One of the most surprising things in the just released SPM is the absence of a best estimate for climate sensitivity. The SPM now says this:

The equilibrium climate sensitivity quantifies the response of the climate system to constant radiative forcing on multi-century time scales. It is defined as the change in global mean surface temperature at equilibrium that is caused by a doubling of the atmospheric CO2 concentration. Equilibrium climate sensitivity is likely in the range 1.5°C to 4.5°C (high confidence), extremely unlikely less than 1°C (high confidence), and very unlikely greater than 6°C (medium confidence)16. The lower temperature limit of the assessed likely range is thus less than the 2°C in the AR4, but the upper limit is the same. This assessment reflects improved understanding, the extended temperature record in the atmosphere and ocean, and new estimates of radiative forcing. {TFE6.1, Figure 1; Box 12.2}

16 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.

So from a footnote we have to learn that no best estimate “can now be given because of a lack of agreement on values across assessed lines of evidence and studies”. How strange this is. Climate sensitivity is one of the most important parameters. It determines largely how much warming we can expect. If there is lack of agreement between different methods/studies, we want to know all about it. However, apart from this footnote, the SPM is silent about it. Hopefully the full report, which will be released on Monday, will give all the details.

Tradition
Ever since the Charney report in 1979, national and international reports about climate have given a best estimate for climate sensitivity. So to speak in IPCC terminology, it is unprecedented not to give one.

In most reports of the past several decades the best estimate was 3°C with a range of 1.5 to 4.5°C. Only in the first two IPCC reports the best estimate was 2.5°C. But a best estimate was always given. What made it so much more difficult this time that IPCC felt it was impossible to give one?

Good news
Here is my best guess. It is true that more methods are now available to estimate climate sensitivity. Traditionally the models (GCMs) had most weight in the value of climate sensitivity. However since early this century it has become possible to estimate climate sensitivity from observations as well. The method is very simple: one only needs the total amount of forcing increase, ocean heat energy increase and temperature increase between two periods. Several papers/letters have done this in the last year (Aldrin et al, Ring et al, Lewis, Otto et al) and they all conclude that climate sensitivity is somewhere between 1.5°C and 2°C.

This is really good news! Our climate definitely seems to be less sensitive than we thought for a long time. The range for climate sensitivity based on these observations is also much more constrained, somewhere between 1.2°C and 2.6°C. Note that the lower bound of this range falls outside the likely range that the SPM now gives of 1.5°C to 4.5°C.

The main reason that climate sensitivity has come down so dramatically is not the slowdown. It’s the fact that estimates for aerosol cooling have come down considerably since AR4 and as a result the total increase in anthropogenic forcing has increased considerably in only a few years. This was mentioned in the SPM:

The total anthropogenic RF for 2011 relative to 1750 is 2.29 [1.13 to 3.33] W m−2 (see Figure SPM.5), and it has increased more rapidly since 1970 than during prior decades. The total anthropogenic RF best estimate for 2011 is 43% higher than that reported in AR4 for the year 2005. This is caused by a combination of continued growth in most greenhouse gas concentrations and improved estimates of RF by aerosols indicating a weaker net cooling effect (negative RF). {8.5}

Now with considerably more forcing and no temperature increase, climate sensitivity has to come down! There is no other possibility. It is the only logical consequence. Unless…much more heat went into the ocean. Now the recent observationally based estimates for climate sensitivity take this into account. The increase of heat in the ocean is just by far not enough to compensate for the huge increase in the forcing.

Now why didn’t IPCC bring us this good news?
IPCC reports rely for a large part on the climate models. All the claims about the future are fully or partly based on the GCMs. These models are also used to determine climate sensitivity. Now here comes the problem. The climate sensitivity of the CMIP5 models (used for AR5) is on average 3°C. Real world observations however indicate climate sensitivity is much lower, between 1.5°C and 2°C. Admitting that these observationally based estimates are more reliable, would be like admitting that the models are less reliable. This would then question all the projections that are mentioned in AR5.

That models are (probably) too sensitive for greenhouse gases is becoming clear already when we are looking at the recent past. Not at the slowdown of 15 years, no a full climatic period of 34 years. This was very well explained in Steve McIntyre’s latest blog article two minutes to midnight where he showed that over the period 1979-2013 models on average warm up 50% faster than the real climate.

The IPCC had an impossible task this week. Their models were already ‘disproven’ by the real climate before the report came out. They have been unable to explain why models warmed up 50% more than the observations show. And they couldn’t be fully transparant about this because then the report would be regarded as outdated at the moment of publication, including al their projections.

So they did the smartest thing they could do given the circumstances, avoid this very difficult issue by not giving a best estimate for climate sensitivity.

At the end of the press conference when co-chair Thomas Stocker was asked why they mentioned the 15 year slowdown at all if they think it was unimportant, he answered that IPCC wants to deal with difficult questions. However they avoided a much more crucial and difficult issue, climate sensitivity, and by doing this they left the policy makers in the dark.