Glacier-Climate Interactions: What Glaciers can tell us about the Changing Climate

Glaciers Signalling Climate Change

Melting glaciers depicted in the media are one of the most iconic symbols of a warming environment. Images of diminishing glaciers are often used to illustrate the urgency of climate change, because of the ability to demonstrate stark contrast over time. For example, the highly acclaimed documentary Chasing Ice has raised awareness about global warming through a time lapse of rapidly melting glaciers.

extract from Chasing Ice

Tropical Glaciers of East Africa

Overall, the surface area of East African glaciers has been receding rapidly over the past century, as shown in the graph below. The mapping of these recessions can be a useful indicator of natural climatic variability, as well as the effects of increased anthropogenic activity exacerbating these changes, given that glaciers, and even more so tropical glaciers, are extremely sensitive to these changes (Kaser, 2004). This means that they are recognised as valuable proxies of climate change (Mölg, 2008).

Time series of glacial surface areas in East Africa - Kilimanjaro (KIL), Rwenzori (RW) and Mount Kenya (MK) (Kaser, 2004)

Glaciers are valuable proxies of climate change, but what exactly does their rapid recession reflect?

The glacier-climate interactions seem to be far more complex in tropical glacier regions than elsewhere in the world. While globally glaciers' mass and extent is considered to strongly correlate with changing air temperatures (Kaser, 2004), the behaviour of tropical glaciers seems to be more complicated.

Kilimanjaro

Let us take Kilimanjaro as an example. Although it was thought that the main mechanisms driving the Kilimanjaro glacier recession were increasing local air temperatures, it was later evaluated that they are in reality most sensitive to changes in the atmospheric moisture content as they exist "far above the mean annual freezing level" (Prinz, 2015). Therefore, the ongoing retreat of the Kilimanjaro glaciers since the 19th century has been mainly due to the drying of the region's atmosphere in the following two centuries; in contrast to the projections of the IPCC suggesting an increase in rainfall in eastern Africa due to a shift towards more El Niño-like conditions. 

This is linked to a shift in the Walker Cell circulation (shown below), caused by changing ocean conditions and resulting in increased activity of the convective branch of the Walker circulation over the Indian Ocean while suppressing convection along the east African continental margin (Williams, 2011). In turn, the reduction of this deep convection in East Africa (Prinz, 2015) and the easterly flow of dry air means that less cloud cover and precipitation is brought onto the glaciated mountain summit - due to the steady decline of convective activity during MAMJ (African rainfall from March to June) over the past 30 years. This decline reduces the build up of snowfall at the summit, promoting recession instead (Williams, 2011).

Diagram showing the mechanism of Walker Circulation - driving longitudinal air flows over the Indian Ocean and transporting moisture to regions including East Africa

Mt Kenya and the Ruwenzori mountains

It is important to bear in mind that the glaciers on Kilimanjaro are found nearly 1000 m higher than Mt Kenya and the Rwenzori. This means that although glaciers on Kilimanjaro are highly influenced by moisture content, glaciers on Mt. Kenya as well as on Rwenzori are a lot more sensitive to increasing air temperatures, as they are situated closer to the "elevation of the mean regional freezing level" (Mölg et al., 2013). However, air temperature variability also coexists with changes in atmospheric moisture - meaning it cannot be taken as a single isolated driver of the changes in the glacier's mass either (Prinz, 2015).

Proxy Potential

Despite the differences within the region, the general trend of rapidly disappearing tropical glaciers over the past century has revealed their high sensitivity to the globally warming climate. Even though regional climatic factors, such as precipitation, have an effect on this disappearance, these local factors are themselves influenced by global climate change (Mölg et al., 2013). It is the large scale tropical ocean circulation systems that redistribute heat and bring moisture into the region - influencing precipitation patterns in East Africa. This demonstrates that the changes faced by East African glaciers are far more complex than a reflection of increased local warming, making them the protagonists for further studies of tropical glaciers as climate proxies (Prinz, 2016).

Although there is uncertainty within the climatological causes of the present day retreats, there is growing concern over the increased role of anthropogenic forcing, including greenhouse gas and aerosol emissions, which increase sea surface temperatures in the Indian Ocean, promoting the current drying of East Africa and exacerbating the natural variability (Kaser, 2005).