A Short History of Nearly Everything-第56章

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　little　furtheron；　may　even　have　created　us。

oceans　are　the　real　powerhouse　of　the　planet’s　surface　behavior。　indeed；　meteorologistsincreasingly　treat　oceans　and　atmosphere　as　a　single　system；　which　is　why　we　must　give　thema　little　of　our　attention　here。　water　is　marvelous　at　holding　and　transporting　heat。　every　day；the　gulf　stream　carries　an　amount　of　heat　to　europe　equivalent　to　the　world’s　output　of　coalfor　ten　years；　which　is　why　britain　and　ireland　have　such　mild　winters　pared　with　canadaand　russia。

but　water　also　warms　slowly；　which　is　why　lakes　and　swimming　pools　are　cold　even　on　thehottest　days。　for　that　reason　there　tends　to　be　a　lag　in　the　official；　astronomical　start　of　aseason　and　the　actual　feeling　that　that　season　has　started。　so　spring　may　officially　start　in　thenorthern　hemisphere　in　march；　but　it　doesn’t　feel　like　it　in　most　places　until　april　at　the　veryearliest。

the　oceans　are　not　one　uniform　mass　of　water。　their　differences　in　temperature；　salinity；depth；　density；　and　so　on　have　huge　effects　on　how　they　move　heat　around；　which　in　turnaffects　climate。　the　atlantic；　for　instance；　is　saltier　than　the　pacific；　and　a　good　thing　too。　thesaltier　water　is　the　denser　it　is；　and　dense　water　sinks。　without　its　extra　burden　of　salt；　theatlantic　currents　would　proceed　up　to　the　arctic；　warming　the　north　pole　but　deprivingeurope　of　all　that　kindly　warmth。　the　main　agent　of　heat　transfer　on　earth　is　what　is　knownas　thermohaline　circulation；　which　originates　in　slow；　deep　currents　far　below　the　surface—aprocess　first　detected　by　the　scientist…adventurer　count　von　rumford　in　1797。

2what　happensis　that　surface　waters；　as　they　get　to　the　vicinity　of　europe；　grow　dense　and　sink　to　greatdepths　and　begin　a　slow　trip　back　to　the　southern　hemisphere。　when　they　reach　antarctica；they　are　caught　up　in　the　antarctic　circumpolar　current；　where　they　are　driven　onward　intothe　pacific。　the　process　is　very　slow—it　can　take　1；500　years　for　water　to　travel　from　the2the　term　means　a　number　of　things　to　different　people；　it　appears。　in　november　2002；　carl　wunsch　of　mitpublished　a　report　in　science；　〃what　is　the　thermohaline　circulation？；〃　in　which　he　noted　that　the　expressionhas　been　used　in　leading　journals　to　signify　at　least　seven　different　phenomena　（circulation　at　the　abyssal　level；circulation　driven　by　differences　in　density　or　buoyancy；　〃meridional　overturning　circulation　of　mass；〃　and　soon）…though　all　have　to　do　with　ocean　circulations　and　the　transfer　of　heat；　the　cautiously　vague　and　embracingsense　in　which　i　have　employed　it　here。

north　atlantic　to　the　mid…pacific—but　the　volumes　of　heat　and　water　they　move　are　veryconsiderable；　and　the　influence　on　the　climate　is　enormous。

（as　for　the　question　of　how　anyone　could　possibly　figure　out　how　long　it　takes　a　drop　ofwater　to　get　from　one　ocean　to　another；　the　answer　is　that　scientists　can　measure　poundsin　the　water　like　chlorofluorocarbons　and　work　out　how　long　it　has　been　since　they　were　lastin　the　air。　by　paring　a　lot　of　measurements　from　different　depths　and　locations　they　canreasonably　chart　the　water’s　movement。）thermohaline　circulation　not　only　moves　heat　around；　but　also　helps　to　stir　up　nutrients　asthe　currents　rise　and　fall；　making　greater　volumes　of　the　ocean　habitable　for　fish　and　othermarine　creatures。　unfortunately；　it　appears　the　circulation　may　also　be　very　sensitive　tochange。　according　to　puter　simulations；　even　a　modest　dilution　of　the　ocean’s　saltcontent—from　increased　melting　of　the　greenland　ice　sheet；　for　instance—could　disrupt　thecycle　disastrously。

the　seas　do　one　other　great　favor　for　us。　they　soak　up　tremendous　volumes　of　carbon　andprovide　a　means　for　it　to　be　safely　locked　away。　one　of　the　oddities　of　our　solar　system　is　thatthe　sun　burns　about　25　percent　more　brightly　now　than　when　the　solar　system　was　young。

this　should　have　resulted　in　a　much　warmer　earth。　indeed；　as　the　english　geologist　aubreymanning　has　put　it；　“this　colossal　change　should　have　had　an　absolutely　catastrophic　effecton　the　earth　and　yet　it　appears　that　our　world　has　hardly　been　affected。”

