W. H. Calvin's THE ASCENT OF MIND (Chapter 5)
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A book by
William H. Calvin
The Ascent of Mind (Bantam 1990) is my book on the ice ages and how human intelligence evolved; the "throwing theory" is one aspect.
   My Scientific American article, "The emergence of intelligence," (October 1994) also discusses ice-age evolution of intelligence. Also see Wallace S. Broecker, "Massive iceberg discharges as triggers for global climate change," Nature 372:421-424 (1 December 1994) and his "Chaotic Climate" Scientific American article (November 1995 issue).
AVAILABILITY is challenging.
Many libraries have it (try the OCLC on-line listing), but otherwise it’s strictly used bookstores (and German and Dutch translations).
The Ascent of Mind
Ice Age Climates and
the Evolution of Intelligence

Copyright ©1990 by William H. Calvin.

You may download this for personal reading but may not redistribute or archive without permission (exception: teachers should feel free to print out a chapter and photocopy it for students).



Surveying the Ice Ages from a Seat in Heaven

The human psyche has frequently been compared to an iceberg. And in the early days of the polar flight from Copenhagen to California, when planes were smaller and still flew low enough and slow enough for the passenger to see something, there was a wonderful sight along the way. Crossing the Denmark Strait between Iceland and Greenland, you looked down on icebergs floating south. Each was a white jewel glittering in the low northern sun, and were you a passenger viewing the icy mountain from a ship's deck, this would be all that you would see. But from one's window in heaven you saw far more. Painted turquoise by the waters, the immense underwater mass of the iceberg spread all about beneath your eyes. Majestic the frosty mountain of ice might be; but hidden in mighty mystery was the force that supported it. And such is the unconscious mind.

the dramatist Robert Ardrey, 1969

Rather than timeless, my "over the pole" flight turned out to be a journey backward in time, as I surveyed the land covered and uncovered by the ice ages. It seems strange to realize that only 150 years ago, hardly anybody knew about the ice ages -- even scientists still talked in terms of a biblical deluge. The great Swiss (and later American) naturalist, Louis Agassiz (1807-1873), established about 1840 that massive ice sheets had pushed their way around Europe, sometime in the not-so-distant past. He also wrote a great work in biology, sorting out the relationships of the fossil fishes to the living species.
      Despite this major discovery about evolution in geology, plus the major theoretical feat in classification that paved the way for evolutionary understanding, Agassiz didn't believe in biological change in the Darwinian sense. He was the last creationist who was also a major biologist, and was bitterly opposed to Charles Darwin's interpretation of how species evolved. Perhaps one major heresy per lifetime (extending the span of life on earth far beyond the biblical scholars' estimate) was all he could manage.

GLACIERS DESCENDING from the north lead most people to think that the center of the ice cap must have been the North Pole (that is, after all, the way it works at the South Pole). It took a while after the discovery of the ice ages before anyone realized that glaciers don't form over open ocean: the pack ice at the North Pole is only a few meters thick (and it's rapidly getting thinner). As a naval officer sitting next to me on another airplane flight once remarked, "If anyone ever builds a house there, they'll get a surprise if they dig a basement!" (his submarine had punched through the ice, and he had gone walking on top of the world). The bottom of the Arctic Ocean is as deep as the Atlantic. It has features such as Nansen Basin, underwater ridges such as the Nansen Cordillera; both are named for the Norwegian scientist Fridtjof Nansen, one of the first neurobiologists. In 1888, Nansen and the Spanish neurobiologist Santiago Ramón y Cajal simultaneously discovered "the neuron doctrine"; Nansen was later an arctic explorer in 1893 to 1896, still later a diplomat who received the Nobel Peace Prize in 1922.
      Although it may snow up on top, the sinking ice changes back into water on the submerged surface of the ice sheet. To build up ice to the thickness of a mountain range, as happens during an ice age, requires a solid foundation such as Greenland. Down at the South Pole, there is a whole continent (9.3 percent of the Earth's land surface) to house glaciers; they have even spread out into some shallow bays and displaced the seawater, e.g., the Ross Ice Shelf. Greenland is about the only such land at high latitudes in the northern hemisphere, although it is smaller than Europe (it usually looks bigger because the high latitudes get stretched on most maps).
      When an ice age really gets going, then the northern hemisphere has a lot more land on which to house glaciers than the southern. Glaciers often came down to 50° latitude (past London and Vancouver), but to 40° in a few places (such as New York City and Woods Hole). Between 50° and 70° latitude, the southern hemisphere has only the tip of South America plus a bit of the Antarctic Peninsula -- but the northern has Greenland, northern Europe, the vast expanse of Siberia, and then Canada too. It also has Alaska, but surprisingly the interior of Alaska north of the coastal mountain ranges had few glaciers, probably due to the "rain shadow."

