Increasing Elevation of the Temperature Constraint on Malaria Transmission in the Ethiopian Highlands

Lower temperatures in the Highlands of East Africa have historically offerred an altitudinal constraint on the transmission of malaria.  A warming climate, however, will serve to increase the elevation at which this constraint applies.  Using a new climatological dataset that calibrates and combines inferred temperatures from satellites with numerous station observations the elevation at which the constraint holds has been analyzed for the country of Ethiopia over the period 1982-2014.  The results show that in a given year there are substantial spatial variations in the elevation where the temperature constraint holds, which is shown to be associated with the relative orientation of orography to the prevailing winds (e.g., upslope and downslope flow patterns).  On interannual timescales there is also a substantial contribution to elevation changes in the temperature constraint related to the El Nino-Southern Oscillation (ENSO) phenomenon, with El Nino (La Nina) events being associated with higher (lower) elevations.  When the influence of ENSO has been statistically removed from the temperature constraint elevation time series, a clear upward trend is identified which is highly statistically significant.  The average magnitude of this trend is > 40 meters per decade.  When gridded population estimates for Ethiopia are utilized, the results show that over the last 3+ decades the elevation zone where the temperature constraint on malaria no longer holds is home to over 6 million people.  Given the complexities of malaria transmission dynamics and current control and elimination programs, the results cannot be used to directly predict that this additional population is now vulnerable to malaria.  What the study does show, however, is that for the first time we can now quantify the effect of increasing temperatures on an important constraint on malaria transmission in the Ethiopian Highlands.