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e 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 airmolecules above。 this creates a sort of membrane strong enough to support insects andskipping stones。 it is what gives the sting to a belly flop。
i hardly need point out that we would be lost without it。 deprived of water； the human bodyrapidly falls apart。 within days； the lips vanish 鈥渁s if amputated； the gums blacken； the nosewithers to half its length； and the skin so contracts around the eyes as to prevent blinking。鈥�
water is so vital to us that it is easy to overlook that all but the smallest fraction of the wateron earth is poisonous to us鈥攄eadly poisonous鈥攂ecause of the salts within it。
we need salt to live； but only in very small amounts； and seawater contains way more鈥攁bout seventy times more鈥攕alt than we can safely metabolize。 a typical liter of seawater willcontain only about 2。5 teaspoons of mon salt鈥攖he kind we sprinkle on food鈥攂ut muchlarger amounts of other elements； pounds； and other dissolved solids； which arecollectively known as salts。 the proportions of these salts and