17 INTO THE TROPOSPHERE
《万物简史英文版》章节: 17 INTO THE TROPOSPHERE,宠文网网友提供全文无弹窗免费在线阅读。!
The most striking thing about our atmosphere is that there isn’t very much of it. It extendsupward for about 120 miles, which might seem reasonably bounteous when viewed fromground level, but if you shrank the Earth to the size of a standard desktop globe it would onlybe about the thickness of a couple of coats of varnish.
For scientific convenience, the atmosphere is divided into four unequal layers: troposphere,stratosphere, mesosphere, and ionosphere (now often called the thermosphere). Thetroposphere is the part that’s dear to us. It alone contains enough warmth and oxygen to allowus to function, though even it swiftly becomes uncongenial to life as you climb up through it.
From ground level to its highest point, the troposphere (or “turning sphere”) is about ten milesthick at the equator and no more than six or seven miles high in the temperate latitudes wheremost of us live. Eighty percent of the atmosphere’s mass, virtually all the water, and thusvirtually all the weather are contained within this thin and wispy layer. There really isn’tmuch between you and oblivion.
Beyond the troposphere is the stratosphere. When you see the top of a storm cloudflattening out into the classic anvil shape, you are looking at the boundary between thetroposphere and stratosphere. This invisible ceiling is known as the tropopause and wasdiscovered in 1902 by a Frenchman in a balloon, Léon-Philippe Teisserenc de Bort. Pause inthis sense doesn’t mean to stop momentarily but to cease altogether; it’s from the same Greekroot as menopause. Even at its greatest extent, the tropopause is not very distant. A fastelevator of the sort used in modern skyscrapers could get you there in about twenty minutes,though you would be well advised not to make the trip. Such a rapid ascent withoutpressurization would, at the very least, result in severe cerebral and pulmonary edemas, adangerous excess of fluids in the body’s tissues. When the doors opened at the viewingplatform, anyone inside would almost certainly be dead or dying. Even a more measuredascent would be accompanied by a great deal of discomfort. The temperature six miles up canbe -70 degrees Fahrenheit, and you would need, or at least very much appreciate,supplementary oxygen.
After you have left the troposphere the temperature soon warms up again, to about 40degrees Fahrenheit, thanks to the absorptive effects of ozone (something else de Bortdiscovered on his daring 1902 ascent). It then plunges to as low as -130 degrees Fahrenheit inthe mesosphere before skyrocketing to 2,700 degrees Fahrenheit or more in the aptly namedbut very erratic thermosphere, where temperatures can vary by a thousand degrees from day to night—though it must be said that “temperature” at such a height becomes a somewhatnotional concept. Temperature is really just a measure of the activity of molecules. At sealevel, air molecules are so thick that one molecule can move only the tiniest distance—aboutthree-millionths of an inch, to be precise—before banging into another. Because trillions ofmolecules are constantly colliding, a lot of heat gets exchanged. But at the height of thethermosphere, at fifty miles or more, the air is so thin that any two molecules will be milesapart and hardly ever come in contact. So although each molecule is very warm, there are fewinteractions between them and thus little heat transference. This is good news for satellitesand spaceships because if the exchange of heat were more efficient any man-made objectorbiting at that level would burst into flame.
Even so, spaceships have to take care in the outer atmosphere, particularly on return trips toEarth, as the space shuttle Columbia demonstrated all too tragically in February 2003.
Although the atmosphere is very thin, if a craft comes in at too steep an angle—more thanabout 6 degrees—or too swiftly it can strike enough molecules to generate drag of anexceedingly combustible nature. Conversely, if an incoming vehicle hit the thermosphere attoo shallow an angle, it could well bounce back into space, like a pebble skipped across water.
But you needn’t venture to the edge of the atmosphere to be reminded of what hopelesslyground-hugging beings we are. As anyone who has spent time in a lofty city will know, youdon’t have to rise too many thousands of feet from sea level before your body begins toprotest. Even experienced mountaineers, with the benefits of fitness, training, and bottledoxygen, quickly become vulnerable at height to confusion, nausea, exhaustion, frostbite,hypothermia, migraine, loss of appetite, and a great many other stumbling dysfunctions. In ahundred emphatic ways the human body reminds its owner that it wasn’t designed to operateso far above sea level.
