Guest Post By John R. Christy of the University of Alabama at Huntsville
The three warm-color time series are taken from Hansen’s published testimony in June 1988 in which global surface air temperatures were projected under three scenarios by his global climate model. The red curve follows a scenario (A) of continued emissions growth based on the previous 20 years before 1988 (which turned out to be an underestimate of actual emissions growth.) The orange represents a scenario (B) of fixed emissions at the rate achieved in the 1980s. The yellow curve portrays a scenario (C) in which “a drastic reduction” in GHG emissions is assumed for 1990-2000. The observations are global tropospheric temperatures adjusted to mimic the magnitude of surface temperature variability and trends according to published climate model simulations (i.e. a reduction in satellite anomalies by 0.83.)
After tying all time series to a 1979-83 reference mean, one can see the significant divergence in the results. (Notes: 1. observed 2010 is Jan-Jul only; 2.) tropospheric temperatures are used as the comparison metric due to many uncertainties and biases in the surface temperature record, i.e. Klotzbach et al. 2009, 2010 ; 3.) both models and observations included the 1982 eruption of El Chichon while B and C scenarios included a volcano in the mid 1990s – not too different from Mt. Pinatubo.)
The result suggests the old NASA GCM was considerably more sensitive to GHGs than is the real atmosphere since (a) the model was forced with lower GHG concentrations than actually occurred and (b) still gave a result that was significantly warmer than observations.