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h an event occurred a little under six millionyears ago and provoked what is known to science as the messinian salinity crisis。 whathappened was that continental movement closed the strait of gibraltar。 as the mediterraneandried； its evaporated contents fell as freshwater rain into other seas； mildly diluting theirsaltiness鈥攊ndeed； making them just dilute enough to freeze over larger areas than normal。
the enlarged area of ice bounced back more of the sun鈥檚 heat and pushed earth into an iceage。 so at least the theory goes。
what is certainly true； as far as we can tell； is that a little change in the earth鈥檚 dynamicscan have repercussions beyond our imagining。 such an event； as we shall see a little furtheron； may even have created us。
oceans are the real powerhouse of the planet鈥檚 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鈥檚 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鈥檛 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鈥攁process 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鈥攊t 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鈥攂ut 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鈥檚 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鈥檚 saltcontent鈥攆rom increased melting of the greenland ice sheet； for instance鈥攃ould 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； 鈥渢his colossal change should have had an absolutely catastrophic effecton the earth and yet it appears that our world has hardly been affected。鈥�
so what keeps the world stable and cool？
life does。 trillions upon trillions of tiny marine organisms that most of us have neverheard of鈥攆oraminiferans and coccoliths and calcareous algae鈥攃apture 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鈥檚 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鈥攁bout half a million years for a typical carbon atom鈥攂ut 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鈥檚not actually all that much。 nature鈥攎ostly through the belchings of volcanoes and the decayof plants鈥攕ends 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 鈥渘atural鈥潯evel of carbon dioxide in theatmosphere鈥攖hat is； before we started inflating it with industrial activity鈥攊s 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鈥檚 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：
鈥渢here is a critical threshold where the natural biosphere stops buffering us from the effects ofour emissions and actually starts to amplify them。鈥潯he fear is that there would be a runawayincrease in the earth鈥檚 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。
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18 THE BOUNDING MAIN
銆婂皬璇磘銆媥t澶╁爞
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 75p