Why global climate models do not give a realistic description of the local climate


RealClimate – May 27, 2007

I have written about this excellent site often (here and here and here and here and here) but I really must encourage all of my regular readers to read this article on climate models.  As you know, I regularly have called for more effort to be done in this area of computer technology as I honestly believe that the climate scientists need a concerted effort for a rapid increase in capability in handling massive amounts of numbers and equations in parallel.

This article describes how many climate models work and why they start to have problems when charged with describing the local climate of a region or city. At issue here is that averages are not truly the problem with global warming. The impact of warming one or two degrees C in St. Louis is not nearly the impact as the warming of 1 or 2 degrees in Greenland or the Antarctica. In fact, some would argue that if St. Louis was a little warmer it could benefit mankind (longer growing season, milder winters, etc.).  However that same change in Greenland could result in more glacial ice melting, more rain/less snow, etc. which might have a devastating impact on the surrounding ocean currents.

There is also the issue of causality that would be better understood if the models were better at the regional level. What is the impact of CO2 and methane concentrations? If our models understood the climate in the Northeast part of the country without the addition of human induced methane and carbon dioxide (and we were confident in the model’s analysis) we could measure the difference between the natural and the human influenced.

Yet, whereas the global climate models (GCMs) tend to describe the global climate statistics reasonably well, they do not provide a representative description of the local climate. Regional climate models (RCMs) do a better job at representing climate on a smaller scale, but their spatial resolution is still fairly coarse compared to how the local climate may vary spatially in regions with complex terrain.

There are several reasons why GCMs do not provide a representative description of the local climate (i.e. exactly where I live). For one, the grid mesh, on which they compute the physical quantities relevant for the climate, is too coarse (typically 200km) to capture the local aspects.

Climate models need boundary conditions describing the surface conditions (e.g. energy and moisture fluxes) in order to yield a realistic representation of the climate system. Often simple parameterisation schemes are employed to provide a reasonable description, but these do not capture the detailed variations associated with small spatial scales.

There is much more to read in the original article, including some excellent visuals to explain the concepts.  Please click through to the article here.

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