Validity of Climate Models

According to Oreskes et al (Science 263, 641-645), climate models are so large and describe so many different processes that it is impossible to independently test and certify that all components of the models are correctly representing physical processes. However, present and past (paleoclimates, to be discussed later) climates do offer several opportunities to validate model performance. From these comparisons, we have the following conclusions: Validation of global climate models (Figure 5) (IPCC 1992 and personal experience)

Models show considerable skill in reproducing large-scale maps of surface pressure, temperature, wind, and precipitation in both summer and winter. On regional scales (sub-continental) all models show significant departures from observations for both temperature and precipitation. Soil moisture comparisons are limited by lack of data, but results (where data are available) are in qualitative agreement. Snow cover is reasonably well simulated, except where temperatures are too high (e.g., at high latitudes in the Northern Hemisphere). Radiative fluxes at the top of the atmosphere are simulated well in some models. Daily and interannual variability is mixed, with most models giving good results in some locations but not good in others. Model response is good for slow changes in forcing such as El Nino, Mount Pinatubo, wet and dry periods in the Sahel, and select periods from the last 18,000 years. Ocean models reproduce large-scale features fairly well. Coupled atmosphere-ocean models do reasonably well in simulations of the last ice age.

It can be concluded that models have enough skill in simulating known climate features that they can be useful tools in trying to estimate the climatic impacts that are likely to occur from the changes in atmospheric chemistry that we discussed earlier in the semester. Recall that we concluded the section on radiative forcing with four scenarios of possible radiative forcing. There are two possible ways the global models can be used to estimate future climatic impacts. The most accurate procedure is to simulate the climate, day-by-day, year-by-year using the increases in radiative forcing previously discussed from the time of the Industrial Revolution until the year 2100 or some other point in the future. Experiments of this computing magnitude are now being done by a limited number of research laboratories having very large computers. Results of these simulations, called transient climate simulations because greenhouse gas concentrations are changing (increasing), will be described in Unit 2-4.

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