《A Short History of Nearly Everything》第141章

t　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’t　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’t　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—none—what　natural　phenomena　could　soswiftly　rattle　earth’s　thermometer。　as　elizabeth　kolbert；　writing　in　the　new　yorker；　hasobserved：　“no　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。”　there　seems　to　be；　she　adds；　“some　vast　and　terrible　feedback　loop；”
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’s　warmth；　the　northern　latitudes　returned　to　chilly　conditions。　but　this　doesn’t　begin　toexplain　why　a　thousand　years　later　when　the　earth　warmed　once　again　the　gulf　stream　didn’tveer　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’stendency　to　plunge　back　into　glacial　conditions。　however；　as　kolbert　has　pointed　out；　whenyou　are　confronted　with　a　fluctuating　and　unpredictable　climate　“the　last　thing　you’d　want　todo　is　conduct　a　vast　unsupervised　experiment　on　it。”　it　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—rising　and　falling　carbon　dioxide　levels；　theshifts　of　continents；　solar　activity；　the　stately　wobbles　of　the　milankovitch　cycles—that　it　is　asdifficult　to　prehend　the　events　of　the　past　as　it　is　to　predict　those　of　the　future。　much　issimply　beyond　us。　take　antarctica。　for　at　least　twenty　million　years　after　it　settled　over　thesouth　pole　antarctica　remained　covered　in　plants　and　free　of　ice。　that　simply　shouldn’t　havebeen　possible。
no　less　intriguing　are　the　known　ranges　of　some　late　dinosaurs。　the　british　geologiststephen　drury　notes　that　forests　within　10　degrees　latitude　of　the　north　pole　were　home　togreat　beasts；　including　tyrannosaurus　rex。　“that　is　bizarre；”　he　writes；　“for　such　a　highlatitude　is　continually　dark　for　three　months　of　the　year。”　moreover；　there　is　now　evidencethat　these　high　latitudes　suffered　severe　winters。　oxygen　isotope　studies　suggest　that　theclimate　around　fairbanks；　alaska；　was　about　the　same　in　the　late　cretaceous　period　as　it　isnow。　so　what　was　tyrannosaurus　doing　there？　either　it　migrated　seasonally　over　enormousdistances　or　it　spent　much　of　the　year　in　snowdrifts　in　the　dark。　in　australia—which　at　thattime　was　more　polar　in　its　orientation—a　retreat　to　warmer　climes　wasn’t　possible。　howdinosaurs　managed　to　survive　in　such　conditions　can　only　be　guessed。
one　thought　to　bear　in　mind　is　that　if　the　ice　sheets　did　start　to　form　again　for　whateverreason；　there　is　a　lot　more　water　for　them　to　draw　on　this　time。　the　great　lakes；　hudson　bay；the　countless　lakes　of　canada—these　weren’t　there　to　fuel　the　last　ice　age。　they　were　createdby　it。
on　the　other　hand；　the　next　phase　of　our　history　could　see　us　melting　a　lot　of　ice　rather　thanmaking　it。　if　all　the　ice　sheets　melted；　sea　levels　would　rise　by　two　hundred　feet—the　heightof　a　twenty…story　building—and　every　coastal　city　in　the　world　would　be　inundated。　morelikely；　at　least　in　the　short　term；　is　the　collapse　of　the　west　antarctic　ice　sheet。　in　the　past　fiftyyears　the　waters　around　it　have　warmed　by　2。5　degrees　centigrade；　and　collapses　haveincreased　dramatically。　because　of　the　underlying　geology　of　the　area；　a　large…scale　collapseis　all　the　more　possible。　if　so；　sea　levels　globally　would　rise—and　pretty　quickly—by　betweenfifteen　and　twenty　feet　on　average。
the　extraordinary　fact　is　that　we　don’t　know　which　is　more　likely；　a　future　offering　us　eonsof　perishing　frigidity　or　one　giving　us　equal　expanses　of　steamy　heat。　only　one　thing　iscertain：　we　live　on　a　knife　edge。
in　the　long　run；　incidentally；　ice　ages　are　by　no　means　bad　news　for　the　planet。　they　grindup　rocks　and　leave　behind　new　soils　of　sumptuous　richness；　and　gouge　out　fresh　water　lakesthat　provide　abundant　nutritive　possibilities　for　hundreds　of　species　of　being。　they　act　as　aspur　to　migration　and　keep　the　planet　dynamic。　as　tim　flannery　has　remarked：　“there　is　onlyone　question　you　need　ask　of　a　continent　in　order　to　determine　the　fate　of　its　people：　‘did　youhave　a　good　ice　age？’　”　and　with　that　in　mind；　it’s　time　to　look　at　a　species　of　ape　that　trulydid。
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