ES-01. A simple economic forecasting approach outperforms the IMF at global and regional scales

Abstract
Authoritative economic forecasts, such as by the International Monetary Fund (IMF), have historically tended to be biased towards over-projecting growth and under-projecting inequality. These forecasts form the basis for projections of other societally important quantities such as greenhouse gas emissions and poverty rates. We develop a simple differential-equation-based approach to forecasting global and regional economic growth, and show that its early-21st-century forecasts would have been more precise and more accurate than the IMF's, especially on longer forecast horizons. Looking to 2100, our approach projects per-capita GDP and poor-to-rich-country income convergence near the lower ends of ranges of IPCC scenarios and expert opinion. All else equal, our projections would imply a world in 2100 with less severe climate change, but more poverty and inequality--and thus potentially less resilience and adaptive capacity--than some other forecasts might suggest.