Saturday, December 02, 2006

Real Climate Audits.....

The climate community, contrary to assertions by the usual suspects, has always engaged in testing of methods and measurements. Eli is most familiar with measurement intercomparisons, where several groups bring their instrumentation to a common site and test each other against ambient conditions and reference samples but there are also intercomparisons of modeling methods and more. Best practice and method recommendations emerge. It is pretty easy to find these, for example, just search under intercomparison in the AGU archive. Since these are critical intercomparisons, they always mention what has to be improved, what should be abandoned and where a lot more work is needed. That does not mean that the tested state of the art is wrong, just that it is not perfect.

Weakening of the thermohaline circulation has been much in the news recently, but over a year ago, AGU published an 18 author paper:

GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L12703, doi:10.1029/2005GL023209, 2005

A model intercomparison of changes in the Atlantic thermohaline circulation in response to increasing atmospheric CO2 concentration

As part of the Coupled Model Intercomparison Project, integrations with a common design have been undertaken with eleven different climate models to compare the response of the Atlantic thermohaline circulation (THC) to time-dependent climate change caused by increasing atmospheric CO2 concentration. Over 140 years, during which the CO2 concentration quadruples, the circulation strength declines gradually in all models, by between 10 and 50%. No model shows a rapid or complete collapse, despite the fairly rapid increase and high final concentration of CO2. The models having the strongest overturning in the control climate tend to show the largest THC reductions. In all models, the THC weakening is caused more by changes in surface heat flux than by changes in surface water flux. No model shows a cooling anywhere, because the greenhouse warming is dominant.
What brings this up is a reminder in EOS of the EPICA Challenge. EPICA is the European Project for Ice Core Analysis, which, at the time the challenge was issued were beavering (rabbits are slower, calmer and altogether cuter, ask Ms. Rabett) away on a long Dome C ice core that would extend back 740,000 years:
The prospect of a substantially longer record poses some fascinating new questions:What will be the CO2 and CH4 concentrations in the weak interglacials of the earlier period? Will CO2 still be at the standard “interglacial level”of 280 ppmv, or will it scale with Antarctic temperature and stand at about 240 ppmv (and similarly for methane)?......

What do the modeling community, and others who are putting forward ideas, believe we will see, and why? The purpose of the “EPICA challenge” is not to find a right answer, and declare a winner; indeed with our present knowledge it is more than likely that someone can get the right answer for the wrong reason.Rather the idea is to provide an impetus for modelers to expose the assumptions and arguments behind their predictions, leading to a more open discussion once the data are revealed.

We therefore invite anyone interested in doing so to predict what carbon dioxide and methane will look like back to at least 800 kyr B.P.,and to explain their reasoning,whether the result comes from a simple concept or from a full model run.Time is short, because it is possible that the first outline data sets will be available for presentation at the AGU Fall Meeting (13–17 December 2004).The data groups involved will endeavor to keep the data under wraps until then. Some modeling groups may like to submit their ideas in full to journals or at meetings. However, the PAGES International Project Office has also offered to collate and summarize responses that are received there before 15 November.
And the results of the EPICA Challenge is....the usual spaghetti graph, but you can find the details here (RTFR) in the final EOS write up and a poster.

OK, this record only goes to 650KY, anyone know where the rest is....modelers would like to know.

1 comment:

paul said...

If carbon isotope half life measurements are only useful back to around 60,000 years, how can these figures be interpolated to 650,000 years?
Are deuterium temperature levels an accurate measure of CO2 levels? How do they know?

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