The climate system is being transformed by human civilization into one that poses much greater risks, with potentially catastrophic consequences for society and ecosystems. The world is warming faster than expected and significant impacts are appearing on all continents. During 2007 and 2008, polar ice thickness and extent reached record low levels (reducing, on average, by 9.1 per cent per year from 1979 to 2006). Mountain glaciers and snow cover have declined in both hemispheres and widespread decreases in glaciers and ice caps have contributed to sea-level rise. In the Southern Hemisphere, climatic extremes are being exacerbated with longer and more profound droughts, more pronounced flood events and fire risks.

Greenhouse gas emissions continue to increase at an ever increasing pace - concentrations of greenhouse gases in the atmosphere have increased faster during the last ten years than at any time since continuous measurements began in 1960 (NASA, 2008).

If trends continue, the Earth's average temperature could increase by a further 4°C to 6°C by the end of this century (Hare, 2009), and based upon the paleo-data, there is a risk of precipitating devastating climatic shifts where the climate system 'flips' into a much different, less habitable state (Lenton et al, 2008). Even if we are moderately successful in containing emissions growth and, hence, global temperature increases to within 3°C, there is still a significant chance of massive human health and welfare impacts, as well as major disruptions to ecosystems. Knowledge about these impacts can aid in efforts to adapt to climate change; but avoidance should always remain the first line of defence in climate policy.

The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC, 2007b) shows that reducing greenhouse gas emissions by 25 to 40 per cent below 1990 levels by 2020, and by 80 per cent by 2050 would give us a 50 per cent chance that the global temperature increase could be kept to within 2°C. However, reducing emissions by 80 per cent by 2050 will not eliminate some serious risks and damages.

Other chapters in this book highlight the consequential security challenges that are associated with the projected impacts of climate change. In this chapter we examine the latest Earth system science to characterize the major risks to society posed by global warming.

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