Although these constraints on the ICARUS program as a part of the USGCRP would be substantial, the benefits would be enormous, including utilization of the existing USGCRP administrative structure and integration with existing USGCRP measurement and modeling capabilities.

INTEGRATION VIA SENSITIVITY/UNCERTAINTY ANALYSES

To define priorities within any research program, we accept and endorse the time-honored rational, quantitative procedure of performing sensitivity analyses and then weighting the results appropriately with feasibility and various other factors (such as cost) imposed by broader systems. However, major difficulties are encountered when sensitivity analysis is applied to climate forcing by aerosols.

To guide decisions associated with potential climate change, sufficiently reliable models of the climate must be developed. To develop reliable climate models, reliable models of climate forcing by airborne particles from future emissions are needed. Therefore, central to the ICARUS program is development of the understanding of aerosol forcing, as embodied in climate models:

  • to describe, realistically, existing concentrations and properties of aerosol particles throughout the global atmosphere, thereby

  • to provide climate models with needed predictions of current and future anthropogenic aerosol radiative forcings, thereby

  • to meet the prime goals of the ICARUS program and the USGCRP, plus

  • to establish and continuously update rational choices of ICARUS research priorities via sensitivity analyses, and

  • to integrate data, theory, and applications.

AN EXAMPLE OF SENSITIVITY ANALYSIS: DIRECT RADIATIVE FORCING

We present here an example that illustrates sensitivity analysis but not uncertainty analysis (uncertainties are not evaluated) and only for direct radiative forcing by sulfate particles. The flux calculations are based on a column version of the National Center for Atmospheric Research (NCAR) CCM2 radiation model. The model atmosphere employed three layers of cloud, a high cloud layer, a midlayer cloud, and a low cloud level. The cloud amounts were adjusted to yield a top of atmosphere planetary albedo of 0.3. The lowest cloud layer is located at 800 millibars (mb). The below-cloud calculation assumed that the aerosol layer was completely below this



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