1C – Vital for low-lying island states but virtually impossible
The Arctic sea ice is already disappearing and, after a 1C global average temperature rise, it would disappear for good in the summer months. Heatwaves and forest fires will become more common in the sub-tropics – worst-hit will be the Mediterranean region, southern Africa, Australia and south-west United States. Most of the world's corals will die, including the Great Barrier Reef. Glaciers that provide crops for 50m people with fresh water begin to melt and 300,000 people are affected every year by climate-related diseases such as malaria and diarrhoea.
2C – The temperature limit the scientists want
The heatwaves seen in Europe during 2003, which killed tens of thousands of people, will come back every year with a 2C global average temperature rise. Southern England will regularly see temperatures around 40C in summer. The Amazon turns into desert and grasslands, while increasing CO2 levels in the atmosphere make the world's oceans too acidic for remaining coral reefs and thousands of other marine lifeforms. More than 60 million people, mainly in Africa, would be exposed to higher rates of malaria. Agricultural yields around the world will drop and half a billion people will be at greater risk of starvation. The West Antarctic ice sheet collapses, the Greenland ice sheet melts and the world's sea level begins to rise by seven metres over the next few hundred years. Glaciers all over the world will recede, reducing the fresh water supply for major cities including Los Angeles. Coastal flooding affects more than 10 million extra people. A third of the world's species will become extinct as the 2C rise changes their habitats too quickly for them to adapt.
3C – Looking increasingly likely
After a 3C global temperature rise, global warming may run out of control and efforts to mitigate it may be in vain. Millions of square kilometres of Amazon rainforest could burn down, releasing carbon from the wood, leaves and soil and thus making the warming even worse, perhaps by another 1.5C. In southern Africa, Australia and the western US, deserts take over. Billions of people are forced to move from their traditional agricultural lands, in search of scarcer food and water. Around 30-50% less water is available in Africa and around the Mediterranean. In the UK, summers of droughts are followed by winter floods. Sea levels rise to engulf small islands and low-lying areas such as Florida, New York and London. The Gulf Stream, which warms the UK all year round, will decline and changes in weather patterns will lead to higher sea levels at the Atlantic coasts.
4C - Possible with an extremely weak deal
At this stage, the Arctic permafrost enters the danger zone. The methane and carbon dioxide currently locked in the soils will be released into the atmosphere. At the Arctic itself, the ice cover would disappear permanently, meaning extinction for polar bears and other native species that rely on the presence of ice. Further melting of Antarctic ice sheets would mean a further 5m rise in the sea level, submerging many island nations. Italy, Spain, Greece and Turkey become deserts and mid-Europe reaches desert temperatures of almost 50C in summer. Southern England's summer climate could resemble that of modern southern Morocco.
EDIT
http://www.guardian.co.uk/environment/2009/dec/18/copenhagen-five-climate-scenarios
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