The Global Warming Challenge

Evidence-based forecasting for climate change

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Armstrong presents testimony at the US Senate Committee on Environment and Public Works

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On January 30, 2008, Scott Armstrong gave a talk presenting his findings to the US Senate Committee on Environment and Public Works’ “Examining Threats and Protections for the Polar Bear”. Click here for full text of the talk. The following are selected excerpts. Full text is available of the paper “Polar Bear Population Forecasts: A Public Policy Forecasting Audit” by Armstrong, Green, and Soon.

We conducted forecasting audits of two of the nine administrative reports that were prepared in 2007 to “…Support U.S. Fish and Wildlife Service Polar Bear Listing Decision.” We selected the reports Amstrup et al. and Hunter et al. as they appeared to be the primary forecasting documents. Our concern was to establish whether the reports’ forecasts of the polar bear population over the balance of the 21st Century were the product of scientific procedures.

We found that the authors of both reports made complex sets of assumptions in order to derive their polar bear population forecasts. Critically, the authors assumed on the basis of general circulation model (GCM) forecasts that summer sea ice will diminish in the Arctic over the rest of the 21st Century. Green and Armstrong (2007) audited the procedures used by GCM climate modelers and found that their forecasts were not scientific. Forecasting sea ice conditions adds an additional level of complexity to the problem and the GCMs fail to reproduce actual sea ice conditions even when historical periods are simulated.

Amstrup et al. and Hunter et al. based their forecasts heavily on their judgments (unaided by forecasting principles) of what will happen over the course of the 21st Century. (I realize that the authors used some elaborate mathematical procedures, but the point remains that the procedures did not amount to forecasting methods that had been subject to rigorous scientific testing). Relying on unaided judgment, while appropriate in some circumstances, is at odds with research findings when the situation is complex and uncertain, as it surely is in the case of long term forecasting of the polar bear population. In such situations, experts who are unaided by scientific forecasting methods are unable to make useful predictions. This finding is astonishing to many people.

Written by climatebet

January 31st, 2008 at 3:53 pm

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