It’s amazing what you can do with Excel. I thought I’d have another quick look before breakfast at my 450ppm stabilisation scenario (hey, kids, you can play this game at home!).
Here’s what I was referring to yesterday (all numbers approximate):
To some extent I’m being optimistic. The 4AR mostly refers to scenarios that we would not now countenance as we’ve come out of denial over the last few years (I suggest they review their approach for the next report, 5AR). But if we look at the Scientific Basis, page 791 (I kid you not – strictly I should also be using a 3 line reference to the chapter – 10, section 4.1 as it happens), we see some discussion of stabilisation scenarios. The IPCC suggest a higher peak in fossil fuel emissions (about 12GtC/yr compared to the 9GtC I’ve shown), but with a steeper reduction. Their scenario allows 596GtC over the 21st century, whereas I came up with 566GtC. But the key point is that the IPCC also calculate some scenarios with positive carbon cycle feedbacks – that is, when we listen to the science and assume that warming will cause ecosystems to release carbon, or in actual fact merely to take it up more slowly than at present – and in these scenarios taking account of carbon cycle feedbacks we are “allowed” to emit 105 to 300GtC less. That is, even an aggressive scenario to stabilise CO2 at 450ppm relies on a get-out-of-jail-free card.
A more rigorous analysis – I would next separate out land use change (deforestation) from the fertilisation effect altogether – is unlikely to give a different conclusion, because the sanity check (total fossil fuel emissions) succeeds. This simple spreadsheet, adding together the main parts of the the carbon cycle is compatible with the sophisticated models cited by the IPCC. And it shows that, at first approximation (as the scientists say) we have to manage both components we can influence – fossil fuel burning and land uptake of CO2.
The critical point is that, if we want to save the planet, we’ve got to make sure that land carbon uptake over the next century – by natural ecosystems, such as forests, wetland and grassland – increases, not decreases. And if we plough them up and plant even more crops, then they will release carbon for a while and then store a roughly constant amount.
This is the macro reason why promoting biofuels is a really, really bad idea. In fact, it’s difficult to think of a worse policy response to the threat of global warming.