The easiest way to look at it is to put all the average world temperatures in a graph, with the world temperatures being plotted against the number of days in the year when the temperatures occur. As with most things in nature, it forms a bell curve, with the largest number of days during the year reaching an average temperature of around 14°C and a few days a year reaching the highest and lowest world temperatures.
While freezing temperatures carry their own problems, it is the highest temperatures we need to worry about when looking at global warming and which also cause, of course, the highly negative affects of melting ice in the north pole, the rising sea levels, draughts (and so famines), loss of ecosystems, and the defrosting of the tundra that locks away vast amounts of methane (a particularly nasty greenhouse gas) to name just a few.
When we look at recent yearly world temperatures we can see that, though there are some days which fall into the 'danger zone', the number is small and not really something we need to worry about. However, this changes significantly when the average temperature rises, even by just 2°C, and we have a much large precentage of days in the high, dangerous temperatures.
2 degrees is seen by many as a 'safe' rise in temperature, though the damage to certain ecosystems is still significant. When we increase to 4°C and 5°C we really are in the danger zone with the high temperatures not only producing unbearable conditions for life on earth but also having a negative feedback affect on the carbon cycle, releasing more carbon dioxide into the atmosphere through natural processes rather than what we're pumping in ourselves.
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