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U. researchers discover inaccuracy of previous climate models

New study suggests current models underestimate temperature changes in high-elevation areas

An analysis of past temperatures on Mount Kenya suggests that current climate models used to study climate change may not be as accurate as previously thought, according to a paper published in January by University researchers in the journal Science Advances.

By comparing lake sediment cores from Lake Rutundu on the top of Mount Kenya to temperature data from lower elevations, climate researchers showed that mountaintop temperatures increased more than temperatures at lower elevations since the last ice age — about 20,000 years ago.

A comparison of these findings to past climates simulated by climate models showed that such models “significantly underestimate high-elevation temperature changes,” said Jim Russell, associate professor of earth, environmental and planetary sciences and an author of the study. Such underestimation could lead to errors in predicting future effects of climate change and global warming, he added.

“People talk a lot about how sensitive and fragile mountain ecosystems are … and yet there’s really not that much data out there” to show that they are more susceptible to climate change, Russell said.

Organic compounds found in these sediment cores provide a proxy for temperature, and the depth at which they were found indicate the period they were produced. “All cells have some sort of cell membrane. But bacteria and archaea actually change their structure based on temperature,” said Shannon Loomis PhD’13, lead author of the study.  These organisms produce molecules that maintain their cell membrane fluidity in low temperatures, and these molecules can survive in the sediment for thousands of years, she added.

“We extract these compounds from the sediment and then measure their relative abundances,” Russell said. The relative abundance of different types of molecules found in the cell wall of  the bacteria correlates to a temperature at a specific time in the past. Tiny differences in the makeup of these compounds, such as the addition of a carbon chain, can give meaningful data about past climates.

Similar studies conducted by Russell’s lab have “observed temperature changes of around two to three degrees” in low-elevation African lakes since the last ice age, Russell said. Slightly higher elevations — around 2,000 meters above sea level — showed temperature increases of around five degrees. At Lake Rutundu — around 3,000 meters above sea level — the researchers observed a temperature increase of about six degrees over the same amount of time.

“What (these findings) mean is that over the past 20,000 years, temperatures have warmed more at high elevations than at low elevations,” Loomis said. This phenomenon of greater temperature change at higher altitudes is called a “lapse rate.”

Models that attempt to simulate the climate during the last ice age incorporate a lapse rate, but not one as steep as the rate Russell’s team found in the study. “These are exactly the same models used to predict how the climate will be a hundred years from now,” Russell said. “The models all underestimated the temperature changes at high elevation” compared to what the data indicate, he added.

“When we think about the climate problem, we really only have 130 (to) 150 years of observations,” said Baylor Fox-Kemper, associate professor of earth, environmental and planetary sciences who was unaffiliated with the study. “What this paper shows is that our guesses — what the climate models do — are not as sensitive as the real system.”


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