Climate sensitivity? What climate sensitivity?

After looking at the attribution certainty differences between AR4 and AR5, this time the differences of the equilibrium climate sensitivity. This is how AR5 defines it:

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.

Basically, the warming we would expect for a doubling of CO2, in most cases determined as doubling of pre-industrial concentration of 280 ppm to 560 ppm (when we continue emissions like this, we will maybe get there, dependent on how much fossil fuels stay available). This is how they were assessed in both reports:

AR4 (SPM p. 12)
Progress since the TAR enables an assessment that climate sensitivity is likely to be in the range of 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values.

AR5 (SPM p. 11)
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).

That’s a step back in sensitivity and no mention anymore of the best estimate (3 °C in AR4). Maybe it is still too high. The pause tells us that something doesn’t add up: the climate sensitivity could be even smaller than presented and/or there are other forces or feedbacks that are not represented in the models. Therefor the big surprise about the declaration of “even more certainty” in AR5, despite the known discrepancy between the observations and the outcome of the climate models.

It is strange that the certainty range is getting larger. The lower limit is 1.5 °C, but the higher limit stays at 4.5 °C and a best estimate can not be given anymore. Really? After 6 years the settled science is even less clear about sensitivity?

Nevertheless, if climate sensitivity could be lower than expected, then less warming could be expected. That’s good news. But no mention of the lowered limit in the press release. To be fair, it is mentioned in the Summary for the Policymakers, also that it is lower than the value in AR4, but because the media doesn’t read the Summary report, it is not communicated to the public. It almost didn’t get in the Summary report. According to the minutes of the IPCC plenary session:

Quantification of Climate System Responses: On equilibrium climate sensitivity, several delegations, including Australia, the Netherlands and others, noted that the message that the lower limit of the assessed “likely” range of climate sensitivity is less than the 2°C in the AR4 can be confusing to policy makers and suggested noting it is the same as in previous assessments. The CLAs explained that comparison to each of the previous IPCC assessments would be difficult, and new language was developed adding that the upper limit of the assessed range is the same as in AR4.

Some representatives in the plenary session found the lower sensitivity to be “confusing” and suggested to note it is the same as previous assessment (it is not). I can imagine vividly why this lower value would be confusing for policymakers.

Finally, when looking at how this lower sensitivity limit is communicated to the public…Nothing…Nada…Niente… only skeptical sites/blogs seem to report on it.


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