In the post A choice of catastrophes: the IPCC budget approach I explained the socalled ‘carbon budget approach’ in some detail. In general terms:
- In a warming world what matters is the total quantum of CO2 in the atmosphere. The ‘climate budget approach’ identifies the total anthropogenic CO2 emitted to cause warming of 2°C. For a 66% chance of staying under 2°C the total CO2 emitted must not exceed 1000Gt, according to calculations done by Malte Meinshausen and others back in 2009. The later we leave cutting the harder we have to cut.
Rahmstorf’s budget was about 1000Gt of CO2 or about 1500GT of CO2 equivalent with other greenhouse gases for a 25% chance of staying within 2°C. Then
- as Giles Parkinson reports, the carbon budget figures have taken a haircut to become 800Gt for a 66% chance of 2°C when “accounting for non-CO2 forcings”. Problem is we’d already used up 543Gt of the budget by 2011.