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W. H. Calvin's THE ASCENT OF MIND (Chapter 5) Home Page || Public Bookmarks || Science Surf column || The Calvin Bookshelf A book byWilliam H. Calvin UNIVERSITY OF WASHINGTONSEATTLE, WASHINGTON 98195-1800 USA 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).Powell’s Books in Portland lists used copies in their web database.All chapters are webbed. The Ascent of MindIce Age Climates and the Evolution of IntelligenceCopyright ©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). 5 OVER THE POLE: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 thantimeless, my "over the pole" flight turned out to be a journeybackward in time, as I surveyed the land covered anduncovered by the ice ages. It seems strange to realize thatonly 150 years ago, hardly anybody knew about the ice ages --even scientists still talked in terms of a biblical deluge. Thegreat Swiss (and later American) naturalist, Louis Agassiz(1807-1873), established about 1840 that massive ice sheetshad pushed their way around Europe, sometime in the not-so-distant past. He also wrote a great work in biology, sortingout the relationships of the fossil fishes to the living species. Despite this major discovery about evolution ingeology, plus the major theoretical feat in classification thatpaved the way for evolutionary understanding, Agassiz didn'tbelieve in biological change in the Darwinian sense. He wasthe last creationist who was also a major biologist, and wasbitterly opposed to Charles Darwin's interpretation of howspecies evolved. Perhaps one major heresy per lifetime(extending the span of life on earth far beyond the biblicalscholars' estimate) was all he could manage. GLACIERS DESCENDING from the north lead mostpeople to think that the center of the ice cap must have beenthe North Pole (that is, after all, the way it works at the SouthPole). It took a while after the discovery of the ice agesbefore anyone realized that glaciers don't form over openocean: the pack ice at the North Pole is only a few metersthick (and it's rapidly getting thinner). As a naval officersitting next to me on another airplane flight once remarked, "Ifanyone ever builds a house there, they'll get a surprise if theydig a basement!" (his submarine had punched through the ice,and he had gone walking on top of the world). The bottom ofthe Arctic Ocean is as deep as the Atlantic. It has featuressuch as Nansen Basin, underwater ridges such as the NansenCordillera; both are named for the Norwegian scientist FridtjofNansen, one of the first neurobiologists. In 1888, Nansen andthe Spanish neurobiologist Santiago Ramón y Cajalsimultaneously discovered "the neuron doctrine"; Nansen waslater an arctic explorer in 1893 to 1896, still later a diplomatwho received the Nobel Peace Prize in 1922. Although it may snow up on top, the sinking icechanges back into water on the submerged surface of the icesheet. To build up ice to the thickness of a mountain range,as happens during an ice age, requires a solid foundation suchas Greenland. Down at the South Pole, there is a wholecontinent (9.3 percent of the Earth's land surface) to house glaciers; they have even spread out into some shallow baysand displaced the seawater, e.g., the Ross Ice Shelf. Greenland is about the only such land at high latitudes in thenorthern hemisphere, although it is smaller than Europe (itusually looks bigger because the high latitudes get stretched onmost maps). When an ice age really gets going, then the northernhemisphere has a lot more land on which to house glaciersthan the southern. Glaciers often came down to 50°latitude (past London and Vancouver), but to 40° in afew places (such as New York City and Woods Hole). Between 50° and 70° latitude, the southernhemisphere has only the tip of South America plus a bit of theAntarctic Peninsula -- but the northern has Greenland, northernEurope, the vast expanse of Siberia, and then Canada too. Italso has Alaska, but surprisingly the interior of Alaska northof the coastal mountain ranges had few glaciers, probably dueto the "rain shadow." FLYING OVER OSLO, one suddenly notices that theflatland appearance of the Baltic has given way to rock -- anancient eroded landscape, the shallow valleys filled withagriculture. Oslo sits at the head of a long fjord; though at60° latitude, higher than Scotland, it is warmed by theNorth 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 isvery thin. There is little topsoil in the uplands, except in someof the grooves (only 1 percent of Norway is agricultural, withanother 2 percent as grassland). This area has been repeatedlyscoured by glaciers, right down to bedrock. This is not onlysome of the oldest rock in Europe, it is some of the earliestrock anywhere on Earth. The deep grooves are not scrapemarks in the manner of the scratches made by boulders carriedalong by the advancing glaciers; rather they are the result ofa billion years of erosion of the granite and gneiss by waterrunoff. Snow remains in some of the shadowed grooves,sheltered from the oblique sunlight at these latitudes, givingthis part of Norway a zebra-striped appearance during someseasons. This is the 65° latitude (the same as Iceland)that Milutin Milankovitch used as the reference latitude for histhesis developed in Budapest: elaborating on the 1842suggestion of the French mathematician Joseph A. Adhémar,Milankovitch proposed that the sunlight reaching such latitudescontrolled the ice ages. He showed that in the warmest times,there was as much sunlight here at 65° as there ispresently at 49° (e.g., Paris). In the coldest periods,65° got about as much sunlight as they get today up