Robert Graham

Abstract:

The “liquid water habitable zone” (HZ) concept is predicated on the ability of the silicate weathering feedback to stabilize climate across a wide range of instellations. However, representations of silicate weathering used in current estimates of the effective outer edge of the HZ do not account for the thermodynamic limit on concentration of weathering products in runoff set by clay precipitation, nor for the energetic limit on precipitation set by planetary instellation. We find that when the thermodynamic limit is included in an idealized coupled climate/weathering model, steady-state planetary climate loses sensitivity to silicate dissolution kinetics, becoming sensitive to temperature primarily through the effect of temperature on runoff and to pCO2 through an effect on solute concentration mediated by pH. This increases sensitivity to land fraction, CO2 outgassing, and geological factors such as soil age and lithology, all of which are found to have a profound effect on the position of the effective outer edge of the HZ. The interplay between runoff sensitivity and the energetic limit on precipitation leads to novel warm states in the outer reaches of the HZ, owing to the decoupling of temperature and precipitation. We discuss strategies for detecting the signature of silicate weathering feedback through exoplanet observations in light of insights derived from the revised picture of weathering.

Abstract:

©2020. The Authors. Simulations show that storm tracks were weaker during past cold, icy climates relative to the modern climate despite increased surface baroclinicity. Previous work explained the weak North Atlantic storm track during the Last Glacial Maximum using dry zonally asymmetric mechanisms associated with orographic forcing. Here we show that zonally symmetric mechanisms associated with the hydrological cycle explain the weak Snowball Earth storm track. The weak storm track is consistent with the decreased meridional gradient of evaporation and atmospheric shortwave absorption and can be predicted following global mean cooling and the Clausius-Clapeyron relation. The weak storm track is also consistent with decreased latent heat release aloft in the tropics, which decreases upper tropospheric baroclinicity and mean available potential energy. Overall, both hydrological cycle mechanisms are reflected in the significant correlation between storm track intensity and the meridional surface moist static energy gradient across a range of simulated climates between modern and Snowball Earth.

Abstract:

According to the Snowball Earth hypothesis, Earth has experienced periods of low‐latitude glaciation in its deep past. Prior studies have used general circulation models (GCMs) to examine the effects such an extreme climate state might have on the structure and dynamics of Earth's troposphere, but the behavior of the stratosphere has not been studied in detail. Understanding the snowball stratosphere is important for developing an accurate account of the Earth's radiative and chemical properties during these episodes. Here we conduct the first analysis of the stratospheric circulation of the Snowball Earth using ECHAM6 general circulation model simulations. In order to understand the factors contributing to the stratospheric circulation, we extend the Statistical Transformed Eulerian Mean framework. We find that the stratosphere during a snowball with prescribed modern ozone levels exhibits a weaker meridional overturning circulation, reduced wave activity, and stronger zonal jets and is extremely cold relative to modern conditions. Notably, the snowball stratosphere displays no sudden stratospheric warmings. Without ozone, the stratosphere displays a complete lack of polar vortex and even colder temperatures. We also explicitly quantify for the first time the cross‐tropopause mass exchange rate and stratospheric mixing efficiency during the snowball and show that our values do not change the constraints on CO2 inferred from geochemical proxies during the Marinoan glaciation (ca. 635 Ma), unless the O2 concentration during the snowball was orders of magnitude less than the CO2 concentration.