SNOWBALL EARTH: THE STORY OF THE GREAT GLOBAL CATASTROPHE THAT SPAWNED LIFE AS WE KNOW IT

SNOWBALL EARTH: THE STORY OF THE GREAT GLOBAL CATASTROPHE THAT SPAWNED LIFE AS WE KNOW IT Gabrielle Walker, 2003, 269 pp., $24.95, hardbound, Crown Publishing Group, ISBN 0-609-60973-4

Five years ago, Paul Hoffman and colleagues at Harvard University published a paper in Science (Hoffman et al. 1998) in which they argued that the Earth experienced global glaciation at least twice during the Neoproterozoic Era, once at 750 million years ago and again at 600 million years ago. This "snowball Earth" hypothesis, which had been suggested by Joe Kirschvink at Caltech several years earlier (Kirschvink 1992), has caught the attention of both Earth scientists and the general public. This book by Gabrielle Walker, a former features editor and current contributing editor with New Scientist magazine, documents the development of this idea and provides interesting insights into the personalities of the major scientific players, particularly Hoffman himself.

The book is an entertaining read for scientists and nonscientists alike. (I have direct anecdotal evidence for this because my father picked it up and read through the whole book during a recent family visit.) It starts with a description of Hoffman's first, remarkably good, attempt at the Boston marathon-he ran 2:28 and finished ninth-and includes travelogues of several field trips on which the author was invited. The most harrowing of these was to Namibia, where Hoffman found the glacial deposits and surrounding strata on which his theory was based, and where the author experienced a close encounter with an African elephant. (She survived, but was left with mixed emotions about the quality of the travel services provided by her scientific tour guide.)

From a scientific point of view, the book provides an excellent introduction to the geologic evidence for snowball Earth and a somewhat less well-researched review of the accompanying climate theory. The primary geologic evidence comes from paleomagnetism. Glacial deposits in Australia and elsewhere are found interbedded with igneous rocks in which the remnant magnetization is parallel to the original bedding plane of the rocks, indicating a low-latitude origin. The Australian geologist George Williams and his colleagues had realized this decades ago, but they focused on Williams's alternative, "high-obliquity hypothesis" (Williams 1975), which has received little support in recent years. During the late 1980s, Joe Kirschvink performed a definitive "fold test" on some of the Australian rock samples. He found that the embedded magnetic field lines were bent along with the rocks themselves, indicating that the magnetization was emplaced before the rocks were folded tectonically. Shortly afterward, he presented his findings at a symposium at UCLA and coined the term snowball Earth to describe how this might have happened (Kirschvink 1992). On a low-obliquity Earth it is difficult, though not necessarily impossible (Hyde et al. 2000), to glaciate continents in the Tropics without having ice everywhere else as well. Kirshvink's story and other interesting anecdotes about his research, such as his finding that southern hemisphere magnetotactic bacteria are smarter than their northern hemisphere cousins, are described in the book. There are other pieces of evidence, as well, that fit the snowball theory, including thick "cap carbonates" overlying the glacial deposits (presumably formed by removal of volcanic CO2 that built up in the atmosphere during the glaciations) and the reappearance of banded-iron formations, or BIFs, formed from ferrous iron that accumulated in an anoxic deep ocean cut off by the ice from the O2-rich atmosphere. Nongeologists will find much to learn about these topics from the discussions in the book.

From a climatologist's standpoint, the theory of how global glaciation might occur is equally interesting. The author cites the pioneering work on ice albedo feedback in energy balance climate models (EBMs) by Budyko (1969) and Sellers (1969), although she misses the earlier work by Eriksson (1968) that Hoffman has been careful to point to in his more recent papers and talks. All of these authors noted that ice albedo feedback could lead to runaway glaciation if the ice line advanced too close to the equator. Walker is less accurate in describing the alternative "slushball Earth" model of Hyde et al. (2000). Of this work she says, "However much the modelers wanted to generate an ice-covered world, their models wouldn't oblige. . . . The modern models stuck at a sort of halfway house, where ice advanced to somewhere near the Tropics, but no farther." In fact, the GENESIS 2 general circulation model (GCM) run by Hyde et al. for 600 million years ago (5% reduced solar constant) is stable for 2.5 times present CO2, but "goes snowball" for CO2 levels lower than this value. This reviewer is particularly attuned to the possibility that Earth could globally glaciate, as I have spent many hours trying to figure out how Earth avoided such a fate earlier in its history when solar luminosity was even lower.

