By Megan Tan ’19

The Earth is warming at about two degrees annually at sea level. Though it is estimated that high-elevation warming occurs more steeply, it is difficult to measure due to environmental factors such as radiation and humidity which have made it challenging to accurately quantify past temperature changes.

Shannon E. Loomis, a postdoctoral fellow at the Department of Earth, Environmental, and Planetary Sciences at Brown University, and her team of researchers reconstructed temperatures over the past 25,000 years using a paleothermometer on a sediment record from Lake Rutundu on Mount Kenya, located 3078 m above sea level. This elevation was chosen because reconstruction was not possible at higher elevations as those lakes were formed after the peak of the last ice age, a time known as the Last Glacial Maximum (LGM). The temperature reconstruction was also done on other lakes, Lake Malawi and Lake Tanganyika, which were closer to sea level on Mount Kenya. The recent developments of temperature climate proxies were based on glycerol dialkyl glycerol tetraethers, GDGTs, which are compounds produced in microbial cell walls. The microbes alter the chemical composition of the GDGTs in response to temperature changes. Calibrating the GDGT composition from the sediment with air temperatures through time aided in temperature reconstruction.

The results indicated that the mean temperature increased by 5.5 °C per year since the LGM at the elevation of Lake Rutundu. Meanwhile, Lake Tanganyika showed a temperature increase of 3.3 °C, while Lake Malawi showed a temperature increase of 2 °C. The lapse rate, or the rate at which air temperature falls with increasing altitude, was then calculated by taking the slope of the linear regression of temperature versus elevation.

These calculations showed that existing climate models, which reproduce low-elevation temperature changes accurately, underestimated high-elevation temperature changes by about 40%. This suggests that future predictions by current climate models likely underestimate future high-elevation temperatures, and so miscalculate future lapse rates. Therefore, more research must be done to further understand the temperature changes at high elevations so that future predictions can be made more accurately.

References:

1. S.E. Loomis, et al., The tropical lapse rate steepened during the last glacial maximum. Science Advances 3, (2017). doi: 10.1126/sciadv.1600815.
2. Image retrieved from: https://upload.wikimedia.org/wikipedia/commons/thumb/7/7c/Mount_Kenya.jpg/280px-Mount_Kenya.jpg