Ist die Jormungandhypothese eine mögliche alternative Erklärung für die Neoproterozoischen Eiszeiten?
Physik, Chemie und Biologie des Meeres
Zusammenfassung der Projektergebnisse
Ample geological evidence signals active tropical glaciers at sea level during the Neoproterozoic (1000 Ma to 541 Ma), which the traditional hard Snowball Snowball Earth hypothesis explains by postulating a completely ice-covered ocean. Waterbelt scenarios with a narrow strip of ice-free tropical ocean are attractive alternative hypotheses as they can in addition explain the survival of complex marine life. Here, we combine simulations with two global climate models and an energy balance model to assess the plausibility of waterbelt scenarios produced by the reliable exposure of snow-free dark sea ice in the subtropics. We show that in contrast to expectations from previous work, the waterbelt scenario is not a robust feature of global climate models, which we trace to a stark model difference in the simulation of low-latitude mixed-phase clouds. Sufficient low-latitude clouds are needed for the waterbelt states to be of geological relevance. Simulations with modified clouds and cloud radiation demonstrate the importance of clouds explicitly. By shielding the surface from shortwave radiation, clouds weaken the ice-albedo feedback and act to decouple the climate from the position of the subtropical sea-ice edge. Given the large uncertainty arising from cloud-radiation interactions, our results challenge the robustness of a Neoproterozoic waterbelt scenario. The high uncertainty associated with representing mixed-phase clouds in general circulation models prohibits to assess whether shortwave cloud reflectivity was high and thus whether a waterbelt climate prevailed during the Neoproterozoic period. The uncertainty associated with representing clouds in general circulation models is generally known to primarily arise from convection parameterizations and aerosol-cloud interactions. Therefore, we further investigate whether reducing the required model assumptions associated with the treatment of convection in atmospheric models helps us to assess the plausibility of a waterbelt scenario. First, we show that unresolved sub-grid scale processes generate substantial differences in Neoproterozoic subtropical cloud reflectivity among general circulation models. Second, we conduct a hierarchy of simulations ranging from general circulation model simulations with grid boxes larger than 100 km to large-eddy simulations with grid boxes smaller than 500 m that explicitly resolve atmospheric convective-scale motions. Our hierarchy of simulations supports the existence of highly reflective subtropical clouds if we apply moderate ice nucleating particle concentrations. Third, we test the sensitivity of cloud reflectivity to the abundance of ice nucleating particles. In the presence of high but justifiable ice nucleating particle concentrations, cloud reflectivity is strongly reduced. Hence, the existence of stable waterbelt states does critically depend on the abundance of ice nucleating particles. We conclude that explicitly resolving convection can help to constrain Neoproterozoic cloud reflectivity, but limited knowledge concerning Neoproterozoic aerosol conditions hampers strong constraints. Overall, given the large uncertainty in mixed-phase clouds and their interaction with radiation, waterbelt states remain an uncertain feature of Earth’s climate. Our results strongly question the idea that waterbelt scenarios can explain the Neoproterozoic geology. Thus, we conclude that Neoproterozoic life likely faced the harsh conditions of a hard Snowball Earth.
Projektbezogene Publikationen (Auswahl)
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2022: Controls on subtropical cloud reflectivity during a waterbelt scenario for the Cryogenian glaciations. Journal of Climate
Braun, C., A. Voigt, C. Hoose, A. M. L. Ekman, and J. G. Pinto
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2022: Ice-free tropical waterbelt during Snowball Earth events questioned by uncertain clouds. Nature Geoscience, 15, 489–493
Braun, C., J. Hörner, A. Voigt, and P. Joaquim
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2022: Snowball Earth initiation and the thermodynamics of sea ice. Journal of Advances in Modeling Earth Systems
Hörner, J., A. Voigt, and C. Braun
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Waterbelt scenario for the Cryogenian glaciations questioned by uncertain mixed-phase clouds, June 2022
Christoph Braun
