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hs 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。
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18THE BOUNDING MAIN
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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