at76° (e.g., Thule, Greenland -- as far north as we'll fly ingoing "over the pole" today). So sometimes Iceland has hadas much sunlight as Paris, and sometimes as little as Thule(that hadn't been "intuitively obvious" until the calculationswere done). The winds blow, and the rivers flow, their patterns andstrengths 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 toour left has a steamer ship heading inland, leaving behind along white wake in the fjord. A road cuts into the borderinghillside of otherwise unrelieved rock. An hour after leaving Copenhagen, we finally departEurope. We're now out over the Norwegian Sea, to be exact. We will miss seeing Iceland, as our route takes us well to thenorth. We have reached the latitude of the Arctic Circle; werethis midwinter, the sun would barely be peeking over thesouthern horizon at noontime. Near midsummer, the sundoesn't set, merely skimming the northern horizon at midnight. We do see the mid-ocean ridge, where the ocean flooris spreading apart as new material upwells from the depths ofthe earth. There are some volcanos along that ridgeline: Iceland's are the best known, but now we see Jan MayenIsland out the right window, its volcano Beeren Berg pokingup through the clouds. Its glaciers have receded most of theway back to the uppermost cone; they can't go very far beforereaching the sea -- except to the south, where the mid-Atlanticridge has poked up above the waters to form a long spit, likethe handle on a frying pan. The island resembles a Hawaiianvolcano, arched like a shield or convex lens -- except for thetop half, whose erect cone sweeps upward like the tip ofJapan's Fujiyama.THE TILT OF THE EARTH'S AXIS of rotation, relativeto the plane of its orbit (in the arcane astronomicalterminology, "the obliquity of the ecliptic"), changes someover the years. It drifts back and forth between 22° and24.5°, taking about 41,000 years to make a completecircuit. Currently the tilt is about 23.4° (and declining)-- and at that latitude the sun stands overhead on the longestday of the year. We northern hemisphere types call thislatitude the Tropic of Cancer; it passes just north of Havana,Cuba. At maximum tilt, which last occurred about 9,600 yearsago, the sun makes it up to Key West, Florida; at minimum,it only makes it up to the Isle of Pines off the southern coastof Cuba. This 2.5° may not seem like much (only thedifference in latitude between New York and Washington,D.C., or between Geneva and the Mediterranean). But if youlive up where the glaciers do, you can get a considerablepercentage improvement in the warmth delivered insummertime. The sun climbs a little higher in the sky atmidday, stays above the horizon a little longer to make thenights even shorter. Trying to reason out the physics of all this? A littleknowledge of physics can be misleading when it comes to iceages. The heat exchange involved in freezing and melting atray of ice cubes is identical -- so a change in tilt thatproduces both hotter summers and cooler winters shouldn'tmake much difference in ice buildup, right? But that analysisassumes nothing moves -- and ice can move. The AtlanticOcean beneath us is full of icebergs, calved off of Greenlandand floating south, the warming job being exported to warmerlatitudes than where the snow fell and froze into ice atopGreenland. And this is the northernmost extreme of the NorthAtlantic Current we're flying over. It is nice and warm,flowing up as it does from the tropics. The North AtlanticCurrent warms up all that cold Arctic air that flows east fromCanada, and so Europe gets much more comfortable weatherthan they get at comparable latitudes in Canada (all of Europenorth of the Paris-Prague line is at Canadian latitudes). TheNorth Atlantic Current makes winter in Oslo tolerable eventhough the sun only stands 7° above the southernhorizon at noon. But the North Atlantic Current was shutdown during the last ice age, starting up only 14,000 years agowhen melting got underway. I wonder what all those icebergs coming off Greenlandduring the end of the last ice age did to the North AtlanticCurrent? Certainly there was that period about 11,000 yearsago, during the most rapid phase of the meltoff, when Europeparadoxically cooled down. The infamous Younger Dryas. The massive melting of the Canadian ice cap might helpexplain its thousand-year duration, but there are a series ofDryas-like "cold spikes" all though the last ice age, especiallyduring the period of 30,000 to 70,000 years ago. Even ifsomeone should "explain" the Younger Dryas in terms ofevents unlikely to be repeated in the coming century, there areall those other shorter snaps to explain. Something besides theMilankovitch rhythms and meltoff deluges seems occasionallyto cause some centuries-long cold snaps, and they haveknowledgeable people worried. GLACIERS CAN BREAK UP in dramatic ways. Whenmelting gets going, some of the water runoff gets beneath theglacier and thaws the glacier's attachment to terra firma --greases the skids, as they say. This allows the glacier to slipsideways -- and if the ice is piled very high and heavy (severalthousand meters or more is not unusual), it may start tocollapse, the edges of the glacier surging outwards andbreaking up as the center tumbles down. Because the icesurface area exposed to the warm summer air is greatlyincreased by fragmentation, melting speeds up further. There is nothing analogous to iceberg deluges andglacial surges in the orderly layer-after-layer buildup of iceduring the cooler wintertime. So warmer summers but coolerwinters suggests net melting of glaciers even if theannual average sunlight doesn't change much. In the modelsthat have been