmosphere and cools the surface. In isolated areas with high soot production, such as rural India, as much as 50% of surface warming due to greenhouse gases may be masked by atmospheric brown clouds. When deposited, especially on glaciers or on ice in arctic regions, the lower surface albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

 can also directly heat the surface. The influences of particulates, including black carbon, are most pronounced in the tropics and sub-tropics, particularly in Asia, while the effects of greenhouse gases are dominant in the extratropics and southern hemisphere.


Solar variation


Variations in solar output have been the cause of past climate change
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...

s. The effect of changes in solar forcing in recent decades is uncertain, but small, with some studies showing a slight cooling effect, while others studies suggest a slight warming effect.*

Greenhouse gases and solar forcing affect temperatures in different ways. While both increased solar activity and increased greenhouse gases are expected to warm the troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

, an increase in solar activity should warm the stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

 while an increase in greenhouse gases should cool the stratosphere.* Radiosonde
Radiosonde
A radiosonde is a unit for use in weather balloons that measures various atmospheric parameters and transmits them to a fixed receiver. Radiosondes may operate at a radio frequency of 403 MHz or 1680 MHz and both types may be adjusted slightly higher or lower as required...

 (weather balloon) data show the stratosphere has cooled over the period since observations began (1958), though there is greater uncertainty in the early radiosonde record. Satellite observations, which have been available since 1979, also show cooling.

A related hypothesis, proposed by Henrik Svensmark
Henrik Svensmark
Henrik Svensmark is a physicist at the Danish National Space Center in Copenhagen who studies the effects of cosmic rays on cloud formation. His work presents hypotheses about solar activity as an indirect cause of global warming; his research has suggested a possible link through the interaction...

, is that magnetic activity of the sun deflects cosmic rays that may influence the generation of cloud condensation nuclei and thereby affect the climate. Other research has found no relation between warming in recent decades and cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...

s. The influence of cosmic rays on cloud cover is about a factor of 100 lower than needed to explain the observed changes in clouds or to be a significant contributor to present-day climate change.
Studies in 2011 have indicated that solar activity may be slowing, and that the next solar cycle could be delayed. To what extent is not yet clear; Solar Cycle 25 is due to start in 2020, but may be delayed to 2022 or even longer. It is even possible that Sol could be heading towards another Maunder Minimum
Maunder Minimum
The Maunder Minimum is the name used for the period roughly spanning 1645 to 1715 when sunspots became exceedingly rare, as noted by solar observers of the time....

. While there is not yet a definitive link between solar sunspot activity and global temperatures, the scientists conducting the solar activity study believe that global greenhouse gas emissions would prevent any possible cold snap.

Feedback


Feedback
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...

 is a process in which changing one quantity changes a second quantity, and the change in the second quantity in turn changes the first. Positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...

 increases the change in the first quantity while negative feedback
Negative feedback
Negative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...

 reduces it. Feedback is important in the study of global warming because it may amplify or diminish the effect of a particular process.

The main positive feedback in the climate system is the water vapor feedback. The main negative feedback is radiative cooling
Radiative cooling
Radiative cooling is the process by which a body loses heat by thermal radiation.- Earth's energy budget :In the case of the earth-atmosphere system it refers to the process by which long-wave radiation is emitted to balance the absorption of short-wave energy from the sun.The exact process by...

 through the Stefan–Boltzmann law, which increases as the fourth power of temperature. Positive and negative feedbacks are not imposed as assumptions in the models, but are instead emergent properties that result from the interactions of basic dynamical and thermodynamic processes.

Imperfect understanding of feedbacks is a major cause of uncertainty and concern about global warming. A wide range of potential feedback process exist, such as Arctic methane release
Arctic methane release
Arctic methane release is the release of methane from seas and soils in permafrost regions of the Arctic, as part of a more general release of carbon from these soils and seas. Whilst a long-term natural process, it may be exacerbated by global warming. This results in a weak positive feedback...

 and ice-albedo feedback
Ice-albedo feedback
Ice-albedo feedback is a positive feedback climate process where a change in the area of snow-covered land, ice caps, glaciers or sea ice alters the albedo. This change in albedo acts to reinforce the initial alteration in ice area...

. Consequentially, potential tipping points
Tipping point (climatology)
A climate tipping point is a point when global climate changes from one stable state to another stable state, in a similar manner to a wine glass tipping over. After the tipping point has been passed, a transition to a new state occurs...

 may exist, which may have the potential to cause abrupt climate change
Abrupt climate change
An abrupt climate change occurs when the climate system is forced to transition to a new state at a rate that is determined by the climate system itself, and which is more rapid than the rate of change of the external forcing...

