Old Rocks
Diamond Member
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- #21
Hansen's prior predictions have been pretty accurate, much more so than his critics, many of whom were predicting a cooling for right now. Now those same critics lie about what they said only a year ago. I guess the 'coming ice age' came and went.
Who are the experts that you are talking about?
Hansen is the guy who is held up as the Lord High Guru of Climate Science. He is wrong and has been wrong for years. Every time he revises his prediction of warming, the extent of the wrming is lessened.
He missed by 300% in his 1988 prediction. How is that "pretty accurate"?
Hansen missed by 10%, and that is pretty damned accurate considering that the science was in it's infancy at that time.
RealClimate: Hansen’s 1988 projections
So which forcing scenario came closest to the real world? Given that were mainly looking at the global mean surface temperature anomaly, the most appropriate comparison is for the net forcings for each scenario. This can be compared with the net forcings that we currently use in our 20th Century simulations based on the best estimates and observations of what actually happened (through to 2003). There is a minor technical detail which has to do with the efficacies of various forcings our current forcing estimates are weighted by the efficacies calculated in the GCM and reported here. These weight CH4, N2O and CFCs a little higher (factors of 1.1, 1.04 and 1.32, respectively) than the raw IPCC (2001) estimate would give.
The results are shown in the figure. I have deliberately not included the volcanic forcing in either the observed or projected values since that is a random element scenarios B and C didnt do badly since Pinatubo went off in 1991, rather than the assumed 1995 but getting volcanic eruptions right is not the main point here. I show three variations of the observed forcings the first which includes all the forcings (except volcanic) i.e. including solar, aerosol effects, ozone and the like, many aspects of which were not as clearly understood in 1984. For comparison, I also show the forcings without solar effects (to demonstrate the relatively unimportant role solar plays on these timescales), and one which just includes the forcing from the well-mixed greenhouse gases. The last is probably the best one to compare to the scenarios, since they only consisted of projections of the WM-GHGs. All of the forcing data has been offset to have a 1984 start point.
Regardless of which variation one chooses, the scenario closest to the observations is clearly Scenario B. The difference in scenario B compared to any of the variations is around 0.1 W/m2 around a 10% overestimate (compared to > 50% overestimate for scenario A, and a > 25% underestimate for scenario C). The overestimate in B compared to the best estimate of the total forcings is more like 5%. Given the uncertainties in the observed forcings, this is about as good as can be reasonably expected. As an aside, the match without including the efficacy factors is even better.