As a scientific aside, the question of exactly what it takes to trigger global glaciation has still not been answered satisfactorily. One critical issue is the albedo of ice and snow. The Neoproterozoic simulation performed by Chandler and Sohl (2000) with the GISS GCM did not go snowball at any plausible CO2 level, but then their spectrally-averaged sea-ice albedo was 0.45, compared to (temperature-dependent) values of 0.58-0.68 in GENESIS 2. Another key issue concerns meridional heat transport rates. EBMs with diffusive heat transport (and GCMs with diffusive slab oceans) predict instability when the ice line moves equatorward of 25�-30�; however, the critical latitude for instability changes when different heat transport mechanisms are considered. The tropics become more resistant to glaciation if meridional heat transport is inhibited so that they cannot export heat to higher latitudes, or when rapid heat exchange occurs within the tropics themselves (Lindzen and Farrell 1977). Recently, Poulsen (2003) has published calculations using a coupled atmosphere-ocean GCM (FOAM version 1.4) that appears highly resistant to global glaciation, apparently because of a combination of low ocean heat transport and low transport of latent heat from the tropical atmosphere to midlatitudes. Whether or not this conclusion is robust remains to be seen.

Snowball Earth also misses the mark slightly in tracing the evolution of ideas. Dan Schrag (the "idea man," according to the text) gets credit for the thought that low-latitude continents predispose the Earth to global glaciation by inhibiting the normal negative feedback between CO2 and silicate weathering, whereas this idea had already been described in considerable detail more than a decade earlier (Marshall et al. 1988). Joe Kirschvink, who truly does deserve credit for the basic hypothesis of Snowball Earth, is also credited with thinking of the way out (volcanic outgassing leads to CO2 buildup), whereas that idea had been published even earlier (Walker et al. 1981) and was rehashed in the same volume in which Kirshvink's paper appeared. The author also makes no mention of the evidence for snowball Earth episodes in the Paleoproterozoic (~2.3 billion years ago) (Evans et al. 1997), a phenomenon that may have been linked to the initial rise in atmospheric O2 (Pavlov et al. 2000).

This is quibbling, however. Snowball Earth is an engaging and scientifically useful book. In my view, the status of this hypothesis is similar to that of the asteroid-impact hypothesis for the extinction of the dinosaurs shortly after the Alvarezes published their seminal paper in 1981. Despite the presence of a global iridium layer that no other theory could explain, many paleontologists remained skeptical until the "smoking gun" (the Chicxulub crater) was found 10 years later. (Some paleontologists remain skeptical, of course, but they are fewer and fewer in numbers, and their skepticism will eventually die with them.) The equivalent of the iridium layer for the snowball Earth hypothesis is the paleomagnetic evidence for low-latitude glaciation. The cap carbonates and BIFs may constitute the smoking gun for this hypothesis, but these pieces of evidence are more subject to interpretation than is a 200-km diameter crater. Gabrielle Walker's book should help to popularize the snowball Earth hypothesis and may ultimately help it to become an accepted part of Earth science.

-JAMES F. KASTING

[Reference]

REFERENCES

Budyko, M. I., 1969: The effect of solar radiation variations on the climate of the Earth. Tellus, 21, 611-619.

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, 20 737-20 756.

Eriksson, E., 1968: Air-ocean-icecap interactions in relation to climatic fluctuations and glaciation cycles. Meteorol. Monogr., 8, 68-92.

Evans, D. A., N. J. Beukes, and J. L. Kirshvink, 1997: Low-latitude glaciation in the Proterozoic era. Nature, 386, 262-266.

Hoffman, P. F., A. J. Kaufman, G. P. Halverson, and D. P. Schrag, 1998: A Neoproterozoic snowball Earth. Science, 281, 1342-1346.

Hyde, W. T., T. J. Crowley, S. K. Baum, and W. R. Peltier, 2000: Neoproterozoic 'snowball Earth' simulations with a coupled climate/ice-sheet model. Nature, 405, 425-429.

Kirschvink, J. L., 1992: Late Proterozoic low-latitude global glaciation: the snowball Earth. The Proterozoic Biosphere: A Multidisciplinary Study, J. W. Schopf and C. Klein, Eds., Cambridge University Press, 51-52.

Lindzen, R. S., and B. Farrell, 1977: Some realistic modifications of simple climate models. J. Atmos. Sci., 34, 1487-1500.

Marshall, H. G., J. C. G. Walker, and W. R. Kuhn, 1988: Long-term climate change and the geochemical cycle of carbon. J. Geophys. Res., 93, 791-802.

Pavlov, A. A., J. F. Kasting, L. L. Brown, K. A. Rages, and R. Freedman, 2000: Greenhouse warming by CH4 in the atmosphere of early Earth. J. Geophys. Res., 105, 11,981-11,990.

Poulsen, C. J., 2003: Absence of a runaway ice-albedo feedback in the Neoproterozoic. Geology, 31, 473-476.

Sellers, W. D., 1969: A climate model based on the energy balance of the Earth-atmosphere system. J. Appl. Meteor., 8, 392-400.

Walker, J. C. G., P. B. Hays, and J. F. Kasting, 1981: A negative feedback mechanism for the long-term stabilization of Earth's surface temperature. J. Geophys. Res., 86, 9776-9782.

Williams, G. E., 1975: Late Precambrian glacial climate and the Earth's obliquity. Geol. Mag., 112, 441-465.

[Author Affiliation]

James Kasting is a professor of geosciences and meteorology at The Pennsylvania State University. His research interests are in atmospheric and climate evolution on Earth and other planets.