so　what　keeps　the　world　stable　and　cool？

life　does。　trillions　upon　trillions　of　tiny　marine　organisms　that　most　of　us　have　neverheard　of—foraminiferans　and　coccoliths　and　calcareous　algae—capture　atmospheric　carbon；in　the　form　of　carbon　dioxide；　when　it　falls　as　rain　and　use　it　（in　bination　with　otherthings）　to　make　their　tiny　shells。　by　locking　the　carbon　up　in　their　shells；　they　keep　it　frombeing　reevaporated　into　the　atmosphere；　where　it　would　build　up　dangerously　as　a　greenhousegas。　eventually　all　the　tiny　foraminiferans　and　coccoliths　and　so　on　die　and　fall　to　the　bottomof　the　sea；　where　they　are　pressed　into　limestone。　it　is　remarkable；　when　you　behold　anextraordinary　natural　feature　like　the　white　cliffs　of　dover　in　england；　to　reflect　that　it　ismade　up　of　nothing　but　tiny　deceased　marine　organisms；　but　even　more　remarkable　when　yourealize　how　much　carbon　they　cumulatively　sequester。　a　six…inch　cube　of　dover　chalk　willcontain　well　over　a　thousand　liters　of　pressed　carbon　dioxide　that　would　otherwise　bedoing　us　no　good　at　all。　altogether　there　is　about　twenty　thousand　times　as　much　carbonlocked　away　in　the　earth’s　rocks　as　in　the　atmosphere。　eventually　much　of　that　limestone　willend　up　feeding　volcanoes；　and　the　carbon　will　return　to　the　atmosphere　and　fall　to　the　earth　inrain；　which　is　why　the　whole　is　called　the　long…term　carbon　cycle。　the　process　takes　a　verylong　time—about　half　a　million　years　for　a　typical　carbon　atom—but　in　the　absence　of　anyother　disturbance　it　works　remarkably　well　at　keeping　the　climate　stable。

unfortunately；　human　beings　have　a　careless　predilection　for　disrupting　this　cycle　byputting　lots　of　extra　carbon　into　the　atmosphere　whether　the　foraminiferans　are　ready　for　it　ornot。　since　1850；　it　has　been　estimated；　we　have　lofted　about　a　hundred　billion　tons　of　extracarbon　into　the　air；　a　total　that　increases　by　about　seven　billion　tons　each　year。　overall；　that’snot　actually　all　that　much。　nature—mostly　through　the　belchings　of　volcanoes　and　the　decayof　plants—sends　about　200　billion　tons　of　carbon　dioxide　into　the　atmosphere　each　year；nearly　thirty　times　as　much　as　we　do　with　our　cars　and　factories。　but　you　have　only　to　look　atthe　haze　that　hangs　over　our　cities　to　see　what　a　difference　our　contribution　makes。

we　know　from　samples　of　very　old　ice　that　the　“natural”　level　of　carbon　dioxide　in　theatmosphere—that　is；　before　we　started　inflating　it　with　industrial　activity—is　about　280　partsper　million。　by　1958；　when　people　in　lab　coats　started　to　pay　attention　to　it；　it　had　risen　to　315parts　per　million。　today　it　is　over　360　parts　per　million　and　rising　by　roughly　one…quarter　of　1percent　a　year。　by　the　end　of　the　twenty…first　century　it　is　forecast　to　rise　to　about　560　partsper　million。

so　far；　the　earth’s　oceans　and　forests　（which　also　pack　away　a　lot　of　carbon）　have　managedto　save　us　from　ourselves；　but　as　peter　cox　of　the　british　meteorological　office　puts　it：

“there　is　a　critical　threshold　where　the　natural　biosphere　stops　buffering　us　from　the　effects　ofour　emissions　and　actually　starts　to　amplify　them。”　the　fear　is　that　there　would　be　a　runawayincrease　in　the　earth’s　warming。　unable　to　adapt；　many　trees　and　other　plants　would　die；releasing　their　stores　of　carbon　and　adding　to　the　problem。　such　cycles　have　occasionallyhappened　in　the　distant　past　even　without　a　human　contribution。　the　good　news　is　that　evenhere　nature　is　quite　wonderful。　it　is　almost　certain　that　eventually　the　carbon　cycle　wouldreassert　itself　and　return　the　earth　to　a　situation　of　stability　and　happiness。　the　last　time　thishappened；　it　took　a　mere　sixty　thousand　years。

。。



18THE　BOUNDING　MAIN

？小｜说网
imagine　trying　to　live　in　a　world　dominated　by　dihydrogen　oxide；　a　pound　that　hasno　taste　or　smell　and　is　so　variable　in　its　properties　that　it　is　generally　benign　but　at　othertimes　swiftly　lethal。　depending　on　its　state；　it　can　scald　you　or　freeze　you。　in　the　presence　ofcertain　organic　molecules　it　can　form　carbonic　acids　so　nasty　that　they　can　strip　the　leavesfrom　trees　and　eat　the　faces　off　statuary。　in　bulk；　when　agitated；　it　can　strike　with　a　fury　thatno　human　edifice　could　withstand。　even　for　those　who　have　learned　to　live　with　it；　it　is　anoften　murderous　substance。　we　call　it　water。

water　is　everywhere。　a　potato　is　80　percent　water；　a　cow　74　percent；　a　bacterium　75percent。　a　tomato；　at　95　percent；　is　little　but　water。　even　humans　are　65　percent　water；making　us　more　liquid　than　solid　by　a　margin　of　almost　two　to　one。　water　is　strange　stuff。　it　isformless　and　transparent；　and　yet　we　long　to　be　beside　it。　it　has　no　taste　and　yet　we　love　thetaste　of　it。　we　will　travel　great　distances　and　pay　small　fortunes　to　see　it　in　sunshine。　andeven　though　we　know　it　is　dangerous　and　drowns　tens　of　thousands　of　people　every　year；　wecan’t　wait　to　frolic　in　it。

because　water　is　so　ubiquitous　we　tend　to　overlook　what　an　extraordinary　substance　it　is。

almost　nothing　about　it　can　be　used　to　make　reliable　predictions　about　the　properties　of　otherliquids　and　vice　versa。　if　you　knew　nothing　of　water　and　based　your　assumptions　on　thebehavior　of　poun