4.2. Large ice sheets first appeared in the Northern Hemisphere nearly 2.75 million years ago and grew and melted at the 41,000-year cycle of orbital tilt until about 0.9 million years ago. Since that time, the major cycle of ice-sheet changes has been at a cycle of 100,000 years.
By the 1980s, a ship funded by an international scientific consortium had begun applying techniques borrowed from the oil industry to drill sediment sequences penetrating thousands of feet into the sea floor and millions of years back in time. From these long sequences came the first complete record of the history of glaciation in the Northern Hemisphere. Several methods were used to date these much longer sequences. One technique relied on magnetic signatures carried by tiny, iron-rich minerals present both in ocean-sediment layers and in layers of basaltic rock formed from cooling lava. The magnetic patterns were initially found in the rock layers and were dated using radioactive decay methods. Then the dating scheme from the rocks on land was directly transferred to similar patterns found in the ocean sediments, providing a time scale for the glacial variations.
Scientists at last had the full history of Northern Hemisphere ice-age cycles shown in figure 4.2. One of the surprises about this record was the number of glacial cycles: at least 40—50 of them, depending on how many of the smaller cycles embedded in larger ones you choose to count. The fragmentary glacial record left on the continental "blackboard" after all those erasings had suggested 4 or 5 glacial cycles. Now it was clear that 10 times that many had occurred.
These records revealed the full history of the northern ice ages. In the Southern Hemisphere, a large ice sheet had been present for at least 14 million years on the pole-centered Antarctic continent. In contrast, the North Pole is located in the Arctic Ocean, and the nearest continents on which ice sheets can form lie at lower latitudes, where the summer Sun is stronger. For many millions of years, it was just too warm for ice to form in the North, even when the orbital changes caused minima in solar radiation. As a result, no ice sheets existed in the Northern Hemisphere 3 million years ago.
But global climate was steadily cooling (chapter 2). The first large ice sheets appeared 2.75 million years ago, when the Northern Hemisphere crossed a new threshold into an ice age, or, more accurately, the start of a long sequence of ice-age cycles. At that time, icebergs began to dump mineral grains and rock fragments from the continents into Atlantic sediments as far south as Newfoundland and France. At the same time, the oxygen-isotope record shows the start of the series of oscillations that have continued right through until the present time (fig. 4.2).
From 2.7 until 0.9 million years ago, most of the glacial cycles occurred at regular intervals of 41,000 years, with a few others at intervals of 22,000 years. Milutin Milankovitch would have been pleased to see this two-thirds of Northern Hemisphere glacial history. His radiation calculations indicated that the 41,000-year and 22,000-year cycles are both strong at the latitudes where northern ice sheets existed, and he had predicted that ice sheets would form during times of low summer radiation at those two cycles. And here they were, some 40 or 50 individual glaciations, each separated by times when the northern ice sheets had melted. The implication of this early pattern was that climate had become cold enough for ice sheets to form when summer radiation was weak yet was still warm enough that the ice melted when summer radiation was strong.
Also evident in figure 4.2 is a very slow drift toward more glacial conditions. The oxygen-isotope technique measures not just ice-sheet size but also ocean temperature. In many regions, the temperature trends are thought to track those of the ice: when the ice sheets grow large, they make climate (and the ocean) colder, and conversely. So the slow drift evident in figure 4.2 is partly an indication that ice sheets were getting larger, but also a sign that the ocean was growing colder.
Nearly 0.9 million years ago, the cooling trend reached another threshold, and a new pattern of ice-sheet variations appeared. Since that time, ice sheets have not melted completely after every individual 41,000-year or 22,000-year cycle as they had previously. Instead, some ice has survived during the weaker summer radiation peaks, providing a base-level amount that could be added to during the next radiation minimum. As a result, ice sheets began to persist for as long as 100,000 years, shrinking back somewhat during the smaller radiation peaks, but then growing even larger. This change in ice response is probably explained by continued cooling of Earth's climate: now the Sun was having trouble melting the ice, when much earlier it had not allowed any ice to grow.
Nevertheless, Northern Hemisphere ice sheets still disappeared for relatively brief intervals. Every 100,000 years or so, the solar radiation peaks are strong enough to melt all the ice on North America and Eurasia very rapidly. These larger radiation maxima occur at times of close alignment of the peaks caused by cycles of tilt (41,000 years) and precession (22,000 years). The most recent of these alignments happened between 16,000 and 6,000 years ago, and the great northern ice sheets melted. Only the small ice sheet on Greenland has endured these major melting episodes.
Over the last 900,000 years, up to nine 100,000-year glacial cycles can be counted in figure 4.2. Superimposed on top of these, and more difficult to see, are 41,000-year and 22,000-year cycles like those that had existed earlier. These shorter cycles did not end when the new cycles developed; instead, they were overprinted by them. Overall, the three cycles have a saw-toothed shape, with a slow buildup to maximum size, but then a very fast interval of melting. Milankovitch would have been surprised to see this pattern. His theory did not predict the longer and larger 100,000-year cycle, and the mechanisms that may link it to changes in orbital eccentricity are still being explored.
