Six

I used to doubt that a 6°C rise global average temperature this century was likely. Now a study indicates it may be a plausible scenario. [1]

CO2 emissions have risen by 29% in the past decade, and 2% in the last year despite the economic downturn.

Concerns such as 'peak oil' look more misplaced than ever as there is more than enough coal, oil (if Dan Yergin is right) and natural gas to send things over the edge.

[1] P.S. Richard Betts says:

Year-to-year changes in the global economy have quite an effect, and it's too early to discern longer term, robust changes. However, if we continue to let emissions rise without mitigation, there's a strong chance we'll hit 4 °C and beyond. If we want to be staying below 2 °C then it's true to say we've only got a few years to curb emissions.