I watched this last night and found it really compelling. It goes some way to explain the science behind three big numbers in Climate Change science.

The three numbers are:
0.85 degrees (the amount of warming the planet has undergone since 1880)

95 per cent (the degree of certainty climate scientists have that at least half the recent warming is man-made)

1 trillion tonnes (the total amount of carbon we can afford to burn - ever - in order to stay below 'dangerous levels' [2deg C] of climate change)

BBC Four - Climate Change by Numbers
http://www.bbc.co.uk/programmes/p02jsdrk