.

For example, the "emission scenarios" used by IPCC in its 2007 report primarily examined greenhouse gas emissions from human sources. In 2011, a joint study by NSIDC-(US) and NOAA-(US) calculated the additional greenhouse gas emissions that would emanate from melted and decomposing permafrost, even if policymakers attempt to reduce human emissions from the currently-unfolding A1FI scenario to the A1B scenario. The team found that even at the much lower level of human emissions, permafrost thawing and decomposition would still result in 190 ± 64 Gt C of permafrost carbon being added to the atmosphere on top of the human sources. Importantly, the team made three extremely conservative assumptions: (1) that policymakers will embrace the A1B scenario instead of the currently-unfolding A1FI scenario, (2) that all of the carbon would be released as carbon dioxide instead of methane, which is more likely and over a 20 year lifetime has 72x the greenhouse warming power of CO2, and (3) their model did not project additional temperature rise caused by the release of these additional gases. These very conservative permafrost carbon dioxide emissions are equivalent to about 1/2 of all carbon released from fossil fuel burning since the dawn of the Industrial Age, and is enough to raise atmospheric concentrations by an additional 87 ± 29 ppm, beyond human emissions. Once initiated, permafrost carbon forcing (PCF) is irreversible, is strong compared to other global sources and sinks of atmospheric CO2, and due to thermal inertia will continue for many years even if atmospheric warming stops. A great deal of this permafrost carbon is actually being released as highly flammable methane instead of carbon dioxide. IPCC 2007's temperature projections did not take any of the permafrost carbon emissions into account and therefore underestimate the degree of expected climate change.

Other research published in 2011 found that increased emissions of methane could instigate significant feedbacks that amplify the warming attributable to the methane alone. The researchers found that a 2.5-fold increase in methane emissions would cause indirect effects that increase the warming 250% above that of the methane alone. For a 5.2-fold increase, the indirect effects would be 400% of the warming from the methane alone.

Climate models


A climate model
Climate model
Climate models use quantitative methods to simulate the interactions of the atmosphere, oceans, land surface, and ice. They are used for a variety of purposes from study of the dynamics of the climate system to projections of future climate...

 is a computerized representation of the five components of the climate system: Atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

, hydrosphere
Hydrosphere
A hydrosphere in physical geography describes the combined mass of water found on, under, and over the surface of a planet....

, cryosphere
Cryosphere
The cryosphere is the term which collectively describes the portions of the Earth’s surface where water is in solid form, including sea ice, lake ice, river ice, snow cover, glaciers, ice caps and ice sheets, and frozen ground . Thus there is a wide overlap with the hydrosphere...

, land surface, and biosphere
Biosphere
The biosphere is the global sum of all ecosystems. It can also be called the zone of life on Earth, a closed and self-regulating system...

. Such models are based on physical principles including fluid dynamics
Fluid dynamics
In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...

, thermodynamics
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...

 and radiative transfer
Radiative transfer
Radiative transfer is the physical phenomenon of energy transfer in the form of electromagnetic radiation. The propagation of radiation through a medium is affected by absorption, emission and scattering processes. The equation of radiative transfer describes these interactions mathematically...

. There can be components which represent air movement, temperature, clouds, and other atmospheric properties; ocean temperature, salt content
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...

, and circulation; ice cover on land and sea; the transfer of heat and moisture from soil and vegetation to the atmosphere; chemical and biological processes; and others.

Although researchers attempt to include as many processes as possible, simplifications of the actual climate system are inevitable because of the constraints of available computer power and limitations in knowledge of the climate system. Results from models can also vary due to different greenhouse gas inputs and the model's climate sensitivity. For example, the uncertainty in IPCC's 2007 projections is caused by (1) the use of multiple models with differing sensitivity to greenhouse gas concentrations
Climate sensitivity
Climate sensitivity is a measure of how responsive the temperature of the climate system is to a change in the radiative forcing. It is usually expressed as the temperature change associated with a doubling of the concentration of carbon dioxide in Earth's atmosphere.The equilibrium climate...

, (2) the use of differing estimates of humanities' future greenhouse gas emissions, (3) any additional emissions from climate feedbacks that were not included in the models IPCC used to prepare its report, i.e., greenhouse gas releases from permafrost.

The models do not assume the climate will warm due to increasing levels of greenhouse gases. Instead the models predict how greenhouse gases will interact with radiative transfer and other physical processes. One of the mathematical results of these complex equations is a prediction whether warming or cooling will occur.

