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

Ice cores and other paleotemperature proxies, together with general
circulation models, have provided information on past surface temperatures
and the atmosphere’s composition in different climates. Little is known,
however, about past temperatures at high altitudes, which play a crucial
role in Earth’s radiative energy budget. Paleoclimate records at
high-altitude sites are sparse, and the few that are available show poor
agreement with climate model predictions. These disagreements could be due
to insufficient spatial coverage, spatiotemporal biases, or model physics;
new records that can mitigate or avoid these uncertainties are needed.
Here, we constrain the change in upper-tropospheric temperature at the
global scale during the Last Glacial Maximum (LGM) using the
clumped-isotope composition of molecular oxygen trapped in polar ice
cores. Aided by global three-dimensional chemical transport modeling, we
exploit the intrinsic temperature sensitivity of the clumped-isotope
composition of atmospheric oxygen to infer that the upper troposphere
(effective mean altitude 10 – 11 km) was 6-9ºC cooler during the LGM than
during the late preindustrial Holocene. A complementary energy balance
approach supports a minor or negligible steepening of atmospheric lapse
rates during the LGM, which is consistent with a range of climate model
simulations. Proxy-model disagreements with other high-altitude records
may stem from inaccuracies in regional hydroclimate simulation, possibly
related to land-atmosphere feedbacks.