pale blue dot -carl sagan-第40章

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！


and　what　happens　when　we　have　too　much　of　one　thing　or　too　little　of　another。　We　learn　how　a　planet　can　go　wrong。　We　gain　a　new　understanding；　foreseen　by　the　spaceflight　pioneer　Robert　Goddard；　called　parative　planetology。

The　exploration　of　other　worlds　has　opened　our　eyes　in　the　study　of　volcanos；　earthquakes；　and　weather。　It　may　one　day　have　profound　implications　for　biology；　because　all　life　on　Earth　is　built　on　a　mon　biochemical　master　plan。　The　discovery　of　a　single　extraterrestrial　organism—even　something　as　humble　as　a　bacterium—would　revolutionize　our　understanding　of　living　things。　But　the　connection　between　exploring　other　worlds　and　protecting　this　one　is　most　evident　in　the　study　of　Earth's　climate　and　the　burgeoning　threat　to　that　climate　that　our　technology　poses。　Other　worlds　provide　vital　insights　about　what　dumb　things　not　to　do　on　Earth。

Three　potential　environmental　catastrophes—all　operating　on　a　global　scale—have　recently　been　uncovered：　ozone　layer　depletion；　greenhouse　warming；　and　nuclear　winter。　All　three　discoveries；　it　turns　out；　have　strong　ties　to　the　exploration　of　the　planets。

（1）　It　was　disturbing　to　find　that　an　inert　material　with　all　sorts　of　practical　applications—it　serves　as　the　working　fluid　in　refrigerators　and　air　conditioners；　as　aerosol　propellant　for　deodorants　and　other　products；　as　lightweight　foamy　packaging　for　fast　foods；　and　as　a　cleaning　agent　in　microelectronics；　to　name　only　a　few…can　pose　a　danger　to　life　on　Earth。　Who　would　have　figured？

The　molecules　in　question　are　called　chlorofluorocarbons　（CFCs）。　Chemically；　they're　extremely　inert；　which　means　they're　invulnerable—until　they　find　themselves　up　in　the　ozone　layer；　where　they're　broken　apart　by　ultraviolet　light　from　the　Sun。　The　chlorine　atoms　thus　liberated　attack　and　break　down　the　protective　ozone；　letting　more　ultraviolet　light　reach　the　ground。　This　increased　ultraviolet　intensity　ushers　in　a　ghastly　procession　of　potential　consequences　involving　not　just　skin　cancer　and　cataracts；　but　weakening　of　the　human　immune　system　and；　most　dangerous　of　all；　possible　harm　to　agriculture　and　to　photosynthetic　organisms　at　the　base　of　the　food　chain　on　which　most　life　on　Earth　depends。

Who　discovered　that　CFCs　posed　a　threat　to　the　ozone　layer？　Was　it　the　principal　manufacturer；　the　DuPont　Corporation；　exercising　corporate　responsibility？　Was　it　the　Environmental　Protection　Agency　protecting　us？　Was　it　the　Department　of　Defense　defending　us？　No；　it　was　two　Ivory…tower；　white…coated　university　scientists　working　on　something　else—Sherwood　Rowland　and　Mario　Molina　of　the　University　of　California；　Irvine。　Not　even　an　Ivy　League　university。　No　one　instructed　them　to　look　for　dangers　to　the　environment。　They　were　pursuing　fundamental　research。　They　were　scientists　following　their　own　interests。　Their　names　should　be　known　to　every　schoolchild。

In　their　original　calculations；　Rowland　and　Molina　used　rate　constants　of　chemical　reactions　involving　chlorine　and　other　halogens　that　had　been　measured　in　part　with　NASA　support。　Why　NASA？　Because　Venus　has　chlorine　and　fluorine　molecules　in　its　atmosphere；　and　planetary　aeronomers　had　wanted　to　understand　what's　happening　there。

Confirming　theoretical　work　on　the　role　of　CFCs　in　ozone　depletion　was　soon　done　by　a　group　led　by　Michael　McElroy　at　Harvard　How　is　it　they　had　all　these　branching　networks　of　halogen　chemical　kinetics　in　their　puter　ready　to　go？　Because　they　were　working　on　the　chlorine　and　fluorine　chemistry　of　the　atmosphere　of　Venus。　Venus　helped　make　and　helped　confirm　the　discovery　that　the　Earth's　ozone　layer　is　in　danger。　An　entirely　unexpected　connection　was　found　between　the　atmospheric　photochemistries　of　the　two　planets。　A　result　of　importance　to　everyone　on　Earth　emerged　from　what　might　well　have　seemed　the　most　blue…sky；　abstract；　impractical　kind　of　work；　understanding　the　chemistry　of　minor　constituents　in　the　upper　atmosphere　of　another　world。