FLYING OVER OSLO, one suddenly notices that the flatland appearance of the Baltic has given way to rock -- an ancient eroded landscape, the shallow valleys filled with agriculture. Oslo sits at the head of a long fjord; though at 60° latitude, higher than Scotland, it is warmed by the North Atlantic Current and its harbor usually remains as ice-free as New York City's (at 40° latitude).
      A little further north at 65°, the vegetation is very thin. There is little topsoil in the uplands, except in some of the grooves (only 1 percent of Norway is agricultural, with another 2 percent as grassland). This area has been repeatedly scoured by glaciers, right down to bedrock. This is not only some of the oldest rock in Europe, it is some of the earliest rock anywhere on Earth. The deep grooves are not scrape marks in the manner of the scratches made by boulders carried along by the advancing glaciers; rather they are the result of a billion years of erosion of the granite and gneiss by water runoff.
      Snow remains in some of the shadowed grooves, sheltered from the oblique sunlight at these latitudes, giving this part of Norway a zebra-striped appearance during some seasons. This is the 65° latitude (the same as Iceland) that Milutin Milankovitch used as the reference latitude for his thesis developed in Budapest: elaborating on the 1842 suggestion of the French mathematician Joseph A. Adhémar, Milankovitch proposed that the sunlight reaching such latitudes controlled the ice ages. He showed that in the warmest times, there was as much sunlight here at 65° as there is presently at 49° (e.g., Paris). In the coldest periods, 65° got about as much sunlight as they get today up at 76° (e.g., Thule, Greenland -- as far north as we'll fly in going "over the pole" today). So sometimes Iceland has had as much sunlight as Paris, and sometimes as little as Thule (that hadn't been "intuitively obvious" until the calculations were done).
      The winds blow, and the rivers flow, their patterns and strengths mostly a matter of seasonal sunlight.

THE NORWEGIAN UPLANDS are interrupted by fjords, where some of the deeper grooves go out to sea. The one to our left has a steamer ship heading inland, leaving behind a long white wake in the fjord. A road cuts into the bordering hillside of otherwise unrelieved rock.
      An hour after leaving Copenhagen, we finally depart Europe. We're now out over the Norwegian Sea, to be exact. We will miss seeing Iceland, as our route takes us well to the north. We have reached the latitude of the Arctic Circle; were this midwinter, the sun would barely be peeking over the southern horizon at noontime. Near midsummer, the sun doesn't set, merely skimming the northern horizon at midnight.
      We do see the mid-ocean ridge, where the ocean floor is spreading apart as new material upwells from the depths of the earth. There are some volcanos along that ridgeline: Iceland's are the best known, but now we see Jan Mayen Island out the right window, its volcano Beeren Berg poking up through the clouds. Its glaciers have receded most of the way back to the uppermost cone; they can't go very far before reaching the sea -- except to the south, where the mid-Atlantic ridge has poked up above the waters to form a long spit, like the handle on a frying pan. The island resembles a Hawaiian volcano, arched like a shield or convex lens -- except for the top half, whose erect cone sweeps upward like the tip of Japan's Fujiyama.

THE TILT OF THE EARTH'S AXIS of rotation, relative to the plane of its orbit (in the arcane astronomical terminology, "the obliquity of the ecliptic"), changes some over the years. It drifts back and forth between 22° and 24.5°, taking about 41,000 years to make a complete circuit. Currently the tilt is about 23.4° (and declining) -- and at that latitude the sun stands overhead on the longest day of the year. We northern hemisphere types call this latitude the Tropic of Cancer; it passes just north of Havana, Cuba. At maximum tilt, which last occurred about 9,600 years ago, the sun makes it up to Key West, Florida; at minimum, it only makes it up to the Isle of Pines off the southern coast of Cuba.
      This 2.5° may not seem like much (only the difference in latitude between New York and Washington, D.C., or between Geneva and the Mediterranean). But if you live up where the glaciers do, you can get a considerable percentage improvement in the warmth delivered in summertime. The sun climbs a little higher in the sky at midday, stays above the horizon a little longer to make the nights even shorter.
      Trying to reason out the physics of all this? A little knowledge of physics can be misleading when it comes to ice ages. The heat exchange involved in freezing and melting a tray of ice cubes is identical -- so a change in tilt that produces both hotter summers and cooler winters shouldn't make much difference in ice buildup, right? But that analysis assumes nothing moves -- and ice can move. The Atlantic Ocean beneath us is full of icebergs, calved off of Greenland and floating south, the warming job being exported to warmer latitudes than where the snow fell and froze into ice atop Greenland.
      And this is the northernmost extreme of the North Atlantic Current we're flying over. It is nice and warm, flowing up as it does from the tropics. The North Atlantic Current warms up all that cold Arctic air that flows east from Canada, and so Europe gets much more comfortable weather than they get at comparable latitudes in Canada (all of Europe north of the Paris-Prague line is at Canadian latitudes). The North Atlantic Current makes winter in Oslo tolerable even though the sun only stands 7° above the southern horizon at noon. But the North Atlantic Current was shut down during the last ice age, starting up only 14,000 years ago when melting got underway.
      I wonder what all those icebergs coming off Greenland during the end of the last ice age did to the North Atlantic Current? Certainly there was that period about 11,000 years ago, during the most rapid phase of the meltoff, when Europe paradoxically cooled down. The infamous Younger Dryas. The massive melting of the Canadian ice cap might help explain its thousand-year duration, but there are a series of Dryas-like "cold spikes" all though the last ice age, especially during the period of 30,000 to 70,000 years ago. Even if someone should "explain" the Younger Dryas in terms of events unlikely to be repeated in the coming century, there are all those other shorter snaps to explain. Something besides the Milankovitch rhythms and meltoff deluges seems occasionally to cause some centuries-long cold snaps, and they have knowledgeable people worried.

