GRID POINT SURFACE AIR-TEMPERATURE CALCULATIONS WITH A FAST TURNAROUND - COMBINING THE RESULTS OF IMAGE AND A GCM

This paper describes a methodology that combines the outputs of (1) th e Integrated Model to Assess the Greenhouse Effect (IMAGE Version 1.0) of the Netherlands National Institute of Public Health and Environmen tal Protection (RIVM) (given a greenhouse gas emission policy, this mo del can estimate the effects such as global mean surface air temperatu re change for a wide variety of policies) and (2) ECHAM-1/LSG, the Glo bal Circulation Model (GCM) of the Max-Planck Institute for Meteorolog y in Hamburg, Germany. The combination enables one to calculate grid p oint surface air temperature changes for different scenarios with a tu rnaround time that is much quicker than that for a GCM. The methodolog y is based upon a geographical pattern of the ratio of grid point temp erature change to global mean values during a certain period of the si mulation, as calculated by ECHAM-1/LSG for the 1990 Scenarios A and D of the Intergovernmental Panel on Climate Change (IPCC). A procedure, based upon signal-to noise ratios in the outputs, enabled us to estima te where we have confidence in the methodology; this is at about 23% t o 83% of the total of 2,048 grid points, depending upon the scenario a nd the decade in the simulation. It was found that the methodology ena bled IMAGE to provide useful estimates of the GCM-predicted grid point temperature changes. These estimates were within 0.5K (0.25K) through out the 100 years of a given simulation for at least 79% (74%) of the grid points where we are confident in applying the methodology. The te mperature ratio pattern from Scenario A enabled IMAGE to provide usefu l estimates of temperature change within 0.5K (0.25K) in Scenario D fo r at least 88% (68%) of the grid points where we have confidence; indi cating that the methodology is transferable to other scenarios. Tests with the Geophysical Fluid Dynamics Laboratory GCM indicated, however, that a temperature ratio pattern may have to be developed for each GC M. The methodology, using a temperature ratio pattern from the 1990 IP CC Scenario A and involving IMAGE, gave gridded surface air temperatur e patterns for the 1992 IPCC radiative-forcing Scenarios C and E and t he RIVM emission Scenario B; none of these scenarios has been simulate d by ECHAM-1/LSG. The simulations reflect the uncertainty range of a f uture warming.