Ocean: Reflections on a Century of Exploration

13. The Ocean's Memory of the Ice Ages: The Endless Cycles of Climate Change

373 Of Fjords and Reefs and Pioneers Discovery of the Great Ice Age Multiple Ice Ages and Astronomy The Enchanting World of Microfossils Discovery of the Ice Age Cycles A Time Scale and Orbital Pacing On the Origin of the 100,000-Year Cycle The Last Ice Age and How it Ended We live in an ice age, geologically speaking. Some considerable portion of the water on this planet is locked up in ice, at high latitudes (fig. 13.1).1 More precisely, we live in a warm period within a long series of ice age fluctuations. Over the last million years, the sea level was about 200 feet (60 meters) lower than today, on average. It was higher than today only for a few percent of this time span. The reason the sea level was normally much lower was the presence of large ice sheets that covered much of Canada and Scandinavia, and of thickened ice on Greenland, as well as increases in ice volume elsewhere. During times of maximum ice volume on land (glacial maxima) ice fields covered the region of the Great Lakes in North America, and the Baltic Sea in northern Europe. Besides, many elevated regions in temperate to high latitudes had substantial mountain glaciers: the Rocky Mountains, the High Sierras, the Alps, and the Himalayas all had enormous ice fields. In the Northern Hemisphere, much of the glacial ice disappeared between 16,000 and 8,000 years ago; but not from the Antarctic continent, which has been in a glacial condition for more than 15 million years (fig. 13.2). Ice age climate fluctuations are prominently a matter of the Northern Hemisphere, the Southern Hemisphere ’s climate being stabilized by the permanent ice cap on Antarctica. Modern landscapes—just about all of them, including coastal landscapes—carry a legacy from the last glacial period and cannot be understood when only present-day processes are considered. By the same token, neither the present distribution of plants on the surface of Earth, nor that of coral reefs and atolls can be understood without reference to ice age history. Furthermore, the same is true for the zoogeography of the sea, inasmuch as the evolution of marine birds and mammals is involved. The evolution of present-day warm-blooded vertebrates, THIRTEEN The Ocean’s Memory of the Ice Ages THE ENDLESS CYCLES OF CLIMATE CHANGE obviously, was closely linked to the history of ice ages and the associated changes in the ocean environment for the last 3 million years. The climate future of the planet hinges largely on the response of ice on Greenland and in Antarctica to ongoing warming. As the Egyptian priest told the Greek traveler Solon—if you do not think on a time scale of many thousands of years, your knowledge and understanding remains that of a child.2 The priest had a point, but his time scale (the last 10,000 years) fell short. It is the last million years that we need to consider. And perhaps more. OF FJORDS AND REEFS AND PIONEERS The fjords of western Norway are among the most stunningly beautiful coastal landscapes on 374 T H E O C E A N ’S M E M O R Y O F T H E I C E A G E S FIGURE 13.1. A glacier calving into a fjord in Svalbard, Norway, near 78 ⬚N. We live in the warm phase of an ice age that began 3 million years ago, when Greenland acquired an ice cap (1). FIGURE 13.2. Icebergs in the waters off the Antarctic Peninsula, Bransfield Strait, near 64 ⬚S. The Antarctic has been in an ice age for some 15 million years. The sea surface is ice free in summer, during the present warm period. our planet (fig. 13.3), while the coral gardens in the Great Barrier Reef are among the most alluring scenes imaginable, below the sea surface . Surprisingly, both environments are shaped by the action of ice. The fjords are glacial valleys, carved by the powerful grinding action of enormous ice tongues forcing a path to the sea from the mountains to the east. The reefs are heaps of rubble made by rapidly growing coral forests, forests that responded to the large fluctuations of sea level that accompanied the buildup and decay of northern ice sheets. An ocean with a steady-state sea level would produce carbonate...