Clumped-isotope constraint on upper-tropospheric cooling during the Last Glacial Maximum

Retrieving a “Weather Balloon” From the Last Ice Age

“…While there is room to further refine this range in the coming years, this pioneering study of Banerjee et al. (2022) clearly rules out the possibility of a major steepening of the LGM lapse rate– an important result for the climate science community. By using Δ 36 in ice core air bubbles as a “weather balloon” from the past, Banerjee et al.…”

“…But the work of Banerjee et al. (2022) is careful and thorough. They obtain identical Δ 36 values of the LGM troposphere from Antarctic and Greenland ice cores across over 100 samples, and they find a convincing cooling signal of the upper troposphere during the LGM.…”

“…In this issue, Banerjee et al. (2022) present the first‐ever constraints on the temperature of the upper troposphere during the LGM, which helps to answer the question, “What was the lapse rate (i.e., the change in temperature with height) of the LGM atmosphere?” Of all the possible questions about the structure of LGM temperature change, this one is particularly relevant for future climate change. This is because the “lapse rate feedback” is a key negative feedback in the climate system (Bony et al., 2006; Colman & Soden, 2021; Soden & Held, 2006).…”

“…Since 18 O is a rare isotope (representing about 1 in 500 O atoms), the “clumping” of two 18 O atoms to form a mass‐36 O 2 molecule is extremely rare: the ratio of 36 O 2 to 32 O 2 is about 4 parts per million (ppm). Banerjee et al.’s (2022) study is focused on Δ 36 , a value that reflects small changes in the ratio of 36 O 2 to 32 O 2 from a purely random distribution. From thermodynamic expectations and laboratory experiments, Δ 36 has been shown to be related to the temperature of isotope exchange reactions that occur in our atmosphere, primarily in the upper troposphere at a mean elevation of ∼10 km.…”

“…By using ice cores from Antarctica and Greenland to access bubbles of ancient tropospheric air from the LGM, Banerjee et al. (2022) measured Δ 36 and employed a state‐of‐the‐art atmospheric model estimate the temperature of the upper troposphere and refine the range of plausible LGM lapse rates.…”