Recent research has called special attention to the need to refine models with respect to the effect of clouds and the carbon cycle
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...

.

Models are also used to help investigate the causes of recent climate change
Attribution of recent climate change
Attribution of recent climate change is the effort to scientifically ascertain mechanisms responsible for recent changes observed in the Earth's climate...

 by comparing the observed changes to those that the models project from various natural and human-derived causes. Although these models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects, they do indicate that the warming since 1970 is dominated by man-made greenhouse gas emissions.*

The physical realism of models is tested by examining their ability to simulate current or past climates.

Current climate models produce a good match to observations of global temperature changes over the last century, but do not simulate all aspects of climate. Not all effects of global warming
Effects of global warming
This article is about the effects of global warming and climate change. The effects, or impacts, of climate change may be physical, ecological, social or economic. Evidence of observed climate change includes the instrumental temperature record, rising sea levels, and decreased snow cover in the...

 are accurately predicted by the climate models used by the IPCC
Intergovernmental Panel on Climate Change
The Intergovernmental Panel on Climate Change is a scientific intergovernmental body which provides comprehensive assessments of current scientific, technical and socio-economic information worldwide about the risk of climate change caused by human activity, its potential environmental and...

. Observed Arctic shrinkage
Arctic shrinkage
Ongoing changes in the climate of the Arctic include rising temperatures, loss of sea ice, and melting of the Greenland ice sheet. Projections of sea ice loss suggest that the Arctic ocean will likely be free of summer sea ice sometime between 2060 and 2080, while another estimate puts this date at...

 has been faster than that predicted. Precipitation increased proportional to atmospheric humidity, and hence significantly faster than current global climate models predict.

Expected effects


"Detection" is the process of demonstrating that climate has changed in some defined statistical
Statistics
Statistics is the study of the collection, organization, analysis, and interpretation of data. It deals with all aspects of this, including the planning of data collection in terms of the design of surveys and experiments....

 sense, without providing a reason for that change. Detection does not imply attribution of the detected change to a particular cause. "Attribution" of causes of climate change is the process of establishing the most likely causes for the detected change with some defined level of confidence. Detection and attribution may also be applied to observed changes in physical, ecological and social systems.


Natural systems


Global warming has been detected in a number of systems. Some of these changes, e.g., based on the instrumental temperature record, have been described in the section on temperature changes. Rising sea levels
Current sea level rise
Current sea level rise potentially impacts human populations and the wider natural environment . Global average sea level rose at an average rate of around 1.8 mm per year over 1961 to 2003 and at an average rate of about 3.1 mm per year from 1993 to 2003...

 and observed decreases in snow and ice extent are consistent with warming. Most of the increase in global average temperature since the mid-20th century is, with high probability, attributable to human-induced changes in greenhouse gas concentrations.

Even with current policies to reduce emissions, global emissions are still expected to continue to grow over the coming decades. Over the course of the 21st century, increases in emissions at or above their current rate would very likely induce changes in the climate system larger than those observed in the 20th century.

In the IPCC Fourth Assessment Report, across a range of future emission scenarios, model-based estimates of sea level rise for the end of the 21st century (the year 2090–2099, relative to 1980–1999) range from 0.18 to 0.59 m. These estimates, however, were not given a likelihood due to a lack of scientific understanding, nor was an upper bound given for sea level rise. On the timescale of centuries to millennia, the melting of ice sheet
Ice sheet
An ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² , thus also known as continental glacier...

s could result in even higher sea level rise. Partial deglaciation of the Greenland ice sheet
Greenland ice sheet
The Greenland ice sheet is a vast body of ice covering , roughly 80% of the surface of Greenland. It is the second largest ice body in the world, after the Antarctic Ice Sheet. The ice sheet is almost long in a north-south direction, and its greatest width is at a latitude of 77°N, near its...

, and possibly the West Antarctic ice sheet
West Antarctic Ice Sheet
The West Antarctic Ice Sheet is the segment of the continental ice sheet that covers West Antarctica, the portion of Antarctica on the side of the Transantarctic Mountains which lies in the Western Hemisphere. The WAIS is classified as a marine-based ice sheet, meaning that its bed lies well...

, could contribute 4–6 metres (13 to 20 ft) or more to sea level rise.

Changes in regional climate are expected to include greater warming over land, with most warming at high northern latitudes, and least warming over the Southern Ocean
Southern Ocean
The Southern Ocean comprises the southernmost waters of the World Ocean, generally taken to be south of 60°S latitude and encircling Antarctica. It is usually regarded as the fourth-largest of the five principal oceanic divisions...

 and parts of the North Atlantic Ocean. Snow cover area and sea ice extent are expected to decrease, with the Arctic expected to be largely ice-free in September by 2037. The frequency of hot extremes, heat waves, and heavy precipitation will very likely increase.