“Even under the most favorable circumstances,” the climber Peter Habeler has written ofconditions atop Everest, “every step at that altitude demands a colossal effort of will. Youmust force yourself to make every movement, reach for every handhold. You are perpetuallythreatened by a leaden, deadly fatigue.” In The Other Side of Everest, the British mountaineerand filmmaker Matt Dickinson records how Howard Somervell, on a 1924 British expeditionup Everest, “found himself choking to death after a piece of infected flesh came loose andblocked his windpipe.” With a supreme effort Somervell managed to cough up theobstruction. It turned out to be “the entire mucus lining of his larynx.”
Bodily distress is notorious above 25,000 feet—the area known to climbers as the DeathZone—but many people become severely debilitated, even dangerously ill, at heights of nomore than 15,000 feet or so. Susceptibility has little to do with fitness. Grannies sometimescaper about in lofty situations while their fitter offspring are reduced to helpless, groaningheaps until conveyed to lower altitudes.
The absolute limit of human tolerance for continuous living appears to be about 5,500meters, or 18,000 feet, but even people conditioned to living at altitude could not tolerate suchheights for long. Frances Ashcroft, in Life at the Extremes, notes that there are Andean sulfurmines at 5,800 meters, but that the miners prefer to descend 460 meters each evening andclimb back up the following day, rather than live continuously at that elevation. People whohabitually live at altitude have often spent thousands of years developing disproportionatelylarge chests and lungs, increasing their density of oxygen-bearing red blood cells by almost athird, though there are limits to how much thickening with red cells the blood supply can stand. Moreover, above 5,500 meters even the most well-adapted women cannot provide agrowing fetus with enough oxygen to bring it to its full term.
In the 1780s when people began to make experimental balloon ascents in Europe,something that surprised them was how chilly it got as they rose. The temperature drops about3 degrees Fahrenheit with every thousand feet you climb. Logic would seem to indicate thatthe closer you get to a source of heat, the warmer you would feel. Part of the explanation isthat you are not really getting nearer the Sun in any meaningful sense. The Sun is ninety-threemillion miles away. To move a couple of thousand feet closer to it is like taking one stepcloser to a bushfire in Australia when you are standing in Ohio, and expecting to smell smoke.
The answer again takes us back to the question of the density of molecules in the atmosphere.
Sunlight energizes atoms. It increases the rate at which they jiggle and jounce, and in theirenlivened state they crash into one another, releasing heat. When you feel the sun warm onyour back on a summer’s day, it’s really excited atoms you feel. The higher you climb, thefewer molecules there are, and so the fewer collisions between them.
Air is deceptive stuff. Even at sea level, we tend to think of the air as being ethereal and allbut weightless. In fact, it has plenty of bulk, and that bulk often exerts itself. As a marinescientist named Wyville Thomson wrote more than a century ago: “We sometimes find whenwe get up in the morning, by a rise of an inch in the barometer, that nearly half a ton has beenquietly piled upon us during the night, but we experience no inconvenience, rather a feeling ofexhilaration and buoyancy, since it requires a little less exertion to move our bodies in thedenser medium.” The reason you don’t feel crushed under that extra half ton of pressure is thesame reason your body would not be crushed deep beneath the sea: it is made mostly ofincompressible fluids, which push back, equalizing the pressures within and without.
But get air in motion, as with a hurricane or even a stiff breeze, and you will quickly bereminded that it has very considerable mass. Altogether there are about 5,200 million milliontons of air around us—25 million tons for every square mile of the planet—a notinconsequential volume. When you get millions of tons of atmosphere rushing past at thirty orforty miles an hour, it’s hardly a surprise that limbs snap and roof tiles go flying. As AnthonySmith notes, a typical weather front may consist of 750 million tons of cold air pinnedbeneath a billion tons of warmer air. Hardly a wonder that the result is at timesmeteorologically exciting.
