"How much carbon can we emit?"

"...while keeping global temperature rise from exceeding 2 degrees Centigrade above pre-industrial levels"?

- asked Myles Allen this afternoon at an open meeting on carbon capture and storage at Imperial College London organised by Jon Gibbins.

Among key points, as I understood them:

• stabilisation of emissions at 1990 levels (as per Princeton wedge study) does not deliver "safe" climate change;
• common assumption is that cutting emissions to 40% of 1990 levels by 2050 would deliver "safe" climate change;
• a "sustainable" per capita emissions quota is scientifically indefensible;
• the maximum forecast warming is constrained by things we can observe if we limit the total C02 injection into the atmosphere
• robust case for total emissions profile consistent with an approx 20% chance of greater than 2 C warming allows for a total of 1,200 gigatonnes of carbon to be burnt. This means we can burn roughly twice what we have burnt to date;
• all scenarios require a greater than 50% cut in emissions at some point in less than fifty years;
• declining C02 in the atmosphere will also present challenges, and will need to be managed.

(related: Allen et al for Stabilisation 2005 here)