Climate sensitivity and what it means in practice

RealClimate: Climate response estimates from Lewis & Curry

The significance of a lowering of the transient climate response (which is what the recent paper from Nic Lewis and Judith Curry suggests) gets a run in this post at Real Climate, which are deals with criticisms of the choices Lewis and Curry made in their paper.

These seem to be the crucial paragraphs:

The median estimate of the TCR from Lewis and Curry (1.3K) is towards the lower end of the IPCC likelyrange and lower than the CMIP5 median value of around 1.8K. A simple
way to understand the importance of the exact TCR value for mitigation policy is via its impact on the cumulative carbon budget to avoid crossing a 2K threshold of global surface temperature warming. Using the Allen and Stocker relationship between TCR and TCRE (the transient climate response to cumulative emissions) we can scale the remaining carbon budget to reflect different values for the TCR. Taking the IPCC CO₂-only carbon budget of 1000 GtC (based on the CMIP5 median TCR of 1.8K) to have a better than 2 in 3 chance of restricting CO₂-induced warming to beneath 2K, means that emissions would have to fall on average at 2.4%/year from today onwards. If instead, we take the Lewis and Curry median estimate (1.3K), emissions would have to fall at 1.2%/year. If TCR is at the 5^th percentile or 95^th percentiles of the Lewis and Curry range, then emissions would need to fall at 0.6%/year and 7.1%/year respectively.

Non-CO₂ emissions also contribute to peak warming. The RCP scenarios have a non-CO₂ contribution to the 2K peak warming threshold of around 0.5K [IPCC AR5 WG1 – Summary
for Policymakers]. Therefore, to limit total warming to 2K, the CO₂-induced contribution to peak warming is restricted to around 1.5K. This restricts the remaining carbon budget further, meaning that emissions would have to fall at 4.5%/year assuming a TCR of 1.8K or 1.9%/year
taking TCR to be equal to the Lewis & Curry median estimate of 1.3K (assuming no mitigation of non-CO₂ emissions).

While of some scientific interest, the impact for real-world mitigation policy of the range of conceivable values for the TCR is small (see also this discussion in Sci. Am.). For targets like the 2 K guide-rail, a TCR on the lower end of the Lewis and Curry and IPCC ranges might
just be the difference between a achievable rate of emissions reduction and an impossible one…

 The take home points (which climate change lukewarmenists do not want to know) seem to be this:

1.      the lower sensitivity estimates that some recent studies suggest do not mean you can burn carbon and have no risk of breaching the nominal 2 degree limit;

2.      the lower estimates make achieving a "safe" limit significantly more do-able, but effort to achieve it is still necessary.