GLACIERS CAN BREAK UP in dramatic ways. When melting gets going, some of the water runoff gets beneath the glacier and thaws the glacier's attachment to terra firma -- greases the skids, as they say. This allows the glacier to slip sideways -- and if the ice is piled very high and heavy (several thousand meters or more is not unusual), it may start to collapse, the edges of the glacier surging outwards and breaking up as the center tumbles down. Because the ice surface area exposed to the warm summer air is greatly increased by fragmentation, melting speeds up further.
      There is nothing analogous to iceberg deluges and glacial surges in the orderly layer-after-layer buildup of ice during the cooler wintertime. So warmer summers but cooler winters suggests net melting of glaciers even if the annual average sunlight doesn't change much. In the models that have been made of this process, a melting rate about four times faster than the buildup rate fits the fluctuations of ocean salinity quite well during the last ice age.
      But the 41,000 year tilt cycle doesn't, by itself, match up with the 100,000 year period between big meltoffs. What else might increase summertime heating at high northern latitudes? Well, the earth's orbit isn't circular but elliptical, a bit elongated. That means that our distance to the sun isn't constant: the earth is closest to the sun ("reaches perihelion") on the third of January. By the fifth of July, we are about three percent farther away from the sun. Sunlight's intensity falls off as the square of our distance from the sun; in January, we get about seven percent more sunlight (averaged over the whole earth) than in July. If we didn't, northern winters would be even colder these days.
      The date on which perihelion occurs is not, however, always the third of January. The date of perihelion drifts because, like other spinning tops, the earth slowly precesses, its axis tracing out a cone. Since that is independent of the elliptical orbit itself, the earth's orientation toward the sun at perihelion changes over time. Only 5,500 years ago, perihelion was about the time of the autumn equinox in late September. And 11,000 years ago, it was coincident with the summer solstice in late June -- and so the northern hemisphere got its maximal heating for the year at the time when its glaciers are most susceptible to melting. The cycle takes between 19,000 and 26,000 years (as tops go, the earth is rather massive and the precession period quite slow).
      Furthermore, the elongation changes as the positions of the other planets pull the earth into an even more elliptical orbit. Those seven percent differences in heating increase, considerably exaggerating the summer-winter differences. The maximum eccentricity occurs every 400,000 years, although there is a minor peak at 100,000 years embedded in it (the tilt cycle also has minor peaks, and perihelion date also doesn't advance uniformly).
      One of the puzzles about the ice ages is that they recur every 100,000 years, but the eccentricity contribution to arriving sunshine seems too weak to be so important; some geophysicists suspect that the earth's crust resonates at about 100,000 year periods, it taking that long for the depressed crust to rebound after sinking under a mountain of ice. Whatever the cause, when two out of the three astronomical factors (tilt, season of perihelion, eccentricity) are going to have major or minor peaks at about the same time (as when tilt peaked 9,600 years ago and perihelion was at the summer solstice 11,000 years ago), northern glaciers melt back substantially. The glacial maximum was about 20,000 years ago; the meltoff was well underway by 14,000 years ago and was mostly complete by 9,000 years ago. When only one of the three astronomical factors is at a peak, there is some meltback. June perihelion date is best correlated with all of the minor meltbacks between the major ones.
      And so, as the relative mix changes, there is lots of back-and-forth movement of glaciers between the major meltbacks, aided and abetted by variations in the sun's nuclear furnace. The augmented summer sunshine not only melted the ice sheets, but it had some effects at more tropical latitudes as well: the Sahara was green about 8,000 years ago (the "Pluvial"), thanks to the way the enhanced monsoons spread into northern Africa, just as they did on earlier occasions when perihelion was in the northern summertime.

There is a good Greenland map (180k JPEG) at the University of Texas, from the CIA's collection.

