Carbon budgets, climate sensitivity and the myth of "burnable carbon"

Arctic modelling underestimates sea-ice loss, albedo change and warming

The IPCC's 2013 carbon budget work uses Coupled Model Intercomparison Project Phase 5 (CMIP5) computer modelling results. Results are given for the RCP2.6 (~2°C warming) scenario, which show a 43% reduction in September Arctic sea-ice extent by 2100 (compared to a 1985–2005 reference period), so that ~ 4 million square kilometres on Arctic sea ice still remains in summer by 2100.

This is so at odds with the reality on the ground as to be not credible. With less than 1°C of global warming, the Arctic sea-ice extent has dropped by half, and the sea-ice volume by three-quarters. At the 2012 summer minimum, sea-ice extent was 3.4 million square kilometres, less than the CMIP5's projection for 2100!

Many Arctic experts think that the Arctic is likely to reach an sea-ice-free state (defined as less than 1 mil. sq. km.) in the northern summer within the next decade or so, and perhaps sooner, with the number of ice-free days growing from then on. Prof. Will Steffen told "The Age" in September 2012 that: “I’m pretty certain that we have now passed the tipping point for Arctic sea ice”. This reflects work by researchers include Livina, Lenton, Wadhams and Maslowski (1).

Climate sensitivity

The mid-range ECS estimate is generally around 3°C (range 2–4.5°C), and it plays out in the first hundred years or so after an injection of carbon dioxide into the atmosphere. The 2013 IPCC report finds that "Equilibrium climate sensitivity is likely in the range 1.5°C to 4.5°C" but "No best estimate for equilibrium climate sensitivity can now be given because of a lack of agreement on values across assessed lines of evidence and studies."

However a recent paper by Sherwood, Bony et al. looking at clouds and atmospheric convective mixing finds that on “the basis of the available data… the new understanding presented here pushes the likely long-term global warming towards the upper end of model ranges.” Taking “the available observations at face value,” they write, “implies a most likely climate sensitivity of about 4°C, with a lower limit of about 3°C” (3).

Conclusion

If climate sensitivity is, in reality, at the high end of the range used for the IPCC's carbon budgets, then as a consequence that means that we must adopt a very low-risk of exceeding the target. As the previous post explained, If a risk-averse (pro-safety) approach is applied – say, of less than 10% probability of exceeding the 2°C target – to carbon budgeting, there is simply no budget available, because it has already been used up.