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Calibration of the MAGICC model; and projection of the Glacier and Small Ice Sheet (GSIC) component of sea level rise

PI: Tom M.L. Wigley
Institution: National Center for Atmospheric Research
Additional Investigators: Sarah Raper, Ben Santer, Doug Nychka
In the IPCC TAR, the Wigley and Raper coupled gas-cycle/energy-balance climate model (MAGICC) was used to produce the primary projections of future global-mean temperature and sea level change and to assess the uncertainties in these projections. To do this, MAGICC was first ‘calibrated’ by Sarah Raper against a range of different AOGCMs using data from 1% compound CO2 increase experiments archived in the CMIP data base. This
involved tuning the main parameters of MAGICC (climate sensitivity, effective oceanic diffusivity, etc.) against AOGCM results for different variables such as global-mean temperature, land-ocean temperature differentials, etc. So-tuned, MAGICC was able to emulate the global-mean temperature and oceanic thermal expansion results from individual AOGCMs with high accuracy, justifying its use to expand these results to cover emissions scenarios not considered directly by the AOGCMs.

The first part of this project will use the new AR4 AOGCM results in the same way so that these new models may also be emulated with MAGICC. This is directed towards the possible use of MAGICC to produce a wider spectrum of global-mean temperature (and sea level) projections for AR4 than would otherwise be available. This work will be carried out by Wigley and Raper. Santer is involved to assist in accessing appropriate data from the AR4 data files. Nychka is involved to assist in applying more rigorous statistical tools than previously to the AOGCM/MAGICC comparisons that underlie the calibration exercise. All investigators will be involved in analysis and interpretation of the results. The plan is to perform the calibrations using 1% CO2 experiment results (accounting for control-run drift if necessary) and to test these calibrations with data from other forcing experiments.

For this work to be carried out effectively we require, ideally, annual-mean, gridpoint data from (at least) 1% CO2 runs and the parallel control runs for: reference height temperature, sea surface temperatures, and ocean temperatures through the full ocean column. The latter are used to determine net changes in ocean heat content, which is the most accurate way to quantify heat flux into the ocean. We also require oceanic thermal expansion data, some indicator of changes in the thermohaline circulation, and the top of the troposphere radiative forcing for 2xCO2 (after stratospheric equilibration). For testing against other forcing experiments we require estimates of the total forcing time series for these experiments and the breakdown of this forcing between land and ocean in each hemisphere.

The second part of this project involves the use of a new GSIC model developed by Sarah Raper to quantify future changes in this component of the ice-melt contribution to sea level rise. The use of this model will be a major conceptual advance on the way GSICs were modeled in the TAR – where an ad hoc correction factor produced an unrealistic upper bound to GSIC melt. These calculations will be carried out on a gridpoint basis over the globe (for regions where GSICs exist) and will use the temperature data requested for the MAGICC calibration exercise. Santer will assist in data extraction, and Nychka will assist in the application of rigorous statistical methods.
  • Raper, S.C.B. and R.J. Braithwaite, 2006: Low sea level rise projections from mountain glaciers and icecaps under global warming. Nature, 439, 311-313, doi:10.1038. Abstract. Edit.

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