CROSSING THE COAST OF GREENLAND, one sees fjords again. The one below the airplane is full of icebergs and broken sheets of floating ice. Long wide roads of ice, furrowed and cracked, come down from the Greenland's highlands and then end abruptly in open sea. There are dozens of glaciers emptying into this labyrinth of fjords on Greenland's east coast; hundreds of white iceberg tips dot the channels. And this is late in an interglacial period when iceberg birth rates are lowest; one wonders what this sight would have been like 13,000 years ago when the big meltoff was getting going, and the iceberg factory was running flat out.
      The eroded red rock lining the fjords is old, probably more than 2.5 billion years (just as is the coast of Norway; back then, Scandinavia, Greenland, and Canada were all connected, before the mid-Atlantic rift did its work separating them). Greenland is part of the Laurentian shield of Canada, recently revealed to be a series of microcontinents fused together by some great lava flows nearly 2 billion years ago. There's not a speck of vegetation to be seen from 10,000 meters up, though there are surely some lichens clinging to those rocks. But hardly enough to get a soil started.
      From my stratospheric perspective, however, I can see a monster of a glacier to the south, staircasing its way down from the highlands, feeding northward out of the prominent mountain range several hundred kilometers away. More familiar locales used to have monster glaciers like that: the one that pushed down out of the north to cover up where Vancouver and Seattle now are, the one that pushed down out of the Alps into the Danube's valley.
      Some great blue spots are visible atop those glaciers beneath the plane's wing; they're ponds of summer melt water. On active glaciers staircasing downhill, a crack will soon open up beneath such a pond and it will drain. The ponds I see are considerably inland from where small blocks of ice are calving off and floating away, so the ponds aren't holes in the ice, of the kind frequented by the surfacing seals that attract both polar bears and Inuit hunters.
      I didn't see any coastal settlements, and there aren't many this far north except for some Inuit ruins. The population of Greenland, about that of a large town elsewhere, is mostly along the west coast of Greenland at lower latitudes.
      Farther inland, the glaciers give way to smoothed snowfields. Endlessly. Greenland is eerie, a high plateau of ice, everywhere. Tips of mountains barely poke through the ice sheets and snowfields. This makes the mountain tops look like a chain of islands in a white sea; the occasional furrowed glacier showing through the wind-smoothed snowfields looks like an offshore barrier reef producing turbulence. But the highest point in Greenland is on one of those plains of snow; that's where a European scientific team is drilling 3,000 meters down to bedrock and, about 30 kilometers to the west, an American scientific team is drilling a comparison core, part of the effort to be sure about what's real climate data, and what's just the noise introduced by ice flow over the millennia.
      Though it seems frozen and static, the ice is pushing and shoving due to its own weight, eventually working its way down to be born as a multitude of little white icebergs poking up through a real sea. The mountain of ice is as much as 3,410 meters thick. In some places the land beneath it has sunk 365 meters below sea level (about the same elevation as the Dead Sea), thanks to the weight of the ice. The glacier could never have gotten started if the land had originally been below sea level, another reason why buildup and melting of ice can be so different.
      It is still noon as we pass over Greenland. The shadows I see are about as short as they ever get; usually they are very long, the mountain peaks casting great shadows for long distances to the north across the frozen snowfields. The purser says that this plane turns around in Seattle and immediately flies back to Copenhagen with a new load of passengers, passing over here again in the early morning hours. At that time, the long shadows will stretch out towards the south, melting ice around the clock. If you were to become lost around here, the Boy Scout lore about moss growing on the north sides of trees wouldn't work: the sun rotates all around the tree! That presumes, of course, that you could find a tree -- when there isn't even soil yet.
      You'd think that Iceland would have been named the "green land" and Greenland the "ice land," rather than vice versa. The reversal in names is due to the reversal in regional climate in the century between their discoveries. When Iceland was first settled by Vikings about A.D. 860, it was during a cold spell (we now know from the ice core's oxygen isotopes, which serve as "frozen thermometers"), causing Iceland's fjords to ice up. Erik the Red, banished from Iceland a century later (a little matter of murder), explored to the west across the Denmark Straits and discovered what he called "Greenland" -- and we now know that things had warmed up considerably in that century since Iceland's settlement.
      During this warmer period, the Norse explored the northeast coast of North America, shipping back timber to Greenland. But it cooled dramatically in the fourteenth century and the fortunes of the Greenland settlers declined. The settlement lasted until about 1540, wiped out by the cooling (and the failure of the settlers, addicted to European styles in clothing, to adopt Eskimo techniques for survival in such climates).
      At Greenland's southern tip, something is actually green these days. Along the coastline, there are small trees: occasional willows as high as a person and, in sheltered spots, dwarf birches only half as high. These, and the mosses and berries that cover the ground, probably gave the place its name among the boat-borne visitors lacking our elevated perspective on the ice. Thus the name "Greenland" commemorates a green facade, shielding the mountain of ice further inland. And a fickle facade at that, varying from century to century with the erratic climate of the North Atlantic.

THE CLOUDS WE ENCOUNTERED over central Greenland now part and I see land below that isn't Greenland because there are no glaciers -- it looks scraped clean, the same kind of reddish Canadian Shield as Greenland, but I think it hasn't seen a glacier for many millennia. If this is Canada, then I missed seeing Thule, Greenland, known to the rest of the world mostly for its cold-war radar installations (and as the American "Siberia" to which unpopular Air Force officers were reassigned). Canada's Baffin Island is north of Hudson's Bay, and that must be it below. There is a lot of ice, but exposed sea lanes as well, with pancakes of ice scattered here and there -- not icebergs, just flat ice. The Inuit live up here too, the last of the ice-age hunters; indeed, there are more groups in the eastern Canadian Arctic than elsewhere. They can be found from Siberia around to Greenland, following the seals and bears.
      Now if this were 1831, the year that the North Magnetic Pole was first located on the Boothia Peninsula, we'd be flying right over it. But it has moved since then, and is now about 1000 kilometers (about 600 miles) out the right window to the northwest. The North Magnetic Pole is the point toward which all those decelerating charged particles from the solar wind converge to cause the aurora borealis; from space, the aurora looks like a fountain, spewing light -- making the Magnetic pole look considerably more exciting than the Geographical North Pole. No northern lights for us today; they're there 24 hours a day, but it's still noon and we can't see them for all the sunlight that reflects off the thin air to produce a blue sky. We missed the North Pole itself by quite a bit, the distance between Miami and New York City. So this over-the-pole flight might be better described as the almost-over-the-magnetic-pole flight.
      If this were 14,000 years ago instead, and I looked out the left window to the south, I would have seen a "mountain range" going all the way to New York City and Cape Cod. The Laurentide ice sheet was truly massive, and so tall that it probably deflected some of the jet stream to more northerly latitudes.
      What are Arctic travelers likely to see out the window a few decades from now? Looking into the future with computer simulations, this tundra beneath us may thaw in a big way: northern Canada is likely to warm up more than anywhere else on earth, as the greenhouse warming progresses. The methane that the thaw releases from the tundra is also likely to make the greenhouse even worse.