Ecological systems


In terrestrial ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

s, the earlier timing of spring events, and poleward and upward shifts in plant and animal ranges, have been linked with high confidence to recent warming.
Future climate change is expected to particularly affect certain ecosystems, including tundra
Tundra
In physical geography, tundra is a biome where the tree growth is hindered by low temperatures and short growing seasons. The term tundra comes through Russian тундра from the Kildin Sami word tūndâr "uplands," "treeless mountain tract." There are three types of tundra: Arctic tundra, alpine...

, mangrove
Mangrove
Mangroves are various kinds of trees up to medium height and shrubs that grow in saline coastal sediment habitats in the tropics and subtropics – mainly between latitudes N and S...

s, and coral reef
Coral reef
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...

s. It is expected that most ecosystems will be affected by higher atmospheric CO2 levels, combined with higher global temperatures.
Future climate change is expected to particularly affect certain ecosystems, including tundra
Tundra
In physical geography, tundra is a biome where the tree growth is hindered by low temperatures and short growing seasons. The term tundra comes through Russian тундра from the Kildin Sami word tūndâr "uplands," "treeless mountain tract." There are three types of tundra: Arctic tundra, alpine...

, mangrove
Mangrove
Mangroves are various kinds of trees up to medium height and shrubs that grow in saline coastal sediment habitats in the tropics and subtropics – mainly between latitudes N and S...

s, and coral reef
Coral reef
Coral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...

s. It is expected that most ecosystems will be affected by higher atmospheric CO2 levels, combined with higher global temperatures. Overall, it is expected that climate change will result in the extinction
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...

 of many species and reduced diversity of ecosystems.

Social systems


Vulnerability of human societies to climate change mainly lies in the effects of extreme weather events rather than gradual climate change. Impacts of climate change so far include adverse effects on small islands, adverse effects on indigenous populations in high-latitude areas, and small but discernable effects on human health. Over the 21st century, climate change is likely to adversely affect hundreds of millions of people through increased coastal flooding, reductions in water supplies, increased malnutrition
Malnutrition
Malnutrition is the condition that results from taking an unbalanced diet in which certain nutrients are lacking, in excess , or in the wrong proportions....

 and increased health impacts.

Future warming of around 3 °C (by 2100, relative to 1990–2000) could result in increased crop yields in mid- and high-latitude areas, but in low-latitude areas, yields could decline, increasing the risk of malnutrition. A similar regional pattern of net benefits and costs could occur for economic (market-sector) effects. Warming above 3 °C could result in crop yields falling in temperate regions, leading to a reduction in global food production. Most economic studies suggest losses of world gross domestic product (GDP) for this magnitude of warming.

Mitigation


Reducing the amount of future climate change is called mitigation of climate change. The IPCC defines mitigation as activities that reduce greenhouse gas (GHG) emissions, or enhance the capacity of carbon sinks to absorb GHGs from the atmosphere. Many countries, both developing
Developing country
A developing country, also known as a less-developed country, is a nation with a low level of material well-being. Since no single definition of the term developing country is recognized internationally, the levels of development may vary widely within so-called developing countries...

 and developed
Developed country
A developed country is a country that has a high level of development according to some criteria. Which criteria, and which countries are classified as being developed, is a contentious issue...

, are aiming to use cleaner, less polluting, technologies. Use of these technologies aids mitigation and could result in substantial reductions in CO2 emissions. Policies include targets for emissions reductions, increased use of renewable energy
Renewable energy
Renewable energy is energy which comes from natural resources such as sunlight, wind, rain, tides, and geothermal heat, which are renewable . About 16% of global final energy consumption comes from renewables, with 10% coming from traditional biomass, which is mainly used for heating, and 3.4% from...

, and increased energy efficiency
Efficient energy use
Efficient energy use, sometimes simply called energy efficiency, is the goal of efforts to reduce the amount of energy required to provide products and services. For example, insulating a home allows a building to use less heating and cooling energy to achieve and maintain a comfortable temperature...

. Studies indicate substantial potential for future reductions in emissions.

To limit warming to the lower range in the overall IPCC's "Summary Report for Policymakers" means adopting policies that will limit emissions to one of the significantly different scenarios described in the full report. This will become more and more difficult, since each year of high emissions will require even more drastic measures in later years to stabilize at a desired atmospheric concentration of greenhouse gases, and energy-related carbon-dioxide (CO2) emissions in 2010 were the highest in history, breaking the prior record set in 2008.

