CMIP - History
Global coupled ocean-atmosphere general circulation models (coupled GCMs) that include interactive sea ice simulate the physical climate system, given only a small number of external boundary conditions such as the solar "constant" and atmospheric concentrations of radiatively active gases and aerosols. These models have been employed for decades in theoretical investigations of the mechanisms of climatic changes. In recent years, coupled GCMs have also been used to separate natural variability from anthropogenic effects in the climate record of the 20th century, and to estimate future anthropogenic climate changes including global warming. A number of coupled GCMs have been developed by different research groups. For some time it has been apparent that these models give somewhat contradictory answers to the same questions -- e.g., a range from roughly 1.5 - 4.5°C in the global mean surface air temperature increase due to a doubling of atmospheric carbon dioxide -- due to subtle differences in their assumptions about clouds and other phenomena at scales smaller than the separation of model grid points (Cess et al. 1989; Mitchell et al. 1989).
In 1995 the JSC/CLIVAR Working Group on Coupled Models, part of the World Climate Research Program, established the Coupled Model Intercomparison Project (CMIP; see Meehl et al. 2000). The purpose of CMIP is to provide climate scientists with a database of coupled GCM simulations under standardized boundary conditions. CMIP investigators use the model output to attempt to discover why different models give different output in response to the same input, or (more typically) to simply identify aspects of the simulations in which "consensus" in model predictions or common problematic features exist. CMIP may be regarded as an analog of the Atmospheric Model Intercomparison Program (AMIP; see Gates et al. 1999). In the AMIP simulations, sea ice and sea surface temperature are prescribed to match recent observations, and the atmospheric response to these boundary conditions is studied; in CMIP, the complete physical climate system including the oceans and sea ice adjust to prescribed atmospheric concentrations of CO2.
Details of the CMIP database, together with access information, may be found on the CMIP Web site at http://www-pcmdi.llnl.gov/cmip/diagsub.php. The first phase of CMIP, called CMIP1, collected output from coupled GCM control runs in which CO2, solar brightness and other external climatic forcing is kept constant. (Different CMIP control runs use different values of solar "constant" and CO2 concentration, ranging from 1354 to 1370 W m-2 and 290 to 345 ppm respectively; for details see http://www-pcmdi.llnl.gov/cmip/Table.php.) A subsequent phase, CMIP2, collected output from both model control runs and matching runs in which CO2 increases at the rate of 1% per year. No other anthropogenic climate forcing factors, such as anthropogenic aerosols (which have a net cooling effect), are included. Neither the control runs nor the increasing-CO2 runs in CMIP include natural varations in climate forcing, e.g., from volcanic eruptions or changing solar brightness.
CMIP thus facilitates the study of intrinsic model differences at the price of idealizing the forcing scenario. The rate of radiative forcing increase implied by 1% per year increasing CO2 is nearly a factor of two greater than the actual anthropogenic forcing in recent decades, even if non-CO2 greenhouse gases are added in as part of an "equivalent CO2 forcing" and anthropogenic aerosols are ignored (see, e.g., Figure 3 of Hansen et al. 1997). Thus the CMIP2 increasing-CO2 scenario cannot be considered as realistic for purposes of comparing model-predicted and observed climate changes during the past century. It is also not a good estimate of future anthropogenic climate forcing, except perhaps as an extreme case in which the world accelerates its consumption of fossil fuels while reducing its production of anthropogenic aerosols. Nevertheless, this idealized scenario generates an easily discernible response in all the CMIP models and thus provides the opportunity to compare and possibly explain different responses arising from different model formulations.
see An Overview of Results from the Coupled Model Intercomparison Project (CMIP) by Curt Covey et al. for more details.
Cess, R. D., and Coauthors, 1989: Interpretation of cloud-climate feedback as produced by 14 atmospheric general circulation models. Science, 245, 513-516.
Hansen, J., M. Sato, A. Lacis, and R. Ruedy, 1997: The missing climate forcing. Phil. Trans. R. Soc. Lond. B, 352, 231-240.
Meehl, G. A., G. J. Boer, C. Covey, M. Latif, and R. J. Stouffer, 2000: The Coupled Model Intercomparison Project (CMIP). Bull. Amer. Meteor. Soc., 81, 313-318.
Mitchell, J. F. B., C. A. Senior, and W. J. Ingram, 1989: CO2 and climate: a missing feedback? Nature, 341, 132-134.