LAKES SEEM TO BE EVERYWHERE in the Northwest Territories, and we've just passed near Great Slave Lake and our first town since leaving Norway, Yellowknife. Far to the right side must be Great Bear Lake, noted for an ancient volcano tipped on its side, erosion exposing the internal plumbing.
      The natives that live up here were also quite successful as emigrants to the United States; the languages spoken by the Apache and Navajo, down south near the Grand Canyon, are closely related to the ones spoken up here, and it appears that the Athabascan-speaking peoples (named for a lake off to our left) hunted and gathered their way down south rather recently -- they barely got there in time to be used as slave labor by the sixteenth-century Spanish in building their churches in the Rio Grande valley. This was, of course, a half century before the Pilgrims arrived in New England in 1620, another fact that my school textbooks somehow omitted.
      We've haven't seen much of the precambrian rock protruding through the tundra, but hereabouts ought to be the end of the Canadian Shield. Southwest of here is more recent geology, late-arriving chunks of North America that sailed across the Pacific Ocean during the last 50 million years.

THE ICE-FREE CORRIDOR (minus the ice) is seen out the right side, as the Rockies come into view. The pioneers might have been able to walk across the Bering Strait from Asia, but Alaska's northern interior was the end of the line. The rugged coastline, augmented by ice sheets on the continental shelf, probably prevented animals (including humans) from moving south, though there might have been a few small "harbors" along the way if boats were available. And the great ice cap sitting atop Canada would have blocked the alternative inland route. But it was a two-part ice sheet, which is why a corridor was possible.
      The Laurentide ice sheet didn't grow down from the Rocky Mountains towards the east -- it spread west from Hudson's Bay (drained empty by the fall in sea level) and actually started to push up into the foothills of the Rockies before it met up with the Cordilleran glaciers flowing down the Rockies (eastern Canada then received much more snowfall because the Gulf Stream shifted). From about 30,000 years ago until 14,000 years ago, the two ice masses pushed against each other; then between 14,000 and 12,000 years ago, as both started to melt back somewhat, a corridor opened up from the north coast of Alaska leading down to eastern Montana.
      As I said, I tend to imagine this as something like the biblical parting of the Red Sea -- a north-south corridor opening up as the ice walls pull back on both the east and west sides. Shortly after the corridor opened, there was a human population explosion in the Americas south of Canada. Besides the relatives of the elephant, there were lions, horses, and camels in North America, back in those days. Many of those species now remain only in Africa; Teddy Roosevelt, early in the century, took a train trip through Africa and called it a "railroad through the Pleistocene," a tour of what America used to be like.
      The corridor ran up the eastern front of the Rockies from the southern limit of the glaciers, at about the Alberta-Montana border, to Dawson Creek and Fort Nelson. We must be over Dawson Creek, as I can now see the Alaskan Highway snaking off to the west en route to Whitehorse and eventually Fairbanks. The mountains continue northwest as the Mackenzie Mountains all of the way up to the Yukon. No glaciers visible now; just the unfurrowed white patches that are permanent snowfields -- the seeds of glaciers.
      So the earliest route to the south required first going north, up above the Arctic Circle, almost (unless the Yukon's valley opened up early) all the way up to the North Slope and the Arctic Ocean coastline, reaching the Mackenzie River delta and then turning southeast and traveling down the eastern front of the Mackenzies and Rockies. There were a lot of lakes along the way, formed by moraines of the Laurentide ice sheet: the bulldozing ice snouts actually dammed up some valleys of the Rockies in a manner not unlike modern reservoirs, with an earthen dam of rubble stretched across the exit to the valley -- except that the rubble was pushed up the valley from below by that monster glacier from Hudson's Bay far to the east. Herds of grazing animals probably worked their way down the corridor, following the new grasslands, followed by the hunters.

THE FIRST AMERICAN POPULATION explosion likely came from those hunting bands that found their way down the ice-free corridor. Or maybe it was the second or third, since there is a lot of argument over whether there were some human inhabitants in both North and South America during the last quarter of the last ice age, more than 31,000 years ago. Like the Vikings who explored the Atlantic coastline centuries before the southern European explorers came and stayed (and attracted the even later but more prolific English), so the earliest human occupation of the Americas may have been a multistep affair.
      Because the corridor east of the Rockies was open before 30,000 years ago, an earlier Bering Strait emigration from Asia could, conceivably, have initially populated the rest of the Americas. But the door closed on the corridor at 30,000 years ago, and didn't reopen until about the time of the Clovis hunters, 11,800 years ago. Of course, the early South American populations presently dated (these numbers are forever being updated, and the radiocarbon dates recalibrated) earlier than 31,000 years might also have arrived by boat from the Pacific islands. Everyone is eagerly awaiting enough bones and cultural artefacts from the early sites to make comparisons to ancient populations of the Asian mainlands that spread into the Pacific islands.
      The present-day natives of North and South America seem fairly closely related, just what one might expect from a population explosion based on some initially successful hunting tribes pouring through the ice-free corridor. Whether or not some humans arrived even earlier, the hunters seen starting at about 11,800 years ago were prolific big-game hunters and left their Clovis-style arrowheads and spear points all over the continent, including in the rib cages of some now-extinct species of megafauna. Some groups certainly came later, such as the Arctic-specialized Aleut and the Inuit perhaps 8,000 years ago.
      There was, of course, a population contraction in more recent centuries, as the native populations were decimated by the diseases imported via the European and African immigrants. That sort of replacement of one hominid population by another is likely how modern-type Homo sapiens, the descendants of the African "Eve" collection of mitochondrial DNA that was around 150,000 years ago, came to dominate the scene. They need not have brutally eliminated Homo erectus and "archaic Homo sapiens", though there probably were incidents of that sort, just as occurred involving the U.S. Cavalry in the nineteenth century, massacring Cheyenne Indian families at Sand Creek. It would suffice to possess an immune system that could cope with a virulent virus that predecessor immune systems could not.
      It is difficult, as Richard Leakey points out, to otherwise account for the widespread disappearance of the predecessors (conquest, despite occasional massacres, tends to lead to interbreeding and thus regional retention of some characteristic features). Even without superior technology, the Europeans could have displaced the American Indians -- just with the smallpox that Europeans could survive better than the Indians. Anthropologists often argue that waves of settlement shouldn't occur without the newcomers having some advantage such as new-model body styles or advanced culture -- but they sometimes forget the pathogens and antibodies that aren't preserved as well as stones and bones.

