Benchmark forecasts for climate change

Climate is complex, uncertain, and, over horizons that are relevant for policy decisions, varies little. Using evidence-based (scientific) forecasting principles, we determined that for such a situation a naïve “no change” extrapolation method was the appropriate benchmark. We tested this benchmark against global mean temperatures. To be useful to policy makers, a proposed forecasting method would have to provide forecasts that were substantially more accurate than the benchmark. We calculated benchmark forecasts against the UK Met Office Hadley Centre’s annual average thermometer data from 1850 through 2007. The accuracy of forecasts from our naïve model is such that even perfect forecasts would be unlikely to help policy makers. For example, mean absolute errors for 20- and 50-year horizons were 0.18°C and 0.24°C. We nevertheless evaluated the Intergovernmental Panel on Climate Change’s 1992 projected warming rate of 0.03°C-per-year. The small sample of errors from ex ante forecasts for 1992 through 2008 was practically indistinguishable from the naïve benchmark errors. To get a larger sample and evidence on longer horizons we backcast successively from 1974 to 1850. Averaged over all horizons, IPCC errors were more than seven-times greater than errors from the benchmark. Relative errors were larger for longer backcast horizons.