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Scenarios Based on Outputs from Climate Models

Climate models at different spatial scales and levels of complexity provide the major source of information for constructing scenarios. GCMs and a hierarchy of simple models produce information at the global scale. At the regional scale there are several methods for obtaining sub-GCM grid scale information. The following topics are discussed in this section:

 

Scenarios from General Circulation Models

The most common method of developing climate scenarios for quantitative impact assessments is to use results from GCM experiments. GCMs are the most advanced tools available for simulating the response of the global climate system to changing atmospheric composition.

The evolving (transient) pattern of climate response to gradual changes in atmospheric composition was introduced into climate scenarios using outputs from coupled AOGCMs in the early 1990s. Recent AOGCM simulations begin by modeling historical forcing by greenhouse gases and aerosols from the late 19th or early 20th century onwards. Climate scenarios based on these simulations are being increasingly adopted in impact studies along with scenarios based on ensemble simulations and scenarios accounting for multi-decadal natural climatic variability from long AOGCM control simulations.

Several limitations restrict the usefulness of AOGCM outputs for impact assessment:

  1. large resources are required to undertake GCM simulations and store their outputs, restricting the range of experiments that can be conducted (e.g., the range of radiative forcings assumed);
  2. their spatial resolution is coarse compared to the scale of many impact assessments;
  3. it is difficult to distinguish an anthropogenic signal from the noise of natural internal model variability; and
  4. climate sensitivity differs across models.

 

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