made of this process, a melting rate about fourtimes faster than the buildup rate fits the fluctuations of oceansalinity quite well during the last ice age. But the 41,000 year tilt cycle doesn't, by itself, matchup with the 100,000 year period between big meltoffs. Whatelse might increase summertime heating at high northernlatitudes? Well, the earth's orbit isn't circular but elliptical, abit elongated. That means that our distance to the sun isn'tconstant: the earth is closest to the sun ("reaches perihelion")on the third of January. By the fifth of July, we are aboutthree percent farther away from the sun. Sunlight's intensityfalls off as the square of our distance from the sun; in January,we get about seven percent more sunlight (averaged over thewhole earth) than in July. If we didn't, northern winters wouldbe even colder these days. The date on which perihelion occurs is not, however,always the third of January. The date of perihelion driftsbecause, like other spinning tops, the earth slowly precesses,its axis tracing out a cone. Since that is independent of theelliptical orbit itself, the earth's orientation toward the sun atperihelion changes over time. Only 5,500 years ago,perihelion was about the time of the autumn equinox in lateSeptember. And 11,000 years ago, it was coincident with thesummer solstice in late June -- and so the northern hemispheregot its maximal heating for the year at the time when itsglaciers are most susceptible to melting. The cycle takesbetween 19,000 and 26,000 years (as tops go, the earth israther massive and the precession period quite slow). Furthermore, the elongation changes as the positionsof the other planets pull the earth into an even more ellipticalorbit. Those seven percent differences in heating increase,considerably exaggerating the summer-winter differences. Themaximum eccentricity occurs every 400,000 years, althoughthere is a minor peak at 100,000 years embedded in it (the tiltcycle also has minor peaks, and perihelion date also doesn'tadvance uniformly). One of the puzzles about the ice ages is that they recurevery 100,000 years, but the eccentricity contribution toarriving sunshine seems too weak to be so important; somegeophysicists suspect that the earth's crust resonates at about100,000 year periods, it taking that long for the depressed crustto rebound after sinking under a mountain of ice. Whateverthe cause, when two out of the three astronomical factors (tilt,season of perihelion, eccentricity) are going to have major orminor peaks at about the same time (as when tilt peaked 9,600years ago and perihelion was at the summer solstice 11,000years ago), northern glaciers melt back substantially. Theglacial maximum was about 20,000 years ago; the meltoffwas well underway by 14,000 years ago and was mostlycomplete by 9,000 years ago. When only one of the threeastronomical factors is at a peak, there is some meltback. Juneperihelion date is best correlated with all of the minormeltbacks between the major ones. And so, as the relative mix changes, there is lots ofback-and-forth movement of glaciers between the majormeltbacks, aided and abetted by variations in the sun's nuclearfurnace. The augmented summer sunshine not only melted theice sheets, but it had some effects at more tropical latitudes aswell: the Sahara was green about 8,000 years ago (the"Pluvial"), thanks to the way the enhanced monsoons spreadinto northern Africa, just as they did on earlier occasions whenperihelion 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 seesfjords again. The one below the airplane is full of icebergsand broken sheets of floating ice. Long wide roads of ice,furrowed and cracked, come down from the Greenland'shighlands and then end abruptly in open sea. There are dozensof glaciers emptying into this labyrinth of fjords onGreenland's east coast; hundreds of white iceberg tips dot thechannels. And this is late in an interglacial period wheniceberg birth rates are lowest; one wonders what this sightwould have been like 13,000 years ago when the big meltoffwas getting going, and the iceberg factory was running flatout. The eroded red rock lining the fjords is old, probablymore than 2.5 billion years (just as is the coast of Norway;back then, Scandinavia, Greenland, and Canada were allconnected, before the mid-Atlantic rift did its work separatingthem). Greenland is part of the Laurentian shield of Canada,recently revealed to be a series of microcontinents fusedtogether by some great lava flows nearly 2 billion years ago. There's not a speck of vegetation to be seen from 10,000meters up, though there are surely some lichens clinging tothose rocks. But hardly enough to get a soil started. From my stratospheric perspective, however, I can seea monster of a glacier to the south, staircasing its way downfrom the highlands, feeding northward out of the prominentmountain range several hundred kilometers away. Morefamiliar locales used to have monster glaciers like that: theone that pushed down out of the north to cover up whereVancouver and Seattle now are, the one that pushed down outof the Alps into the Danube's valley. Some great blue spots are visible atop those glaciersbeneath the plane's wing; they're ponds of summer melt water. On active glaciers staircasing downhill, a crack will soon openup beneath such a pond and it will drain. The ponds I see areconsiderably inland from where small blocks of ice are calvingoff and floating away, so the ponds aren't holes in the ice, ofthe kind frequented by the surfacing seals that attract bothpolar bears and Inuit hunters. I didn't see any coastal settlements, and there aren'tmany this far north except for some Inuit