Did the Snowball Earth Have a Slushball Ocean?
In 1964, Brian Harland of Cambridge University took stock of the widespread occurrence of certain ancient glacial deposits in the geologic record and suggested something nearly unthinkable: at some time in Earth's distant past there was an ice age so extreme that the world virtually froze over. The debate over this astonishing idea continued over the decades, but it gained new life and credibility in 1992 when Caltech's Joseph Kirschvink re-assessed the most recent geologic data and arrived at a similar conclusion. By this time, however, it was apparent that the phenomenon had happened more than once! Kirschvink dubbed his view of this snow- and ice-laden world the "snowball Earth" hypothesis.
We know a good deal more now about the snowball Earth intervals than in Harland's day. Between 543 and 800 million years ago, during the Neoproterozoic Era, the Earth twice dipped into — and then apparently recovered rapidly from — deep freezes that most geologists consider to have been among the coldest climates in Earth's history. The older interval, called the Sturtian glaciation, occurred about 750 million years ago, and the younger interval, called the Varanger glaciation, took place roughly 590 million years ago.
Using a special lab technique that measures the paleomagnetism of ancient glacial sediments, researchers have determined that continental-scale ice sheets once extended to latitudes as low as 10° (the latitude of modern Costa Rica). However, questions remain about the detailed timing of glacial advances and retreats and the maximum extent of the snow and ice cover. Especially controversial is whether the tropical oceans were also totally covered by sea ice, as suggested by Paul Hoffman and Daniel Schrag of Harvard University, a factor that would have had enormous consequences for the continued existence and evolution of early multi-cellular life.
In order to better understand the climatic processes that may have been responsible for producing the snowball Earth intervals and the subsequent recovery, we used the GISS GCM to conduct a series of climate simulations of the Varanger glaciation. Paleoclimate modeling presents some unique challenges not posed by climate studies of the near future, such as a reduced luminosity of the Sun. Other climate forcings such as the amount of atmospheric carbon dioxide are not well constrained by the geologic record, and so for the major climate forcings we wanted to examine, we selected arbitrary values based upon other climatological or geological considerations.
The key climate forcings we altered in our simulations were:
- Solar Luminosity: The energy output of the Sun was about 4% less 600 million years ago than it is now.
- Geographic Distribution: The locations and shapes of the continents were quite different 600 million years ago. We used a continental distribution based upon the available geologic evidence.
- Atmospheric CO2: The extreme nature of the snowball Earth intervals suggests that these periods were times of reduced greenhouse gases such as carbon dioxide. We ran simulations with atmospheric CO2 levels set to 315 parts per million (the value measured in 1958), 140 ppm (half the pre-industrial value), and 40 ppm (an extreme example).
- Ocean Heat Transport: We simulated the potential effects of decreased and increased heat transports by the ocean from the tropics toward the poles using values 50% less and 50% greater than the modern global average.
We found that no single one of these forcings yielded surface air temperatures and snowfall rates that would allow snow to accumulate on low-latitude continents, as the geologic record indicates. The reduced solar luminosity and 40 ppm carbon dioxide simulations did, however, show that snow and ice accumulations on land in mid-latitudes could have occurred during a snowball Earth interval if either of these conditions actually existed.
Combining climate forcings causes further cooling and extends the annual average freeze line, as well as snow and ice accumulations, into the outskirts of the tropics. For example, experiments combining reduced solar luminosity with either 140 ppm atmospheric CO2 or reduced ocean heat transports produced cooling patterns similar to the experiment using just the 40 ppm atmospheric CO2 altered forcing.
Ultimately, only the most extreme scenarios in our study yielded tropical conditions that were cold enough to permit significant snow and ice accumulation on the low latitude continents. With atmospheric CO2, solar luminosity and ocean heat transports all reduced together, sea ice never completely covered the tropical oceans. With as much as 30% of the oceans remaining ice-free, the snowball Earth may instead have been more of a slushball.
In the last few years other researchers using different climate models have found similar, but not identical, results. In general, though, it appears that the more explicitly the study represents the ocean physics in the model, the more difficult it becomes to freeze over the oceans at lower latitudes. Modeling results suggest, therefore, that during periods of extreme cold, low-latitude continental ice sheets may be easier to produce than a global ocean ice cover — leaving breathing space for life in an almost unimaginably cold time.
Chandler, M.A., and L.E. Sohl 2000. Climate forcings and the initiation of low-latitude ice sheets during the Neoproterozoic Varanger glacial interval. J. Geophys. Res. 105, 20737-20756.
Harland, W.B. 1964. Critical evidence for a great Infracambrian glaciation. Geologische Rundschau 54, 45-61.
Hoffman, P.F., and D.P. Schrag 2000. Snowball Earth. Scientific American 282, 68-75.
Kirschvink, J.L., 1992. Late Proterozoic low-latitude global glaciation: The snowball Earth. In The Proterozoic Biosphere: A Multidisciplinary Study (J.W. Schopf and C. Klein, Eds.), pp. 51-52. Cambridge University Press.
Sohl, L.E., N. Christie-Blick, and D.V. Kent 1999. Paleomagnetic polarity reversals in Marinoan (ca. 600 Ma) glacial deposits of Australia: Implications for the duration of low-latitude glaciation in Neoproterozoic time. Geol. Soc. Amer. Bull. 111, 1120-1139.