In the tropical climate of East Africa, glaciers are an unexpected, yet vitally important part of the ecosystem. Since 1900, African glaciers have lost a staggering 80 percent of their surface area, contributing to regional water shortages.
While rising temperatures may seem like an obvious cause of global glacier retreat in many regions, the glaciers of east Africa are a unique exception. A study published in Cryosphere earlier this year has found that the largest glacier on Mount Kenya, the Lewis Glacier, is melting because of decreasing atmospheric moisture rather than increasing temperatures.
African glaciers have all but disappeared, except for three locations in East Africa: Mount Kilimanjaro in Tanzania, Mount Kenya in Kenya, and the Rwenzori Range in Uganda. Scientists have been studying the few remaining African glaciers in hopes of preserving what is left of the rapidly melting ice. While headway had been made in understanding the causes of melting on Kilimanjaro, the melting on Mount Kenya, Africa’s second tallest mountain, has remained a mystery until now.
The complex climatic features of Mount Kenya, combined with the lack of observational data, has made it difficult to pinpoint an exact cause of Lewis Glacier’s retreat. Lindsey Nicholson, a researcher at the Institute of Atmospheric and Cryospheric Sciences, led a study in 2013 that concluded a combination of causes was responsible for the melt, rather than one factor in particular.
Building on her previous work, the team, led by University of Graz’s Rainer Prinz and Lindsey Nicholson, set out to collect the data they needed to gain a more accurate understanding of why Lewis Glacier was melting. They installed an automatic weather station on the glacier at an elevation of 4,828 meters, and collected 773 days of data over the course of two-and-a-half years.
In conjunction with the data from the weather station, the team used a model to predict how much Lewis Glacier would melt under a range of different scenarios. By manipulating variables, including precipitation, air temperature, air pressure, and wind speed, in the model, the team was able to see which factors played the biggest role in glacier melt.
The team found that moisture had the biggest impact on Lewis Glacier’s surface area, rather than air temperature or a combination of other climatic factors. Despite differences in location and elevation, the glaciers of Mount Kenya and Kilimanjaro are melting for the same reason: East Africa is getting progressively drier, and the lack of water is impacting much more than just the glaciers.
The glaciers on the peak of Kilimanjaro lie significantly above the regional freezing point—year round, the peak is cold enough to maintain its ice levels, even as surface temperatures in East Africa have steadily increased. Yet, Kilimanjaro’s glaciers continue to retreat and are projected to disappear completely by 2020. Temperature changes fail to explain the severity of the mountain’s glacier retreat.
Observational studies have showed that Kilimanjaro is receiving less cloud cover that leads to increased radiation from the sun, and less precipitation, causing infrequent snowfall. The IPCC has projected a 10% decrease in rainfall during the already dry season from June through August, amplifying the impacts of regional dryness and drought.
The impact of a drying climate has greatly impacted Kilimanjaro, and caused its glaciers to retreat from sublimation–a process by which the ice changes directly into water vapor rather than melting into water. The theory that moisture is the main factor impacting glacier melt on Kilimanjaro has, up until now, been assumed to be a product of the mountain’s height and not generalizable to all East African glaciers. Prinz and Nicholoson’s findings suggest that drying may be the main reason for glacier melt throughout the region as a whole.
Mount Kenya’s glaciers are at lower elevations compared to Kilimanjaro’s, and lie much closer to the regional freezing level. It was therefore expected that rising temperatures would affect the glaciers of Mount Kenya, and no scientific studies had proved or disputed this assumption.
Droughts, desertification, and crop failure have become increasingly common in tropical Africa, and according to the study this is primarily caused by shifting ocean conditions that are preventing moisture from circulating over East Africa. The lack of moisture means there is not enough precipitation—either as rain over the savannas or snow on the mountain peaks—to sustain the glaciers or the populations that rely on them. In order to preserve the last remaining African glaciers, it will be necessary to understand and prevent changes in water, rather than only changes in temperature.