THE ROCKY MOUNTAINS take a brief respite and we see interior valleys for a few minutes until we cross the Fraser River and we're into the coastal mountains, looking every bit as rugged as the Rockies. I can see why it would be hard to walk down the coast from Alaska: the mountains continue to the coastline; further north along the Gulf of Alaska, glaciers extend right out to the water's edge, contributing more meltwater to the oceans than any other glaciers in the world, outside Greenland and Antarctica. But then we suddenly pop out of the mountains and are over a real metropolis, complete with a large river delta. It's Vancouver, British Columbia, and that is the Fraser River emptying into the Strait of Georgia.
      We are flying right down the strait between the mainland and Vancouver Island to the west. I know we just passed into United States airspace because we are over the San Juan Islands, one of my favorite places; I spotted the Friday Harbor Labs (at least its dock; the buildings blend so well with the natural setting that I can't distinguish them). To the west I can see the Strait of Juan de Fuca, separating Vancouver Island from the Olympic Peninsula, and opening out into the vast Pacific Ocean. The Atlantic for lunch, the Pacific for dinner.
      To the south is Puget Sound, not a sound at all (since it is deadend) but rather a very long bay with only the one narrow exit to the ocean. The world's larger "bays" include the Mediterranean Sea and the Red Sea. They all have an interesting salt economy -- not of the kind associated with the camel caravans of centuries past, but a salt economy associated with the bay's gains and losses of fresh water. The Red Sea is an extreme example: it loses quite a lot of fresh water by evaporation, but gains essentially none from rivers (or melting icebergs!). It doesn't dry up into salt flats because less salty Indian Ocean water is attracted in through the Strait of Bab al Mandab -- and so the Red Sea's salinity has stabilized at about ten percent higher than the oceans. Puget Sound has lots of rivers coming down from the mountains to the east, south, and west; no danger of Red-Sea-style hypersalinity here. Except, perhaps, if it really turned cold and the winter snows turned into glaciers rather than runoff.
      The Mediterranean gets fresh water from some big rivers such as the Nile and Rhone, but it also has quite a lot of surface area for evaporating fresh water. As the Mediterranean starts to get hypersaline, it attracts ocean water of normal salinity in through the Strait of Gibraltar. This creates an interesting circulation pattern. Hypersaline water is heavy, and so it sinks to the bottom of the eastern Mediterranean, the fresher waters from the rivers and the normal salinity seawater from Gibraltar replacing it on the surface. The deep salty water tends to escape, creeping along the bottom and out into the Atlantic, just as the extra salt flushes out of the bottom of the Red Sea into the Indian Ocean.
      During the pluvial period about 8,000 years ago when the greatly augmented monsoons were watering Africa and turning the Sahara green, a lot more fresh water was delivered to the Mediterranean via the Nile (and some large North African rivers that can no longer be seen, their dry beds filled in with sand). And the Mediterranean's salty circulation pattern became the exact reverse (rather like Puget Sound today): the fresher water stayed on the surface and flowed out to sea, and some deep salty water was attracted into the bottom of the Strait of Gibraltar. So while it is a salt economy, it's really all a matter of fresh water runoff into, and evaporation from, a basin.
      While bays illustrate the principles more readily, the same principles apply to regions of the oceans too, should you have areas (such as the Northern Atlantic) with more fresh water loss than gain. This principle was recognized several centuries ago:

But if the water of the ocean, which, on being deprived of a great part of its Heat by cold winds [evaporation], descends to the bottom of the sea, cannot be warmed where it descends, as its specific gravity [density] is greater than that of water at the same depth in warmer latitudes, it will immediately begin to spread on the bottom of the sea, and to flow towards the equator, and this must necessarily produce a current at the surface in an opposite direction.
Benjamin Thompson (Count Rumford), 1800

Just imagine the North Atlantic Current as the equivalent of that normal salinity surface current flowing into the Mediterranean at Gibraltar, nice and warm. To balance it, you get a deep salty current heading south from Iceland; actually, it flows from North Atlantic to the tip of Africa, east through the Indian Ocean, around Australia and up into the North Pacific.
      It's a somewhat exaggerated version of the Mediterranean's story: the water sinks like a stone around Iceland because it is already hypersaline when it arrives:

Every winter at about the latitude of Iceland, water of relatively high salinity, flowing northward at intermediate depths (perhaps 800 meters), rises as winds sweep the surface waters aside. Exposed to the chill air, the water releases heat, cooling from perhaps 10 degrees C. to two degrees [50° to 36°F]. The water's high salinity together with the drop in temperature makes it unusually dense, and it sinks again, this time all the way to the ocean bottom. The formation of the North Atlantic deep water, as it is called, gives off a staggering amount of heat. Equal to about 30 percent of the yearly direct input of solar energy to the surface of the northern Atlantic, this bonus accounts for the surprisingly mild winters of Western Europe. (The warming is often mistakenly ascribed to the Gulf Stream, which ends well to the south).... [During the ice age, the conveyor was shut down but resumed during the melting; during the Younger Dryas], the conveyor had shut down once again. Deep-water formation had stopped, and so the warm intermediate-depth water that supplies Europe's bonus of heat could no longer flow northward. The chill over this region was dispelled only when the conveyor began running again 1,000 years later.... [One theory for the stoppage is that meltwater] poured into the North Atlantic close to the site of deep-water formation. There it reduced the salinity of surface waters (and hence their density) by so much that, in spite of severe winter cooling, they could not sink into the abyss.
Wallace S. Broecker and George H. Denton, 1990

Cause-and-effect reasoning can be tricky because nonlinear systems often chase their tails. This is a particularly apt description of the North Atlantic Current: it even does a vertical U-turn. The Current -- now so cold and hypersaline that it is denser than any layer of underlying water -- plunges from the surface to the abyss. There may not be a giant North Atlantic whirlpool or waterfall to gaze down upon, but this "deep water production" is equal in magnitude to 20 times the combined flow of all the rivers of the world. Once the dense water has sunk under its own weight to the sea floor, it flows south -- and so attracts even more warm currents north to replace it.
      Why did this Current falter? On the model of the Mediterranean in the last Pluvial, the obvious candidates would be all those analogies to the augmented Nile: the salt-free icebergs calving off of Greenland, that fresh water coming out of the St. Lawrence River from eastern Canada's massive ice sheet, and the meltwater from the Scandinavian ice sheet emerging from the Baltic and from Norwegian fjords. The North Atlantic got fresh water from all sides except the south. With sufficient dilution of the ocean surface waters, there wouldn't have been an "attraction" of warm tropical waters northward to replace the hypersaline water that otherwise sinks around Iceland. There may not be any major sources of meltwater left in Canada or Scandinavia, but Greenland has enormous supplies -- and its east coast fjords are located close to the current focus of deep water production, south of Iceland. A greenhouse-encouraged glacial surge into the fjords, or the sudden emptying of a meltwater lake, might have effects on climate far out of proportion to their effects on rising sea level.
      And remember the "White Earth Catastrophe", where the ice cover prevented rewarming? It could well have happened to the North Atlantic in another sense. As wind and evaporation are essential to the deep water production, ice cover would limit evaporation and deep-water formation. An iceberg deluge might have shut off the northerly movement of (warm) replacement water, but also (by raising the freezing point of the sea water) allowed winter ice to form much farther south. Indeed, the southern border of the sea-ice islands floating in the wintertime Atlantic descended from Scandinavian to Iberian latitudes (55°N to 35°N) as the Dryas started. This wintertime "cap" on the North Atlantic would have delayed the resumption of the salt conveyor. Ice matters.

