Following my recent post, a number of commenters suggested that I ought to respond more directly to the arguments of James Hansen and others for a CO2 target of 350 parts per million, as opposed to the 450 ppm that forms the basis of much current policy discussion. I’m using this paper as a basis, and take the following two points as its central claims
* To avoid unacceptable risk of passing a point of no return beyond which explosive feedbacks (icecaps melting etc) are inevitable, we should aim to reduce CO2 concentrations to 350 ppm by 2100. This is below current levels and won’t be achieved simply by ending net emissions
* We can achieve part of this (maybe a reduction of 60 ppm) through reforestation, biochar and similar measures
* Further reductions will require expensive technological solutions, estimated cost $200/tonne or $20 trillion to remove 50 ppm. Given a maximum point around 450 ppm and 50 ppm from reforestation, that’s about the amount required.
What then should we do? In particular, how much should we be willing to pay now, to avoid high costs in the second half of this century?