Friday, November 25, 2011

Schmittner and Climate Sensitivity: no cheer for the sceptics

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.

More here on Real Climate


Steve said...

Richard, trouble is, only very recently you were saying:
"Climate science has applied a great deal of effort to the question of CS. Multiple lines of evidence - from models, observations of known temperature changes, and proxy records from ice cores - show remarkable convergence towards a value of 3ºC (+/- 1.5º) C for a doubling of CO2 in the atmosphere. In particular, it looks as if a value less than 1.5ºC is very unlikely."

You have also mentioned the 3 deg C (+/- 1.5 deg) several times in other posts. Now someone is saying it CS more like 1.7 to 2.6 deg C. Another slight tweaking and you're down into your 'very unlikely' 1.5 deg C territory. You can see why some people are sceptical...

We should of course still be doing everything we can as individuals (I try but it's hard), and as a nation (we're pretty dismal), to reduce CO2 emissions wherever possible just in case you're both wrong and the CS figure is actually somewhat higher - which would also not surprise me all that much!

Steve (I am the anonymous that posted the original link but I pressed send before putting my id in, sorry).

DocRichard said...

All scientific figures come with a range representing the margin of error.

The IPCC figure is 1.5 - 4.5, with 3 the most likely.
Schmittner's figure is 1.7 - 2.6. So his range is fully within the IPCC range.

His range is smaller than the others, but as I wrote, given the limitations of knowing fully what was going on in the climate 21000 years ago, this is pretty good concordance, and crucially, it completely falsifies the sceptics' theory.

And yes, we are agreed on the major point, that we all - as individuals, communities, businesses and nations - need to decarbonise our economies. Urgently.