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ing the gap between north and south america；disrupting the flows of warming currents between the pacific and atlantic； and changingpatterns of precipitation across at least half the world。 one consequence was a drying out ofafrica； which caused apes to climb down out of trees and go looking for a new way of livingon the emerging savannas。
at all events； with the oceans and continents arranged as they are now； it appears that icewill be a long…term part of our future。 according to john mcphee； about fifty more glacialepisodes can be expected； each lasting a hundred thousand years or so； before we can hope fora really long thaw。
before fifty million years ago； earth had no regular ice ages； but when we did have themthey tended to be colossal。 a massive freezing occurred about 2。2 billion years ago； followedby a billion years or so of warmth。 then there was another ice age even larger than the first鈥攕o large that some scientists are now referring to the age in which it occurred as thecryogenian； or super ice age。 the condition is more popularly known as snowball earth。
鈥渟nowball；鈥潯owever； barely captures the murderousness of conditions。 the theory is thatbecause of a fall in solar radiation of about 6 percent and a dropoff in the production （orretention） of greenhouse gases； earth essentially lost its ability to hold on to its heat。 itbecame a kind of all…over antarctica。 temperatures plunged by as much as 80 degreesfahrenheit。 the entire surface of the planet may have frozen solid； with ocean ice up to half amile thick at higher latitudes and tens of yards thick even in the tropics。
there is a serious problem in all this in that the geological evidence indicates iceeverywhere； including around the equator； while the biological evidence suggests just asfirmly that there must have been open water somewhere。 for one thing； cyanobacteriasurvived the experience； and they photosynthesize。 for that they needed sunlight； but as youwill know if you have ever tried to peer through it； ice quickly bees opaque and after onlya few yards would pass on no light at all。 two possibilities have been suggested。 one is that alittle ocean water did remain exposed （perhaps because of some kind of localized warming ata hot spot）； the other is that maybe the ice formed in such a way that it remained translucent鈥攁 condition that does sometimes happen in nature。
if earth did freeze over； then there is the very difficult question of how it ever got warmagain。 an icy planet should reflect so much heat that it would stay frozen forever。 it appearsthat rescue may have e from our molten interior。 once again； we may be indebted totectonics for allowing us to be here。 the idea is that we were saved by volcanoes； whichpushed through the buried surface； pumping out lots of heat and gases that melted the snowsand re…formed the atmosphere。 interestingly； the end of this hyper…frigid episode is marked bythe cambrian outburst鈥攖he springtime event of life鈥檚 history。 in fact； it may not have been astranquil as all that。 as earth warmed； it probably had the wildest weather it has everexperienced； with hurricanes powerful enough to raise waves to the heights of skyscrapersand rainfalls of indescribable intensity。
throughout all this the tubeworms and clams and other life forms adhering to deep oceanvents undoubtedly went on as if nothing were amiss； but all other life on earth probably cameas close as it ever has to checking out entirely。 it was all a long time ago and at this stage wejust don鈥檛 know。
pared with a cryogenian outburst； the ice ages of more recent times seem pretty smallscale； but of course they were immensely grand by the standards of anything to be found onearth today。 the wisconsian ice sheet； which covered much of europe and north america；was two miles thick in places and marched forward at a rate of about four hundred feet a year。
what a thing it must have been to behold。 even at their leading edge； the ice sheets could benearly half a mile thick。 imagine standing at the base of a wall of ice two thousand feet high。
behind this edge； over an area measuring in the millions of square miles； would be nothingbut more ice； with only a few of the tallest mountain summits poking through。 wholecontinents sagged under the weight of so much ice and even now； twelve thousand years afterthe glaciers鈥櫋ithdrawal； are still rising back into place。 the ice sheets didn鈥檛 just dribble outboulders and long lines of gravelly moraines； but dumped entire landmasses鈥攍ong islandand cape cod and nantucket； among others鈥攁s they slowly swept along。 it鈥檚 little wonderthat geologists before agassiz had trouble grasping their monumental capacity to reworklandscapes。
if ice sheets advanced again； we have nothing in our armory that could deflect them。 in1964； at prince william sound in alaska； one of the largest glacial fields in north americawas hit by the strongest earthquake ever recorded on the continent。 it measured 9。2 on therichter scale。 along the fault line； the land rose by as much as twenty feet。 the quake was soviolent； in fact； that it made water slosh out of pools in texas。 and what effect did thisunparalleled outburst have on the glaciers of prince william sound？ none at all。 they justsoaked it up and kept on moving。
for a long time it was thought that we moved into and out of ice ages gradually； overhundreds of thousands of years； but we now know that that has not been the case。 thanks toice cores from greenland we have a detailed record of climate for something over a hundredthousand years； and what is found there is not forting。 it shows that for most of its recenthistory earth has been nothing like the stable and tranquil place that civilization has known；but rather has lurched violently between periods of warmth and brutal chill。
toward the end of the last big glaciation； some twelve thousand years ago； earth began towarm； and quite rapidly； but 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鈥檛 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鈥檛 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鈥攏one鈥攚hat natural phenomena could soswiftly rattle earth鈥檚 thermometer。 as elizabeth kolbert； writing in the new yorker； hasobserved： 鈥渘o 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。鈥潯here seems to be； she adds； 鈥渟ome vast and terrible feedback loop；鈥�
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鈥檚 warmth； the northern latitudes returned to chilly conditions。 but this doesn鈥檛 begin toexplain why a thousand years later when the earth warmed once again the gulf stream didn鈥檛veer 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鈥檚tendency to plunge back into glacial conditions。 however； as kolbert has pointed out； whenyou are confronted with a fluctuating and unpredictable climate 鈥渢he last thing you鈥檇 want todo is conduct a vast unsupervised experiment on it。鈥潯t 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鈥攔ising and falling carbon dioxide levels； theshifts of continents； solar activity； the stately wobbles of the milankovitch cycles鈥攖hat it is asdifficult to prehend the events of the past as it is to predict