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THE PROBLEM IN A NUTSHELL
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uman activities are increasing the concentrations of naturally-occurring 'greenhouse gases' in the atmosphere. These gases, which include carbon dioxide (CO2) and methane, trap solar heat in the atmosphere, so warming the surface of the planet. The IPCC forecasts that, under reasonable assumptions about future human activities - its mid-range 'business as usual scenario' - concentrations of greenhouse gases will rise sufficiently over the next century to cause a worldwide average temperature rise of between 1.5 ° C and 4° C. | ||||||
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The cooling effect of other pollution, through shading the planet's surface, may reduce this to perhaps 1° C-3.5° C, says the IPCC. But this would still be enough to trigger major changes in local climates and ecosystems and disruption of many human activities, including causing famine and widespread flooding of inhabited areas. Global mean surface temperatures have already increased by between 0.3° C and 0.6° C, probably as a result of human activity.
The main human source of greenhouse gases is the burning of fossil fuels, such as coal, oil and gas, which releases CO2. Other sources include deforestation, which also releases CO2, and agriculture, which through a variety of routes releases methane and other gases such as nitrous oxide. Carbon dioxide, the main man-made greenhouse gas, has a lifetime in the atmosphere of a century or more. Even if global carbon dioxide emissions were to be stabilised near their current levels, concentrations would increase throughout the 21st century and would continue to increase slowly for several hundred years afterwards. Because of this, substantial cuts in emissions will be necessary to stabilise concentrations in the atmosphere. | ||||||
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As the world's climate begins to change, the planet will respond, sometimes in ways that will accelerate warming, sometimes in ways that will slow it down. Critical factors include the response to warming of clouds, forests, ice cover and ocean circulation. The overall effect of these 'feedbacks' remains uncertain. Scientists are not yet certain how sensitive the climate is to the direct 'radiative forcing' of increased greenhouse gases. This gap in knowledge explains the large margins of error surrounding current predictions of climate change. But there is a consensus that there will be significant warming.
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