Since even in the most optimistic scenario, fossil fuel
Fossil fuel
Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...

s are going to be used for years to come, mitigation may also involve carbon capture and storage
Carbon capture and storage
Carbon capture and storage , alternatively referred to as carbon capture and sequestration, is a technology to prevent large quantities of from being released into the atmosphere from the use of fossil fuel in power generation and other industries. It is often regarded as a means of mitigating...

, a process that traps CO2 produced by factories and gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...

 or coal power station
Power station
A power station is an industrial facility for the generation of electric energy....

s and then stores it, usually underground.

Adaptation



Other policy responses include adaptation
Adaptation to global warming
Adaptation to global warming and climate change is a response to climate change that seeks to reduce the vulnerability of natural and human systems to climate change effects. Even if emissions are stabilized relatively soon, climate change and its effects will last many years, and adaptation will...

 to climate change. Adaptation to climate change may be planned, e.g., by local or national government, or spontaneous, i.e., done privately without government intervention. The ability to adapt
Adaptive capacity
Adaptive capacity is the capacity of a system to adapt if the environment where the system exists is changing. It is applied to e.g., ecological systems and human social systems.As applied to ecological systems, the adaptive capacity is determined by :...

 is closely linked to social and economic development
Economic development
Economic development generally refers to the sustained, concerted actions of policymakers and communities that promote the standard of living and economic health of a specific area...

. Even societies with high capacities to adapt are still vulnerable to climate change. Planned adaptation is already occurring on a limited basis. The barriers, limits, and costs of future adaptation are not fully understood.

Geoengineering


Another policy response is geoengineering
Geoengineering
The concept of Geoengineering refers to the deliberate large-scale engineering and manipulation of the planetary environment to combat or counteract anthropogenic changes in atmospheric chemistry The Intergovernmental Panel on Climate Change concluded in 2007 that geoengineering options, such...

 of the climate. Geoengineering encompasses a range of techniques to remove from the atmosphere or to reflect incoming sunlight. Little is known about the effectiveness, costs or potential side effects
Unintended Consequences
Unintended Consequences is a novel by John Ross, first published in 1996 by Accurate Press. The story chronicles the history of the gun culture, gun rights and gun control in the United States from the early 1900s through the late 1990s...

 of geoengineering options. As most geoengineering techniques would affect the entire globe, deployment would likely require global public acceptance and an adequate global legal and regulatory framework, as well as significant further scientific research.

Views on global warming


There are different views over what the appropriate policy response to climate change should be.
These competing views weigh the benefits of limiting emissions of greenhouse gases against the costs. In general, it seems likely that climate change will impose greater damages and risks in poorer regions.

Global warming controversy


The global warming controversy refers to a variety of disputes, significantly more pronounced in the popular media
Media coverage of climate change
Media coverage of climate change has significant effects on public opinion on climate change, as it mediates the scientific opinion on climate change that the global instrumental temperature record shows increase in recent decades and that the trend is caused mainly by human-induced emissions of...

 than in the scientific literature, regarding the nature, causes, and consequences of global warming. The disputed issues include the causes of increased global average air temperature
Instrumental temperature record
The instrumental temperature record shows fluctuations of the temperature of the global land surface and oceans. This data is collected from several thousand meteorological stations, Antarctic research stations and satellite observations of sea-surface temperature. Currently, the longest-running...

, especially since the mid-20th century, whether this warming trend is unprecedented or within normal climatic variations, whether humankind has contributed significantly to it
Attribution of recent climate change
Attribution of recent climate change is the effort to scientifically ascertain mechanisms responsible for recent changes observed in the Earth's climate...

, and whether the increase is wholly or partially an artifact of poor measurements. Additional disputes concern estimates of climate sensitivity
Climate sensitivity
Climate sensitivity is a measure of how responsive the temperature of the climate system is to a change in the radiative forcing. It is usually expressed as the temperature change associated with a doubling of the concentration of carbon dioxide in Earth's atmosphere.The equilibrium climate...

, predictions of additional warming, and what the consequences of global warming will be.

In the scientific literature, there is a strong consensus
Scientific opinion on climate change
The predominant scientific opinion on climate change is that the Earth is in an ongoing phase of global warming primarily caused by an enhanced greenhouse effect due to the anthropogenic release of carbon dioxide and other greenhouse gases...

 that global surface temperatures have increased in recent decades and that the trend is caused mainly by human-induced emissions of greenhouse gases. No scientific body of national or international standing disagrees with this view, though a few organisations hold non-committal positions.

