Global warming is the observed increase in the average temperature of the Earth's atmosphere and oceans in recent decades and its projected continuation. In principle, global warming is neutral as to the period or causes, but in both common and scientific usage the term generally refers to recent warming and implies a human influence. Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations, which leads to warming of the surface and lower atmosphere by increasing the greenhouse effect caused by gases which are released by activities such as the burning of fossil fuels, land clearing, and agriculture. The predicted effects of global warming for the environment and for human life are numerous and varied. The main effect is an increasing global average temperature. From this flow a variety of resulting effects, namely, rising sea levels, altered patterns of agriculture, increased extreme weather events, and the expansion of the range of tropical diseases. In some cases, the effects may already be occurring, although it is generally difficult to attribute specific natural phenomena to long-term global warming. Examples of projected climate changes include, significant slowing of the ocean circulation that transports warm water to the North Atlantic, large reductions in the Greenland and West Antarctic Ice Sheets, accelerated global warming due to carbon cycle feedbacks in the terrestrial biosphere, and releases of terrestrial carbon from permafrost regions and methane from hydrates in coastal sediments.
Global warming controversy
The global warming controversy is a debate about the causes of observed global warming since the mid-20th century, as well as the expected magnitude and consequences of future warming. A major part of the debate centers around what actions, if any, society should take in response to the prospect of future warming. Some of the main areas of controversy include: 1. Whether the climate is changing beyond natural variations in the historical temperature record 2. Whether human/industrial activity is responsible for the change and if so, to what extent 3. The effect of predicted depletion of fossil fuels, both individually as e.g. oil runs out and users turn to the higher polluting coal and overall as to whether there are sufficient available reserves to cause the more extreme climate change scenarios 4. The effectiveness of policies to reduce CO2 emissions
5. The size of future changes in climate
6. The regional effects of climate change
7. The consequences of climate change
Among climate scientists there is little disagreement that global warming is primarily anthropogenic, but the debate continues in the popular media and on a policy level. Questions include whether there is a scientific consensus on the extent and rate of anthropogenic global warming, and in particular whether there is sufficient evidence to justify immediate and far-reaching actions to ameliorate its effects. Those who believe such a consensus exists express a wide range of opinions: some merely recognize the validity of the observed increases in temperature, while others support measures such as the Kyoto Protocol which are intended to reduce the magnitude of future global warming. Still others believe that environmental damage will be so severe that immediate steps must be taken to reduce carbon dioxide and methane emissions, even if the precise results are unknown, and even if there are substantial economic costs to doing so. One example of an attempt to force action is the Sierra Club suing the U.S. government over failure to raise automobile fuel efficiency standards, and thereby decrease carbon dioxide emissions. Most of the consequences of global warming would result from one of three physical changes: sea level rise, higher local temperatures, and changes...
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