Chill out

Natural History, July, 2005 by Robert Anderson

Summer heat getting you down? Then imagine setting your time machine for a geologic era more than half a billion years ago, when the entire globe, not just the polar regions, was enveloped in ice. Average surface temperatures reached minus fifty-eight degrees Fahrenheit. Geologists have been kicking the "snowball Earth" hypothesis around for more than a decade, ever since the discovery that glacial features occur in Precambrian rocks over a wide area of the planet. According to the theory, Earth was exposed to as many as four freeze-over episodes, each followed by melting brought on by volcanoes [see "The Longest Winter," by Gabrielle Walker, April 2003]. Some scientists think the catastrophic climate swings may have sparked the so-called Cambrian explosion, the geologically sudden evolution of complex animal life on Earth.

Try the snowball Earth theory on for size by listening first to a five-minute audio explanation that covers the basics, at the North Country Public Radio online site (go to www.northcountry publicradio.org/news/natural.php and scroll down to "Natural Selections: Return to Snowball Earth"). Or go straight to a lengthy but informative text, "'Snowball' Scenarios of the Cryogenian," posted in 2002 on an impressive Web site called "Palaeos: The Trace of Life on Earth" (www.palaeos.com/Pro terozoic/Snowballs.html).

To get a good sense of the extreme climatic fluctuations conjectured by the theory, visit the site of the "Paleomap Project" (www.scotese.com/). From the menu at the top left of the page, click on "Climate History." Scroll down here to the chart titled "Ice House or Hot House." Earth's climate history is a fascinating topic, and the National Oceanic and Atmospheric Administration has posted information about it in a highly accessible form, organized as a time line (www.ngdc.noaa. gov/paleo/ctl/resourcebeyond.html). On the home page, scroll down to view a time chart of Earth's ice ages. You can click on "About CTL," on the menu at the top of the page, to learn how best to take advantage of the site's many resources.

In the 550 million years since the end of the Precambrian era, geologic processes have melted, twisted, reoriented, deeply buried, or destroyed much of the oldest rock record. Yet through painstaking detective work, geologists have assembled an accurate map of the continents at the time of the Cambrian explosion. To view a map titled "Late Precambrian Supercontinent and Ice House World," return to the "Paleomap Project" (www.scotese. com/), and from the list at the left of the home page, click on "Earth History." Once there, click "Precambrian" on the menu at the left of the page.

Joseph L. Kirschvink, a geologist at Caltech specializing in paleomagnetics, coined the term "snowball Earth" in 1992. At his Web site (www.gps.cal tech.edu/users/jkirschvink), you can access information about past and present projects of Kirschvink and his laboratory group. One fascinating feature of his site is an animated reconstruction of continental displacements at about the time of the Cambrian explosion (scroll down the page and click on the blue hypertext "iitpw.mov"). The Internet is also the place to go for dissident views. At the Web site of NASA's Goddard Institute for Space Studies (www.giss.nasa.gov/research/ briefs/sohl_01/), you'll find an article by two geologists at Columbia University. They simulated climatic conditions during the late Precambrian era, and found that, even in worst-case scenarios, with reduced solar radiation and low levels of greenhouse gases, the oceans would have remained ice free near the equator. They dubbed their model "slushball Earth."

ROBERT ANDERSON is a freelance science writer living in Los Angeles.