The Snowball Earth
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The Snowball Earth
Many lines of evidence support a theory that the entire Earth was ice-covered for long periods 600-700 million years ago. Each glacial period lasted for millions of years and ended violently under extreme greenhouse conditions. These climate shocks triggered the evolution of multicellular animal life, and challenge long-held assumptions regarding the limits of global change.
by Paul F. Hoffman and Daniel P. Schrag
August 8, 1999
CONTENTS
Introduction
Frozen and fried
Sea ice at the equator
The acid test
Survival and redemption of life
Snowball episodes and Earth history
Further reading
Illustrations
Introduction
Geology tells us that the Earths climate is subject to change on various timescales, but what are the limits to climatic variability? Over the last million years that constitute the Pleistocene epoch, the time in which humans evolved, continents bordering the North Atlantic Ocean were periodically glaciated at intervals governed by changes in the Earths orbit around the Sun. At the height of the last ice age, a mere 21,000 years ago, much of North America and Europe were covered by glaciers over 2 kilometers thick, causing sea level to drop by 120 meters. The chill was global: land and sea ice combined to cover 30 percent of the Earths surface, more than at any other time in the last 500 million years. Although these are dramatic examples of the variability of Earths climate, they pale by comparison with climatic events near the end of the Neoproterozoic eon (1000-543 million years ago), events that immediately preceded the first appearance of recognizable animal life around 600 million years ago.
In 1964, Brian Harland at Cambridge University postulated that the Earth had experienced a great Neoproterozoic ice age. He pointed out that Neoproterozoic glacial deposits, similar in type to those of the Pleistocene, are widely distributed on virtually every continent. Harland could only speculate on the positions of continents in Neoproterozoic time and could not rule out the possibility that various continents were glaciated at different times as they drifted close to the poles. Nevertheless, he inferred that ice lines penetrated the tropics from the occurrence of glacial deposits within types of marine sedimentary strata characteristic of low latitudes. What could cause glaciers to reach sea level near the Equator? Climate physicists were just developing mathematical models of the Earths climate, providing a new perspective on the limits to glaciation. The Earths climate is fundamentally controlled by the way that solar radiation interacts with the Earths surface and atmosphere. We receive ~343 watts per square meter of radiation from the Sun. Some of this is reflected back to space by clouds and by the Earths surface, but approximately two thirds is absorbed by the Earths surface and atmosphere, increasing the average temperature. Earths surface emits radiation at longer wavelengths (infrared), balancing the energy of the radiation that has been absorbed. If more of the solar radiation were reflected back to space, then less radiation would be absorbed at the surface and the Earths temperature would decrease. The surface albedo is a measure of how much radiation is reflected; snow has a high albedo (~0.8), seawater has a low albedo (~0.1), and land surfaces have intermediate values that vary widely depending mainly on the types and distribution of vegetation. When snow falls on land or ice forms at sea, the increase in the albedo causes greater cooling, stabilizing the snow and ice. This is called ice-albedo feedback, and it is an important factor in the waxing (and waning) of ic e sheets.
At the same time that Harland was examining Neoproterozoic glacial deposits, Mikhail Budyko at the Leningrad Geophysical Observatory, working with simple two-dimensional energy-balance climate models, found that the ice-albedo feedback created an instability in the Earths climate system. Budyko showed that if the Earths climate were to cool, and ice were to form at lower and lower latitudes, the planetary albedo would rise at a faster and faster rate because there is more surface area per degree of latitude as one approaches the Equator. In his model, once ice formed beyond a critical latitude (around 30 degrees north or south, equivalent to half the Earths surface area), the positive feedback became so strong that temperatures of the surface plummeted, yielding a completely frozen planet. The relatively small amount of heat escaping from the Earths interior is sufficient to prevent the oceans from freezing to the bottom, but would still allow a kilometer thick cap of sea ice to form, thicker at the poles and thinner at the Equator.
Frozen and Fried
Budykos model results helped stimulate interest in the science of climate modeling, but few believed that the Earth had ever actually experienced a runaway ice albedo. First, it was assumed that such a catastrophe would have extinguished all life, contrary to microscopic evidence of extant life forms in rocks as old as 3500 million years. Second, once the Earth became totally ice-covered, the high albedo would drive surface temperatures so low that there seemed no means of escape. Had such a glaciation ever occurred, Budyko reasoned, it would have been permanent. The first of these objections began to fade in the late 1970s with the discovery of remarkable communities of organisms living in deep-sea hydrothermal (hot water) vents, and later in the extremely cold, dry, mountain valleys of East Antarctica. Some of these organisms appeared capable of surviving global glaciation and their existence in the Neoproterozoic was unquestioned—molecular studies showed that they disproportionately represent the oldest branches in the universal tree of life.
The key to the second problem—reversing the ice-albedo feedback—is plate tectonics. Climate scientists have long known that the amount of carbon dioxide in the atmosphere plays an important role in determining the Earths temperature because it is a “greenhouse” gas, meaning that it absorbs infrared radiation emitted from the Earths surface. Over time scales of human lifetimes, the amount of atmospheric carbon dioxide