September 27, 2013, by Warren Pearce
Just one number: has the IPCC changed its supply of evidence?
As I have researched online climate scepticism over the last year, its become clear that climate sensitivity has been one of the biggest topics for discussion. This is perhaps one of the easier parts of climate science to understand. Put simply, sensitivity measures the amount by which the Earth’s temperature increases when atmospheric carbon dioxide doubles (more detail here). This is a key question for climate sceptics, and explains how those who accept the existence of the greenhouse effect (most of them) can still be unconvinced about the severity of future climate change. To use a simple analogy, one could imagine sitting on a trolley going down a slope. With a high climate sensitivity the slope is very steep and we roll down very quickly, with low sensitivity we roll much more slowly. All agree hat the trolley is rolling, but the speed of the rolling is likely to affect how quickly we try and put the brakes on.
Today, the Intergovernmental Panel on Climate Change hit the headlines with the release of its Summary for Policymakers from its new Assessment Report (AR5). Section D2 of the Summary tells us that the lower limit of the climate sensitivity range has fallen from 2°C to 1.5°C, while the upper limit remains unchanged at to 4.5°C. This would seem to constitute a small degree of good news. However, perhaps more interesting is the change in approach revealed in the footnote:
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.
This is a notable change from the approach in the previous report (AR4):
‘Equilibrium climate sensitivity’, is likely to lie in the range 2°C to 4.5°C, with a most likely value of about 3°C. Equilibrium climate sensitivity is very likely larger than 1.5°C.
So the IPCC have broken with their previous approach, opting not to present a single number as the most likely figure for sensitivity, instead relying on reporting a range of likely figures.
The AR5 Summary doesn’t go into any further detail about the new developments which render cross-study comparisons of sensitivity studies invalid. We will have to wait until Monday for that. However, what does the absence of this number mean for policy decisions?
‘Just one number’
In his presentation at STEPS Centre earlier this year, Roger Pielke Jr. discussed a couple of examples of how policymakers express a preference for ‘just one number’ rather than the range of certainties and error bars which scientists are most comfortable with. In particular he quotes from the Danish Prime Minister speaking to scientists before the Copenhagen Climate Conference, expressing a desire for “fixed targets and certain figures, and not too many considerations on uncertainty and risk and things like that.” In this context, one can see how decision-makers are likely to overlook the IPCC’s careful discussion of various climate models and their range of climate sensitivities and concentrate on their supply of a single number:
Pielke Jr. implies that it is unsurprising that politicians seek to shift accountability from themselves to the expert community through demands for just one number. After all, a decision-maker finds themselves with a lot more criteria to factor in to their deliberations if they are presented with a range of future possibilities, than if they are supplied with a ‘best estimate’. Pielke Jr. argues it is the responsibility of experts to resist decision-makers’ calls to make their choices appear simpler than they are in reality. So while many of the headlines have been grabbed by ‘increasing certainty over human-caused climate change’, perhaps decision-makers find themselves with more policy decisions to take today, as the IPCC has kicked away the ‘best estimate’ crutch.
Enter the sceptics
Monitoring social media chatter from the sceptic community this morning, climate sensitivity has been one of the big topics of interest. Marcel Crok notes that the omission of a ‘best estimate’ is a case of “hiding good news from policy makers” based on the presumption that such a number would have fallen along with the lower limit of the sensitivity range. Andrew Montford was very critical of the decision to omit the estimate, calling it ‘disreputable’ in the absence of any discussion of how climate models have been performing against observed temperatures.
@WarrenPearce Without discussion of model vs obs, it represented attempt to "hide the decline". Disreputable.
— Bishop Hill (@aDissentient) September 27, 2013
Montford provides a good summary of his argument on this here: in short, the models are running hotter than reality. Given this argument, it’s perhaps easy to see why sceptics smell a rat on the sudden non-appearance of the best estimate, as lowering the figure cold be interpreted by some as an admission that climate models have not proved up to the task of predicting temperature change accurately.
More certainty about uncertainty?
However, maybe the real story is that the IPCC should never have published such a best estimate in the first place. How much more does one number tell us about what to do (or not do) about climate change, than a range of numbers? And how justifiable is the process of simplification down to that one number? While sceptics may question some of the circumstances around the disappearance of the best estimate, they may welcome it in the long run as an attempt by the scientific community to claim back standards of scientific reporting around uncertainty, and hand back to politicians the decisions about what to do within those parameters.