If we take a step back from the many wiggles in this long ice-age history, the basic message from figure 4.2 is that the Northern Hemisphere (and the planet as a whole) has been gradually drifting toward a more refrigerated state for the last 3 million years. Before 2.7 million years ago, no large ice sheets existed in the North. From then and until 0.9 million years ago, ice sheets appeared in cycles but then melted away completely; they were probably present during less than 50% of that interval. Since 0.9 million years ago, ice sheets have been present more than 90% of the time and more difficult to melt. Our ice-free condition in the Northern Hemisphere today (excluding Greenland) is part of a very short break in a mostly glaciated world. If this long-term cooling trend were to continue into the distant future, northern Canada and Scandinavia could at some point reach a condition more like that in modern-day Antarctica or Greenland, with permanent ice sheets persisting through any and all orbitally driven fluctuations in solar radiation.
These glacial cycles are superimposed on the longer-term cooling trend in much the same way that daily cycles of heating and cooling are superimposed on the seasonal drift from summer to winter in the higher midlatitudes. As temperatures slowly fall during autumn, birdbaths or ponds may freeze at night, but the ice melts in the midday sunshine. By early winter, the freezes may persist through colder days but thaw during occasional warmer spells. In midwinter, the freezes are deeper and feel permanent. By this analogy, the high northern latitudes have gradually reached the chill of early winter and are slowly heading toward the deep freeze of midwinter. Of course, in the short term (the next several centuries), we face the prospect of rapid greenhouse warming and a big thaw, rather than a slow drift into permanent glacial refrigeration.
Given that the warmth of the last several thousand years is relatively unusual in a world where northern ice sheets are present more than 90 percent of the time, we might well ask what a more typical glacial world is like. The full-glacial world was cold, dusty, and windy, especially in the Northern Hemisphere at latitudes north of about 40°, and especially near the ice sheets. The ice sheet in North America reached as far south as 42°N, more than halfway to the equator, and it accounted for half or more of the "extra" ice present on Earth (the amount in excess of that today). Overall, ice sheets covered 25 percent of Earth's total land surface, compared to about 10 percent today. Most of northern Canada has slowly been scraped clean of its ancient cover of soil by the thick masses of ice that again and again carried off sediment and soil. Toward their margins, the melting ice thinned and flowed down valleys created in preglacial times by running water, but sometimes it rearranged the old drainage systems by scraping out new valleys. The ice also used boulders, cobbles, and pebbles trapped in its lower layers as crude chisels to gouge the hard bedrock.
To the south, in the northern plains and the northern Midwest of the United States, the bulldozing ice sheets heaped up ridges of moraine rubble as high as 40 to 50 meters (150 feet). During summer, melt water from the ice margins flowed in steams and rivers to the south and east. The water picked up sand, silt, and clay and carried them south but left behind coarser gravel, cobbles, and boulders. In the cold of winter, the meltwater streams slowed to a trickle or stopped. Cold winter and early spring winds blew across the debris near and south of the ice margins, picked up the fine sand, the silt, and some of the clay, and blew it across the Midwest. Today these glacial deposits are the legendary sandy loams of the farmlands of the Midwest, among the richest soils in the world.
Europe just south of the Scandinavian ice sheet was similar, yet different. The southern margin of the European ice sheet reached only to 52°N, 10° north of the limit in North America, but it covered all of Scandinavia and Scotland as well as the northern parts of Denmark, Germany, France, England, and Ireland. A smaller ice sheet covered the high Alps of Switzerland, France, Austria, and Italy.
Today the climate of Europe is moderated in winter by large amounts of heat released from the North Atlantic Ocean. The northward extensions of the Gulf Stream carry warm, subtropical water to unusually high latitudes, and these warm waters release almost as much heat in winter as the Sun can deliver through the nearly persistent cloud cover. But when the ice sheets were large, this warm ocean current flowed east toward Portugal rather than north toward Scandinavia, and the cold North Atlantic Ocean filled with sea ice in the glacial winters and melting icebergs in summer.
This icy ocean, aided by cold winds blowing south from the ice sheet in Scandinavia, created polarlike conditions that eliminated the rich forests typical of modern-day Europe southward all the way to the Alps. Only tundralike vegetation survived across the region south of the ice: grass, moss, lichen, and herbs adapted to ground that was hard-frozen in winter but thawed to a depth of several feet in summer. The underlying soil was permanently frozen (permafrost) to great depths. Farther south and east, Europe was grassy steppe, with no trees. Much of central Europe 20,000 years ago was like present-day Siberia.
At least one area became wetter during the glacial maximum rather than drier. Those "youthful-looking" beach deposits in the American Southwest that had hinted at lake levels much higher than today (chapter 3) turned out to be the same age as the ice sheet and a direct result of the ice's effect on atmospheric circulation. Today the wettest weather on the Pacific coast of North America occurs where the winter jet stream intercepts the coast between Oregon and British Columbia (or even Alaska), bringing powerful winter storms and lots of snow. But at the last glacial maximum, the North American ice sheet was so large an obstacle to atmospheric flow that the main path of the jet stream, along with its winter storms, was displaced south to the American Southwest. More winter snow, along with cooler temperatures that slowed evaporation in summer, allowed enormous lakes like the one at Salt Lake City to form.
The subtropics and tropics, although far from the north polar ice sheets, were generally cooler and drier. Deserts expanded, and strong winter winds blew thick clouds of dust westward from the Sahara Desert over the Atlantic Ocean and across to the Americas, southeastward from the Arabian Desert into the Indian Ocean, and eastward from Asia into the Pacific Ocean and on to Greenland, where dust particles accumulated in the ice sheet in layers that survive today. And throughout these large climatic changes in the North, our ancestors continued to evolve toward modern human form.
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