The threat of human-induced climate change, popularly known as global warming, presents a difficult challenge to society over the coming decades. The production of so-called "greenhouse gases" (GHGs) as a result of human activity, mainly due to the burning of fossil fuels such as coal, oil, and natural gas, is expected to lead to a generalized warming of the Earth’s surface, rising sea levels, and changes in precipitation patterns. The potential impacts of these changes are many and varied – more frequent and intense heat waves, changes in the frequency of droughts and floods, increased coastal flooding, and more damaging storm surges – all with attendant consequences for human health, agriculture, economic activity, biodiversity, and ecosystem functioning.

Because the impacts of climate change are expected to be global and potentially severe, and because energy production from fossil fuels is a fundamental component of the world economy, the stakes in the issue are high. At the same time, a number of aspects of climate change complicate the problem. First, while much is known about the factors governing climate, considerable uncertainty remains in projections of how much climate will change, how severe the impacts will be, and how costly it would be to reduce GHG emissions. Second, because the impacts of today’s GHG emissions will be felt for decades into the future, it is not possible to wait and see how severe impacts turn out to be before taking preventive action. Therefore, if emissions are reduced now, the costs will be borne in the near term while the benefits, which will depend on uncertain projections of future impacts, will be realized largely in the long term. Third, sources of GHG emissions are widely dispersed among nations; no single country could significantly reduce future global climate change just by reducing its own emissions. Any solution to the problem must eventually be global.

The ultimate source of energy driving the Earth’s climate is the sun. Sunlight warms the Earth’s surface, which responds by re-emitting energy in the form of infrared radiation (heat). If the Earth had no atmosphere, the sun would warm the surface to only about –18 degrees Celsius (°C; or about 0 degrees Fahrenheit (°F)), a temperature well below the freezing point of water and probably unable to support life. But observations show that the average surface temperature is actually about 15 °C (59 °F). This 33 °C (59 °F) difference is due to a natural atmospheric phenomenon known as the greenhouse effect.

The greenhouse effect results from a number of gases in the atmosphere that are transparent to incoming sunlight but absorb the heat emitted from the Earth’s surface and reradiate it in all directions, including back toward the surface. As a result, the surface is warmed more than it would have been in the absence of these gases, which serve a purpose similar to that of the glass walls of a greenhouse. The greenhouse effect forms the backbone of current understanding of the Earth’s climatic history, as well as the understanding of climates of other planets such as Venus and Mars.

Global warming refers to the enhancement of Earth's natural greenhouse effect due to rising atmospheric concentrations of GHGs resulting from human activities. That the Earth will warm because of the enhanced greenhouse effect is not in dispute. Debate centers on how much and how fast warming will occur, and how serious the consequences of such changes might be.

Although GHGs play a critical role in maintaining the planet's surface temperature, they make up less than 1% of the atmosphere by volume. The most abundant GHG is water vapor. Although it is responsible for most of the Earth’s natural greenhouse effect, its global abundance is not directly influenced by human activity and so it is not considered an anthropogenic (human-generated) GHG. The most important anthropogenic GHGs are carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and halocarbons.

Carbon dioxide is the anthropogenic GHG that is of most concern. Large natural flows of CO₂ between the atmosphere and the oceans, land plants, and soils have maintained relatively constant atmospheric concentrations over the past ten thousand years. However, over the past two centuries human activity has released CO₂ at increasing rates. The "pioneer effect" (deforestation in the Northern Hemisphere) was probably the first significant contributor, but with the beginning of the Industrial Revolution in the 19th century the main source quickly became the burning of fossil fuels such as coal, oil, and natural gas. Over the past several decades, tropical deforestation has also become a significant source, making up about 20% of present global emissions.

Measurements show that atmospheric CO₂ concentrations have increased about 30% from their pre-industrial level as a result of human activity, to a level higher than at any time in at least the past 420,000 years. Carbon dioxide is removed from the atmosphere slowly by being absorbed by the oceans and by land plants. Some portion of the effect of CO₂ emitted into the atmosphere is removed in a century or less, but a quarter to a half of the effect is removed so slowly it may be considered essentially permanent.

