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?

It’s important to note a big shift in focus here. Most of the discussion so far has been along the lines “What do we need to do by 2050 to avoid unacceptable damage to the climate later this century”. Looking ahead for a century is challenging, to put it mildly. But the questions raised by Hansen shift the time-scale for action out another 50 years, and the consequences are centuries into the future. That means there are huge uncertainties that are difficult to reason about. As a starting point, I’m going to follow Hansen and co-authors in treating the problem as if it were deterministic, with a known target of 350 ppm and costs as stated.

With these drastic simplifications, the problem is not all that hard, and can be made a bit simpler with the right choice of parameters. The question is, how much would we pay (in $/tonne) today (I’ll say 2010) and around 2050 (I’ll say 2045) to avoid a cost of $200/tonne in 2080 (all in constant value dollars). With a 2 per cent discount rate (I’ve argued at length that this is a good choice), the answer is given by the rule of 70: values double every 35 years. So, we ought to be willing to pay $50/tonne now and around $100/tonne in 2045.

I’ll come back to this a bit later and discuss less simplified estimates. The main point that would suggest a higher current price is a lower trajectory with the same endpoint implies less residence time for CO2 in the atmosphere and therefore less risk.