Written by Climate Skeptic
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Backcasting with Computer Climate Models
I found the chart below in the chapter Global Climate Change of the NOAA/NASA CCSP climate change report. (I discuss this report more here). I thought it was illustrative of some interesting issues:
The Perfect Backcast
What they are doing is what I call "backcasting," that is, taking a predictive model and running it backwards to see how well it preforms against historical data. This is a perfectly normal thing to do.
And wow, what a fit. I don't have the data to do any statistical tests, but just by eye, the red model output line does an amazing job at predicting history. I have done a lot of modeling and forecasting in my life. However, I have never, ever backcast any model and gotten results this good. I mean it is absolutely amazing.
Of course, one can come up with many models that backcast perfectly but have zero predictive power.
A recent item of this ilk maintains that the results of the last game played at home by the NFL's Washington Redskins (a football team based in the national capital, Washington, D.C.) before the U.S. presidential elections has accurately foretold the winner of the last fifteen of those political contests, going back to 1944. If the Redskins win their last home game before the election, the party that occupies the White House continues to hold it; if the Redskins lose that last home game, the challenging party's candidate unseats the incumbent president. While we don't presume there is anything more than a random correlation between these factors, it is the case that the pattern held true even longer than claimed, stretching back over seventeen presidential elections since 1936.
And in fact, our confidence in the climate models based on their near-perfect back-casting should be tempered by the fact that when the models first were run backwards, they were terrible at predicting history. Only a sustained effort to tweak and adjust and plug them has resulted in this tight fit (we will return to the subject of plugging in a minute).
In fact, it is fairly easy to demonstrate that the models are far better at predicting history than they are at predicting the future. Like the Washington Redskins algorithm, which failed in 2004 after backcasting so well, climate models have done a terrible job in predicting the first 10-20 years of the future. This is the reason that neither this nor any other global warming alarmist report every shows a chart grading how model forecasts have performed against actual data: Because their record has been terrible. After all, we have climate model forecasts data all the way back from the late 1980's -- surely 20+ years is enough to get a test of their performance.
Below is the model forecasts James Hansen, whose fingerprints are all over this report, used before Congress in 1988 (in yellow, orange, and red), with a comparison to the actual temperature record (in blue). (source)
Here is the detail from the right side:
CONTINUE TO FULL ARTICLE COMPLETE WITH GRAPHS AND ILLUSTRATIONS