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 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 asglobal warming, while referring to other changes caused by increased CO₂ as climate change

Global warming became more widely popular after 1988 when NASA scientist James Hansen 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.

Other views

Most scientists accept 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, conservative commentators, and some companies such as ExxonMobil have challenged IPCC climate change scenarios, funded scientists who disagree with the scientific consensus, and provided their own projections of the economic cost of stricter controls. In the finance industry, Deutsche Bank 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.

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, 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 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.

Politics

Developed and developing countries have made different arguments over who should bear the burden of economiccosts for cutting emissions. Developing countries often concentrate on per capita emissions, that is, the total emissions of a country divided by its population. Per capita emissions in the industrialized countries are typically as much as ten times the average in developing countries. This is used to make the argument that the real problem of climate change is due to the profligate and unsustainable lifestyles of those living in rich countries.

On the other hand, commentators from developed countries point out that total carbon emissions,carrying capacity, efficient energy use and civil and political rights are very important issues. World population is the number of humans per unit area. However the land is not the same everywhere. Not only the quantity of fossil fuel use but also the quality of energy use is a key debate point. For example, efficient energy use supporting technological change might help reduce excess carbon dioxide in Earth's atmosphere. The use of fossil fuels for conspicuous consumption and excessiveentertainment are issues that can conflict with civil and political rights. People in developed countries argue that history has proven the difficulty of implementing fair rationing programs in different countriesbecause there is no global system of checks and balances or civil liberties.

The Kyoto Protocol, which came into force in 2005, sets legally binding emission limitations for most developed countries. Developing countries are not subject to limitations. This exemption led the U.S. and Australia to decide not to ratify the treaty, although Australia did finally ratify the treaty in December 2007. Debate continued at the Copenhagen climate summit and the Cancún climate summit.

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

UNFCCC

Most countries are Parties to the United Nations Framework Convention on Climate Change(UNFCCC).^[99] 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 GHGs are stabilized in the atmosphere at a level where ecosystems can adapt naturally to climate change, food production is not threatened, and economic development can proceed in a sustainable fashion.

The UNFCCC recognizes differences among countries in their responsibility to act on climate change. In the Kyoto Protocol to the UNFCCC, most developed countries (listed in Annex I of the treaty) took on legally binding commitments to reduce their emissions. Policy measures taken in response to these commitments have reduced emissions. For many developing (non-Annex I) countries, reducing poverty is their overriding aim.

At the 15^th UNFCCC Conference of the Parties, held in 2009 at Copenhagen, several UNFCCC Parties produced the Copenhagen Accord. Parties agreeing with the Accord aim to limit the future increase in global mean temperature to below 2 °C. The 16^th Conference of the Parties (COP16) was held at Cancún in 2010. It produced an agreement, not a binding treaty, that the Parties should take urgent action to reduce greenhouse gas emissions to meet the 2 °C goal. It also recognized the need to consider strengthening the goal to a global average rise of 1.5 °C.

Adaptation & Geoengineering

Other policy responses include adaptation 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 is closely linked to social and economic development. 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.

Another policy response is engineering of the climate (geoengineering). This policy response is sometimes grouped together with mitigation. Geoengineering is largely unproven, and reliable cost estimates for it have not yet been published. Geoengineering encompasses a range of techniques to remove CO₂ from the atmosphere or to block incoming sunlight.

Responses: 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 ofcarbon sinks to absorb GHGs from the atmosphere. Many countries, both developing anddeveloped, are aiming to use cleaner, less polluting, technologies.^:192 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, and increased energy efficiency. Studies indicate substantial potential for future reductions in emissions. Since even in the most optimistic scenario, fossil fuels are going to be used for years to come, mitigation may also involve carbon capture and storage, a process that traps CO₂ produced by factories and gas or coal power stationsand then stores it, usually underground

Ecological & Social systems

In terrestrial ecosystems, 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, mangroves, and coral reefs. 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 of many species and reduced diversity of ecosystems.

There is some evidence of regional climate change affecting systems related to human activities, including agricultural and forestry management activities at higher latitudes in the Northern Hemisphere. Future climate change is expected to particularly affect some sectors and systems related to human activities. Low-lying coastal systems are vulnerable to sea level rise and storm surge. Human health will be at increased risk in populations with limited capacity to adapt to climate change. It is expected that some regions will be particularly affected by climate change, including theArctic, Africa, small islands, and Asian and African megadeltas. In some areas the effects on agriculture, industry and health could be mixed, or even beneficial in certain respects, but overall it is expected that these benefits will be outweighed by negative effects.

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 ontemperature changes. Rising sea levels 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. Over the course of centuries to millennia, the melting of ice sheets could result in sea level rise of 4–6 m or more.

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 and parts of the North Atlantic Ocean. Snow cover area and sea ice extent are expected to decrease. The frequency of hot extremes, heat waves, and heavy precipitation will very likely increase.

Climate models

The main tools for projecting future climate changes aremathematical models based on physical principles including fluid dynamics, thermodynamics and radiative transfer. Although they 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. All modern climate models are in factcombinations of models for different parts of the Earth. These include an atmospheric model for air movement, temperature, clouds, and other atmospheric properties; an ocean model that predicts temperature, salt content, and circulation of ocean waters; models for ice cover on land and sea; and a model of heat and moisture transfer from soil and vegetation to the atmosphere. Some models also include treatments of chemical and biological processes. Warming due to increasing levels of greenhouse gases is not an assumption of the models; rather, it is an end result from the interaction of greenhouse gases with radiative transfer and other physical processes. Although much of the variation in model outcomes depends on the greenhouse gas emissions used as inputs, the temperature effect of a specific greenhouse gas concentration (climate sensitivity) varies depending on the model used. The representation of clouds is one of the main sources of uncertainty in present-generation models.

