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d as little as eightthousand years。 our own has already passed its ten thousandth anniversary。
the fact is； we are still very much in an ice age； it鈥檚 just a somewhat shrunken one鈥攖houghless shrunken than many people realize。 at the height of the last period of glaciation； aroundtwenty thousand years ago； about 30 percent of the earth鈥檚 land surface was under ice。 tenpercent still is鈥攁nd a further 14 percent is in a state of permafrost。 three…quarters of all thefresh water on earth is locked up in ice even now； and we have ice caps at both poles鈥攁situation that may be unique in earth鈥檚 history。 that there are snowy winters through much of the world and permanent glaciers even in temperate places such as new zealand may seemquite natural； but in fact it is a most unusual situation for the planet。
for most of its history until fairly recent times the general pattern for earth was to be hotwith no permanent ice anywhere。 the current ice age鈥攊ce epoch really鈥攕tarted about fortymillion years ago； and has ranged from murderously bad to not bad at all。 ice ages tend towipe out evidence of earlier ice ages； so the further back you go the more sketchy the picturegrows； but it appears that we have had at least seventeen severe glacial episodes in the last 2。5million years or so鈥攖he period that coincides with the rise of homo erectus in africafollowed by modern humans。 two monly cited culprits for the present epoch are the riseof the himalayas and the formation of the isthmus of panama； the first disrupting air flows；the second ocean currents。 india； once an island； has pushed two thousand kilometers into theasian landmass over the last forty…five million years； raising not only the himalayas； but alsothe vast tibetan plateau behind them。 the hypothesis is that the higher landscape was notonly cooler； but diverted winds in a way that made them flow north and toward northamerica； making it more susceptible to long…term chills。 then； beginning about five millionyears ago； panama rose from the sea； closing 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 old