Climate Model Evaluation

Over the last decade climate model evaluation became an activity of globally co-ordinated research programmes especially the World Climate Research Programme (WCRP). A thorough evaluation helps to understand deficiencies of such coupled atmosphere/ocean/land models and thus shapes their suitability for answers to certain scientific questions.

The following steps of climate model evaluation should be successfully done before the model can be used for projections of future climate that goes well beyond climate states known from the instrumental period.

1. Reproduction of present day climate including observed variability and thus extremes.

2. Reproduction of the climate of the 20th century for which radiative forcing and global climate have been derived or measured.

3. Reproduction of the last millennium in which forcing by volcanoes and solar flux density variations could be reconstructed from ice cores, lake sediments, tree-rings, corals, etc. at least for the northern hemisphere.

4. Reproduction of an abrupt climate change event in recent climate history using palaeoclimatic evidence in so-called proxy data.

While models of all major climate research centres have successfully passed the first two tests only few have passed the third and the fourth has been attempted only with so-called Earth system models of intermediate complexity (EMICs) that strongly simplify modules for the atmosphere, the ocean and the vegetated land surface.

Figure 3.3 Modelled (red) and measured (black) global mean near surface air temperature since 1860. Using the interactive aerosol particle module the green curve results. Because of internal variability on inter-annual and decadal time scales the curves must not coincide but should show similar long-term trends (MPI, 2006b)

Figure 3.3 Modelled (red) and measured (black) global mean near surface air temperature since 1860. Using the interactive aerosol particle module the green curve results. Because of internal variability on inter-annual and decadal time scales the curves must not coincide but should show similar long-term trends (MPI, 2006b)

As an example for model validation the global mean temperature change since 1860 is shown as observed and modelled in figure 3.3 (MPI-M, 2006), whereby the model input also contained natural climate change factors like major volcanic eruptions and changes of solar irradiance at the top of the atmosphere. When the interactive aerosol module has been used (green curve) the temperature decrease after the major volcanic eruption of Krakatao in 1883 became too strong, probably a consequence of the insufficient knowledge about emitted SO2.

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