Global warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation. There is strong evidence that significant global warming is occurring; this evidence comes from direct measurements of rising surface air temperatures and subsurface ocean temperatures and from phenomena such as increases in average global sea levels, retreating glaciers, and changes to many physical and biological systems. It is likely that most of the warming in recent decades is attributable to human activity, particularly the burning of fossil fuels and deforestation.
Global average air temperature near the Earth's surface rose by 0.74 ± 0.18 °C (1.33 ± 0.32 °F) from 1906 to 2005. The prevailing scientific view,
 as represented by the science academies of the major industrialized nations and the Intergovernmental Panel on Climate Change, it is very likely that most of the temperature increase since the mid-20th century has been caused by increases in atmospheric greenhouse gas concentrations produced by human activity. Climate models predict that average global surface temperatures will increase by a further 1.1 to 6.4 °C (2.0 to 11.5 °F) by the end of the century, relative to 1980–1999. The range of values reflects differing assumptions of future greenhouse gas emissions and results of models that differ in their sensitivity to increases in greenhouse gases.
Scientists have not yet quantitatively assessed the potential self-accelerating effects of global-warming itself, either on threshold or rate. Melting of permafrost, for example, causes increased production and atmospheric release of such newly produced as well as anciently stored methane gas, which “….packs a far greater warming punch than [carbon dioxide] (CO2),” possibly as much as 25 times that of CO2 per unit mass.
An increase in global temperatures will cause the sea level to rise, glaciers to retreat, sea ice to melt, and changes in the amount, geographical distribution and seasonal pattern of precipitation. There may also be changes in the frequency and intensity of extreme weather events. These will have many practical consequences, including changes in agricultural yields and impacts on human health. Scientific uncertainties include the extent of climate change expected in the future, and how changes will vary around the globe. There is political and public debate about what action should be taken to reduce future warming or to adapt to its consequences. The Kyoto Protocol, an international agreement aimed at reducing greenhouse gas emissions, was adopted by 169 nations.
The weather is the day-by-day temperature, humidity, wind and rainfall in a given region; climate encompasses long- term weather patterns. The Earth's atmosphere is heated by radiation from the sun, and how much of that heat is retained rather than reflected depends critically on the composition of the atmosphere. In particular, the burning of fossil fuels releases "greenhouse gases" into the atmosphere that are causing "climate change."
The United Nations Framework Convention on Climate Change (UNFCCC) uses this term, "climate change", for human-caused change, and "climate variability" for other changes. The terms "anthropogenic global warming" and "anthropogenic climate change" are sometimes also used for human-induced changes.
The Intergovernmental Panel on Climate Change (IPCC) is a scientific body that was established by the United Nations Environment Programme  and the World Meteorological Organization (WMO), a specialized agency of the UN. It was established to provide a clear scientific view on the current state of knowledge in climate change, and its potential environmental and socio-economic impacts. The IPCC works by reviewing and assessing current scientific, technical and socio-economic information, and thousands of scientists from all over the world contribute to this, encompassing a range of views and expertise. Governments participate in the review process and in the plenary sessions, where decisions about the IPCC work programme are taken and where reports are presented, revised and approved. The IPCC is endorsed by the UN General Assembly, and currently has 194 member countries. In 2007, the IPCC was awarded the Nobel Peace Prize jointly with Al Gore "for their efforts to build up and disseminate greater knowledge about man-made climate change, and to lay the foundations for the measures that are needed to counteract such change"
The Kyoto Protocol is an agreement that set binding targets for 37 industrialized countries, including the European community, for reducing greenhouse gas emissions. Acknowledging that developed countries are mainly responsible for the current high levels of greenhouse gas emissions, the Protocol placed a greater burden on them under the principle of "common but differentiated responsibilities.". Under the Bush Administration, the USA refused to ratify the protocol, and the Obama administration followed suit With the Kyoto Protocol set to expire in 2012, in December 2009, governments met at an international Climate Conference in Copenhagen to negotiate a continuation of international efforts to minimise global warming. That conference concluded with the Copenhagen accord which asserted a "strong political will to urgently combat climate change in accordance with the principle of common but differentiated responsibilities and respective capabilities." Signatories agree that deep cuts in global emissions are required, recognize the crucial role of reducing emission from deforestation, and agree that developed countries shall provide financial resources, technology and capacity-building to support adaptation in developing countries. The USA has signed, along with most other countries of the world.
