The dwindling icecap atop Africa's Mount Kilimanjaro cannot be attributed to human induced global warming, an American and an Austrian researcher duo have said in a recent study.
"There are dozens, if not hundreds, of photos of midlatitude glaciers you could show where there is absolutely no question that they are declining in response to the warming atmosphere," said climatologist Philip Mote, a University of Washington research scientist.
Mote and Georg Kaser, a glaciologist at the University of Innsbruck in Austria, write in their study in the journal American Scientist, that the decline in Mt. Kilimanjaro's ice has been going on for more than a century and that most of it occurred before 1953.
Evidence of atmospheric warming there before 1970 is inconclusive. As such, using the mountain as a "poster child" for climate change is inaccurate, they say.
According to Mote, in the tropics - particularly on Mt. Kilimanjaro - processes very different from those that have diminished glacial ice in temperate regions closer to the poles are at work.
The ice decline is primarily due to complex interacting factors, including the vertical shape of the ice's edge, which allows it to shrink but not expand.
They also cite decreased snowfall, which reduces ice buildup and determines how much energy the ice absorbs - because the whiteness of new snow reflects more sunlight, the lack of new snow allows the ice to absorb more of the sun's energy, for this shrinkage.
"Unlike midlatitude glaciers, which are warmed and melted by surrounding air in the summer, the ice loss on Mt. Kilimanjaro is driven strictly by solar radiation. Since air near the mountain's ice almost always is well below freezing, there typically is no melting. Instead ice loss is mainly through a process called sublimation, which requires more than eight times as much energy as melting," the study says.
"Sublimation occurs at below-freezing temperatures and converts ice directly to water vapour without going through the liquid phase," Mote said, adding that the process is much similar to moisture-sapping conditions that cause food to suffer freezer burn.
According to the duo, fluctuating weather patterns related to the Indian Ocean could also affect the shifting balance between the ice's increase, which might have occurred for decades before the first explorers reached Mt. Kilimanjaro's summit in 1889, and the shrinking that has been going on since.
The level of nearby Lake Victoria, the world's largest tropical freshwater lake, declined in the late 19th century, when the decline of Kibo's icecap began. The volcano Kibo is the highest point on Kilimanjaro, about 19,340 feet above sea level.
The researchers say the lake and the icecap likely suffer from a precipitation decline caused by Indian Ocean variability, which could also have caused the icecap to vary in size and shape over millennia.
"It is certainly possible that the icecap has come and gone many times over hundreds of thousands of years. But for temperate glaciers there is ample evidence that they are shrinking, in part because of warming from greenhouse gases," said Mote.
The researchers write that the same factors that have contributed to ice loss in the South Cascade Glacier in Washington State, do not apply to Mt. Kilimanjaro's icecap, even though its decline has been cited in forums such as the Academy Award-winning documentary film "An Inconvenient Truth".
"There is no evidence to support that assertion. It's not that it is impossible, but rather the decline is most likely associated with processes dominated by sublimation and with an energy balance dominated by solar radiation, rather than by a warmer troposphere," Mote added.