Richard B. Alley - The Two-Mile Time Machine

THE TWO-MILE TIME MACHINE

PRINCETON UNIVERSITY PRESS Princeton and Oxford

Copyright 2000 by Princeton University Press

Published by Princeton University Press, 41 William Street,

Princeton, New Jersey 08540

In the United Kingdom: Princeton University Press,

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Library of Congress Cataloging-in-Publication Data

Alley, Richard B.

The two-mile time machine : ice cores, abrupt climate change, and our future / Richard B. Alley.

p. cm.

Includes bibliographical references and index.

ISBN 0-691-00493-5 (alk. paper)

1. Paleoclimatology. 2. Climatic changes. 3. IceGreenland

Analysis. I. Title.

QC884.A55 2000

551.60901dc21 00-036730

This book has been composed in ITC Garamond Light with Gill Sans display.

The paper used in this publication meets the minimum requirements of ANSI/NISO Z39.48-1992 (R 1997) (Permanence of Paper)

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Printed in the United States of America

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Why we might care what happened to Earths climate in the past, and what might happen in the future

We live with familiar weatherski areas are snowy, deserts are parched, rain forests drip. But what if our climate jumped to something totally unexpected? What if you went to bed in slushy Chicago, but woke up with Atlantas mild weather? Or worse, what if your weather jumped back and forth between that of Chicago and Atlanta: a few years cold, a few years hot? Such crazy climates would not doom humanity, but they could pose the most momentous physical challenge we have ever faced, with widespread crop failures and social disruption.

Large, rapid, and widespread climate changes were common on Earth for most of the time for which we have good records, but were absent during the few critical millennia when humans developed agriculture and industry. While our ancestors were spearing woolly mammoths and painting cave walls, the climate was wobbling wildly. A few centuries of warm, wet, calm climate alternated with a few centuries of cold, dry, windy weather. The climate jumped between cold and warm not over centuries, but in as little as a single year. Often, conditions flickered back and forth between cold and warm for a few decades before settling down.

The history of this climatic craziness is written in cave formations, ocean and lake sediments, and other places. But the record is probably clearest and most convincing in the ice of Greenland. This incomparable, 110,000-year archive provides year-by-year records of how cold and snowy Greenland was, how strong the storms were that blew dust from Asia and salt from the ocean, and even how extensive the wetlands of the world were.

These records show clearly that Earths climate normally involves larger, faster, more widespread climate changes than any experienced by industrial or agricultural humans. The 110,000 years of history in Greenland ice cores tell of a 90,000-year slide from a warm time much like ours into the cold, dry, windy conditions of a global ice age, a 10,000-year climb back to warmth, and the 10,000 years of the modern warm period. But the ice cores also show that the ice age came and went in a drunken stagger, punctuated by dozens of abrupt warmings and coolings. The best known of the abrupt climate changes, the Younger Dryas event, nearly returned Earth to ice-age conditions after the cold seemed to be in full retreat. The Younger Dryas ended about 11,500 years ago, when Greenland warmed about 15F in a decade or less. A little more, slower warming then led to our current 10,000 years of climate stability, agriculture, and industry.

But smaller and slower climate changes during recent millennia have affected human civilizations in many waysand these small climate changes seem to have been getting bigger. The Little Ice Age cooling that changed settlement patterns in Europe a few centuries ago was tiny compared to the Younger Dryas or the global ice age, but seems to have been the biggest change for thousands of years.

Records from many places beyond Greenland provide a longer, if fuzzier, view of climate history. Over the last million years, the pattern recorded in cores of Greenland ice has occurred over and over: a long stagger into an ice age, a faster stagger out of the ice age, a few millennia of stability, repeat. The current stable interval is among the longest in the record. Nature is thus likely to end our friendly climate, perhaps quite soon; the Little Ice Age may have been the first unsteady step down that path.

In our climate, great ocean currents sweep north along the surface of the Atlantic, are warmed by the tropical sun, and release that heat into the winters of northern Europe, allowing Europeans to grow roses farther north than Canadians meet polar bears. The ocean waters that cool in the north Atlantic then sink into the deep ocean and flow south on the first stage of a globe-girdling journey before returning. This conveyor belt circulation is delicately balancedadd a little too much fresh water to the north Atlantic from rain or melting icebergs, and the wintertime ocean surface will freeze to produce floating sea ice rather than sinking to make room for more hot water. Much evidence shows that the abrupt coolings and warmings occurred when the conveyor circulation suddenly shut off or turned on again, triggering other changes that spread across Earth.

Human-induced greenhouse warming appears capable of triggering a conveyor shutdown, by increasing precipitation in the far north and by melting some of the remaining ice sheet on Greenland. Strange as it seems, global warming may actually freeze some regions! But, if we slow down the warming, it is just possible that we can avoid an abrupt change and even help stabilize the climate.

This book is a progress report on abrupt climate changes. We will discuss what has been learned, how this knowledge was gained, and what it might mean to us. The existence of abrupt climate changes casts a very different light on the debate about global warming, so we will examine the greenhouse arguments under this new light. We wont find all of the answersmany are not known yetbut we will frame the questions, and we may gain some clues to our future.

Climate matters. It mattered to the Vikings, who settled Iceland, explored the New World, and were lured north to Greenland during a period of unusually warm weather a millennium ago. But the warmth did not last, and Viking settlements on Greenland slowly contracted as the climate cooled into the Little Ice Age (see farm animals into their houses during the cold winters. Eventually, the settlers ate their farm animals, then their dogs, then disappeared themselves. Climate mattered to Oklahoma farmers during the Dust Bowl years of the 1930s, when many people headed west as much of their soil headed east on withering winds. Today, with floods and drought, feast and famine, climate matters to many of us much of the time.

To be fair, climate is not everything. The victims of the Dust Bowl and of the cooling in Greenland may have contributed to their own plights through farming practices that promoted soil erosion, and the Oklahomans were fleeing a great economic depression as well as a change in the weather. While the Vikings froze out of Greenland, their Eskimo neighbors, the Thule Inuit, readily survived the cooling.