There is no serious dispute that there has been a large and continuing increase in atmospheric CO2 concentrations since the middle of the twentieth century; there is no dispute that there has been a parallel increase in fossil fuel use over this time, and there is no dispute that there has been an increase in mean global temperature over the same period. Although some skeptics still doubt that the rise in CO2 is substantially a consequence of man's activities, all national science academies that have issued statements on the matter accept the IPCC's conclusion that they probably are, and that future man-made climate change is likely.
The climate system varies through internal processes and in response to external forcing. External forcing includes fossil fuel emissions, but also solar activity, volcanic emissions, variations in the Earth's orbit, and variations in atmospheric composition. The scientific consensus is that most of the warming observed since the mid-twentieth century is due to increased atmospheric concentrations of greenhouse gases produced by human activity. Alternative mechanisms seem able to account for only a small part of the observed increase in global temperatures. These other mechanisms include warming as a result of natural fluctuations in the climate, and warming as a result of variations in solar radiation, or that warming is caused by changes in cloud cover due to variations in galactic cosmic rays.
The effects of external forcing on the climate are not instantaneous, due to the thermal inertia of the oceans and the slow responses of some feedback processes. Climate models indicate that, even if greenhouse gases were stabilized at present day levels, there would be a further warming of about 0.5 °C (0.9 °F) as the climate continued to adjust toward equilibrium.
The greenhouse effect is the process by which emission of infrared radiation by atmospheric gases warms a planet's atmosphere and surface. Naturally occurring greenhouse gases warm the Earth by about 33 °C (59 °F); without this, the average temperature of the Earth would be about -18 °C (0 °F) making the planet uninhabitable. The major natural greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%.
The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume. From geological evidence, it is believed that CO2 values this high were last attained 20 million years ago. About three-quarters of man-made CO2 emissions over the past 20 years have come from the burning of fossil fuels; most of the rest is due to land-use change, mainly deforestation. Measured trends in atmospheric composition and isotope ratios (namely the simultaneous depletion of 13C, 14C, and O2) confirm that the increased atmospheric CO2 mainly comes from fossil fuels and not from other sources such as volcanos or the oceans.
Future CO2 concentrations depend on uncertain economic, sociological, technological, and natural developments. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100. Fossil fuel reserves are sufficient to reach these levels and continue emissions past 2100, if coal, tar sands, or methane clathrates are used extensively. Positive feedback effects such as the release of methane from the melting of permafrost peat bogs in Siberia (possibly up to 70,000 million tonnes) may lead to significant additional sources of greenhouse gas emissions not included in climate models cited by the IPCC.
The warming due to atmospheric CO2 from burning fossil fuels releases methane from the Arctic surface in at least three ways: (1) by forming lakes of melted ice whose waters melts the underlying permafrost, allowing methane-producing microbes to increase methane production by exposing thawing vegetative and animal matter for them to consume; (2) by opening channels in the attenuated permafrost cap for release into the atmosphere of old, trapped methane hydrates; (3) by thawing offshore layers of permafrost capping methane hydrates.
The effects of forcing agents on the climate are modified by feedback processes, one of the most important of which is caused by the evaporation of water. Increased greenhouse gases from human activity cause a warming of the Earth's atmosphere and surface, which increases the evaporation of water into the atmosphere. As water vapor is itself a greenhouse gas, this causes further warming, causing yet more water vapor to be evaporated, and so on. Eventually a new dynamic equilibrium concentration of water vapor is reached at a slight increase in humidity and with a much larger greenhouse effect than that due to CO2 alone.
