按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
of ice ages。 no one had ever thought before to consider an astronomicalexplanation for variations in earth鈥檚 weather。 thanks almost entirely to croll鈥檚 persuasivetheory； people in britain began to bee more responsive to the notion that at some formertime parts of the earth had been in the grip of ice。 when his ingenuity and aptitude wererecognized； croll was given a job at the geological survey of scotland and widely honored：
he was made a fellow of the royal society in london and of the new york academy ofscience and given an honorary degree from the university of st。 andrews； among much else。
unfortunately； just as agassiz鈥檚 theory was at last beginning to find converts in europe； hewas busy taking it into ever more exotic territory in america。 he began to find evidence forglaciers practically everywhere he looked； including near the equator。 eventually he becameconvinced that ice had once covered the whole earth； extinguishing all life； which god hadthen re…created。 none of the evidence agassiz cited supported such a view。 nonetheless； inhis adopted country his stature grew and grew until he was regarded as only slightly below adeity。 when he died in 1873 harvard felt it necessary to appoint three professors to take hisplace。
yet； as sometimes happens； his theories fell swiftly out of fashion。 less than a decade afterhis death his successor in the chair of geology at harvard wrote that the 鈥渟o…called glacialepoch 。 。 。 so popular a few years ago among glacial geologists may now be rejected withouthesitation。鈥�
part of the problem was that croll鈥檚 putations suggested that the most recent ice ageoccurred eighty thousand years ago； whereas the geological evidence increasingly indicatedthat earth had undergone some sort of dramatic perturbation much more recently than that。
without a plausible explanation for what might have provoked an ice age； the whole theoryfell into abeyance。 there it might have remained for some time except that in the early 1900sa serbian academic named milutin milankovitch； who had no background in celestial motionsat all鈥攈e was a mechanical engineer by training鈥攄eveloped an unexpected interest in thematter。 milankovitch realized that the problem with croll鈥檚 theory was not that it wasincorrect but that it was too simple。
as earth moves through space； it is subject not just to variations in the length and shape ofits orbit； but also to rhythmic shifts in its angle of orientation to the sun鈥攊ts tilt and pitch andwobble鈥攁ll affecting the length and intensity of sunlight falling on any patch of land。 inparticular it is subject to three changes in position； known formally as its obliquity；precession； and eccentricity； over long periods of time。 milankovitch wondered if there mightbe a relationship between these plex cycles and the ings and goings of ice ages。 thedifficulty was that the cycles were of widely different lengths鈥攐f approximately 20；000；40；000； and 100；000 years； but varying in each case by up to a few thousand years鈥攚hichmeant that determining their points of intersection over long spans of time involved a nearlyendless amount of devoted putation。 essentially milankovitch had to work out the angleand duration of ining solar radiation at every latitude on earth； in every season； for amillion years； adjusted for three ever…changing variables。
happily this was precisely the sort of repetitive toil that suited milankovitch鈥檚temperament。 for the next twenty years； even while on vacation； he worked ceaselessly withpencil and slide rule puting the tables of his cycles鈥攚ork that now could be pleted ina day or two with a puter。 the calculations all had to be made in his spare time； but in1914 milankovitch suddenly got a great deal of that when world war i broke out and he wasarrested owing to his position as a reservist in the serbian army。 he spent most of the next four years under loose house arrest in budapest； required only to report to the police once aweek。 the rest of his time was spent working in the library of the hungarian academy ofsciences。 he was possibly the happiest prisoner of war in history。
the eventual oute of his diligent scribblings was the 1930 book mathematicalclimatology and the astronomical theory of climatic changes。 milankovitch was right thatthere was a relationship between ice ages and planetary wobble； though like most people heassumed that it was a gradual increase in harsh winters that led to these long spells ofcoldness。 it was a russian…german meteorologist； wladimir k？ppen鈥攆ather…in…law of ourtectonic friend alfred wegener鈥攚ho saw that the process was more subtle； and rather moreunnerving； than that。
the cause of ice ages； k？ppen decided； is to be found in cool summers； not brutal winters。
if summers are too cool to melt all the snow that falls on a given area； more ining sunlightis bounced back by the reflective surface； exacerbating the cooling effect and encouraging yetmore snow to fall。 the consequence would tend to be self…perpetuating。 as snow accumulatedinto an ice sheet； the region would grow cooler； prompting more ice to accumulate。 as theglaciologist gwen schultz has noted： 鈥渋t is not necessarily the amount of snow that causes icesheets but the fact that snow； however little； lasts。鈥潯t is thought that an ice age could startfrom a single unseasonal summer。 the leftover snow reflects heat and exacerbates the chillingeffect。 鈥渢he process is self…enlarging； unstoppable； and once the ice is really growing itmoves；鈥潯ays mcphee。 you have advancing glaciers and an ice age。
in the 1950s； because of imperfect dating technology； scientists were unable to correlatemilankovitch鈥檚 carefully worked…out cycles with the supposed dates of ice ages as thenperceived； and so milankovitch and his calculations increasingly fell out of favor。 he died in1958； unable to prove that his cycles were correct。 by this time； write john and mary gribbin；鈥測ou would have been hard pressed to find a geologist or meteorologist who regarded themodel as being anything more than an historical curiosity。鈥潯ot until the 1970s and therefinement of a potassium…argon method for dating ancient seafloor sediments were histheories finally vindicated。
the milankovitch cycles alone are not enough to explain cycles of ice ages。 many otherfactors are involved鈥攏ot least the disposition of the continents； in particular the presence oflandmasses over the poles鈥攂ut the specifics of these are imperfectly understood。 it has beensuggested； however； that if you hauled north america； eurasia； and greenland just threehundred miles north we would have permanent and inescapable ice ages。 we are very lucky； itappears； to get any good weather at all。 even less well understood are the cycles ofparative balminess within ice ages； known as interglacials。 it is mildly unnerving toreflect that the whole of meaningful human history鈥攖he development of farming； the creationof towns； the rise of mathematics and writing and science and all the rest鈥攈as taken placewithin an atypical patch of fair weather。 previous interglacials have lasted 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 pac