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.

The impact of climate change induced sea-level rise and storm surges upon coastal groundwater reserves

The African continent lies between the Atlantic and Pacific/Indian ocean – and thus this post discusses how changes in the level of these oceans may affect coastal freshwater resources. It is now generally observed (mean global rate of rise 1993-2009 was 3.3 ± 0.4 mm/year) and predicted that under any of the future emission scenarios we will experience a significant rise in global temperatures, causing the melting of polar and glacial ice and warming the world’s oceans – all contributing to the rise in global sea levels. The magnitude of this sea-level rise is still uncertain, as are the regional differences to be expected (Nicholls and Cazenave, 2010). To give a rough estimate, IPCC AR4 projections show that future ice dynamics discharge could produce about a rise of 80cm by 2100. On top of this, local changes such as land subsidence (often exacerbated by human activities in densely populated areas) will further allow the impact of salt water upon the coastal water.

As I have shown in previous blogs, a large fraction of Africa’s low-income population is strictly reliant on groundwater for its freshwater supply. These aquifers can be relatively thin in the low-lands and form adjacent to the coastline. The Ghyben-Herzberg function explains the existence of a freshwater lense “floating” on top of seawater, separated by the saltwater-freshwater interface (for 1 feet of freshwater above the interface, 40 feet of saltwater lie below).  This interface is vital in determining the quantity of freshwater storage in the coastal aquifer, and the mixing of the layers effects the quality. Many studies have evaluated the pressures existing on these vulnerable freshwater reserves, leading to sea water intrusion (the encroachment of the interface) and excessive chloride contamination of the freshwater.

While the unsustainable abstraction for human use has been evaluated to be a greater threat in many coastal aquifers than climate change (e.g. Ferguson and Gleeson, 2012), I want to highlight the ways climate change may exacerbate this.

The diagram below shows how a rise in sea-level affects a coastal aquifers’ characteristic by reducing the freshwater lens (Figure 1). An area identified as being under significant pressures from this mechanism is Southern Africa. A modelling study was conducted by Ranjan et al (2006) that estimated the loss of freshwater by the encroachment of the interface – also taking into account predicted changes in aquifer recharge. The rate of change of groundwater loss (% per year until 2100) in South African coastal aquifers was named to be 0.027 (A2 scenario) – 0.022 (B2 scenario).

Figure 1: click for Source

Besides this reduction in reserves, climate change also is predicted to increase cyclonic activity and storm events. Storm surges cause coastal flooding, salinisation of surface waters and ecosystems but also will result in instantaneous events of saltwater intrusion to coastal aquifers from above into the freshwater lens (Anderson, 2002). Groundwater chemistry is thus adversely affected by increases in chloride concentrations, adding to the stress on coastal water reserves.

In a comparative impact study of the climate change effects described above (sea-level rise and storm surges), Dasgupta et al (2009) from the World Bank Development Reasearch group, find what I had expected while examining the impacts on Africa. The vulnerability to these threats is found to be concentrated to populations and large cities at the lower end of the international income distribution (for Africa e.g. Djibouti, Tanzania, Mozambique). Once again, this raises the point that the most socio-economically vulnerable are those to expect the largest negative impacts of anthropogenic climate change, while having contributed least to it. A very recent publication by Barbier (2015) effectively highlights the threat climate change poses to African low-elevation coastal zone populations by increasingly pushing these into the ‘poverty-environmental trap’ and calls for a rethinking of policy and development strategies.