From 1990-1997 in the United States, conservative think tanks mobilized to undermine the legitimacy of global warming as a social problem. They challenged the scientific evidence; argued that global warming will have benefits; and asserted that proposed solutions would do more harm than good.

Politics


Most countries are Parties to the United Nations Framework Convention on Climate Change
United Nations Framework Convention on Climate Change
The United Nations Framework Convention on Climate Change is an international environmental treaty produced at the United Nations Conference on Environment and Development , informally known as the Earth Summit, held in Rio de Janeiro from June 3 to 14, 1992...

 (UNFCCC).
The ultimate objective of the Convention is to prevent "dangerous" human interference of the climate system.
As is stated in the Convention, this requires that GHG concentrations are stabilized in the atmosphere at a level where ecosystems
Climate change and ecosystems
This article is about climate change and ecosystems.-Impacts:Unchecked global warming could affect most terrestrial ecoregions. Increasing global temperature means that ecosystems will change; some species are being forced out of their habitats because of changing conditions, while others are...

 can adapt naturally to climate change, food production
Climate change and agriculture
Climate change and agriculture are interrelated processes, both of which take place on a global scale. Global warming is projected to have significant impacts on conditions affecting agriculture, including temperature, carbon dioxide, glacial run-off, precipitation and the interaction of these...

 is not threatened, and economic development
Economics of global warming
-Definitions:In this article, the phrase “climate change” is used to describe a change in the climate, measured in terms of its statistical properties, e.g., the global mean surface temperature. In this context, “climate” is taken to mean the average weather. Climate can change over period of time...

 can proceed in a sustainable fashion. The Framework Convention was agreed in 1992, but since then, global emissions have risen. During negotiations, the G77
Group of 77
The Group of 77 at the United Nations is a loose coalition of developing nations, designed to promote its members' collective economic interests and create an enhanced joint negotiating capacity in the United Nations. There were 77 founding members of the organization, but the organization has...

 (a lobbying group in the United Nations representing 133 developing nations) pushed for a mandate requiring developed countries to "[take] the lead" in reducing their emissions. This was justified on the basis that: the developed world's emissions had contributed most to the stock of GHGs in the atmosphere; per-capita emissions (i.e., emissions per head of population) were still relatively low in developing countries; and the emissions of developing countries would grow to meet their development needs. This mandate was sustained in the Kyoto Protocol
Kyoto Protocol
The Kyoto Protocol is a protocol to the United Nations Framework Convention on Climate Change , aimed at fighting global warming...

 to the Framework Convention, which entered into legal effect in 2005.

In ratifying the Kyoto Protocol, most developed countries accepted legally binding commitments to limit their emissions. These first-round commitments expire in 2012.
US President George W. Bush
George W. Bush
George Walker Bush is an American politician who served as the 43rd President of the United States, from 2001 to 2009. Before that, he was the 46th Governor of Texas, having served from 1995 to 2000....

 rejected the treaty on the basis that "it exempts 80% of the world, including major population centers such as China and India, from compliance, and would cause serious harm to the US economy."

At the 15th UNFCCC Conference of the Parties
2009 United Nations Climate Change Conference
The 2009 United Nations Climate Change Conference, commonly known as the Copenhagen Summit, was held at the Bella Center in Copenhagen, Denmark, between 7 December and 18 December. The conference included the 15th Conference of the Parties to the United Nations Framework Convention on Climate...

, held in 2009 at Copenhagen
Copenhagen
Copenhagen is the capital and largest city of Denmark, with an urban population of 1,199,224 and a metropolitan population of 1,930,260 . With the completion of the transnational Øresund Bridge in 2000, Copenhagen has become the centre of the increasingly integrating Øresund Region...

, several UNFCCC Parties produced the Copenhagen Accord
Copenhagen Accord
The Copenhagen Accord is a document that delegates at the 15th session of the Conference of Parties to the United Nations Framework Convention on Climate Change agreed to "take note of" at the final plenary on 18 December 2009....

.
Parties associated with the Accord (140 countries, as of November 2010)
aim to limit the future increase in global mean temperature to below 2 °C. A preliminary assessment published in November 2010 by the United Nations Environment Programme
United Nations Environment Programme
The United Nations Environment Programme coordinates United Nations environmental activities, assisting developing countries in implementing environmentally sound policies and practices. It was founded as a result of the United Nations Conference on the Human Environment in June 1972 and has its...

