Life on ice

Natural History, April, 2003 by Peter Brown

I know, the picture on our cover this month makes it look as if we re all about to collide with an iceberg. And, truth be told, there seems plenty to be anxious about. A few weeks ago, those of us who live or work in Manhattan (the editorial staff of this magazine, for instance) thought that we had gone from yellow alert to orange, along with the rest of the country. Then the mayor reminded us that we'd been there, done that--New York City had been stuck in orange ever since the code went into effect.

Maybe it's small consolation--but things could be worse, much worse. Compared with what the Earth has undergone in its geological past, even the many human insults to our planet seem puny and fundamentally insubstantial. A few weeks ago Gabrielle Walker stopped by our offices to show us her latest report about what's hot on the geological front. A grand idea, first conceived many years ago but rejected soon afterward, has now returned with such compelling vitality--and is so well supported by the evidence of rocks all over the world--that it is stimulating new work and new thinking across an entire scientific community. Walker's story, with apologies to Laura Ingalls Wilder, is called "The Longest Winter" (page 44).

Walker isn't kidding. The "winter" in question lasted as long as 10 million years. The average annual temperature at the surface of the Earth hovered around 40 degrees below zero. Conditions were antarctic.

Most ice ages--certainly the ones people are most familiar with--are self-limiting: the ice advances, then retreats once again. The retreats may be the result of global warming by atmospheric greenhouse gases, among them carbon dioxide (C[O.sub.2]). Exposed rock continually draws C[O.sub.2] out of the atmosphere and chemically locks up the carbon. During an ice age, however, the more the Earth's landmasses get covered by ice, the less rock is exposed to C[O.sub.2], and so the more C[O.sub.2] remains in the atmosphere. The atmospheric C[O.sub.2] eventually warms the Earth and reverses the march of ice.

But about 750 million years ago the continental tectonic plates haphazardly arranged themselves around the equator. That seems to have turned an "ordinary" ice age into a runaway catastrophe. Even after the polar ice began advancing, continental rock remained exposed, and it continued sucking carbon out of the atmosphere. The warming effect of atmospheric C[O.sub.2] steadily diminished. By the time the ice reached the tropics, it was too late. Ice quickly covered what was left of the Earth. Only the slow release of C[O.sub.2] by volcanoes eventually restored the greenhouse warming and enabled life to get a fresh start.

There's clearly a hopeful message in that fresh start. April, at least in the temperate zone of the Northern Hemisphere, brings mud, blossoms, new life--in short, the promise of spring. So, lest winter seem too prominent a topic for an April issue of Natural History, two photographers bring us their contrasting visions of renewal. Subhankar Banerjee portrays the robust glory of the vernal Arctic, which must gather all its life forces in the short months between breakup and freeze up (see "Arctic Covenant," page 58). Christian Ziegler, at the beginning of the Panamanian rainy season, documents the fragile beauty of falling blossoms that retain their color for just a few hours (see "So Fleeting a Spring," page 6).

And there it is, the simplest, most bracing antidote nature has for all our anxieties: Spring will come again. Count on it.