Certainly there is no shortage of energy in the world above our heads. One thunderstorm, ithas been calculated, can contain an amount of energy equivalent to four days’ use ofelectricity for the whole United States. In the right conditions, storm clouds can rise to heightsof six to ten miles and contain updrafts and downdrafts of one hundred miles an hour. Theseare often side by side, which is why pilots don’t want to fly through them. In all, the internalturmoil particles within the cloud pick up electrical charges. For reasons not entirelyunderstood the lighter particles tend to become positively charged and to be wafted by aircurrents to the top of the cloud. The heavier particles linger at the base, accumulating negativecharges. These negatively charged particles have a powerful urge to rush to the positivelycharged Earth, and good luck to anything that gets in their way. A bolt of lightning travels at270,000 miles an hour and can heat the air around it to a decidedly crisp 50,000 degreesFahrenheit, several times hotter than the surface of the sun. At any one moment 1,800thunderstorms are in progress around the globe—some 40,000 a day. Day and night across theplanet every second about a hundred lightning bolts hit the ground. The sky is a lively place.
Much of our knowledge of what goes on up there is surprisingly recent. Jet streams, usuallylocated about 30,000 to 35,000 feet up, can bowl along at up to 180 miles an hour and vastlyinfluence weather systems over whole continents, yet their existence wasn’t suspected untilpilots began to fly into them during the Second World War. Even now a great deal ofatmospheric phenomena is barely understood. A form of wave motion popularly known asclear-air turbulence occasionally enlivens airplane flights. About twenty such incidents a yearare serious enough to need reporting. They are not associated with cloud structures oranything else that can be detected visually or by radar. They are just pockets of startlingturbulence in the middle of tranquil skies. In a typical incident, a plane en route fromSingapore to Sydney was flying over central Australia in calm conditions when it suddenlyfell three hundred feet—enough to fling unsecured people against the ceiling. Twelve peoplewere injured, one seriously. No one knows what causes such disruptive cells of air.
The process that moves air around in the atmosphere is the same process that drives theinternal engine of the planet, namely convection. Moist, warm air from the equatorial regionsrises until it hits the barrier of the tropopause and spreads out. As it travels away from theequator and cools, it sinks. When it hits bottom, some of the sinking air looks for an area oflow pressure to fill and heads back for the equator, completing the circuit.
At the equator the convection process is generally stable and the weather predictably fair,but in temperate zones the patterns are far more seasonal, localized, and random, whichresults in an endless battle between systems of high-pressure air and low. Low-pressuresystems are created by rising air, which conveys water molecules into the sky, forming cloudsand eventually rain. Warm air can hold more moisture than cool air, which is why tropical andsummer storms tend to be the heaviest. Thus low areas tend to be associated with clouds andrain, and highs generally spell sunshine and fair weather. When two such systems meet, itoften becomes manifest in the clouds. For instance, stratus clouds—those unlovable,featureless sprawls that give us our overcast skies—happen when moisture-bearing updraftslack the oomph to break through a level of more stable air above, and instead spread out, likesmoke hitting a ceiling. Indeed, if you watch a smoker sometime, you can get a very goodidea of how things work by watching how smoke rises from a cigarette in a still room. Atfirst, it goes straight up (this is called a laminar flow, if you need to impress anyone), and thenit spreads out in a diffused, wavy layer. The greatest supercomputer in the world, takingmeasurements in the most carefully controlled environment, cannot tell you what forms theseripplings will take, so you can imagine the difficulties that confront meteorologists when theytry to predict such motions in a spinning, windy, large-scale world.
What we do know is that because heat from the Sun is unevenly distributed, differences inair pressure arise on the planet. Air can’t abide this, so it rushes around trying to equalizethings everywhere. Wind is simply the air’s way of trying to keep things in balance. Airalways flows from areas of high pressure to areas of low pressure (as you would expect; thinkof anything with air under pressure—a balloon or an air tank—and think how insistently thatpressured air wants to get someplace else), and the greater the discrepancy in pressures thefaster the wind blows.