 (UNEP) suggests a possible "emissions gap" between the voluntary pledges made in the Accord and the emissions cuts necessary to have a "likely" (greater than 66% probability) chance of meeting the 2 °C objective.
The UNEP assessment takes the 2 °C objective as being measured against the pre-industrial global mean temperature level. To having a likely chance of meeting the 2 °C objective, assessed studies generally indicated the need for global emissions to peak before 2020, with substantial declines in emissions thereafter.

The 16th Conference of the Parties
2010 United Nations Climate Change Conference
The 2010 United Nations Climate Change Conference was held in Cancún, Mexico, from 29 November to 10 December 2010. The conference is officially referred to as the 16th session of the Conference of the Parties to the United Nations Framework Convention on Climate Change and the 6th session of the...

 (COP16) was held at Cancún
Cancún
Cancún is a city of international tourism development certified by the UNWTO . Located on the northeast coast of Quintana Roo in southern Mexico, more than 1,700 km from Mexico City, the Project began operations in 1974 as Integrally Planned Center, a pioneer of FONATUR Cancún is a city of...

 in 2010. It produced an agreement, not a binding treaty, that the Parties should take urgent action to reduce greenhouse gas emissions to meet a goal of limiting global warming to 2 °C above pre-industrial temperatures. It also recognized the need to consider strengthening the goal to a global average rise of 1.5 °C.

Public opinion


In 2007–2008 Gallup Polls surveyed 127 countries. Over a third of the world's population was unaware of global warming, with people in developing countries less aware than those in developed
Developed country
A developed country is a country that has a high level of development according to some criteria. Which criteria, and which countries are classified as being developed, is a contentious issue...

, and those in Africa the least aware. Of those aware, Latin America leads in belief that temperature changes are a result of human activities while Africa, parts of Asia and the Middle East, and a few countries from the Former Soviet Union lead in the opposite belief. In the Western world, opinions over the concept and the appropriate responses are divided. Nick Pidgeon of Cardiff University
Cardiff University
Cardiff University is a leading research university located in the Cathays Park area of Cardiff, Wales, United Kingdom. It received its Royal charter in 1883 and is a member of the Russell Group of Universities. The university is consistently recognised as providing high quality research-based...

 said that "results show the different stages of engagement about global warming on each side of the Atlantic", adding, "The debate in Europe is about what action needs to be taken, while many in the U.S. still debate whether climate change is happening." A 2010 poll by the Office of National Statistics found that 75% of UK respondents were at least "fairly convinced" that the world's climate is changing, compared to 87% in a similar survey in 2006. A January 2011 ICM poll in the UK found 83% of respondents viewed climate change as a current or imminent threat, while 14% said it was no threat. Opinion was unchanged from an August 2009 poll asking the same question, though there had been a slight polarisation of opposing views.

A survey in October, 2009 by the Pew Research Center for the People & the Press showed decreasing public perception in the United States that global warming was a serious problem. All political persuasions showed reduced concern with lowest concern among Republicans, only 35% of whom considered there to be solid evidence of global warming. The cause of this marked difference in public opinion between the United States and the global public is uncertain but the hypothesis has been advanced that clearer communication by scientists both directly and through the media would be helpful in adequately informing the American public of the scientific consensus and the basis for it. The U.S. public appears to be unaware of the extent of scientific consensus regarding the issue, with 59% believing that scientists disagree "significantly" on global warming.

By 2010, with 111 countries surveyed, Gallup determined that there was a substantial decrease in the number of Americans and Europeans who viewed Global Warming as a serious threat. In the United States, a little over half the population (53%) now viewed it as a serious concern for either themselves or their families; a number 10 percentage points below the 2008 poll (63%). Latin America had the biggest rise in concern, with 73% saying global warming was a serious threat to their families. That global poll also found that people are more likely to attribute global warming to human activities than to natural causes, except in the USA where nearly half (47%) of the population attributed global warming to natural causes.

On the other hand, in May 2011 a joint poll by Yale and George Mason Universities found that nearly half the people in the USA (47%) attribute global warming to human activities, compared to 36% blaming it on natural causes. Only 5% of the 35% who were "disengaged", "doubtful", or "dismissive" of global warming were aware that 97% of publishing US climate scientists agree global warming is happening and is primarily caused by humans.

Researchers at the University of Michigan have found that the public's belief as to the causes of global warming depends on the wording choice used in the polls.

In the United States, according to the Public Policy Institute of California's (PPIC) eleventh annual survey on environmental policy issues, 75% said they believe global warming is a very serious or somewhat serious threat to the economy and quality of life in California.

