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Walking on bare ground through a passage that had probably not been ice-free for 10,000 years on Mt Kilimanjaro is something Dunedin geographer Nicolas Cullen will not forget.
"It was very special," he said of his recent trip to the glaciers on Africa's highest peak and the highest free-standing mountain in the world.
It was his sixth trip to the glaciers since 2005 and it confirmed the research he and surveyor Dr Pascal Sirguey have recently completed, remapping the glaciers to discover the extent of the their retreat in the past century.
Published online this week in The Cryosphere Discuss, the paper "A century of ice retreat on Kilimanjaro: the mapping reloaded" discussed how reinterpreting historical maps of the area and comparing those with satellite images removed uncertainty about the location and extent of past and present ice bodies.
By doing this, they had been able to show that between 1912 and 2011 the glaciers had retreated from their former extent of 11.40sq km to 1.76sq km, which represented an about 85% loss in ice cover, Dr Cullen, of the University of Otago, said.
That was consistent with other recent assessments but they had identified differences from previously published maps.
His trip to Kilimanjaro had meant he could validate what the team had discovered by re-mapping.
He discovered that in the past 12 months the northern ice field, Kilimanjaro's largest glacier, had split into two parts. In a satellite image a year ago, the glacier was still connected.
"There is now a passage you can walk through that is ice-free for possibly the first time in 10,000 years."
Kilimanjaro's height made it an ideal example of climate variability affecting glaciers, he said.
While there was ongoing debate about the role temperature and moisture played in the retreat of the glaciers, Dr Cullen believed their reduction was just as much due to them being "starved of snow than a rise in air temperature".
"Kilimanjaro is useful for reconstructing climate variability and change, but is not necessarily directly linked to global warming."
Dr Sirguey was putting together an animation of the research for National Geographic.