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t then abruptly plunged back into bitter cold for a thousand yearsor so in an event known to science as the younger dryas。 （the name es from the arcticplant the dryas； which is one of the first to recolonize land after an ice sheet withdraws。 therewas also an older dryas period； but it wasn’t so sharp。） at the end of this thousand…year onslaught average temperatures leapt again； by as much as seven degrees in twenty years；which doesn’t sound terribly dramatic but is equivalent to exchanging the climate ofscandinavia for that of the mediterranean in just two decades。 locally； changes have beeneven more dramatic。 greenland ice cores show the temperatures there changing by as much asfifteen degrees in ten years； drastically altering rainfall patterns and growing conditions。 thismust have been unsettling enough on a thinly populated planet。 today the consequenceswould be pretty well unimaginable。
what is most alarming is that we have no idea—none—what natural phenomena could soswiftly rattle earth’s thermometer。 as elizabeth kolbert； writing in the new yorker； hasobserved： “no known external force； or even any that has been hypothesized； seems capableof yanking the temperature back and forth as violently； and as often； as these cores haveshown to be the case。” there seems to be； she adds； “some vast and terrible feedback loop；”
probably involving the oceans and disruptions of the normal patterns of ocean circulation； butall this is a long way from being understood。
one theory is that the heavy inflow of meltwater to the seas at the beginning of theyounger dryas reduced the saltiness （and thus density） of northern oceans； causing the gulfstream to swerve to the south； like a driver trying to avoid a collision。 deprived of the gulfstream’s warmth； the northern latitudes returned to chilly conditions。 but this doesn’t begin toexplain why a thousand years later when the earth warmed once again the gulf stream didn’tveer as before。 instead； we were given the period of unusual tranquility known as theholocene； the time in which we live now。
there is no reason to suppose that this stretch of climatic stability should last much longer。
in fact； some authorities believe that we are in for even worse than what went before。 it isnatural to suppose that global warming would act as a useful counterweight to the earth’stendency to plunge back into glacial conditions。 however； as kolbert has pointed out； whenyou are confronted with a fluctuating and unpredictable climate “the last thing you’d want todo is conduct a vast unsupervised experiment on it。” it has even been suggested； with moreplausibility than would at first seem evident； that an ice age might actually be induced by arise in temperatures。 the idea is that a slight warming would enhance evaporation rates andincrease cloud cover； leading in the higher latitudes to more persistent accumulations of snow。
in fact； global warming could plausibly； if paradoxically； lead to powerful localized cooling innorth america and northern europe。
climate is the product of so many variables—rising and falling carbon dioxide levels； theshifts of continents； solar activity； the stately wobbles of the milankovitch cycles—that it is asdifficult to prehend the events of the past as it is to predict those of the future。 much issimply beyond us。 take antarctica。 for at least twenty million years after it settled over thesouth pole antarctica remained covered in plants and free of ice。 that simply shouldn’t havebeen possible。
no less intriguing are the known ranges of some late dinosaurs。 the british geologiststephen drury notes that forests within 10 degrees latitude of the north pole were home togreat beasts； including tyrannosaurus rex。 “that is bizarre；” he writes； “for such a highlatitude is continually dark for three months of the year。” moreover； there is now evidencethat these high latitudes suffered severe winters。 oxygen isotope studies suggest that theclimate around fairbanks； alaska； was about the same in the late cretaceous period as it isnow。 so what was tyrannosaurus doing there？ either it migrated seasonally over enormousdistances or it spent much of the year in snowdrifts in the dark。 in australia—which at that time was more polar in its orientation—a retreat to warmer climes wasn’t possible。 howdinosaurs managed to survive in such conditions can only be guessed。
one thought to bear in mind is that if the ice sheets did start to form again for whateverreason； there is a lot more water for them to draw on this time。 the great lakes； hudson bay；the countless lakes of canada—these weren’t there to fuel the last ice age。 they were createdby it。
on the other hand； the next phase of our history could see us melting a lot of ice rather thanmaking it。 if all the ice sheets melted； sea levels would rise by two hundred feet—the heightof a twenty…story building—and every coastal city in the world would be inundated。 morelikely； at least in the short term； is the collapse of the west antarctic ice sheet。 in the past fiftyyears the waters around it have warmed by 2。5 degrees centigrade； and collapses haveincreased dramatically。 because of the underlying geology of the area； a large…scale collapseis all the more possible。 if so； sea levels globally would rise—and pretty quickly—by betweenfifteen and twenty feet on average。
the extraordinary fact is that we don’t know which is more likely； a future offering us eonsof perishing frigidity or one giving us equal expanses of steamy heat。 only one thing iscertain： we live on a knife edge。
in the long run； incidentally； ice ages are by no means bad news for the planet。 they grindup rocks and leave behind new soils of sumptuous richness； and gouge out fresh water lakesthat provide abundant nutritive possibilities for hundreds of species of being。 they act as aspur to migration and keep the planet dynamic。 as tim flannery has remarked： “there is onlyone question you need ask of a continent in order to determine the fate of its people： ‘did youhave a good ice age？’ ” and with that in mind； it’s time to look at a species of ape that trulydid。
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28THE MYSTERIOUS BIPED
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