Snowball Earth: algae triggered cooling millions of years ago

27. August 2015
The advance of certain algae was probably one key contribution to an almost complete glaciation of the Earth millions of years ago. The consequent rise in emissions of organic cloud-condensation nuclei led to increased cloudiness. Thereby, they likely contributed crucially to the cooling of the climate, because clouds reduce solar radiation on the Earth´s surface. This was discovered by a team of scientists in a new study to be published today in the renowned journal Nature Geoscience.

“There are many examples for the close correlation of life and climate in Earth history,” says Georg Feulner of the Potsdam Institute for Climate Impact Research, lead author of the study. “The expansion of certain algae about 800 million years ago cooled the climate considerably and probably made the ensuing global glaciations – we call them Snowball Earth events – possible in the first place. With our study we discovered a new piece of the puzzle to better understand one of the most fascinating episodes of climate history.” Currently, the Earth heats up due to man-made greenhouse-gas emissions – a cooling like in the past can be ruled out for a very long time.

Until now, the glaciations that occurred 700 million years ago after about a billion years of relative stability (the “boring billion”) could not be explained satisfactory. The new study closes an important knowledge gap. The scientists used new perceptions of the evolution of so-called eukaryotic algae. These algae are thought to be the principal source of cloud-condensation nuclei over the oceans that contribute to clouding: when the cells of the algae die, they are decomposed by bacteria, and microscopic organic material is released into the air. There it oxidizes to sulfur compounds that serve as cloud-condensation nuclei where cloud droplets can form and thereby clouds. In addition to solar intensity, the concentration of greenhouse gases and the Earth´s surface reflectivity, clouds are crucial for the planetary energy balance, and their increase can cause a cooling of the climate.

In several computer simulations with a coupled climate model, the scientists tested how much an increase of cloud-condensation nuclei due to the expansion of algae could have contributed to the cooling. The results show that the rise of algae and their emissions must have played a role in the cooling of the atmosphere at the time.

“Our model offers a plausible explanation why there hadn´t been an ice age in the more than one billion years before the so-called snowball glaciation,” says co-author Christian Hallmann of the Max Planck Institute for Biogeochemistry and MARUM Center for Marine Environmental Sciences of the University Bremen. “The first great emergence of marine algae probably had crucial consequences for the further evolution of environmental conditions and life on Earth – a complex process whose details we are only beginning to understand.”
(Text: PIK, Potsdam)

Original publication
Feulner, G., Hallmann, C., Kienert, H. (2015): Snowball cooling after algal rise. Nature Geoscience (online)

Christian Hallmann
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