WE ARE HEADING SOUTH into the Sound-that-isn't. I just heard someone use the correct French pronunciation of "Puget" -- but she was quickly corrected by another European who explained that Americans make it rhyme with "fidget" for some obscure reason. I hope that she hasn't heard about how they pronounce Goethe Street in Chicago.
      The Strait of Georgia and Puget Sound were also emptied out by the drop in sea level during an ice age, making this a possible path for a glacier. We are flying right down the route of the glacier that sat atop this area 15,000 years ago, the southernmost "Puget Lobe" of the Coastal-plus-Rockies ice sheet known as the Cordilleran. Icebergs set sail out of the Strait of Juan de Fuca back then, just as they do now in the Denmark Strait east of Greenland. The San Juan Islands were scraped down to 350-million-year-old bedrock. The glacier was a mile high (1,600 meters) here, half the height of Mount Baker, the local volcano over to the east.
      The massive tongue plowed down to the south end of Puget Sound, backed up, advanced again, and generally rearranged the land. Whidbey Island, which I see stretched out on the left, is all glacial, sediments deposited by one glacier or another, and carved by the silt-laden runoff from the last meltback. I once encountered some brick fragments on the beach at Double Bluff, near the south end of Whidbey, and thought that they were surely of recent human origins, just as are the plastics that have drifted ashore. But no, the geologists tell me: the warm times of the last interglacial, 120,000 years ago, produced a peat bog resting atop an older layer of clay. And the peat bog dried up and caught on fire, perhaps due to lightning, and so baked the clay beneath it! As it erodes out of the cliff, the beach becomes littered with red brick fragments.
      All the north-south valleys in the Seattle area are probably drainage channels that formed beneath the Puget Lobe. We even got our own fjord out of the deal, Hood Canal snaking along in its fishhook shape out the right window (unlike the Norwegian fjords carved into hard rock, it looks like a runoff channel from the lobe melting).
      Below on our left, atop another glacier-shaped landform, is Paine Field, the birthplace of our airplane -- together with all the other Boeing 747s in the world. Some glacial landforms, such as Long Island and Cape Cod, are rather like ancient landfills, plowed into place by the snout of a glacier that then retreated. Actually, those Whidbey Island bluffs were underwater during the meltback; their tops are the sediments that accumulated in the lakes that formed south of the retreating glacier about 13,000 years ago. The land had been sinking slowly under the weight of the ice, but slowly rebounded over the next few thousand years. And so now these postglacial sediments are above sea level; though sea level has risen during the interglacial, these rebounding sediments have risen even more. This did not happen in southern Puget Sound, as it was covered too briefly by glaciers to sink very much. North of Seattle, the rebound has been more than the sea level rise.
      The rapid melting about 13,000 years ago left even more dramatic evidence in eastern Washington state: a large lake of meltwater formed east of the Idaho-Montana border, but was held in place only by a dam of ice. When that dam broke, the lake emptied suddenly, a great flood sweeping westward. It carved a broad swath across the state until channeled down the Columbia River along the Washington-Oregon border. It sculpted deep valleys in a matter of days. Similar events must have happened as the eastern Canadian and Greenland ice sheets melted, so that the North Atlantic was episodically flooded with fresh water, disrupting the formation of the deep salty current that had attracted the warm North Atlantic Current northward (and promoting winter ice that "capped" the evaporation needed for resumption of the salt cycle).
      Climate change isn't always gradual, and reversals in such salty streams may be among the reasons; still, my physiologist's training makes me worry about the more subtle reasons. All of this salt exchange reminds me of the early days of our physiological understanding of the kidney (the major player in another salt economy, that of our bodies). Since then, we've discovered some of the more subtle regulation, learned how to influence it (and high blood pressure) with medications such as diuretics. Meltwater deluges and ice-capping the salt conveyor may only be part of the story, the equivalent of binge and hangover in the body's salt economy (alcohol dehydrates the body unless a lot of alcohol-free water is also consumed at the same time).

SEATTLE IS OUT THE LEFT WINDOW and I can almost see home. Certainly I can see, in profile, that glacial relic south of the University of Washington known as Capitol Hill. The "Capitol Hill that isn't" was so named a century ago, in hopes of getting the state legislature to locate the Washington state capital there, but Olympia won. I look for its tallest point (about 35 stories uphill from the university) and a towering redwood tree with a perfect conical shape; my favorite "park bench" is just below the redwood. They are in a cemetery not far from home, a place where I often go walking while thinking out some problem.
      This white granite bench, you come to realize, is actually a tombstone. Indeed, the most useful of tombstones, inviting the visitor by its very placement to pause for a while. Even on a typical Seattle day, you can see both Puget Sound to the west and Lake Washington to the east. When the clouds part, you see beyond the waters to the Olympic Mountains and the Cascade Mountains, which together formed a north-south channel for the Puget Lobe. On a clear winter day after the leaves have fallen, the bench has a horizon-to-horizon panoramic view, blocked only by that magnificent redwood just south of it.
      Deeply chiseled into the edges of the top slab of this bench is a characteristically Seattle epitaph. As you walk around the bench, it reads:

West face: West lies the Sound, South a great tree
North face: North is the University
East face: East the mighty Cascades run free
South face: All these places were loved by me.

And this unusual tombstone also offers no name, no dates -- just an evocative reply to "What shall I build or write / Against the fall of night?"

MOUNT RAINIER now appears majestic in the southern sky as the plane banks over Tacoma to turn back north. This massive white volcano stands about four times as high as the Puget Lobe reached in Seattle (at 1,100 meters thickness in Seattle, the glacier would have covered a building 260 stories tall). The lowlands south of Tacoma and Olympia are where the glacier stopped 14,000 years ago, though on earlier advances it had gone slightly further before backing up. One can see the deep valleys extending radially outwards from Mount Rainier, like spokes from a wheel, carved by Rainier's glaciers before they withdrew. Here and there, the radial valleys meet the north-south valleys (some filled with long lakes such as Lake Washington on Seattle's eastern border) formed by the Puget Lobe.
      Not only couldn't anyone make this over-the-pole journey a few decades ago, but it's only in the last century-and-a-half that we've even known the ice ages existed. And the Ice Age still lives here, with nearly a thousand glaciers in this state alone: about 40 glaciers cover Mount Rainier, though some have receded as much as a kilometer in the last century.
      The Seattle-Tacoma International Airport is atop still another assortment of glacial till; it's about as tall as Capitol Hill but has been reshaped to look like a mesa. In the process, they uncovered the skeleton of a giant ground sloth, common in the area during the ice ages. Thomas Jefferson was the one who discovered this species of sloth two centuries ago (scientific literacy among American politicians used to be somewhat better than it is today).
      The airport runways now extend to the very edges of the flattened hilltop. And so after the gradual descent on our final approach, the ground suddenly seems to rise up to meet us, like a slow kiss which accelerates.
      It is about noon here, the end of a timeless journey spanning the ice ages.

Deeds need time,
even after they are done,
to be seen and heard.

Friedrich Nietzsche

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