Incidentally, wind speeds, like most things that accumulate, grow exponentially, so a windblowing at two hundred miles an hour is not simply ten times stronger than a wind blowing attwenty miles an hour, but a hundred times stronger—and hence that much more destructive.
Introduce several million tons of air to this accelerator effect and the result can be exceedingly energetic. A tropical hurricane can release in twenty-four hours as much energy as a rich,medium-sized nation like Britain or France uses in a year.
The impulse of the atmosphere to seek equilibrium was first suspected by EdmondHalley—the man who was everywhere—and elaborated upon in the eighteenth century by hisfellow Briton George Hadley, who saw that rising and falling columns of air tended toproduce “cells” (known ever since as “Hadley cells”). Though a lawyer by profession, Hadleyhad a keen interest in the weather (he was, after all, English) and also suggested a linkbetween his cells, the Earth’s spin, and the apparent deflections of air that give us our tradewinds. However, it was an engineering professor at the école Polytechnique in Paris,Gustave-Gaspard de Coriolis, who worked out the details of these interactions in 1835, andthus we call it the Coriolis effect. (Coriolis’s other distinction at the school was to introducewatercoolers, which are still known there as Corios, apparently.) The Earth revolves at a brisk1,041 miles an hour at the equator, though as you move toward the poles the rate slopes offconsiderably, to about 600 miles an hour in London or Paris, for instance. The reason for thisis self-evident when you think about it. If you are on the equator the spinning Earth has tocarry you quite a distance—about 40,000 kilometers—to get you back to the same spot. If youstand beside the North Pole, however, you may need travel only a few feet to complete arevolution, yet in both cases it takes twenty-four hours to get you back to where you began.
Therefore, it follows that the closer you get to the equator the faster you must be spinning.
The Coriolis effect explains why anything moving through the air in a straight line laterallyto the Earth’s spin will, given enough distance, seem to curve to the right in the northernhemisphere and to the left in the southern as the Earth revolves beneath it. The standard wayto envision this is to imagine yourself at the center of a large carousel and tossing a ball tosomeone positioned on the edge. By the time the ball gets to the perimeter, the target personhas moved on and the ball passes behind him. From his perspective, it looks as if it has curvedaway from him. That is the Coriolis effect, and it is what gives weather systems their curl andsends hurricanes spinning off like tops. The Coriolis effect is also why naval guns firingartillery shells have to adjust to left or right; a shell fired fifteen miles would otherwisedeviate by about a hundred yards and plop harmlessly into the sea.
Considering the practical and psychological importance of the weather to nearly everyone,it’s surprising that meteorology didn’t really get going as a science until shortly before theturn of the nineteenth century (though the term meteorology itself had been around since1626, when it was coined by a T. Granger in a book of logic).
Part of the problem was that successful meteorology requires the precise measurement oftemperatures, and thermometers for a long time proved more difficult to make than you mightexpect. An accurate reading was dependent on getting a very even bore in a glass tube, andthat wasn’t easy to do. The first person to crack the problem was Daniel Gabriel Fahrenheit, aDutch maker of instruments, who produced an accurate thermometer in 1717. However, forreasons unknown he calibrated the instrument in a way that put freezing at 32 degrees andboiling at 212 degrees. From the outset this numeric eccentricity bothered some people, and in1742 Anders Celsius, a Swedish astronomer, came up with a competing scale. In proof of theproposition that inventors seldom get matters entirely right, Celsius made boiling point zeroand freezing point 100 on his scale, but that was soon reversed.
The person most frequently identified as the father of modern meteorology was an Englishpharmacist named Luke Howard, who came to prominence at the beginning of the nineteenthcentury. Howard is chiefly remembered now for giving cloud types their names in 1803.
