Clouds played an important role in climate history

Clouds have a strong influence on ice ages in earth's history. The simulation shows a snapshot of the cloud cover in the lowest layer of the atmosphere (up to approx. three kilometers above the surface of the earth) in an assumed water belt climate. The color scale ranges from "no cloud cover" (dark blue) to "fully covered" (white)
Credit Graphic: IMK-TRO

Were the world's oceans in cryogenium - around 700 million years ago - completely covered with ice or an ice-free water belt stretched around the equator, in which sponges and other life forms could survive? A research team from the Karlsruhe Institute of Technology (KIT) and the University of Vienna has now been able to show in global climate models that a climate condition with a water belt is rather unlikely and therefore not a reliable explanation for the persistence of life in the cryogenium. The reason for this is the uncertain influence of clouds on the climate at that time. The team presents the results of the study in the journal Nature geoscience.

From space, Earth might have looked like a big snowball during the global ice ages in the Cryogenium. Geoscience therefore describes this assumption of a closed sea ice sheet established in research as a snowball earth theory. It is still particularly unclear how sponges - of which fossil finds testify - could have survived in the very cold snowball earth climate. Therefore, some researchers have proposed an ice-free water belt around the equator as an alternative theory.

Life despite probably icy oceans

Climate researchers at KIT, together with colleagues from the University of Vienna, examined the climatic conditions during the cryogenium with global climate models and an idealized energy balance model. They expected to find a climate state with a water belt in the simulated scenarios to investigate the conditions under which it remains stable. "We were surprised that this condition is not robust in the models," says Christoph Braun from the KIT's Institute for Meteorology and Climate Research - Department of Tropospheric Research (IMK-TRO). Life in the Cryogenium was therefore probably exposed to the harsh evolutionary conditions of globally icy oceans.

The study gave new insights into the role of clouds: "Clouds and their radiation reflection are important for the stability of a water belt condition - this strong influence was not yet known," emphasizes the doctoral student and first author of the study. With the cloud reflectivity mechanism proposed in the publication, the results of previous studies could be reinterpreted and possibly linked to a more coherent picture.

Clouds make it difficult to see the climate past

"With the global climate models and an idealized climate balance model, we can show the influence of the reflectivity of clouds and explain the underlying processes," says Braun. “However, it cannot be used to assess how strong the reflectivity of the clouds in the cryogenium was, because the uncertainty in the representation of clouds in global climate models is great.“The decisive factor for the reflectivity is how efficiently water droplets are converted into ice, which depends, among other things, on the type and number of aerosols acting as ice germs. These processes take place on a millimeter scale, while the calculation grids of the models have so far been in the order of more than 100 kilometers. The results show that clouds are crucial to predict climate change and understand the dynamics of geological glaciation. "Clouds make it difficult for us not only to look into the future, but also into the past," says Braun.

Assessment of the habitability of planets outside of our solar system

The researchers' findings could also be useful in the future to assess whether planets can be inhabited outside of our solar system. "This becomes interesting, for example, when future observations of the James Webb space telescope allow views of clouds in the atmospheres of extrasolar planets," says Braun. The KIT researchers carried out the simulations on the Mistral high-performance computer at the German Climate Change Center in Hamburg. “The next step was to simulate clouds on finer arithmetic grids under the climatic conditions of the cryogenium. This enables us to investigate whether and how the uncertainty associated with the clouds can be reduced,” says Braun.

The German Research Foundation (DFG) has the research project completed in May 2022 within the DFG project “Is the Jormungand hypothesis a possible alternative explanation for the Neoproterozoic Ice Ages?“Funded for a total of 202,000 euros for three years. The ICON atmospheric model was used for the simulations, in the development of which KIT is involved in a consortium with the Max Planck Institute for Meteorology and the German Weather Service.

Source/Credit: Karlsruhe Institute of Technology

en061022_01