万物简史英文版_比尔·布莱森-第111章

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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�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�withdrawal；　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