ruins. Thepopulation 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 smoothedsnowfields. Endlessly. Greenland is eerie, a high plateau ofice, everywhere. Tips of mountains barely poke through theice sheets and snowfields. This makes the mountain tops looklike a chain of islands in a white sea; the occasional furrowedglacier showing through the wind-smoothed snowfields lookslike an offshore barrier reef producing turbulence. But thehighest point in Greenland is on one of those plains of snow;that's where a European scientific team is drilling 3,000 metersdown to bedrock and, about 30 kilometers to the west, anAmerican scientific team is drilling a comparison core, part ofthe effort to be sure about what's real climate data, and what'sjust the noise introduced by ice flow over the millennia. Though it seems frozen and static, the ice is pushingand shoving due to its own weight, eventually working its waydown to be born as a multitude of little white icebergs pokingup through a real sea. The mountain of ice is as much as3,410 meters thick. In some places the land beneath it hassunk 365 meters below sea level (about the same elevation asthe Dead Sea), thanks to the weight of the ice. The glaciercould never have gotten started if the land had originally beenbelow sea level, another reason why buildup and melting ofice can be so different. It is still noon as we pass over Greenland. Theshadows I see are about as short as they ever get; usually theyare very long, the mountain peaks casting great shadows forlong distances to the north across the frozen snowfields. Thepurser says that this plane turns around in Seattle andimmediately flies back to Copenhagen with a new load ofpassengers, passing over here again in the early morning hours. At that time, the long shadows will stretch out towards thesouth, melting ice around the clock. If you were to becomelost around here, the Boy Scout lore about moss growing onthe north sides of trees wouldn't work: the sun rotates allaround the tree! That presumes, of course, that you could finda 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 viceversa. The reversal in names is due to the reversal in regionalclimate in the century between their discoveries. WhenIceland was first settled by Vikings about A.D. 860, it wasduring a cold spell (we now know from the ice core's oxygenisotopes, which serve as "frozen thermometers"), causingIceland's fjords to ice up. Erik the Red, banished from Icelanda century later (a little matter of murder), explored to the westacross the Denmark Straits and discovered what he called"Greenland" -- and we now know that things had warmed upconsiderably in that century since Iceland's settlement. During this warmer period, the Norse explored thenortheast coast of North America, shipping back timber toGreenland. But it cooled dramatically in the fourteenthcentury and the fortunes of the Greenland settlers declined. The settlement lasted until about 1540, wiped out by thecooling (and the failure of the settlers, addicted to Europeanstyles in clothing, to adopt Eskimo techniques for survival insuch climates). At Greenland's southern tip, something is actuallygreen 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 andberries that cover the ground, probably gave the place its nameamong the boat-borne visitors lacking our elevated perspectiveon the ice. Thus the name "Greenland" commemorates agreen facade, shielding the mountain of ice further inland. And a fickle facade at that, varying from century to centurywith the erratic climate of the North Atlantic.THE CLOUDS WE ENCOUNTERED over centralGreenland now part and I see land below that isn't Greenlandbecause there are no glaciers -- it looks scraped clean, thesame kind of reddish Canadian Shield as Greenland, but Ithink it hasn't seen a glacier for many millennia. If this isCanada, then I missed seeing Thule, Greenland, known to therest of the world mostly for its cold-war radar installations(and as the American "Siberia" to which unpopular Air Forceofficers were reassigned). Canada's Baffin Island is north ofHudson's Bay, and that must be it below. There is a lot of ice,but exposed sea lanes as well, with pancakes of ice scatteredhere and there -- not icebergs, just flat ice. The Inuit live uphere too, the last of the ice-age hunters; indeed, there are moregroups in the eastern Canadian Arctic than elsewhere. Theycan be found from Siberia around to Greenland, following theseals and bears. Now if this were 1831, the year that the NorthMagnetic Pole was first located on the Boothia Peninsula, we'dbe flying right over it. But it has moved since then, and isnow about 1000 kilometers (about 600 miles) out the rightwindow to the northwest. The North Magnetic Pole is thepoint toward which all those decelerating charged particlesfrom 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 excitingthan the Geographical North Pole. No northern lights for ustoday; they're there 24 hours a day, but it's still noon and wecan't see them for all the sunlight that reflects off the thin airto produce a blue sky. We missed the North Pole itself byquite a bit, the distance between Miami and New York City. So this over-the-pole flight might be better described as thealmost-over-the-magnetic-pole flight. If this were 14,000 years ago instead, and I looked outthe left window to the south, I would have seen a "mountainrange" going all the way to New York City and Cape Cod. The Laurentide ice sheet was truly massive, and so tall that itprobably deflected some of the jet stream to more northerlylatitudes. What are Arctic travelers likely to see out the windowa few decades from now? Looking into the