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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 pro