on 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 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鈥檛 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 pounds most chemically akin to it鈥攈ydrogen selenide or hydrogen sulphidenotably鈥攜ou would expect it to boil at minus 135 degrees fahrenheit and to be a gas at roomtemperature。
most liquids when chilled contract by about 10 percent。 water does too； but only down to apoint。 once it is within whispering distance of freezing； it begins鈥攑erversely； beguilingly；extremely improbably鈥攖o expand。 by the time it is solid； it is almost a tenth morevoluminous than it was before。 because it expands； ice floats on water鈥斺�渁n utterly bizarreproperty；鈥潯ccording to john gribbin。 if it lacked this splendid waywardness； ice would sink；and lakes and oceans would freeze from the bottom up。 without surface ice to hold heat in；the water鈥檚 warmth would radiate away； leaving it even chillier and creating yet more ice。
soon even the oceans would freeze and almost certainly stay that way for a very long time；probably forever鈥攈ardly the conditions to nurture life。 thankfully for us； water seemsunaware of the rules of chemistry or laws of physics。
everyone knows that water鈥檚 chemical formula is h2o； which means that it consists of onelargish oxygen atom with two smaller hydrogen atoms attached to it。 the hydrogen atomscling fiercely to their oxygen host； but also make casual bonds with other water molecules。
the nature of a water molecule means that it engages in a kind of dance with other watermolecules； briefly pairing and then moving on； like the ever…changing partners in a quadrille；to use robert kunzig鈥檚 nice phrase。 a glass of water may not appear terribly lively； but everymolecule in it is changing partners billions of times a second。 that鈥檚 why water moleculesstick together to form bodies like puddles and lakes； but not so tightly that they can鈥檛 be easily separated as when； for instance； you dive into a pool of them。 at any given moment only 15percent of them are actually touching。
in one sense the bond is very strong鈥攊t is why water molecules can flow uphill whensiphoned and why water droplets on a car hood show such a singular determination to beadwith their partners。 it is also why water has surface tension。 the molecules at the surface areattracted more powerfully to the like molecules beneath and beside them than to the airmolec