A July 2011 Rasmussen Reports
Rasmussen Reports
Rasmussen Reports is an American media company that publishes and distributes information based on public opinion polling. Founded by pollster Scott Rasmussen in 2003, the company updates daily indexes including the President's job approval rating, and provides public opinion data, analysis, and...

 poll found that 69% of adults in the USA believe it is at least somewhat likely that some scientists have falsified global warming research.

A September 2011 Angus Reid Public Opinion
Angus Reid Public Opinion
Angus Reid Public Opinion is an international public affairs practice. It was established in 2006 under the name Angus Reid Strategies by Dr Angus Reid, a Canadian sociologist who founded his first research company in 1979. Reid sold the Angus Reid Group to Paris-based Ipsos SA in 2000...

 poll found that Britons (43%) are less likely than Americans (49%) or Canadians (52%) to say that "global warming is a fact and is mostly caused by emissions from vehicles and industrial facilities." The same poll found that 20% of Americans, 20% of Britons and 14% of Canadians think "global warming is a theory that has not yet been proven."

Other views


Most scientists agree that humans are contributing to observed climate change. National science academies have called on world leaders for policies to cut global emissions.
However, some scientists and non-scientists question aspects of climate-change science.

Organizations such as the libertarian Competitive Enterprise Institute
Competitive Enterprise Institute
The Competitive Enterprise Institute is a non-profit think tank founded on March 9, 1984 in Washington, D.C. by lobbyist Fred L. Smith, Jr to advance economic liberty and fight over-regulation by big government...

, conservative commentators, and some companies such as ExxonMobil
ExxonMobil
Exxon Mobil Corporation or ExxonMobil, is an American multinational oil and gas corporation. It is a direct descendant of John D. Rockefeller's Standard Oil company, and was formed on November 30, 1999, by the merger of Exxon and Mobil. Its headquarters are in Irving, Texas...

 have challenged IPCC climate change scenarios, funded scientists who disagree with the scientific consensus
Scientific opinion on climate change
The predominant scientific opinion on climate change is that the Earth is in an ongoing phase of global warming primarily caused by an enhanced greenhouse effect due to the anthropogenic release of carbon dioxide and other greenhouse gases...

, and provided their own projections of the economic cost of stricter controls.
In the finance industry, Deutsche Bank
Deutsche Bank
Deutsche Bank AG is a global financial service company with its headquarters in Frankfurt, Germany. It employs more than 100,000 people in over 70 countries, and has a large presence in Europe, the Americas, Asia Pacific and the emerging markets...

 has set up an institutional climate change investment division (DBCCA),
which has commissioned and published research
on the issues and debate surrounding global warming.
Environmental organizations and public figures have emphasized changes in the current climate and the risks they entail, while promoting adaptation to changes in infrastructural needs and emissions reductions. Some fossil fuel companies have scaled back their efforts in recent years, or called for policies to reduce global warming.

Etymology


The term global warming was probably first used in its modern sense on 8 August 1975 in a science paper by Wally Broecker in the journal Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....

called "Are we on the brink of a pronounced global warming?". Broecker's choice of words was new and represented a significant recognition that the climate was warming; previously the phrasing used by scientists was "inadvertent climate modification," because while it was recognized humans could change the climate, no one was sure which direction it was going. The National Academy of Sciences first used global warming in a 1979 paper called the Charney Report, it said: "if carbon dioxide continues to increase, [we find] no reason to doubt that climate changes will result and no reason to believe that these changes will be negligible." The report made a distinction between referring to surface temperature changes as global warming, while referring to other changes caused by increased CO2 as climate change.

Global warming became more widely popular after 1988 when NASA climate scientist James Hansen
James Hansen
James E. Hansen heads the NASA Goddard Institute for Space Studies in New York City, a part of the Goddard Space Flight Center in Greenbelt, Maryland. He has held this position since 1981...

 used the term in a testimony to Congress. He said: "global warming has reached a level such that we can ascribe with a high degree of confidence a cause and effect relationship between the greenhouse effect and the observed warming." His testimony was widely reported and afterward global warming was commonly used by the press and in public discourse.

See also


  • Glossary of climate change
    Glossary of climate change
    This article serves as a glossary of climate change terms. It lists terms that are related to global warming.- 0-9 :* 100,000-year problem - a discrepancy between the climate response and the forcing from the amount of incoming solar radiation....

  • History of climate change science
    History of climate change science
    The history of the scientific discovery of climate change began in the early 19th century when natural changes in paleoclimate were first suspected and the natural greenhouse effect first identified. In the late 19th century, scientists first argued that human emissions of greenhouse gases could...

  • Index of climate change articles

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