future withcomputer simulations, this tundra beneath us may thaw in abig way: northern Canada is likely to warm up more thananywhere else on earth, as the greenhouse warming progresses. The methane that the thaw releases from the tundra is alsolikely to make the greenhouse even worse.LAKES SEEM TO BE EVERYWHERE in the NorthwestTerritories, and we've just passed near Great Slave Lake andour first town since leaving Norway, Yellowknife. Far to theright side must be Great Bear Lake, noted for an ancientvolcano tipped on its side, erosion exposing the internalplumbing. The natives that live up here were also quitesuccessful as emigrants to the United States; the languagesspoken by the Apache and Navajo, down south near the GrandCanyon, are closely related to the ones spoken up here, and itappears that the Athabascan-speaking peoples (named for alake off to our left) hunted and gathered their way down southrather recently -- they barely got there in time to be used asslave labor by the sixteenth-century Spanish in building theirchurches in the Rio Grande valley. This was, of course, a halfcentury 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 rockprotruding through the tundra, but hereabouts ought to be theend of the Canadian Shield. Southwest of here is more recentgeology, late-arriving chunks of North America that sailedacross the Pacific Ocean during the last 50 million years. THE ICE-FREE CORRIDOR (minus the ice) is seen outthe right side, as the Rockies come into view. The pioneersmight have been able to walk across the Bering Strait fromAsia, but Alaska's northern interior was the end of the line. The rugged coastline, augmented by ice sheets on thecontinental shelf, probably prevented animals (includinghumans) from moving south, though there might have been afew small "harbors" along the way if boats were available. And the great ice cap sitting atop Canada would have blockedthe 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 theRocky Mountains towards the east -- it spread west fromHudson's Bay (drained empty by the fall in sea level) andactually started to push up into the foothills of the Rockiesbefore it met up with the Cordilleran glaciers flowing downthe Rockies (eastern Canada then received much more snowfallbecause the Gulf Stream shifted). From about 30,000 yearsago until 14,000 years ago, the two ice masses pushed againsteach other; then between 14,000 and 12,000 years ago, as bothstarted to melt back somewhat, a corridor opened up from thenorth coast of Alaska leading down to eastern Montana. As I said, I tend to imagine this as something like thebiblical parting of the Red Sea -- a north-south corridoropening up as the ice walls pull back on both the east andwest sides. Shortly after the corridor opened, there was ahuman 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 ofthose species now remain only in Africa; Teddy Roosevelt,early in the century, took a train trip through Africa and calledit a "railroad through the Pleistocene," a tour of what Americaused to be like. The corridor ran up the eastern front of the Rockiesfrom the southern limit of the glaciers, at about the Alberta-Montana border, to Dawson Creek and Fort Nelson. We mustbe over Dawson Creek, as I can now see the Alaskan Highwaysnaking off to the west en route to Whitehorse and eventuallyFairbanks. The mountains continue northwest as theMackenzie Mountains all of the way up to the Yukon. Noglaciers visible now; just the unfurrowed white patches that arepermanent snowfields -- the seeds of glaciers. So the earliest route to the south required first goingnorth, up above the Arctic Circle, almost (unless the Yukon'svalley opened up early) all the way up to the North Slope andthe Arctic Ocean coastline, reaching the Mackenzie River deltaand then turning southeast and traveling down the eastern frontof the Mackenzies and Rockies. There were a lot of lakesalong the way, formed by moraines of the Laurentide icesheet: the bulldozing ice snouts actually dammed up somevalleys of the Rockies in a manner not unlike modernreservoirs, with an earthen dam of rubble stretched across theexit to the valley -- except that the rubble was pushedup the valley from below by that monster glacier fromHudson's Bay far to the east. Herds of grazing animalsprobably worked their way down the corridor, following thenew grasslands, followed by the hunters.THE FIRST AMERICAN POPULATION explosion likelycame from those hunting bands that found their way down theice-free corridor. Or maybe it was the second or third, sincethere is a lot of argument over whether there were somehuman inhabitants in both North and South America during thelast quarter of the last ice age, more than 31,000 years ago. Like the Vikings who explored the Atlantic coastline centuriesbefore the southern European explorers came and stayed (andattracted the even later but more prolific English), so theearliest human occupation of the Americas may have been amultistep affair. Because the corridor east of the Rockies was openbefore 30,000 years ago, an earlier Bering Strait emigrationfrom Asia could, conceivably, have initially populated the restof the Americas. But the door closed on the corridor at 30,000years ago, and didn't reopen until about the time of the Clovishunters, 11,800 years ago. Of course, the early SouthAmerican populations presently dated (these numbers areforever being updated, and the radiocarbon dates recalibrated)earlier than 31,000 years might also have arrived by boat fromthe Pacific islands. Everyone is eagerly awaiting enoughbones and cultural artefacts from the early sites to makecomparisons to ancient populations of the Asian mainlands