The dramatic melting pattern of African mountain glaciers

The seasonal snow melt of mountain glaciers, like those from Mount Kilimanjaro and Mount Kenya, is a key supply of water to out flowing rivers of mountain ranges during warm and dry periods. These glaciers act as a water tower, and several rivers have now become seasonal.

One case study that has gotten large-scale attention is the gradual, yet drastic retreat of glaciers on Mount Kilimanjaro. The thinning and lateral retreat of these ice-caps has caused the 85% loss since 1912. Some studies have assigned this to be a cause of climate change. The rate of glacial decline has been increasing since 1970 (start of industrial revolution and pronounced human impact on the climate system) and can be explained by increased dryness, reduced moisture convergence over East Africa and thus lack of snowfall on the mountain to replenish the seasonal melting (e.g. Ward, 2012). In fact, most of the ice is shown to sublimate to the atmosphere directly. These climatic drivers of glacial retreat are expected to increase further with expected climate change scenarios in the future.

However, the very start of the retreat predates the start of anthropogenic impact on the climate by several decades. Opinions of scientists have thus been divided on the exact cause of Mount Kilimanjaro glacial retreat. This discussion is only ongoing as neither side can be proved completely wrong due to the lack of high altitude data in East Africa. It is making it hard to find a baseline against which to analyse changes of temperature and dryness and relate those to historical trends with forcing factors.

The recent examination of ice cores from the glacier itself shines a new light on the issue. A 30mm thick layer of dust (indicating prolonged dry period in regional climate) ≈4,200 years ago has not been accompanied by any decline in the glacial thickness, as would have been expected if dryness and decreased precipitation really would be the dominant forcing for Kilimanjaro glacial retreat. The core further shows that the recent melting pattern is unique within the whole 49m core, supporting that recent earth-system changes (by human impact!) must be pushing the ice-cap shrinking.

The pattern seen in this case study is not a singular occurrence across Africa: the melting of Mount Kenya, as well as in the Rwezori mountains, illustrate the common pattern of “glacier mass loss, shrinkage, and retreat at high elevations (>5,000 m above sea level) in lower latitudes (30° N to 30° S), particularly in the thermally homogeneous tropics”(Thompson et al, 2009). Coming back to the discussion on the main drivers of this change, this uniform pattern suggests a common underling driver upon which the local influences (e.g. land use change) may be superimposed to accelerate it.