In 1936, Ernest Hemingway described the top of Kilimanjaro as being “as wide as all the world, great, high, and unbelievably white in the sun.” However, the mountain’s ice cap has been shrinking in recent decades because of warming temperatures, and only fragments of it remain today.
In new observations of those fragments, researchers have revealed the thickness and volume of Kilimanjaro’s largest remaining ice field with ground-penetrating radar. Several previous studies documenting the retreat of Kilimanjaro’s ice had used aerial imaging that showed changes in only two dimensions.
Using ground-penetrating radar, “you basically see straight down into the glacier,” said physicist Pascal Bohleber of the Institute for Interdisciplinary Mountain Research of the Austrian Academy of Sciences in Innsbruck, who led the team that brought ground-penetrating radar to Kilimanjaro’s nearly 6000 meter high summit for the first time. “It’s like an X-ray of the glacier.”
In addition to providing a new perspective on the vanishing ice, periodic radar measurements of the mountain’s ice in three dimensions could enhance scientists’ understanding of how the remaining ice fields are changing, Bohleber said.
Carried Up, Up, Up
Although researchers regularly use ground-penetrating radar to peer inside of glaciers, “it’s never really been done on Kilimanjaro,” Bohleber told Eos. That’s largely because of Kilimanjaro’s relative isolation, he explained. “In the Alps, we can fly the equipment in [using a helicopter].” But finding a helicopter and pilot able to fly above 5000 meters in eastern Tanzania isn’t easy, he said, so “everything has to be carried on the back of a porter up the mountain.”
In September 2015, after receiving funding from the National Geographic Global Exploration Fund, he and four other scientists met at Kilimanjaro International Airport. They drove to Kilimanjaro National Park and then hiked the Umbwe trail up the mountain’s flanks, sleeping in tents each evening. Roughly 45 porters accompanied the scientists, carrying the group’s equipment, which included ground-penetrating radar antennae and receivers, batteries, solar panels and chargers, and laptops.
After 6 days of climbing through progressively thinner air, the group reached the broad slope of Kilimanjaro’s summit and set up camp near one edge of the Northern Ice Field, the largest remaining ice field atop Kilimanjaro. Bohleber brushes off the difficulties he and his colleagues encountered, which included unexpected delays and altitude sickness. “You’re so focused on work up there that you hardly think about anything else,” he said.
A Peek Inside
Ground-penetrating radar emits radio waves that travel down through the ice, bounce off internal structures or the bed of loose, silty sand below, and return to the receiver. Using the known speed of radio waves through glacial ice, researchers can then calculate the depths of where those reflections occurred and map out the size and shape of the glacier.
Over the course of 3 days, Bohleber and his colleagues obtained thousands of measurements while dragging their radar equipment over the surface of the Northern Ice Field. The team then calculated the depth of the bed below the ice at many different locations and extrapolated the data to create a map of ice depth. The team found that the ice ranged in thickness from roughly 6 to 54 meters, with the thickest section running along a ridge oriented roughly east–west, the researchers reported on 9 February in The Cryosphere.
The team confirmed the accuracy of their ground-penetrating radar measurements using a simple, low-tech test: They showed that the depth calculated along ice cliffs at one edge of the ice field was consistent with the depth measured by hanging a rope off the same edge.
Tracking the Loss
“This study provides a strong complement to previous airborne, satellite, modeling, and ice core studies of the Kilimanjaro ice fields,” said Kimberly Casey, a glaciologist affiliated with NASA’s Goddard Space Flight Center and the U.S. Geological Survey who was not involved in the study.
In fact, by comparing their new map of ice depth with data from ice cores sampled by other researchers in 2000, Bohleber and his colleagues found that, on average, the ice field had fallen in height by 6 meters over 15 years. Other glaciers at lower elevations on Kilimanjaro have decreased in height even more substantially, said Bohleber.
Using their depth measurements and the known extent of the ice, the team calculated the total ice volume of the Northern Ice Field to be roughly 12 million cubic meters. That’s enough ice to fill all of Manhattan’s Central Park to a depth of 4 meters. This volume estimate is useful for monitoring ice loss, said Bohleber.
An Edge Below the Ice
Besides shedding light on the depth of Kilimanjaro’s ice, the new measurements also revealed the topography of the underlying sandy bed. Kilimanjaro is a dormant volcano, and the team detected an edge of one of its crater rims below the ice.
“We’re measuring what the bed topography looks like,” said Bohleber. That’s an improvement over previous studies, which simply assumed that the bed was flat, he explained.
The team also demonstrated that the layering of ice within the Northern Ice Field was very regular and horizontal. “At least for the upper 30 meters in the central plateau area it’s like a layered cake,” said Bohleber. That discovery means that ice samples from the Northern Ice Field’s exposed walls are representative of its interior, good news for scientists who are studying the ice structure from the outside.
Bohleber and his colleagues are looking forward to using their methods to trace the evolution of the Northern Ice Field in three dimensions. He said his team hopes to take measurements on the summit again in a couple of years to create a new three-dimensional map. Comparing it with the current one would help the researchers track the rate of ice loss in detail, he explained.