Feedback

Feedback 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 amplifies the change in the first quantity while negative feedback 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 global warming is the tendency of warming to increase the amount of water vapor in the atmosphere, a significant greenhouse gas. The main negative feedback is radiative cooling, which increases as the fourth power of temperature; the amount of heat radiated from the Earth into space increases with the temperature of Earth's surface and atmosphere. 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 and ice-albedo feedback. Consequentially, potential tipping points may exist, which may have the potential to cause abrupt climate change.

Solar variation

Variations in solar output have been the cause of pastclimate changes. 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 thetroposphere, an increase in solar activity should warm thestratosphere while an increase in greenhouse gases should cool the stratosphere. Observations show that temperatures in the stratosphere have been cooling since 1979, when satellite measurements became available. Radiosonde (weather balloon) data from the pre-satellite era show cooling since 1958, though there is greater uncertainty in the early radiosonde record.

A related hypothesis, proposed by Henrik Svensmark, 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 rays. 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

Aerosols and soot

Global dimming, a gradual reduction in the amount of global direct irradiance at the Earth's surface, has partially counteracted global warming from 1960 to the present.The main cause of this dimming is aerosols produced by volcanoes and pollutants. These aerosols exert 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 aerosols is temporally limited due to wet deposition which causes aerosols to have anatmospheric lifetime of one week. Carbon dioxide has a lifetime of a century or more, and as such, changes in aerosol concentrations will only delay climate changes due to carbon dioxide.

In addition to their direct effect by scattering and absorbing solar radiation, aerosols have indirect effects on the radiation budget. Sulfate aerosols act as cloud condensation nuclei 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. 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.Indirect effects are most noticeable in marine stratiform clouds, and have very little radiative effect on convective clouds. Aerosols, particularly their indirect effects, represent the largest uncertainty in radiative forcing.

Soot may cool or warm the surface, depending on whether it is airborne or deposited. Atmosphericsoot aerosols 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. Atmospheric soot always contributes additional warming to the climate system.

Greenhouse gases

The greenhouse effect is the process by which absorption andemission of infrared radiation by gases in the atmospherewarm a planet's lower atmosphere and surface. It was proposed by Joseph Fourier in 1824 and was first investigated quantitatively by Svante Arrhenius in 1896.Naturally occurring greenhouse gases have a mean warming effect of about 33 °C (59 °F). The major greenhouse gases are water vapor, which causes about 36–70 percent of the greenhouse effect; carbon dioxide (CO₂), which causes 9–26 percent; methane (CH₄), which causes 4–9 percent; andozone (O₃), which causes 3–7 percent. Clouds also affect the radiation balance, but they are composed of liquid water or ice and so have different effects on radiation from water vapor.Human activity since the Industrial Revolution has increased the amount of greenhouse gases in the atmosphere, leading to increased radiative forcing from CO₂, methane, troposphericozone, CFCs and nitrous oxide. The concentrations of CO₂and methane have increased by 36% and 148% respectively since 1750. These levels are much higher than at any time during the last 650,000 years, the period for which reliable data has been extracted from ice cores. Less direct geological evidence indicates that CO₂ values higher than this were last seen about 20 million years ago. Fossil fuelburning has produced about three-quarters of the increase in CO₂ from human activity over the past 20 years. Most of the rest is due to land-use change, particularly deforestation.

Global Warming

Global warming is the increase in the average temperature of Earth's near-surface air and oceans since the mid-20th century and its projected continuation. According to the 2007 Fourth Assessment Report by theIntergovernmental Panel on Climate Change (IPCC), global surface temperature increased 0.74 ± 0.18 °C(1.33 ± 0.32 °F) during the 20th century. Most of the observed temperature increase since the middle of the 20th century has been caused by increasing concentrations ofgreenhouse gases, which result from human activity such as the burning of fossil fuel and deforestation. Global dimming, a result of increasing concentrations of atmospheric aerosols that block sunlight from reaching the surface, has partially countered the effects of warming induced by greenhouse gases.

Climate model projections summarized in the latest IPCC report indicate that the global surface temperature is likely to rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century. The uncertainty in this estimate arises from the use of models with differing sensitivity to greenhouse gas concentrations and the use of differing estimates of future greenhouse gas emissions. An increase in global temperature will cause sea levels to rise and will change the amount and pattern of precipitation, probably including expansion of subtropical deserts. Warming is expected to be strongest in the Arctic and would be associated with continuing retreat of glaciers, permafrost and sea ice. Other likely effects include changes in the frequency and intensity of extreme weather events, species extinctions, and changes in agricultural yields. Warming and related changes will vary from region to region around the globe, though the nature of these regional variations is uncertain. As a result of contemporary increases in atmospheric carbon dioxide, the oceans have becomemore acidic, a result that is predicted to continue

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 most common measure of global warming is the trend in globally averaged temperature near the Earth's surface. Expressed as a linear trend, this temperature rose by 0.74 ± 0.18 °C over the period 1906–2005. The rate of warming over the last half of that period was almost double that for the period as a whole (0.13 ± 0.03 °C per decade, versus 0.07 °C ± 0.02 °C per decade). The urban heat island effect is estimated to account for about 0.002 °C of warming per decade since 1900. Temperatures in the lowertroposphere have increased between 0.13 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 regionally varying fluctuations such as the Medieval Warm Periodand the Little Ice Age.

Estimates by NASA's Goddard Institute for Space Studies (GISS) and the National Climatic Data Center show that 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 19th century, exceeding the previous record set in 1998 by a few hundredths of a degree. Estimates prepared by the World Meteorological Organization and theClimatic Research Unit show 2005 as the second warmest year, behind 1998. Temperatures in 1998 were unusually warm because the strongest El Niño 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