There's　also　a　Mars　connection。　With　Viking　we　found　the　surface　of　Mars　to　be　apparently　lifeless　and　remarkably　deficient　even　in　simple　organic　molecules。　But　simple　organic　molecules　ought　to　be　there；　because　of　the　impact　of　organic…rich　meteorites　from　the　nearby　asteroid　belt。　This　deficiency　is　widely　attributed　to　the　lack　of　ozone　on　Mars。　The　Viking　microbiology　experiments　found　that　organic　matter　carried　from　Earth　to　Mars　and　sprinkled　on　Martian　surface　dust　is　quickly　oxidized　arid　destroyed。　The　materials　in　the　dust　that　do　the　destruction　are　molecules　something　like　hydrogen　peroxide—which　we　use　as　an　antiseptic　because　it　kills　microbes　by　oxidizing　there；　Ultraviolet　light　from　the　Sun　strikes　the　surface　of　Mars　unimpeded　by　an　ozone　layer；　if　any　organic　matter　were　there；　it　would　be　quickly　destroyed　by　the　ultraviolet　light　itself　and　its　oxidation　products。　Thus　part　of　the　reason　the　topmost　layers　of　Martian　soil　are　antiseptic　is　that　Mars　has　an　ozone　hole　of　planetary　dimensions—by　itself　a　useful　cautionary　tale　for　us；　who　are　busily　thinning　and　puncturing　our　ozone　layer。

（2）　Global　warming　is　predicted　to　follow　from　the　increasing　greenhouse　effect　caused　largely　by　carbon　dioxide　generated　in　the　burning　of　fossil　fuels—but　also　from　the　buildup　of　other　infrared…absorbing　gases　（oxides　of　nitrogen；　methane；　those　same　CFCs；　and　other　molecules）。

Suppose　that　we　have　a　three…dimensional　general　circulation　puter　model　of　the　Earth's　climate。　Its　programmers　claim　it's　able　to　predict　what　the　Earth　will　be　like　if　there's　more　of　one　atmospheric　constituent　or　less　of　another。　The　model　does　very　well　at　〃predicting〃　the　present　climate。　But　there　is　a　nagging　worry：　The　model　has　been　〃tuned〃　so　it　will　e　out　right—that　is；　certain　adjustable　parameters　are　chosen；　not　from　first　principles　of　physics；　but　to　get　the　right　answer。　This　is　not　exactly　cheating；　but　if　we　apply　the　same　puter　model　to　rather　different　climatic　regimes—deep　global　warming；　for　instance—the　tuning　might　then　be　inappropriate。　The　model　might　be　valid　for　today's　climate；　but　not　extrapolatable　to　others。

One　way　to　test　this　program　is　to　apply　it　to　the　very　different　climates　of　other　planets。　Can　it　predict　the　structure　of　the　atmosphere　on　Mars　and　the　climate　there？　The　weather？　What　about　Venus？　If　it　were　to　fail　these　test　cases；　we　would　be　right　in　mistrusting　it　when　it　makes　predictions　for　our　own　planet。　In　fact；　climate　models　now　in　use　do　very　well　in　predicting　from　first　principles　of　physics　the　climates　on　Venus　and　Mars。

On　Earth；　huge　upwellings　of　molten　lava　are　known　and　attributed　to　superplumes　convecting　up　from　the　deep　mantle　and　generating　vast　plateaus　of　frozen　basalt。　A　spectacular　example　occurred　about　a　hundred　million　years　ago；　and　added　perhaps　ten　times　the　present　carbon　dioxide　content　to　the　atmosphere；　inducing　substantial　global　warming。　These　plumes；　it　is　thought；　occur　episodically　throughout　Earth's　history。　Similar　mantle　upwelling　seem　to　have　occurred　on　Mars　and　Venus。　There　are　sound　practical　reasons　for　us　to　want　to　understand　how　a　major　change　to　the　Earth's　surface　and　climate　could　suddenly　arrive　unannounced　from　hundreds　of　kilometers　beneath　our　feet。

Some　of　the　most　important　recent　work　on　global　warming　has　been　done　by　James　Hansen　and　his　colleagues　at　the　Goddard　Institute　for　Space　Sciences；　a　NASA　facility　in　New　York　City。　Hansen　developed　one　of　the　major　puter　climate　models　and　employed　it　to　predict　what　will　happen　to　our　climate　as　the　greenhouse　gases　continue　to　build　up。　He　has　been　in　the　forefront　of　testing　these　models　against　ancient　climates　of　the　Earth。　（During　the　last　ice　ages；　it　is　of　interest　to　note；　more　carbon　dioxide　and　methane　are　strikingly　correlated　with　higher　temperatures。）　Hansen　collected　a　wide　range　of　weather　data　from　this　century　and　last；　to　see　what　actually　happened　to　the　global　temperature；　and　then　pared　it　to　the　puter　model's　predictions　of　what　should　have　happened。　The　two　agree　to　within　the　errors　of　measurement　and　calculation；　respectively。　He　courageously　testified　before　Congress　in　the　face　of　a　politically　generated　order