Hydrologically-informed estimates of future malaria suitability in Africa

Future climate changes will alter the geographic locations that are environmentally suitable for malaria transmission. The primary driver for these shifts is often considered to be the thermal constraints and dependencies of both the Anopheles mosquitoes that act as malaria vectors and of the Plasmodium spp. malaria parasites themselves. The availability of surface water for vector breeding sites is also a critical requirement for malaria transmission; without surface water bodies, there would be no malaria. However, continental scale analyses typically lack any representation of hydrology, instead relying on simple rainfall thresholds that are a poor proxy for water body availability.

Here we incorporate more robust estimates of breeding site availability into estimates of areas of malaria suitability across Africa. We go beyond the use of a single hydrological model by presenting a multi-model, multi-scenario ensemble of global hydrological models and global climate models, weighted based on model performance when applied to preindustrial (i.e., pre-intervention) conditions. We then use this ensemble to estimate changes in malaria transmission season length across Africa.

Including hydrology results in a much more complex pattern of malaria suitability across Africa and identifies river corridors as foci of endemic malaria. This is particularly important given the concentration of human populations around such river corridors. Models predict a net decrease in areas environmentally suitable for malaria transmission from 2025 as the climate warms and dries, though the geographical locations of suitability shift. Notable decreases in length of transmission season are observed across West Africa. Conversely, increases are observed in the Ethiopian highlands, in Lesotho and also along waterways through South Africa, particularly the Orange River. Compared with models that use rainfall as a proxy for water body availability, future malaria suitability changes cover a smaller area, but are associated with greater changes in season length. Hydrologically-informed estimates are also more sensitive to the choice of emissions scenario.

Despite the net decrease in suitable areas, the projected growth in human population means that the number of people living in malaria suitable areas will increase by over 80 million to 2100. Including hydrology emphasises this increase: the number of people estimated to live in a potentially malaria endemic area (with a transmission season of over nine months) by 2100 will be over four times greater than estimated by rainfall driven models. However, we note that malaria is a complex disease and driven by more than climate alone.