The Discovery of Global Ice Ages by Louis Agassiz

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The Discovery of Global Ice Ages by Louis Agassiz

Overview

Today, the concept of thick ice sheets covering large portions of the globe is a familiar one. We now know that ice sheets advance and retreat, altering landscape and climate as they do so. This knowledge, however, is relatively recent, the result of a great deal of geological deduction on the part of the Swiss scientists Johann von Charpentier (1786-1855) and Jean Louis Agassiz (1807-1873). Charpentier first advanced a reasonable scientific explanation of a recent ice age to explain many of the phenomena found in the Alps, while Agassiz fought to have the idea win acceptance in both the scientific and popular arenas. Acceptance of this theory has changed the way we view Earth and its climate, including recent debates on global warming.

Background

For centuries it had been noticed by scientists and local residents that many areas of northern Europe and North America possessed unusual jumbles of sand, gravel, mud, and silt that showed no consistent layering. Many of these jumbles contained large rocks that were obviously brought from some other location and left, presumably deposited by some outside agent.

Initial explanations ranged widely and included some ideas that now seem silly. The most widespread explanations referred to the biblical flood and assumed that these deposits were simply left behind when the flood waters receded from the face of the Earth. A variant on this theory held that many glacial phenomena resulted from icebergs afloat in the post-flood waters, gouging rocks as they floated in shallow waters and depositing rocks and other sediments frozen into their undersides. In fact, the term "glacial drift" as a synonym for what is now known as "till" is a carry-over from such theories. Other explanations included suggestions that glacial erratics (the large rocks apparently brought from some distance) were launched by underground pressure, as though shot from a cannon; that water had suddenly issued forth from now-lost caverns; or that large amounts of water had recently condensed out of the atmosphere and left these deposits. Finally, in the 1820s, several Swiss men began to formulate some other ideas.

The first of these was a chamois hunter named Jean-Pierre Perraudin, who convinced engineer Ignace Venetz (1788-1859) that glacial marks in Alpine valleys were, indeed, left by previous larger glaciers. Venetz expanded on this idea over the next several years, albeit without much success, but was able to convince Charpentier of its essential accuracy. In 1836, Charpentier took Agassiz on a trip through the Alps, which proved utterly convincing. Agassiz not only accepted Venetz's glacial theories, but expanded them further to suggest that, in addition to the expansion of Alpine glaciers, other glaciers descended from the north to cover virtually all of Europe and North America. Unlike Charpentier, Agassiz was an active and ultimately convincing advocate of the glacial theory, although it took many years for the scientific community to fully accept the idea of nearly two miles (3.2 km) of ice sitting atop most of Europe and North America. Since that time, geologists have also come to realize that Earth has undergone many ice ages, going back more than two billion years.

Impact

The theory of ice ages has impacted both the science community and the public in a number of ways. For scientists, understanding ice ages has provided useful tools for better understanding geology, climate, and our current world. The public has shown a great interest in the concept of ice ages and has gained a better appreciation for the variations in climate that the Earth undergoes, even without human intervention. This, in turn, has led to an increased awareness of the possible impacts of humanity on the environment.

Understanding that Earth had undergone one relatively recent ice age helped to explain many problematic geologic deposits found in the Alps, northern Europe, and North America. It did not explain many other, similar deposits found elsewhere. Accepting a single ice age, however, made it easier to accept multiple ice ages, and this has become the standard explanation for the great many glacier-related deposits found on Earth. In fact, it is now assumed that at least four major glaciations have occurred in the northern hemisphere over the past million years or so. In addition, ancient glacial deposits and other evidence suggest strongly that ice ages are not limited to the current era but have, instead, occurred periodically throughout the history of the Earth. This, in turn, causes scientists to wonder why.

Questioning the origins of ice ages has been a fruitful endeavor. The periodicity of northern hemisphere ice advances led to the suggestion by the Serbian geophysicist Milutin Milankovich (1879-1958) that regular variations in the inclination of the Earth's axis coupled with regular orbital variations and other factors periodically coincide to lower global temperature long enough to start a glacial advance. Geologists—once the theory of plate tectonics was accepted—suggested that large continents periodically congregate near one of the poles, causing the land to cool and spurring glacial advances. Others feel that long-term variations in weather may follow changes in solar activity, the passage of the solar system through interstellar dust clouds, or other extraterrestrial events. All of these suggestions have resulted in research that, even if inconclusive with respect to ice ages, has led to a better understanding of our climate and factors that may influence it. Perhaps the single most important outcome of all these research efforts, however, is the realization that Earth's climate does change dramatically over time.

Research efforts at the present time are aimed at determining reasons for sudden climatic shifts, because of fears of global warming caused by human activities. The reasoning is that, if we can understand what caused temperatures to change, we can better understand whether we can cause global warming. However, with all of these studies, it must first be acknowledged that the typical temperature of Earth is much warmer than present global temperatures. In fact, we are currently in what is known as an interglacial period, meaning that the glaciers have temporarily retreated, but there is no reason to assume that they will not again advance in the future. On the other hand, it is also entirely possible that the most recent ice age has, in fact, ended, in which case we would expect global temperatures to begin rising, ice caps to begin melting, and glaciers to be retreating. While scientists have not agreed on the time, magnitude, or direction of these changes (i.e., whether the glaciers will advance again or continue their recent retreat), they unanimously agree that there will again be a major climatic change. There is a growing realization that the climate changes and that human actions may be responsible. This, in turn, is likely to encourage further policies, such as low-emissions vehicles, substitute fuels, and so forth, that are designed to reduce the likelihood and magnitude of future global warming.

Whether the glaciers will melt or advance a fifth time is still not known with any degree of certainty. Similarly, whether human activities will hasten an advance, prevent it, or have no impact remains unknown. What is known is that ice once covered large parts of the Earth and is now gone, leaving irrefutable proof of its existence and power.

P. ANDREW KARAM