The future of the planetís climate has been projected by the Met Office using two scenarios, one with increasing greenhouse gas emissions (left) and one with decreasing emissions (right). The models start in the past, as a way of testing their accuracy.
The scenario on the left (A1B) assumes a business as usual approach to carbon dioxide emissions, with growth in the population, the economy and in fossil fuel use, leading to an approximate rise of 4C by 2100.
The scenario on the right (E1) represents what may happen if swift action is taken to constrain emissions. It assumes that they will start to fall from 2015 onwards and that the global average temperature will rise by about 2C by 2100.
An average global temperature rise of 2C will cause major problems in many parts of the world, but is considered relatively safe compared with the impacts associated with a rise of 4C.
Temperature change will vary widely in different parts of the world. For example in eastern China the hottest days of the year could be up 6C, if future emissions are high, and 4C if they are low.
This variation in temperature rise is seen most dramatically in the projections for the polar regions, with the Arctic expected to see the most change.
The two projections begin to diverge dramatically after the middle of the century. By 2055 they are already more than a degree apart.
Marine life will be affected by increasing acidification of the seas which is projected to have a big impact on coral reefs and the fisheries that they support.
A 4C rise threatens permafrost across the Northern Hemisphere, with the Greenland ice sheet also under pressure.
As temperatures increase, the availability of water is reduced. At higher levels, the Amazon could face the twin problems of forest fire and drought.
By the end of the century, sea levels could be up by 80cm (4C scenario) or 40cm (2C scenario) putting coastal populations at greater risk of flooding.
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