Other greenhouse gases have been accumulating in the atmosphere as well. For example, the CH₄ concentration has more than doubled since the 1700s, and the N₂O concentration has risen by about 15%.

Temperature measurements have been compiled from land- and ship-based weather stations located around the world into a 130-year record of the globally and annually averaged temperature of the surface of the Earth. This record shows that the average surface temperature has risen by a little more than half a degree Celsius. When compared with indirect estimates of surface temperature over the past several centuries, this record shows that the 20th century has been the warmest of any since at least the 15th century and possibly for several thousand years.

However, climate varies naturally and confidently attributing observed changes to human causes is difficult. Nonetheless, the Intergovernmental Panel on Climate Change (IPCC), an international scientific body charged by the United Nations with providing comprehensive surveys of climate change science, concluded recently that "the balance of evidence suggests a discernible human influence on global climate". The question of how much of the observed temperature rise has been due to human influence cannot yet be determined. Over time, detection and attribution of the greenhouse signal are expected to become less ambiguous.

The IPCC also developed several scenarios of climate change that might result over the 21^st century if no action is taken to reduce GHG emissions. Scenarios were developed for future emissions of CO₂ and other GHGs, as well as sulfate aerosols (which reflect sunlight and therefore tend to cool the climate), and were used to drive computer models of the Earth's climate system. Models project increases in global average temperature by the year 2100 of 1–3.5 °C relative to today, with a "best guess" estimate of about 2 °C, under such "business as usual" conditions. It might be thought that a world in which the average temperature is a few degrees warmer would not differ greatly from the world in which we now live. However, average surface temperature is an index of the overall state of the Earth’s climate, and a change of a few degrees is a major global event. For example, during the last ice age, global average temperature was probably just 5 °C lower than it is today, but this difference was enough to cause ice sheets to cover vast areas of Europe and North America that are currently ice free. The high end of the range of IPCC projections, especially beyond 2100, approaches a warming of a magnitude similar to that of the glacial cooling.

The prominence of global warming as a political issue grew slowly in the 1970s and most of the 1980s. By the end of the 1980s, concern was sufficient to create a demand for an international agreement on global warming and a negotiation process was begun through the UN to produce a treaty on climate change. This process led to the signing of the Framework Convention on Climate Change (FCCC) by representatives of 165 governments at the United Nations Conference on Environment and Development (the "Earth Summit") in Rio de Janeiro in 1992. The FCCC calls for the "stabilization of GHG concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system." It stipulates that parties should be guided by a number of principles in achieving this objective, including equity (specifically, that developed countries should "take the lead" in climate change mitigation), the precautionary principle (when in doubt, to err on the side of caution), cost-effectiveness, and sustainable development. The FCCC has now been signed and ratified by more than 175 countries and legally entered into force in March 1994.

The FCCC suggests that industrialized countries aim to stabilize their emissions at 1990 levels by the year 2000. Most countries do not anticipate achieving this goal. Furthermore, stabilizing emissions of GHGs will not lead to a stabilization of atmospheric concentrations, the goal of the convention; deeper cuts in emissions would be required to meet this goal. In recognition of these shortcomings, representatives of over 150 governments signed the Kyoto Protocol in 1997. If the protocol is ratified, it will require industrialized countries to reduce aggregate emissions of six GHGs – CO₂, CH₄, N₂O, HFCs, PFCs, and SF₆ (sulfur hexafluoride) – by about 5% relative to 1990 emission levels. This goal would be achieved over the compliance period 2008–2012. Individual countries accepted different caps; Australia, for example, would be allowed to increase emissions 8% above 1990 levels, while the United States and the European Union would need to decrease emissions by 7% and 8%, respectively. The protocol envisions a system of emissions trading in which countries can make deeper reductions than required and sell excess allowances or emit at rates above their cap and buy allowances for the difference. The aim of such a system is to reduce the overall costs of emission reductions. In principle, a country in which domestic reductions would be expensive could reduce costs by "buying" (in other words, financing the achievement of) cheaper reductions elsewhere.

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