A believer’s faith falters

The Economist’s recent 3-page update on climate sensitivity will be read and understood by many more people than any voluminous IPCC report. Although both sources are notoriously affected by confirmation bias, the former will deservedly carry more weight with discerning readers.

Those who favour short articles might have missed some key conclusions in the closing section of the report:

So what does all this amount to? The scientists are cautious about interpreting their findings. As Dr Knutti puts it, “the bottom line is that there are several lines of evidence, where the observed trends are pushing down, whereas the models are pushing up, so my personal view is that the overall assessment hasn’t changed much.”

Uh-oh. So, when the observed data is inconsistent with the modelled theory, you just split the difference – and/or ignore both? This must be a post-modern iteration of post-normal science.

But given the hiatus in warming and all the new evidence, a small reduction in estimates of climate sensitivity would seem to be justified: a downwards nudge on various best estimates from 3°C to 2.5°C, perhaps; a lower ceiling (around 4.5°C), certainly. If climate scientists were credit-rating agencies, climate sensitivity would be on negative watch. But it would not yet be downgraded.

If a reduction “would seem to justified” already, what further data will it take to trigger the downgrade? If climate sensitivity is “on negative watch”, should it continue to be the basis of policies costing billions of dollars?

Equilibrium climate sensitivity is a benchmark in climate science. But it is a very specific measure. It attempts to describe what would happen to the climate once all the feedback mechanisms have worked through; equilibrium in this sense takes centuries—too long for most policymakers. As Gerard Roe of the University of Washington argues, even if climate sensitivity were as high as the IPCC suggests, its effects would be minuscule under any plausible discount rate because it operates over such long periods. So it is one thing to ask how climate sensitivity might be changing; a different question is to ask what the policy consequences might be.

This makes the important point that policy action could never be justified on the basis of a high ECS – which keeps on evolving for hundreds of years in the fure The benefit-to-cost relationship will invariably be negative,if “any plausible discount rate” is used to calculate present value.

So all the analyses during the past 20 years, which HAVE been based on ECS assumptions, are clearly suspect. A more practical and relevant metric is the TPR:

For that, a more useful measure is the transient climate response (TCR), the temperature you reach after doubling CO₂ gradually over 70 years. Unlike the equilibrium response, the transient one can be observed directly; there is much less controversy about it. Most estimates put the TCR at about 1.5°C, with a range of 1-2°C. Isaac Held of America’s National Oceanic and Atmospheric Administration recently calculated his “personal best estimate” for the TCR: 1.4°C, reflecting the new estimates for aerosols and natural variability.

The argument seems pretty convincing. Few would bet much on futurist estimates about economic conditions, technology, lifestyle, etc more than 70 years in the future – and any such estimates are properly discounted to zero anyway. Also, the ECS is a purely cerebral construct, which may not even happen and can never be observed.

The best-estimate TCR is just half of the ECS previously used, so the 70-year (ie 2080) impacts of CO2e will be barely 50% of the scary outcomes projected in the IPCC’s 4AR. And we know from Dr Richard Tol^[1] and others that initial warming up to about 2°C produces net benefits rather than “danger” – which is why a ceiling of 2°C was chosen as the global target at the COP15 (Copenhagen) conference of the UNFCCC.

That sounds reassuring: the TCR is below estimates for equilibrium climate sensitivity. But the TCR captures only some of the warming that those 70 years of emissions would eventually generate because carbon dioxide stays in the atmosphere for much longer. As a rule of thumb, global temperatures rise by about 1.5°C f or each trillion tonnes of carbon put into the atmosphere. The world has pumped out half a trillion tonnes of carbon since 1750, and temperatures have risen by 0.8°C. At current rates, the next half -trillion tonnes will be emitted by 2045; the one after that before 2080.

We have already noteded that any post-2080 waring effects would be discounted away and are therefore irrelevant to policy decisions. Put differently, the suppression today of guessed effects in future centuries would be hopelessly cost-ineffective and make little sense.

The relationship of 1.5TT to the 0.8°C which coincided with the 0.5TT accrued post-1750 will depend upon: (a) how much of that 0.8°C was greenhouse-related (remember the Little Ice Age); and (b) the logarithmic progression of the greenhouse effect.

Since CO₂ accumulates in the atmosphere, this could increase temperatures compared with pre-industrial levels by around 2°C even with a lower sensitivity and perhaps nearer to 4°C at the top end of the estimates. Despite all the work on sensitivity, no one really knows how the climate would react if temperatures rose by as much as 4°C. Hardly reassuring.

The 2°C figure is justified above, but the “perhaps nearer to 4°C” involves reverting to the ECS – which clearly runs counter to the arguments presented by the Economist itself. Further, the top of the ECS range is a speculative and remote threat that is centuries away. We cannot sensibly assess risk without a confident probability figure.

In a nutshell: The Economist argues convincingly that the TCR, rather than the ECS, should be used for modelled temperature projections and for the cost:benefit assessments that drive climate policy decisions.

And TCR-based projections show a 21st-century temperature rise which is safely below the target ceiling fixed by the UNFCCC. They are a better fit for the observed 16-year temperature standstill, and for the consequences noted by Nic Lewis of reduced aerosol offsets, and for the recent dicoveries regarding the enhanced role of black carbon. Such projections are altogether more plausible.

This switch to TCR might even drive the ‘paradigm shift’ or ‘scientific revolution’ predicted by Thomas Kuhn in 1955 and long overdue in climate science.

The Economist article is a highly compelling summary for policymakers. Let’s hope incoming Australian legislators are considering it with care!

Hon Barry Brill OBE is a New Zealand barrister and solicitor. He is a former Minister of Science & Technology, and Minister of Energy, and is currently chairman of the New Zealand Climate Science Coalition