Although he was an active and respected member of the Linnaean Society and employedLinnaean principles in his new scheme, Howard chose the rather more obscure AskesianSociety as the forum to announce his new system of classification. (The Askesian Society,you may just recall from an earlier chapter, was the body whose members were unusuallydevoted to the pleasures of nitrous oxide, so we can only hope they treated Howard’spresentation with the sober attention it deserved. It is a point on which Howard scholars arecuriously silent.)Howard divided clouds into three groups: stratus for the layered clouds, cumulus for thefluffy ones (the word means “heaped” in Latin), and cirrus (meaning “curled”) for the high,thin feathery formations that generally presage colder weather. To these he subsequentlyadded a fourth term, nimbus (from the Latin for “cloud”), for a rain cloud. The beauty ofHoward’s system was that the basic components could be freely recombined to describe everyshape and size of passing cloud—stratocumulus, cirrostratus, cumulocongestus, and so on. Itwas an immediate hit, and not just in England. The poet Johann von Goethe in Germany wasso taken with the system that he dedicated four poems to Howard.
Howard’s system has been much added to over the years, so much so that the encyclopedicif little read International Cloud Atlas runs to two volumes, but interestingly virtually all thepost-Howard cloud types—mammatus, pileus, nebulosis, spissatus, floccus, and mediocris area sampling—have never caught on with anyone outside meteorology and not terribly muchthere, I’m told. Incidentally, the first, much thinner edition of that atlas, produced in 1896,divided clouds into ten basic types, of which the plumpest and most cushiony-looking wasnumber nine, cumulonimbus.
1That seems to have been the source of the expression “to be oncloud nine.”
For all the heft and fury of the occasional anvil-headed storm cloud, the average cloud isactually a benign and surprisingly insubstantial thing. A fluffy summer cumulus severalhundred yards to a side may contain no more than twenty-five or thirty gallons of water—“about enough to fill a bathtub,” as James Trefil has noted. You can get some sense of theimmaterial quality of clouds by strolling through fog—which is, after all, nothing more than acloud that lacks the will to fly. To quote Trefil again: “If you walk 100 yards through a typicalfog, you will come into contact with only about half a cubic inch of water—not enough togive you a decent drink.” In consequence, clouds are not great reservoirs of water. Only about0.035 percent of the Earth’s fresh water is floating around above us at any moment.
Depending on where it falls, the prognosis for a water molecule varies widely. If it lands infertile soil it will be soaked up by plants or reevaporated directly within hours or days. If itfinds its way down to the groundwater, however, it may not see sunlight again for manyyears—thousands if it gets really deep. When you look at a lake, you are looking at acollection of molecules that have been there on average for about a decade. In the ocean theresidence time is thought to be more like a hundred years. Altogether about 60 percent of1If you have ever been struck by how beautifully crisp and well defined the edges of cumulus clouds tend to be,while other clouds are more blurry, the explanation is that in a cumulus cloud there is a pronounced boundarybetween the moist interior of the cloud and the dry air beyond it. Any water molecule that strays beyond the edgeof the cloud is immediately zapped by the dry air beyond, allowing the cloud to keep its fine edge. Much highercirrus clouds are composed of ice, and the zone between the edge of the cloud and the air beyond is not soclearly delineated, which is why they tend to be blurry at the edges.
water molecules in a rainfall are returned to the atmosphere within a day or two. Onceevaporated, they spend no more than a week or so—Drury says twelve days—in the skybefore falling again as rain.
Evaporation is a swift process, as you can easily gauge by the fate of a puddle on asummer’s day. Even something as large as the Mediterranean would dry out in a thousandyears if it were not continually replenished. Such 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—indeed, making them just dilute enough to freeze over larger areas than normal.
The enlarged area of ice bounced back more of the Sun’s 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’s 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’s 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’s output of coalfor ten years, which is why Britain and Ireland have such mild winters compared 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’t 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—aprocess 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—it 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—but 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 compoundsin the water like chlorofluorocarbons and work out how long it has been since they were lastin the air. By comparing a lot of measurements from different depths 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 computer 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 combination 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 compressed 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 compressed 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 situat