The radiative effects of clouds are a source of uncertainty in climate projections. Seen from below, clouds emit infrared radiation to the surface, and so have a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so have a cooling effect. The cloud feedback effect is influenced not only by the amount of clouds but also by their distribution; high clouds are colder than low clouds, and thus radiate less energy to space. Increased global water vapor content may or may not cause an increase in global or regional cloud cover, since cloud cover is affected by relative humidity rather than the absolute concentration of water vapor. Cloud feedback is second only to water vapor feedback and has a net warming effect in all the models that contributed to the IPCC Fourth Assessment Report.
Global warming also leads to melting of ice near the poles, exposing more land surface. Land (and open water) are less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and the cycle continues (the "ice albedo feedback"). Sea ice also has an important role in moderating heat exchange between the ocean and atmosphere at high latitudes, and by feedbacks involving ice growth and melt and the fresh water balance at the ocean surface The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.
Variations in solar output, possibly amplified by cloud feedbacks, may have been a secondary contributor to recent global warming.  Natural phenomena, such as solar variation and volcanoes, probably had a net warming effect from pre-industrial times to 1950 and a small cooling effect since 1950. Some research indicate that the Sun's contribution may have been underestimated, and that the Sun may have contributed about 40–50% of the global surface warming between 1900 and 2000 and about 25–35% of the warming between 1980 and 2000. Some authors suggest that climate models overestimate the relative effect of greenhouse gases compared to solar forcing, and that the cooling effects of volcanic dust and sulfate aerosols have been underestimated. Nevertheless, they conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming during the latest decades is attributable to the increases in greenhouse gases.
Climate change since the Industrial Revolution
According to the instrumental temperature record, mean global temperatures (both land and sea) have increased by 0.75 °C (1.35 °F) since the period 1860–1900. This increase is not significantly affected by the urban heat island effect. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade). Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.
Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable measurements became available in the late 1800s. Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.  Global temperatures in 1998 were exceptionally warm because the strongest El Niño in the instrumental record occurred in that year.
Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the 20th century, though the cooling may also be due in part to natural variability.
The Earth's climate is an extremely complex system: the atmosphere, oceans, and land masses are tightly coupled subsystems and consequently the energy and mass exchanges between them must be studied simultaneously. Further, the electromagnetic radiation balance between energy absorption and back radiation by the Earth plays a crucial role. Encompassing all these factors, scientists have created computer models of the climate, based on physical principles of fluid dynamics, radiative transfer, and other processes. These models predict that the net effect of adding greenhouse gases is to produce a warmer climate. However, the amount of projected warming varies between models and there is a considerable range of climate sensitivity. Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC report projects global surface temperatures averaged over 2090-2099 are likely to be 1.1 to 6.4 °C (2.0 to 11.5 °F) hotter than the average temperatures from 1980-1999.
Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human derived causes. Climate models can produce a good match to observations of global temperature changes over the last century, but cannot yet simulate all aspects of climate. These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects, but they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.
Global climate model projections of future climate are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may also include a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback.
The representation of clouds is one of the main sources of uncertainty in present models. There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.
Attributed and expected effects
Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming. While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects.
And water expands as it warms: around a third of the continuing rise in sea levels is due to water expansion. That sea level rise is expected to accelerate, putting many of the world's greatest cities at risk.
A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II. The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records before routine satellite observations. The summary also states that there is no clear trend in the annual number of tropical cyclones.
Other anticipated effects include a sea level rise of 110-770 mm (0.36 - 2.5 ft) between 1990 and 2100, repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity and frequency of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on climate projections. McLaughlin et al. have documented two populations of Bay checkerspot butterfly being threatened by precipitation change.
Mitigation and adaptation
The broad agreement among climate scientists that global temperatures will continue to increase has led nations, states, corporations, and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. There has been business action on climate change, including efforts at increased energy efficiency and moves to alternative fuels. One innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.
