Sunday, 26 February 2012

Twenty-three climate model comparison

The blog has a thread posted by Barry Bickmore related to an article which appeared in the Wall Street Journal (WSJ). The original article was written by a group of eminent scientists with little specific expertise in the science of climate change. To summarise in over-simplistic terms they said that they could accept Anthropogenic Global Warming (AGW) but not Catastrophic Anthropogenic Global Warming (CAGW).  The WSJ published a response by a group of equally eminent climate scientists who supported CAGW.

The posting of Barry Bickmore looks at some of the claims in more detail than would be possible in a newspaper column. One point the first group of scientists had made was that climate models had not captured the recent stasis in temperatures. Bickmore’s response was that “individual models actually predict that the temperature will go up and down for a few years at a time, but the long-term slope (30 years or more) will be about what those straight lines say.” Below we show the annual temperatures expressed as degrees Celsius for 23 models (downloaded from the ClimateExplorer site) the maximum, minimum and average for these 23 models and the temperature from the HadCRU3 data series. As the HadCRU3 series only gives temperature anomalies we have adjusted it to give the same mean as the models for the period of overlap.

First of all it can be seen the chart supports Bickmore’s point. The average of the models follows the general trend of temperature from 1900 to the present and many of the individual models have periods with little or no increase even after the effect of CO2 kicked in from the mid-1970s onward. The current period may have some similarities to the period 1910 to 1970 when a strong Atlantic Multidecadal Oscillation led to an enhanced temperature increase for the first half of that period and to period of stasis for the second half. As the period of stasis, but with wide variations, lasted for about a quarter of a century we may have another decade or so of level temperatures with it still being possible to defend the models. In that period it is likely that models will improve and that the models themselves will be better able to represent such periods.

What is surprising is the difference between the models. The average temperature of the ‘hottest’ model is 15.4 °C and of the ‘coolest’ is 12.4 °C. This is, if my maths is correct, equivalent to a difference of 15.92 W/m², an order of magnitude larger than typical anthropogenic forcing estimates.

Although Bickmore doesn’t mention temperature we have produced a similar chart for precipitation.

This also shows a large difference between the models but, unlike temperature simulation, little evidence that the underlying trend has been captured. In this case the ‘wettest’ model has an average precipitation of 1184 mm/year and the ‘driest’ a precipitation of 918 mm/year. This is equivalent to 19.0 W/m², again large compared to anthropogenic forcing.


Donald Harris said...

You suggest that the currrent period might be somewhat similar to 1910 - 1970. There was then a strong Atlantic Multidecadal Oscillation (AMO) accompanied by a peak in temperatures in 1940: so that (given that the present AMO seems to be near its peak) we might expect another decade or so of level temperatures.

It seems to me quite reasonable to consider the possibility that the pattern is repeating itself and that there could be a causal connection between multi-decadal changes in AMO and global temperatures.

I suggest it would be interesting to compare trends in global temperatures, AMO and any other oscillation (e.g PDO) which might conceivably have a causal relationship with temperature. I think a moving average of at least 5 years/30 months would be necessary to eliminate the short term fluctuauions.

Administrator said...

The point you make is valid and I am actually trying to work through some of the implication of the degree to which temperature responds to pseudo-oscillations.