countless millions. And some of these rises could be extremely rapid; the collapse of the Ross Ice Sheet could cause the world’s oceans to rise 15 feet in a week. If all the Arctic and Greenland ice sheets melt, the oceans would rise about 180 feet.
What about the Late Pleistocene? The most dramatic comparison to today’s situation might have been the rising ocean. At the onset of the previous global warming period, known as the Bølling-Allerød, 14,700 years ago and before, when the glaciers were still at their maximum extent, the sea level off British Columbia was 300 to 450 feet lower than today. By 9,000 years ago, the ocean had risen to current levels and, after some local sloshing up and down, settled to where it is now is by 5,500 years ago.
A few scientists are now suggesting that the onset of the warming period following the last glacial maximum might have begun a couple thousand years earlier. The research, some of it from lakes in Alaska, is recent and ongoing. Future research may push that 14,700-date deeper into the Late Pleistocene, say maybe 17,000 years ago, along with earlier dates for the feasibility of navigating the Pacific Coast or the opening of the ice-free corridor. But for the purposes of this book, 14,700 years ago marks the beginning of the warming period.
That amount of ocean rise seems enormous, though moving a shellfish camp might have been easier than relocating skyscrapers. The Pleistocene people would have seen the effects of sea rise as waters slowly drowned the forest of the Pacific continental shelf and glacial flour colored the milky deltas. They would not notice the actual rise of inches per century unless a great chunk of ice, like the Ross, broke off a sheet and fell into the ocean. But they would likely have a collective notion that the climate was changing, much as we have today.
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The end of the Late Pleistocene came suddenly. The global warming that began 14,700 years ago ceased abruptly with a sudden and relatively short-lived cold reversal known as the Younger Dryas (named for an ivory-colored alpine flower with a yellow center that thrived in the cooler air). This was a prolonged cold snap not a re-advance of the glaciers. Isotopes in the Greenland ice indicate the Younger Dryas (YD) began 12,880 years ago and lasted for around 1,300 years, when the warmer modern epoch called the Holocene began.
Causes of the Younger Dryas are hotly debated. One argument posits that the giant freshwater lake perched upon the surface of the North American ice sheet (Lake Agassiz) burst its ice-dam and dumped vast amounts of cold water through the St. Lawrence Valley into the North Atlantic Ocean interrupting the conveyor belt of warm surface water from the south—causing the sudden cooling. Others contend the amount of melt water from Lake Agassiz was insufficient to disrupt the heat conveyor, to the extent that it would alter climate, and that evidence of the eastward flood—flood debris, terraces or an outlet channel—is lacking. Besides, ice cores from the southern hemisphere show that the cooling was worldwide. Likewise, an asteroid theory—an extraterrestrial body smashing into the glacial ice north of the Great Lakes (but leaving no impact crater)—has played to a limited audience, and has been largely, but not entirely, debunked as the precise trigger that brought on the Younger Dryas and drove into extinction the last sabertooths, mammoths, mastodons, dire wolves, horses and short-faced bears (to name a few) in a heartbeat of geologic time.
Whatever the cause, the Younger Dryas cooling was a very big deal in America, and elsewhere; it appeared to precipitate the disappearance of the Clovis culture and their exquisite elephant-hunting spear points, along with the final extinction of the megafauna. The role of climate and human hunting on this great extinction is discussed in Chapter 9.
Worldwide, very close on the heels of the Younger Dryas, the first efforts at agriculture were germinating on an east-west axis emanating from the Fertile Crescent. Somewhere among a dozen or so places in the Middle East or Asia, someone noticed a plant she wanted to eat growing from a place where she had previously spilled wild seeds.
Another revolution was on its way, probably our biggest—the transition from hunting and foraging to farming—one in which we are still floundering, that was born of that last great blast of climate change, its progress unchecked until the burning heat of present day global warming threatens to bake agriculture out of Africa, out of Asia and banish those crops to the gulags of industrial farming in Siberia.
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A ubiquitous stumbling block in telling the story of the Great Adventure is speculating how and if humans could have lived in North America during periods from which there is no archaeological record. Eastern and western Beringia (Alaska and Siberia) before the last glacial maximum (LGM), about 20,000 years ago, is such a place and time. Ecologists attempt to reconstruct Pleistocene environments by analyzing ancient pollen. Beringia about 30,000 years ago was relatively mild, as indicated from lake sediment samples from Siberia, consisting of bogs and larch-birch forests amid a mosaic of tundra. About 3,000 years later, it apparently turned cold and dry.
Some caution is advised here. Scientists sampling selected lakes for spores and pollen amid a mosaic of varied landscapes don’t always get the big picture right. The plant people may conclude the Late Pleistocene habitat was incapable of supporting people or animals, while at the same time paleontologists are finding fossils of big animals all over the place—suggesting the unproductive tundra looked much like an American Serengeti with its vast herds of hoofed critters. Professionals call the contradiction a paradox. The same kind of critical eye should also focus on the interpretation of the Ice Free Corridor as barren and uninhabitable (Chapter 8) or the use of fungal spores to explain Pleistocene extinction (Chapter 9). Some of the claims are specious.
Thus, scientists debate whether the tundra-steppe of eastern Beringia was too cold and dry for people or animals to survive. Palynologists studying ancient pollen cores concluded the Beringian steppe was sparse, tundra-like vegetation, more polar desert than rich grassland. But fossils dredged up by Alaskans sluicing for gold indicate an abundant animal community and contradict this notion: Bison, antelope, musk ox, mammoth, horses, bears and huge cats thrived in this landscape. Finally, botanists suggested, it might have been cold dry tundra but, unlike the mossy tundra of today, rich in grasses, sedges and forbs.
Topographically, if you subtract the glaciers, the Late Pleistocene landforms looked much like they do today. The ice sculpts the mountains into great cirques and knife-edge arêtes and, as it retreats, deposits terminal and lateral moraine that rivers outwash as broad alluvial fans. The rivers melting out of the glaciers were bigger, wider and more braided than today.
Along the southern limits of the ice sheets you might expect to find a thin ribbon of tundra next to the glaciers and along the tops of mountain ranges. Next to the tundra would be a belt of trees—spruce, fir and pine—and then temperate forests of oak, beech and hickory. Up north, the open tundra and steppes would yield to boreal forests or maybe birch and Populus species.
But this is only a most generalized view; in some places oak forests grew almost at the foot of the ice. Winds blew off the glaciers, picking up sand and silt from the outwash, depositing loess throughout the Midwest U.S. and elsewhere. At the edge of the big glaciers, it was windy and cold but no more inhospitable than today’s Arctic.
Like our present day, species of plants and animals tend to migrate up higher on the mountain and northward as the climate warms. That is, if they can: Five-needle stone pines (like whitebark) clinging to the very tops of mountain ranges today have no place higher to go. Neither does the grizzly bear when the corridor to the next productive habitat is a valley blocked by human development and intolerance.
Maintaining corridors, wild areas and wildlife linkages is absolutely critical if we wish to save species of large animals and mitigate a few of the disastrous effects of the Sixth Great Extinction event—the one we are experiencing today.
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The paleozoologist Valerius Geist called our home: “The predator hellhole
