Snowball Earth

  • Gabrielle Walker
Crown Publishers: 2003. 256 pp. $24.95 Bloomsbury: 2003. £16.99
Credit: ILLUSTRATION BY DAVID NEWTON

Snowballs can be fun. But when they have rocks in them, the arguments start, just as between Calvin and Susie in the comic strip Calvin and Hobbes. Snowball Earth is a Hobbes-eye view of the succession of missiles, ambushes and strategies in the long-running skirmishes over the idea that the Earth may have become so cold 600 million years ago that even the tropics froze over. The scene is superbly described, and each throw is meticulously followed from the gathering of the snow to the splat on the target. But a warning: it gets personal and gossipy — snowballologists will need many measures of generous spirit to read this.

The snowball debate first rumbled around the darker recesses of the Spitsbergen room of Cambridge University's Sedgwick Museum 30–40 years ago. Brian Harland had suggested that there had been a great glaciation just before the start of the Cambrian period. Few believed him beyond the confines of the Sedgwick Museum, and even within I recall scepticism (including my own). The new ideas of plate tectonics could surely explain most things, even glacial sediments closely juxtaposed to tropical deposits. We knew that rapid continental motion occurred: India's march northwards showed that. Nevertheless, Harland serenely held to his views, supported later by Mike Hambrey. Then Joe Kirschvink published the briefest of notes on 'Snowball Earth', lost in a fine book, The Proterozoic Biosphere, which was widely read but so vast that its most common use is as a doorstop. There languished the notion.

The Canadian government intervened, deciding in its wisdom to deconstruct what was then arguably the finest (and economically most useful) national geological survey on Earth. Paul Hoffman, a distinguished Arctic field geologist, was unable to work as usual. He departed south and turned his attention to Namibian rocks laid down in the latest Proterozoic, the time just before the Cambrian. What he found were rocks deposited in glacial conditions. But puzzlingly, these rocks were closely juxtaposed with carbonate sediments laid down in warm conditions. Moreover, the succession was very similar to the Spitsbergen rocks that Harland's group had studied.

The 'snowball' idea states that around 600 million years ago there were episodes when even in the tropics the ocean surface froze. The white, icy planet then reflected so much sunlight that the condition persisted. Life luckily survived, presumably in Noah's Arks of habitat in warm water around hot springs or photosynthesizing in open channels in the icy ocean. Eventually, as carbon dioxide continued to degas from volcanoes, the atmospheric greenhouse became so strong that tropical melting began. As ice melted, the newly darkening planet would have absorbed heat, a positive feedback leading to runaway warming. This would have been exacerbated by the release of dissolved carbon dioxide from the heating oceans and methane from vast hydrate stores, so the snowball suddenly flipped to a hothouse hell.

Gabrielle Walker recounts in racy detail the chain of discovery. Hidden in the social froth, she provides nuggets of gold: simple explanations of complex evidence. Carbon isotopes, facies analysis, methane hydrates, metazoan evolution — complicated topics all — slip down with spoonfuls of scandal sugar. The book lucidly exposes the observations that shaped the hypothesis and its testing, the critics and the bases of their criticisms, and the subsequent growth of the idea as the debate provoked more and more insights. Just as Sir Harold Jeffreys' opposition to continental drift was crucial in provoking a rigorous theory of plate tectonics, so Walker shows clearly how disputes about the snowball (for example, the contrary views of Nick Christie-Blick), brought deeper understanding.

Did Snowball Earth really happen? Sitting next to a scientific knight of excellent judgement at the end of Hoffman's peroration on receiving the Wegener Medal in 2001, I noted his warm applause (did he stamp his sandals? Was he clapping act or fact?), while I was still weighing the balance. Snowball Earth is entering the textbooks. Walker quotes a wildly improbable (but splendidly diverting) speculation that Hoffman will become a lord. In reality, like most scientists, geologists welcome but do not seek applause or rewards (or they'd have chosen easier and more remunerative jobs): their thrill is the chase. The details (snowball or slushball) remain hotly argued, and there are many false alarms and excursions, but there is an emerging consensus that something odd is being hunted down.

Is it important? The notion that Earth's climate has alternative stable states and can flip between them carries a warning for today. The stable climate of the past 10,000 years may be a freak — the hidden nightmare monsters that made the snowball may still be out there, especially methane. The Arctic sea floor and the continental shelves worldwide have widespread methane hydrates that will be released by warming. We are already turning Spitsbergen into fried ice-cream. Is it time to learn from geology's nightmares? Kyoto began the estrangement between the United States and Europe: maybe it could also begin the reconciliation. To get technical, either by a stand-alone methane agreement, or by resetting Kyoto's timescale for calculating global-warming potential from 100 years to a 'poor-nation-friendly' 20 years, and hence by placing more emphasis on vigorous cuts in methane emission, we may find a cheap-to-implement accord, acceptable to the United States, the European Union and poorer nations, that actually tackles radiative warming now, within a few decades.

Noah's rainbow was a covenant for all living species, not just humanity. Humanity's emissions now wilfully block the infrared part of that promise. If our nations corrupt the rainbow, is it time to search out supplies of gopher wood and start collecting eukaryotes two-by-two, trying to keep our genetic company afloat while the greedy world goes into liquidation? In the meantime, Snowball Earth makes good spring reading. And when you are done, pass it on to any handy teenager, who will be enthralled, if not by the science, then by the storytelling.