Snowball Earth: Joe Kirschvink

Earth's global climate has been relatively stable over the past 10,000 years, a fact that has permitted human civilization to develop and spread across the globe. From the record of climatic signals in rocks, we know that this recent interval of calm was unusual, and that during the past 500 million years Earth experienced the buildup and decay of polar ice caps, abrupt heating pulses, and even prolonged greenhouse conditions where the poles were warm enough for dinosaurs to roam.

However, studies of even older rocks reveal that Earth experienced a series of extraordinarily severe glaciations, in which the advancing ice fronts actually marched over the carbonate platforms, and reached the equator. These rocks give strong hints that floating ice sheets several kilometers thick covered the oceans, sealing them off from exchange with the atmosphere, curtailing photosynthesis, and shutting down the global hydrological cycle nearly completely.

Dr. Kirschvink called this condition the "Snowball Earth" back in 1992. Publication of this hypothesis triggered a "Grand Debate" in the Earth Sciences, and led to substantial modification of global climate models. However, the value of a scientific hypothesis depends upon how well it can explain a variety of different observations, and whether or not it leads to testable predictions.

Rather than melting away during the past 15 years, the Snowball Earth hypothesis has grown considerably in its power to account for a host of new observations about these Precambrian glacial intervals, and has led to new insights about the rise of atmospheric oxygen and major events in the evolution of life.

Joe Kirschvink is the Nico and Marilyn Van Wingen Professor of Geobiology at Caltech.

Joe Kirschvink
www.snowballearth.org

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