thatspread into the Pacific islands. The present-day natives of North and South Americaseem fairly closely related, just what one might expect from apopulation explosion based on some initially successfulhunting tribes pouring through the ice-free corridor. Whetheror not some humans arrived even earlier, the hunters seenstarting at about 11,800 years ago were prolific big-gamehunters and left their Clovis-style arrowheads and spear pointsall over the continent, including in the rib cages of some now-extinct species of megafauna. Some groups certainly camelater, such as the Arctic-specialized Aleut and the Inuit perhaps8,000 years ago. There was, of course, a population contraction in morerecent centuries, as the native populations were decimated bythe diseases imported via the European and Africanimmigrants. That sort of replacement of one hominidpopulation by another is likely how modern-type Homosapiens, the descendants of the African "Eve" collection ofmitochondrial DNA that was around 150,000 years ago, cameto dominate the scene. They need not have brutally eliminatedHomo erectus and "archaic Homo sapiens",though there probably were incidents of that sort, just asoccurred involving the U.S. Cavalry in the nineteenth century,massacring Cheyenne Indian families at Sand Creek. It wouldsuffice to possess an immune system that could cope with avirulent virus that predecessor immune systems could not. It is difficult, as Richard Leakey points out, tootherwise account for the widespread disappearance of thepredecessors (conquest, despite occasional massacres, tends tolead to interbreeding and thus regional retention of somecharacteristic features). Even without superior technology, theEuropeans could have displaced the American Indians -- justwith the smallpox that Europeans could survive better than theIndians. Anthropologists often argue that waves of settlementshouldn't occur without the newcomers having some advantagesuch as new-model body styles or advanced culture -- but theysometimes forget the pathogens and antibodies that aren'tpreserved as well as stones and bones. THE ROCKY MOUNTAINS take a brief respite and wesee interior valleys for a few minutes until we cross the FraserRiver and we're into the coastal mountains, looking every bitas rugged as the Rockies. I can see why it would be hard towalk down the coast from Alaska: the mountains continue tothe coastline; further north along the Gulf of Alaska, glaciersextend right out to the water's edge, contributing moremeltwater to the oceans than any other glaciers in the world,outside Greenland and Antarctica. But then we suddenly popout of the mountains and are over a real metropolis, completewith a large river delta. It's Vancouver, British Columbia, andthat is the Fraser River emptying into the Strait of Georgia. We are flying right down the strait between themainland and Vancouver Island to the west. I know we justpassed into United States airspace because we are over the SanJuan Islands, one of my favorite places; I spotted the FridayHarbor Labs (at least its dock; the buildings blend so well withthe natural setting that I can't distinguish them). To the westI can see the Strait of Juan de Fuca, separating VancouverIsland from the Olympic Peninsula, and opening out into thevast Pacific Ocean. The Atlantic for lunch, the Pacific fordinner. To the south is Puget Sound, not a sound at all (sinceit is deadend) but rather a very long bay with only the onenarrow exit to the ocean. The world's larger "bays" includethe Mediterranean Sea and the Red Sea. They all have aninteresting salt economy -- not of the kind associated with thecamel caravans of centuries past, but a salt economy associatedwith the bay's gains and losses of fresh water. The Red Seais an extreme example: it loses quite a lot of fresh water byevaporation, but gains essentially none from rivers (or meltingicebergs!). It doesn't dry up into salt flats because less saltyIndian Ocean water is attracted in through the Strait of Bab alMandab -- and so the Red Sea's salinity has stabilized at aboutten percent higher than the oceans. Puget Sound has lots ofrivers coming down from the mountains to the east, south, andwest; no danger of Red-Sea-style hypersalinity here. Except,perhaps, if it really turned cold and the winter snows turnedinto glaciers rather than runoff. The Mediterranean gets fresh water from some bigrivers such as the Nile and Rhone, but it also has quite a lot ofsurface area for evaporating fresh water. As the Mediterranean starts to get hypersaline, it attracts ocean water of normalsalinity in through the Strait of Gibraltar. This creates aninteresting circulation pattern. Hypersaline water is heavy, andso it sinks to the bottom of the eastern Mediterranean, thefresher waters from the rivers and the normal salinity seawaterfrom Gibraltar replacing it on the surface. The deep saltywater tends to escape, creeping along the bottom and out intothe Atlantic, just as the extra salt flushes out of the bottom ofthe Red Sea into the Indian Ocean. During the pluvial period about 8,000 years ago whenthe greatly augmented monsoons were watering Africa andturning the Sahara green, a lot more fresh water was deliveredto the Mediterranean via the Nile (and some large NorthAfrican rivers that can no longer be seen, their dry beds filledin with sand). And the Mediterranean's salty circulationpattern became the exact reverse (rather like Puget Soundtoday): the fresher water stayed on the surface and flowed outto sea, and some deep salty water was attracted into thebottom of the Strait of Gibraltar. So while it is a salteconomy, it's really all a matter of fresh water runoff