Clumped-isotope constraint on upper-tropospheric cooling during the Last
Glacial Maximum
Abstract
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 (5 – 15 km altitude, effective mean 10
– 11 km) was 4 – 10°C cooler during the LGM than during the late
preindustrial Holocene. These results support 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.