The main international agreement on combating global warming is the Kyoto Protocol, negotiated in 1997. The Protocol covered more than 160 countries accounting for 55% of global greenhouse gas emissions. However, the biggest emitter, the USA never ratified the treaty. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed it.
Bjorn Lomborg is a Danish-based scientist and author of The Skeptical Environmentalist (2001); in that influential book, he accepted that global warming is occurring as a result of man's activities, but argued that the costs of drastic, short-term action are too high. He argued that the world's poorest - those likely to be hardest hit by global warming - would benefit more from rich countries honouring pledges on aid, opening up their markets and investing in providing universal access to clean water than they would from aggressive reductions of greenhouse gas emissions.  By 2010, Lomborg had changed his mind, and declared that global warming is "undoubtedly one of the chief concerns facing the world today" and "a challenge humanity must confront",
Related climatic issues
Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans. CO2 dissolved in the ocean reacts with water to form carbonic acid resulting in acidification. Ocean surface pH is estimated to have decreased from approximately 8.25 to 8.14 since the beginning of the industrial era, and it is estimated that it will drop by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2. As organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services.
A factor that may have mitigated global warming in the late twentieth century is global dimming - a gradual reduction in the amount of global direct irradiance at the Earth's surface. From 1960 to 1990, human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.
Although it is widely believed that there is widespread disagreement amongst scientists about the role of man in global warming, in fact there seems to be very little disagreement amongst Earth Scientists. Summarising the results of a survey of over 3,000 Earth Scientists, Doran (2009) wrote: "It seems that the debate on the authenticity of global warming and the role played by human activity is largely nonexistent among those who understand the nuances and scientific basis of long-term climate processes. The challenge, rather, appears to be how to effectively communicate this fact to policy makers and to a public that continues to mistakenly perceive debate among scientists."
Nevertheless, a well-funded campaign by skeptical scientists, free-market think tanks and industry, through advertisements, lobbying and media attention, has argued first that the world is not warming; then that warming is not caused by human activities, now that warming will be harmless. This campaign has had a major effect in the USA, on both the public and Congress; the Tea Party is at the forefront of climate change denial.
A few skeptical scientists believe that the present climate change is not man-made and is unavoidable. 
Most accept that there has been a large increase of CO2 in the atmosphere due to the use of fossil fuels, but doubt that there is any immediate danger and dispute any need for large reductions of CO2 emissions. Their skepticism is based on the complex problems associated with the underlying science and the uncertainty of available climate data. Put most simply, the skeptical view is that we don't understand climate well enough to make reliable predictions, and that there have been large variations of climate in the past, so perhaps the observed changes are just part of a natural cycle that we don't understand.  They argue that we don't have good enough data from the past to develop and test computer models of the climate, and object to the use of proxy data - indirect data on temperature, such as tree rings and the isotopic content of Arctic and Antarctic ice. Proxy data are used to construct historical temperature profiles, yielding, for instance, the hockey stick shaped graph, but have some problems - tree ring thickness for example is affected by both temperature and precipitation.
There is also concern about the lack of transparency of analyses purporting global warming and inaccessibility to data to allow independent analyses.  Specifically, skeptics have requested access to data from the University of East Anglia's Climatic Research Unit (CRU) and Penn State Department of Meteorology. The CRU reports that 95% of their data is available for the public
When an FPS (Forum on Physics & Society) editor of the American Physical Society wrote: "There is a considerable presence within the scientific community of people who do not agree with the IPCC conclusion that anthropogenic CO2 emissions are very probably likely to be primarily responsible for the global warming that has occurred since the Industrial Revolution", the FPS Executive Committee hastened to declare that his statement does not represent their views. Nonetheless, in 2007 a U.S. Senate committee identified more than 400 prominent scientists from more than two dozen countries as objecting to aspects of the "consensus" on man-made global warming. Some of those scientists were participants in the IPCC..