into, andevaporation from, a basin. While bays illustrate the principles more readily, thesame principles apply to regions of the oceans too, should youhave areas (such as the Northern Atlantic) with more freshwater loss than gain. This principle was recognized severalcenturies 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), 1800Just imagine the North Atlantic Current as the equivalentof that normal salinity surface current flowing into theMediterranean at Gibraltar, nice and warm. To balance it, youget a deep salty current heading south from Iceland; actually,it flows from North Atlantic to the tip of Africa, east throughthe Indian Ocean, around Australia and up into the NorthPacific. It's a somewhat exaggerated version of theMediterranean's story: the water sinks like a stone aroundIceland 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 becausenonlinear systems often chase their tails. This is a particularlyapt description of the North Atlantic Current: it even does avertical U-turn. The Current -- now so cold and hypersalinethat it is denser than any layer of underlying water -- plungesfrom the surface to the abyss. There may not be a giant NorthAtlantic whirlpool or waterfall to gaze down upon, but this"deep water production" is equal in magnitude to 20 times thecombined flow of all the rivers of the world. Once the densewater has sunk under its own weight to the sea floor, it flowssouth -- and so attracts even more warm currents north toreplace it. Why did this Current falter? On the model of theMediterranean in the last Pluvial, the obvious candidates wouldbe all those analogies to the augmented Nile: the salt-freeicebergs calving off of Greenland, that fresh water coming outof the St. Lawrence River from eastern Canada's massive icesheet, and the meltwater from the Scandinavian ice sheetemerging from the Baltic and from Norwegian fjords. TheNorth Atlantic got fresh water from all sides except the south. With sufficient dilution of the ocean surface waters, therewouldn't have been an "attraction" of warm tropical watersnorthward to replace the hypersaline water that otherwise sinksaround Iceland. There may not be any major sources ofmeltwater left in Canada or Scandinavia, but Greenland hasenormous supplies -- and its east coast fjords are located closeto the current focus of deep water production, south of Iceland. A greenhouse-encouraged glacial surge into the fjords, or thesudden emptying of a meltwater lake, might have effects onclimate far out of proportion to their effects on rising sea level. And remember the "White Earth Catastrophe", wherethe ice cover prevented rewarming? It could well havehappened to the North Atlantic in another sense. As wind andevaporation are essential to the deep water production, icecover would limit evaporation and deep-water formation. Aniceberg deluge might have shut off the northerly movement of(warm) replacement water, but also (by raising the freezingpoint of the sea water) allowed winter ice to form much farthersouth. Indeed, the southern border of the sea-ice islandsfloating in the wintertime Atlantic descended fromScandinavian to Iberian latitudes (55°N to 35°N)as the Dryas started. This wintertime "cap" on the NorthAtlantic would have delayed the resumption of the saltconveyor. Ice matters. WE ARE HEADING SOUTH into the Sound-that-isn't. Ijust heard someone use the correct French pronunciation of"Puget" -- but she was quickly corrected by another Europeanwho explained that Americans make it rhyme with "fidget" forsome obscure reason. I hope that she hasn't heard about howthey pronounce Goethe Street in Chicago. The Strait of Georgia and Puget Sound were alsoemptied out by the drop in sea level during an ice age, makingthis a possible path for a glacier. We are flying right downthe route of the glacier that sat atop this area 15,000 years ago,the southernmost "Puget Lobe" of the Coastal-plus-Rockies icesheet known as the Cordilleran. Icebergs set sail out of theStrait of Juan de Fuca back then, just as they do now in theDenmark Strait east of Greenland. The San Juan Islands werescraped down to 350-million-year-old bedrock. The glacierwas a mile high (1,600 meters) here, half the height of MountBaker, the local volcano over to the east. The massive tongue plowed down to the south end ofPuget Sound, backed up, advanced again, and generallyrearranged the land. Whidbey Island, which I see stretched outon the left, is all glacial, sediments deposited by one glacier oranother, and carved by the silt-laden runoff from the lastmeltback. I once encountered some brick fragments on thebeach at Double Bluff, near the south end of Whidbey, andthought that they were surely of recent human origins, just asare the plastics that have drifted ashore. But no, the geologiststell me: the warm times of the last interglacial, 120,000 yearsago, produced a peat bog resting atop an older layer of clay. And the peat bog dried up and caught on fire, perhaps due tolightning, and so baked the clay beneath it! As it erodes outof the cliff, the beach becomes littered with red brickfragments. All the north-south valleys in the Seattle area areprobably drainage channels that formed beneath the PugetLobe. We even got our own fjord out of the deal, Hood Canalsnaking along in its fishhook shape out the right window(unlike the Norwegian fjords carved into hard rock, it lookslike a runoff channel from the lobe melting). Below on our left, atop another glacier-shapedlandform, is Paine Field, the birthplace of our airplane --together with all the other Boeing 747s in the world. Someglacial landforms, such as Long Island and Cape Cod, arerather like ancient landfills, plowed into