A new paper in Science by Schmittner looks at surface temperatures in the last Ice Age 21,000 years ago and has concluded that climate sensitivity (CS) to doubled CO2 is 1.7-2.6*C, with a median of 2.3*C.
Regular readers of the Mabinogogiblog know better than most that climate sensitivity is the key to climate science. There is no reasonable doubt that CO2 is a greenhouse gas, that it has increased due to human activity, and that the increase will commit the planet to around 1*C of warming. This is all textbook physics, and you have to be a fundamentalist of some sort to deny it.
The remaining uncertainty is over Climate Sensitivity - the degree to which the planetary system will warm in response to this 1*C.
I use the Three Sleeping Babies test to illustrate it.
Climate "sceptics" are desperate to get a low figure for CS, because without it their case is without merit. So they may be expected to trumpet Schmittner's work as the final final death blow to the last ever nail in climate sciences coffin, as is their wont.
In fact, it is not, and if anything, it is a further blow for the sceptics, who posit a CS value of about 0.5*C. Schmittner gives a median value of 2.3, with a range of 1.7 - 2.6.
The 1.7* is way beyond the needs of the sceptics, while the 2.6 has a comfortable overlap with the IPCC figure of 2 - 4.5.
Schmittner's work is tight, but there are necessary limitations on study of the planet at a remove of 21,000 years that mean his is not the last word on the subject. He was necessarily not able to study all the factors operating to long ago, and CS in a cold planet may not be the same as in todays warmer planet. And he has not yet been subject to critical scrutiny of his maths &c.
Given that the IPCC figure is confirmed by several independent lines of inquiry, the latest work serves to firm up certainty for CS, and serves also to stuff the climate sceptic position of 0.5*C further into the bin reserved for failed hypotheses.
If only journalists and commentators could get their heads around the scientific method and the issue of CS, we could move forward with protecting our children's lives.
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