Documents, Reanalysis, and Global Circulation Models: A New Method for Reconstructing Historical Climate Focusing on Present-day Inland Tanzania, 1856–1890

Abstract. This article proposes a novel methodology for reconstructing past climatic conditions in regions and time-periods for which there is limited evidence from documentary and natural proxy sources. Focusing on present-day inland Tanzania during the period 1856–1890, it integrates evidence from qualitative documentary sources with quantitative outputs from climate reanalysis and global circulation models (GCMs), which enables the creation of interdisciplinary seasonal time-series of rainfall variability for three distinct locales. It does so by indexing each dataset to the same 7-point scale and weighting each output according to a predefined level of confidence in the documentary data. This process challenges the subjectivity of nineteenth-century Europeans in Africa, whose reports form the basis of the documentary material, and adds evidence from the region, which is currently lacking from the latest reanalysis products and GCMs. The result is a more scientifically grounded interpretation of documentary materials and a more locally grounded estimation of rainfall that would otherwise be gained from referring to reanalysis or GCMs alone. The methodology is validated with reference to observed long-term trends gathered from (paleo)limnological studies, and it is shown to provide marked insights into four periods of environmental stress in the region’s late-nineteenth-century past. Future challenges may involve integrating evidence from oral traditions and adapting the methodology for other regions and time-periods.