Monday, November 12, 2012

Reflections on claims of low climate sensitivity

As a post script to my piece yesterday on empirical studies of climate sensitivity (CS), a few more observations on low CS:

  1. Some results give a value of less than unity. For example, Schwartz gives a CS range of 0.6 - 1.6*C. In the lower section of his probabilities (0.6 - 0.99) this means that he can envisage that heating the planet by 1.2*C in any way will actually cool it. That  needs some explanation and thought. Does cooling the planet likewise result in warming? How does that work? If CS is in the region of unity, there would be very little natural variation in climate, which contradicts the contrarians' constant assertion that recent observed climate change is not due to AGW, but to natural variation.
  2. The low results given by empirical observations depend very much on the time period under consideration. Most current observational studies are necessarily time limited, and therefore will not pick up on slow feedbacks. This might explain why palaeoclimate studies tend to return higher values, since they cover much longer time periods. Incidentally, Bishop Hill tells me that he discounts palaeolimatology because they tend to use computer models.
    The excellent Tamsin Edwards (@flimsin) has a paper on this topic here.
  3. The blanket rejection of computer modelling by contrarians is irrational. Models deserve to be treated with caution, because they are only approximations to reality, but they do have a use. The global climate is a system, and systems need computers to represent their processes. The models are in essence composed of millions of calculations, which used, in the infancy of climatology to be carried out by hand. Would contrarians be happy if climatologists went back to doing each calculation by hand? Yes or no? If no, they must accept models. If yes, they are simply trying to put an end to climatology, in a very similar way to the Jesuit who tried to put an end to astronomy by refusing to look through Galileo's telescope.
  4. The final and most important point is to look at the actual physical reactions in the climate system, which I have set out here. I identify 8 positive feedbacks, and 3 negatives. Simple arithmetical accounting of their values points to a CS in the region of 3.
In conclusion, the contrarians' hypothesis, which is that the impact of man-made CO2 is small, is not borne out by observations.


crandles said...

A climate sensitivity less than unity does not mean heating results in cooling. Sensitivity is the response to doubling CO2 not the response to heating.

The radiative effect of a doubling of CO2 is about 1.1C. If sensitivity was less than that( which it isn't), it would mean that negative feedbacks were stronger than positive feedbacks. Maybe that was what you were trying to convey?

DocRichard said...

Thanks Crandles. I stand corrected. I think.

I understand climate sensitivity to be the response of the total climate to any given forcing. Doubled CO2 is accepted as the standard value because that is what we are all interested in.

But agreed, the claim by contrarians that sensitivity is, say, 0.5C is an expression of their belief that negative feedbacks exceed positive. Which is of course the case in some states of the planet's Ice Age cycles, because the planet's temperature has always topped out at about 18C (so far).

My machine is at the disposal of ClimatePrediction, but it has not yet been given any work.

crandles said...

To compare different forcings, they are usually converted to W/m^2 units. Climate sensitivity is specifically to a doubling of CO2 (except when it is used in a different way).

Certainly the sensitivity does vary depending on state. For example if everywhere had over 100m of ice then there would be no ice albedo feedback. Similarly if ice reached mid latitudes so that there was a few tens of millions of km^2 with seasonal ice then it would be higher than now because of the greater seasonal area and greater effect of albedo at lower latitudes.

However I am not sure it gets down to 0.5C in any state I know about. Not that I am any expert.

The direct radiative effect is about 1.1C and water is a more important greenhouse gas that can double that to 2.2C but perhaps that doubling factor varies with the state. Even if most water freezes out of the atmosphere, there will still be some (partial pressures even from ice etc). Any areas of seasonal ice cover will likely increase that 2.2C further but as noted above there are some states when this doesn't apply. I think other feedbacks tend to be small adjustments.

>"because the planet's temperature has always topped out at about 18C (so far)."

Isn't that because the GHG levels didn't go higher rather than the sensitivity being particularly low?

Yes contrarians want to claim the sensitivity is low. The sort of thing seen is basically a handwaving claim that some of the temperature claim is natural ie down to cycles.

In reality cycles require forcings to make them happen. Scientists have a pretty good handle on the main forcings and they add up to produce reasonable reconstructions. Where there are differences, they are often resolved in the way the models claim with the data having bias not the model being wrong. This situation is extremely remote in likelihood if there are large unknown forcings.

This is all just my impression as an amateur interested in climate science.

Sorry about the lack of CPDN work, I know there are a few problems at the moment but I am a little out of touch.

crandles said...

Perhaps these links were also needed

DocRichard said...

Thanks Crandles.
My main post on CS is here:
I'd be grateful for any improvements you may be able to suggest. We are both amateurs, but I think we have an important role in conveying the science to people who are interested but not as obsessed as ourselves.

I am preparing a post on model projections and solar cycles. It is interesting that the maxima achieved in the Ice Age cycles are fairly constant. I agree, maybe with more GHGs it will be larger.