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What is global warming?
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.
Global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.3 ± 0.32 °F) during the past century. The Intergovernmental Panel on Climate Change (IPCC) concludes, "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. Natural phenomena such as solar variation combined with volcanoes have probably had a small warming effect from pre-industrial times to 1950, but a small cooling effect since 1950. These basic conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. The American Association of Petroleum Geologists is the only scientific society that rejects these conclusions. A few individual scientists disagree with some of the main conclusions of the IPCC.
Climate models referenced by the IPCC project that global surface temperatures are likely to increase by 1.1 to 6.4 °C (2.0 to 11.5 °F) between 1990 and 2100. The range of values reflects the use of differing scenarios of future greenhouse gas emissions and results of models with differences in climate sensitivity. Although most studies focus on the period up to 2100, warming and sea level rise are expected to continue for more than a millennium even if greenhouse gas levels are stabilized. This reflects the large heat capacity of the oceans.
An increase in global temperatures can in turn cause other changes, including sea level rise, and changes in the amount and pattern of precipitation resulting in floods and drought. There may also be changes in the frequency and intensity of extreme weather events, though it is difficult to connect specific events to global warming. Other effects may include changes in agricultural yields, glacier retreat, reduced summer streamflows, species extinctions and increases in the ranges of disease vectors.
Remaining scientific uncertainties include the exact degree of climate change expected in the future, and how changes will vary from region to region around the globe. There is ongoing political and public debate regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences. Most national governments have signed and ratified the Kyoto Protocol aimed at combating greenhouse gas emissions.
STERN REVIEW
Climate change fight 'can't wait'
The world cannot afford to wait before tackling climate change, the UK prime minister has warned.
A report by economist Sir Nicholas Stern suggests that global warming could shrink the global economy by 20%.
But taking action now would cost just 1% of global gross domestic product, the 700-page study says.
Tony Blair said the Stern Review showed that scientific evidence of global warming was "overwhelming" and its consequences "disastrous".
International response
The review coincides with the release of new data by the United Nations showing an upward trend in emission of greenhouse gases - a development for which Sir Nicholas said that rich countries must shoulder most of the responsibility.
International response
The review coincides with the release of new data by the United Nations showing an upward trend in emission of greenhouse gases - a development for which Sir Nicholas said that rich countries must shoulder most of the responsibility.
Graph: How new CO2 targets could curb emissions
And Chancellor Gordon Brown promised the UK would lead the international response to tackle climate change.
The BBC's Nick Robinson said that, while the Stern Review did not recommend specific tax rises, upping the cost of flying - both people and goods - and driving was on the agenda of all three main political parties.
Environment Secretary David Miliband said the Queen's Speech would now feature a climate bill to establish an independent Carbon Committee to "work with government to reduce emissions over time and across the economy".
The report says that without action, up to 200 million people could become refugees as their homes are hit by drought or flood.
"Whilst there is much more we need to understand - both in science and economics - we know enough now to be clear about the magnitude of the risks, the timescale for action and how to act effectively," Sir Nicholas said.
"That's why I'm optimistic - having done this review - that we have the time and knowledge to act. But only if we act internationally, strongly and urgently."
Mr Blair said the consequences for the planet of inaction were "literally disastrous".
"This disaster is not set to happen in some science fiction future many years ahead, but in our lifetime," he said.
"Investment now will pay us back many times in the future, not just environmentally but economically as well."
"For every £1 invested now we can save £5, or possibly more, by acting now.
"We can't wait the five years it took to negotiate Kyoto - we simply don't have the time. We accept we have to go further (than Kyoto)."
Large risks
Sir Nicholas, a former chief economist of the World Bank, told BBC Radio 4's Today programme: "Unless it's international, we will not make the reductions on the scale which will be required."
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