A Short History of Nearly Everything-第93章

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hedifficulty　was　that　the　cycles　were　of　widely　different　lengths—of　approximately　20；000；40；000；　and　100；000　years；　but　varying　in　each　case　by　up　to　a　few　thousand　years—whichmeant　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’stemperament。　for　the　next　twenty　years；　even　while　on　vacation；　he　worked　ceaselessly　withpencil　and　slide　rule　puting　the　tables　of　his　cycles—work　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—father…in…law　of　ourtectonic　friend　alfred　wegener—who　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：　“it　is　not　necessarily　the　amount　of　snow　that　causes　icesheets　but　the　fact　that　snow；　however　little；　lasts。”　it　is　thought　that　an　ice　age　could　startfrom　a　single　unseasonal　summer。　the　leftover　snow　reflects　heat　and　exacerbates　the　chillingeffect。　“the　process　is　self…enlarging；　unstoppable；　and　once　the　ice　is　really　growing　itmoves；”　says　mcphee。　you　have　advancing　glaciers　and　an　ice　age。

in　the　1950s；　because　of　imperfect　dating　technology；　scientists　were　unable　to　correlatemilankovitch’s　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；“you　would　have　been　hard　pressed　to　find　a　geologist　or　meteorologist　who　regarded　themodel　as　being　anything　more　than　an　historical　curiosity。”　not　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—not　least　the　disposition　of　the　continents；　in　particular　the　presence　oflandmasses　over　the　poles—but　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—the　development　of　farming；　the　creationof　towns；　the　rise　of　mathematics　and　writing　and　science　and　all　the　rest—has　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’s　just　a　somewhat　shrunken　one—thoughless　shrunken　than　many　people　realize。　at　the　height　of　the　last　period　of　glaciation；　aroundtwenty　thousand　years　ago；　about　30　percent　of　the　earth’s　land　surface　was　under　ice。　tenpercent　still　is—and　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—asituation　that　may　be　unique　in　earth’s　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—ice　epoch　really—started　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—the　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—so　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。

“snowball；”　however；　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—a　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—the　springtime　event　of　life’s　histo