A 2010 assessment of the IPCC report by the Netherlands Environmental Assessment Agency found no significant errors in the report, but its authors reported that they had been unable to find the provenance of some of its conclusions, that the report had not taken account of the positive effects of climate change and that there was a tendency to highlight the upper ends of uncertainty ranges..
The concentration of CO2 in the Earth's atmosphere at the beginning of the Industrial Age (1750) was about 278 ppm; in 2008, it was about 385 ppm. The concept of climate sensitivity arose when the IPCC members asked how much the temperature on Earth would change by an increase of CO2 in the atmosphere. To answer that, the IPCC adopted this definition:
- Climate sensitivity is the equilibrium temperature change, , in the surface temperature, TS, caused by the doubling of the pre-industrial CO2 concentration.
Put more simply, the IPCC defined climate sensitivity as the temperature change, in the Earth's surface temperature, , that would be caused by doubling the pre-1750 atmospheric CO2 concentration of 278 ppm to 556 ppm which is currently expected to occur later in this century.
The IPCC estimated the climate sensitivity to be 3.26 °C. In other words, when the atmospheric concentration of CO2 reaches 556 ppm (expected later this century), the IPCC predicts that the Earth's surface temperature will be 3.26 °C higher than it was more than 250 years ago (1750).
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- Caldeira, K; Wickett ME (2005). "Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean". J Geophys Res 110: 1–12.
- Raven JA et al. (2005). Ocean acidification due to increasing atmospheric carbon dioxide. Royal Society.
- The Truth About Denial Newsweek Education site
- The denial industry George Monbiot The Guardian 19 September 2006
- During the United Nations Climate Conference on Bali in 2007, more than 100 scientists wrote an open letter to Ban Ki-Moon, the Secretary-General of the United Nations expressing their opinion that "the 2007 UN climate conference [is] taking the World in entirely the wrong direction". (Letter to Ban Ki-Moon) They recognized that climate change is occurring, but state that it is a natural phenomenon which is impossible to stop and express doubts that "it is possible to significantly alter global climate through cuts in human greenhouse gas emissions." The letter criticised the IPCC Assessment Reports of 2001 and 2007, claiming that they "are prepared by a relatively small core writing team with the final drafts approved line-by-line by government representatives".
- Climate Scientists’ Perceptions of Climate Change Science Interviews among 558 scientists working in the field of climate change (mainly climatologist and meteorologists) by Dennis Bray and Hans von Storch
- Issues in the Current State of Climate Science The Center for Science and Public Policy, Washington, DC, March 2006 (from the website of the Florida Gulf Coast University)
- ME Mann et al. (1998) Nature:779-87 A plot of mean temperature over the last 1,000 years which is flat on average from the years 1000 to 1900. The flat part forms the hockey stick's shaft. After 1900, and especially after 1980, temperatures appear to shoot up, forming the hockey stick's blade.
- Revkin AC (November 27, 2009) A Climate Scientist Who Engages Skeptics The New York Times
- Curry J (November 27, 2009) An open letter to graduate students and young scientists in fields related to climate research from Dr Judith Curry regarding hacked CRU emails. Climate Progress
- McIntyre S Climate Audit
- CRU data; Anonymous. (November 24, 2009) CRU climate data already ‘over 95%’ available (28 November). University of East Anglia - Communications Office
- The RealClimate blog also maintains a list of data sources at http://www.realclimate.org/index.php/data-sources/
- Editor's Comment, Forum on Physics & Society of the American Physical Society, July 2008
- Forum on Physics & Society of the American Physical Society, July 2008.
- The APS's 2007 statement on Climate Change, which urges an enhanced effort to understand the effects of human activity on the climate, is here.
- U. S. Senate Report: Over 400 Prominent Scientists Disputed Man-Made Global Warming Claims in 2007 Note that some individuals listed objected to their inclusion and declared that the statement did not reflect their views.
- Assessing an IPCC assessment. An analysis of statements on projected regional impacts in the 2007 report', Netherlands Environmental Assessment Agency, 2010