place by the snout ofa glacier that then retreated. Actually, those Whidbey Islandbluffs were underwater during the meltback; their tops are thesediments that accumulated in the lakes that formed south ofthe retreating glacier about 13,000 years ago. The land hadbeen sinking slowly under the weight of the ice, but slowlyrebounded over the next few thousand years. And so nowthese postglacial sediments are above sea level; though sealevel has risen during the interglacial, these reboundingsediments have risen even more. This did not happen insouthern Puget Sound, as it was covered too briefly by glaciersto sink very much. North of Seattle, the rebound has beenmore than the sea level rise. The rapid melting about 13,000 years ago left evenmore dramatic evidence in eastern Washington state: a largelake of meltwater formed east of the Idaho-Montana border,but was held in place only by a dam of ice. When that dambroke, the lake emptied suddenly, a great flood sweepingwestward. It carved a broad swath across the state untilchanneled 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 Canadianand Greenland ice sheets melted, so that the North Atlanticwas episodically flooded with fresh water, disrupting theformation of the deep salty current that had attracted the warmNorth Atlantic Current northward (and promoting winter icethat "capped" the evaporation needed for resumption of the saltcycle). Climate change isn't always gradual, and reversals insuch salty streams may be among the reasons; still, myphysiologist's training makes me worry about the more subtlereasons. All of this salt exchange reminds me of the earlydays of our physiological understanding of the kidney (themajor player in another salt economy, that of our bodies). Since then, we've discovered some of the more subtleregulation, learned how to influence it (and high bloodpressure) with medications such as diuretics. Meltwaterdeluges and ice-capping the salt conveyor may only be part ofthe story, the equivalent of binge and hangover in the body'ssalt economy (alcohol dehydrates the body unless a lot ofalcohol-free water is also consumed at the same time).SEATTLE IS OUT THE LEFT WINDOW and I canalmost see home. Certainly I can see, in profile, that glacialrelic south of the University of Washington known as CapitolHill. The "Capitol Hill that isn't" was so named a century ago,in hopes of getting the state legislature to locate theWashington state capital there, but Olympia won. I look forits tallest point (about 35 stories uphill from the university)and a towering redwood tree with a perfect conical shape; myfavorite "park bench" is just below the redwood. They are ina cemetery not far from home, a place where I often gowalking while thinking out some problem. This white granite bench, you come to realize, isactually 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 Soundto the west and Lake Washington to the east. When theclouds part, you see beyond the waters to the OlympicMountains and the Cascade Mountains, which together formeda north-south channel for the Puget Lobe. On a clear winterday after the leaves have fallen, the bench has a horizon-to-horizon panoramic view, blocked only by that magnificentredwood just south of it. Deeply chiseled into the edges of the top slab of thisbench is a characteristically Seattle epitaph. As you walkaround the bench, it reads: West face: West lies the Sound, South a greattree North face: North is the University East face: East the mighty Cascades run free South face: All these places were loved byme.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 southernsky as the plane banks over Tacoma to turn back north. Thismassive white volcano stands about four times as high as thePuget Lobe reached in Seattle (at 1,100 meters thickness inSeattle, the glacier would have covered a building 260 storiestall). The lowlands south of Tacoma and Olympia are wherethe glacier stopped 14,000 years ago, though on earlieradvances it had gone slightly further before backing up. Onecan see the deep valleys extending radially outwards fromMount Rainier, like spokes from a wheel, carved by Rainier'sglaciers before they withdrew. Here and there, the radialvalleys meet the north-south valleys (some filled with longlakes such as Lake Washington on Seattle's eastern border)formed by the Puget Lobe. Not only couldn't anyone make this over-the-polejourney a few decades ago, but it's only in the last century-and-a-half that we've even known the ice ages existed. Andthe Ice Age still lives here, with nearly a thousand glaciers inthis state alone: about 40 glaciers cover Mount Rainier,though some have receded as much as a kilometer in the lastcentury. The Seattle-Tacoma International Airport is atop stillanother assortment of glacial till; it's about as tall as CapitolHill but has been reshaped to look like a mesa. In the process,they uncovered the skeleton of a giant ground sloth, commonin the area during the ice ages. Thomas Jefferson was the onewho discovered this species of sloth two centuries ago(scientific literacy among American politicians used to besomewhat better than it is today). The airport runways now extend to the very edges ofthe flattened hilltop. And so after the gradual descent on ourfinal approach, the ground suddenly seems to rise up to meetus, like a slow kiss which accelerates. It is about noon here, the end of a timeless journeyspanning the ice ages. Deeds need time, even after they are done, to be seen and heard. Friedrich NietzscheEmail || Home Page || The Calvin Bookshelf || Table of Contents || to End Notes || Continue reading next chapter |
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