For the study, Russell and his colleagues looked at sediment cores taken from the bottom of Lake Rutundu, a volcanic lake on Mount Kenya at an elevation of around 10,000 feet. The cores preserve the signature of GDGT chemistry dating back more than 25,000 to the ice age. The data suggested that mean annual temperatures at Lake Rutundu increased about 5.5 degrees Celsius since the last ice age — a figure consistent with the previous high elevation temperature proxies. Meanwhile, temperature data from two lakes closer to sea level — Lake Tanganyika and Lake Malawi — suggest much more modest temperature changes of about 3.3 degrees and 2 degrees respectively.

Climate models are able to reproduce the temperature changes at low elevations, but they underestimate the high-elevation change by 40 percent, Russell says. That suggests there’s something amiss in the way the models simulate changes in the atmospheric lapse rate — the rate at which air temperature varies with altitude.

“All climate models calculate a lapse rate — it’s integral to the output of the model,” Russell said. “What this work shows is that there’s a problem in the way the models make that calculation.”

Implications for future climate change

It’s difficult to diagnose exactly what that problem is, Russell says, but it likely has something to do with the way models treat atmospheric water vapor content. Water vapor content is the strongest controlling factor in governing the lapse rate (moist air cools more slowly with altitude).

“We would argue that there’s probably a problem in the water vapor concentrations and therefore the feedback,” Russell said.

Whatever the source of the problem, the ramifications for tropical mountains may be significant. The models miss almost half the temperature change at high elevations in the past, and they may be underestimating future change as well.

“These are very fragile ecosystems that house extraordinary biodiversity and unique environments such as tropical glaciers,” Russell said. “Our results suggest future warming in these environments could be more extreme than we predict.”

The work was supported by the National Science Foundation (EAR-1226566). Russell’s coauthors were Shannon Loomis (Brown), Dirk Verschuren (Ghent University) Carrie Morrill (University of Colorado, Boulder), Gijs De Cort (Ghent), Jaap S. Sinninghe Damsté (University of Utrecht), Daniel Olago (University of Nairobi), Hilde Eggermont (Ghent) F. Alayne Street-Perrott (Swansea University) and Meredith A. Kelly (Dartmouth).

Source: NSF, Brown University