Back to the "maybe climate sensitivity is at the lower end" argument

It's been ages since I've posted much about climate change, but a climate change mega death post is probably due soon.  (I've started mucking around with fonts lately, because I have an urge to feel shoutier, given the current numbskullery on abundant display in the world.)

But before I get to that, it's time to re-visit the Nic Lewis/Judith Curry revised attempt at showing that their energy budget/observational take on Equilibrium Climate Sensitivity shows that it is at the lower end of the range given by all the other methods.   (They suggest possible medians of 1.5, 1.66 or 1.76 degrees.  The last is said to allow for "time varying climate feedbacks, which sounds to me like something which ought to be assumed, so I would take their highest median as the most likely.)

There's a good discussion of the paper (particularly in comments, where Nic Lewis condescending joins in) over at And Then There's Physics.   As someone notes, Judith Curry's involvement in this work seems just a tad inconsistent:

It is interesting that Judith has, in the past, argued that internal variability could explain a lot of the observed warming, but now authors a paper essentially suggesting it plays no/little role.

But it's all grist for the mill with climate inactionists, isn't it?   Any argument will do, damn the inconsistency, as long as it ends at "we should not be doing anything now."  

Which brings me to the point of this post.   Even if one takes the optimistic (but not particularly well justified) view that the Nic Lewis estimate of  ECS is (say) 1.7 degrees turns out to be the correct figure, what does that really mean if you hope for the planet to not go over the guesstimate that over 2 degrees would be dramatically dangerous?

This was addressed in a paper I seem not to have linked to before:  Implications of potentially lower climate sensitivity on climate projections and policy.  

The answer:   not as much as one might guess.   I would strongly suggest reading at least the end discussion section, from which I extract this (my bold):

Drawing upon the combined information of these multiple lines of evidence shows that there is no scientific support to diminish the urgency of emission reductions if warming is to be kept below 1.5 or 2 °C, the two temperature limits currently being discussed within the United Nations (UNFCCC 2010). Even the lowest ECS estimate assumed in this study only results in a delay of less than a decade in the timing of when the 2 °C threshold would be crossed when emission trends from the past 10 years are continued. Alternatively, if significantly lower ECS estimates were to be confirmed, following a low emissions trajectory (consistent with RCP3-PD) would become consistent with limiting warming below 1.5 °C by the end of the century with high probability (>80%) instead of only low probabilities (around 40%), and limiting warming to 1.5 °C would require about the same emission reductions as are now consistent with 2 °C when assuming the current IPCC ECS assessment.

Ah, why stop there, the rest of the discussion is so good I may as well cut and paste that too:

Relatively small shifts of ECS distributions towards lower values have a small influence on the temperature outcome and on compatible emissions, when compared to the overall uncertainty. As international climate policy is concerned about limiting warming below 2 °C with a 'likely' chance (UNFCCC 2011) ('likely' denoting and 'at least 66% probability' (Mastrandrea et al 2010)), shifts that robustly constrain the high end of the ECS or TCR distributions would be most important.

With this study we show that betting on the optimistic message of a few recent studies is risky at this point for two important reasons. First, as pointed out above, recent low ECS estimates are only part of the story. Alternative, and equally convincing methods point to higher values of ECS and only looking at the lower estimates would thus obfuscate an important part of the available scientific evidence. Second, not taking into account the combined evidence and delaying emission reductions in the coming decades would lead to lock-in into energy- and carbon-intensive infrastructure. This would thus not only result in a lower remaining carbon budget for the rest of the century, but the world would also be on a much more costly path by 2030 (Rogelj et al 2013b, 2013a, Luderer et al 2013, Riahi et al 2013). If current policies would bet on the optimistic end of the range, and more pessimistic estimates turn out to better capture the Earth system's behavior, limiting warming to low levels (like 2 °C) might well become unattainable (Rogelj et al 2013a, 2013b, Luderer et al 2013).

In conclusion, in light of the large uncertainties that still exist, the lack of consensus across different studies and lines of evidence, and the weak constraint that the observations provide, we argue that the possibility of lower values for ECS and TCR does not reduce the urgency for climate mitigation. On the contrary, a risk-averse strategy points to more ambitious reductions compared to what countries presented so far (Rogelj et al 2013a, UNEP 2013, Riahi et al 2013). Hedging against this uncertainty can be done by reducing global carbon emissions without delay, as to limit cumulative carbon emissions to within a budget in line with medium and higher climate response estimates that currently cannot be excluded. For our current generation, early and deep reductions of carbon emissions will undoubtedly be an important global societal challenge, despite the multiple opportunities and benefits that they bring along, such as reduced air pollution, energy security etc (McCollum et al 2013). However, those challenges are likely small compared to what future generations otherwise might possibly face: high climate impacts or emission reduction rates and associated costs that are substantially higher than the ones that would be necessary, if mitigation action commenced today.

I will also take the opportunity to link back to my 2013 post that discussed the first Nic Lewis paper, and pointed to papers arguing that some very slow feedbacks may well mean a long term  "earth system sensitivity" that could be double the fast feedback ECS.