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..::News::..
Piekielny czysciec
Mamy miliard lat - może znacznie mniej - zanim słońce zmieni Ziemię w piekielny czyściec.
Artykuł Thomas'a Hayden'a - tłumaczenie Michał Tomaszewski
To zdjęcie słońca wykonano w maju, w 1998 roku obrazuje temperaturę powierzchni (korony) (niebieski to 1 million °C, zielony 1.5 millionów, a czerwony 2 milliony).
NASA, SOHO, EIT Consortium
Wieczność to pojęcie względne. Wiemy już od pokoleń, że nasz czas na Ziemi dojdzie do nieuniknionego, przewidywalnego katastroficznego końca, kiedy słońce ostatecznie wyczerpie swoje zapasy wodoru i urośnie do monstrualnych rozmiarów, pochłaniających planety, czerwonego olbrzyma. Ale wodoru starczy jeszcze na kolejne 5 miliardów lat. Nasza zgodność ze słońcem wydaje się nie mieć końca. Dla społeczeństwa cieszącego się przetrwaniem już 2K lat, określony, ale odległy koniec w 5G roku lub coś koło tego jest zdecydowany. To Nie Nasz Problem. Nadal, za każdym razem, gdy astrofizycy badają funkcjonowanie słońca, klimatolodzy korygują, niestety w dół, pozostały żywot na naszej nadającej się do mieszkania planecie. Zbyt wiele niepewności powoduje obniżenie progu apokalipsy słońca. To wszystko powoduje, że nasz 4.6 miliardowo letni taniec z dobrodusznym słońcem zakończy się szybciej niż sądzimy.
Jak długo będzie trwał nasz kosmiczny bal - miliard, pół miliarda, 200 milionów lat? - ale łabędzi śpiew odbędzie się na długo przed brakiem wodoru. Może to i nie nasz problem, ale do czasu nim umilknie muzyka, miejmy nadzieję, że ktoś opanuje międzygwiezdne podróże, ponieważ Ziemia zacznie się stawać gorętsza i gorętsza, paląc wszystko na powierzchni, powodując, że oceany będą wrzeć, aż wyparują. Końcowym rezultatem będzie widok Ziemi, jako obecnego, pokrytego kraterami Merkurego, z "upieczoną" powierzchnią, ukazującą ukształtowanie dna pradawnego oceanu.
Obecnie fizycy słoneczni opisują życie słońca jako nudne. Odkąd mgławica słoneczna spłodziła niemowlęce słońce i wewnętrzna temperatura i gęstość zwiększyła się do momentu, w którym wodór przeistacza się w hel - około 15 milionów ° C, przy gęstości 160 krotnej gęstości wody (na Ziemi) - jest to stan wypalania wodoru. Stan ten trwa 11 miliardów lat, na 12 całkowitego życia słońca. Ostatecznie przejdzie przez początkowe i drugie stadium czerwonego olbrzyma, i zakończy żywot powolnym rozszerzaniem mglistej otoczki, aż do gorącego, białego karła, małej pozostałości - reszta zostaję rozrzucana stopniowo wokół. Słońce ma obecnie 4.6 miliarda lat. Obecnie jest w stanie stabilnym - pod względem temperatury, luminacji... Stan ten potrwa, aż nie wyczerpią się zapasy wodoru. Wówczas jasność wzrośnie o 10% przez następne 1.1 miliarda lat. Korona ociepli się o 1% przez ten czas z 5500° C do 5560° C.
Dla astrofizyków jest to zmiana praktycznie nieistotna. Co innego dla kilmatologów. 10% wzrost jest zdumiewająca ilością.
"Obecnie martwimy się o 10%", mówi Judith Lean z Naval Research Laboratory w Waszyngtonie. "Nawet zmiana o 1/4% może być wystarczająca". J.L. bada zmiany w iluminacji słońca w powiązaniu z 11-letnim okresem zmian cyklu plam słonecznych i jego wpływu na klimat. Nawet krótkotrwała wzmożoność aktywacji plam słonecznych może mieć ogromny wpływ - większy niż stopniowy, ale powolny wzrost luminacji słońca. Obecnie możliwe jest, że aktywność plam słonecznych będzie wzrastać.
"Na tym poziomie aktywności powierzchni słońca, ani teoria, ani obserwacje nie są dobrym wyznacznikiem" - mówi astrofizyk John Bahcall z Institute for Advanced Study w Princeton, New Jersey, "plamy słoneczne nie wpływają na całościową ewolucję". Nie, ale zmiany w cyklach słonecznych, na stałym poziomie w luminacji, mogą mieć wpływ na atmosferę ziemską w głęboki, niemiły sposób.
SŁONECZNY PIEC
Earth's atmosphere acts as security blanket, suit of armor, and source of raw materials all in one. Solar photons shoot through the gases of the upper atmosphere like innumerable steel balls caroming down the board of a pinball machine. Almost a third are bounced back into space. The rest are absorbed in the atmosphere or into the surface, adding warmth, setting up the temperature gradients that drive winds and storms, powering photosynthesis, and, if you're not careful, causing sunburns.
Solar radiation raises our planet's temperature by 450° F (250° C), to 0° F (18° C). Greenhouse gases in the atmospherecarbon dioxide, water vapor, and methane, for exampleraise it further, to a more life-friendly average of about 60° F (15° C) by absorbing infrared radiation that would otherwise be lost back into space.
Anyone questioning whether small changes in solar output can significantly impact Earth's climate would do well to check historical records from the mid-16th through the 17th centuries. For as yet unknown reasons, the sunspot cycle essentially shut down during that period. Astronomers call it the Maunder minimum. Historians refer to the period as the Little Ice Age, the coldest period of time since the last great ice age 10,000 years ago. Glaciers in Norway and Switzerland expanded down mountain valleys, overrunning farms. The Thames River in England froze for the first time in history. Danish astronomer Tycho Brahe recorded winter temperatures that were 2.7° F (1.5° C) below average during the last two decades of the 16th century. Europe's population stalled, crops failed, and grain prices escalated. Sea ice cut Iceland off from the mainland. Why? "It's speculative," says Lean, because there is no historical record chronicling solar radiation. Fortunately, changes in the sun's activity are recorded in such proxies as the amount of radioactive carbon-14 in tree rings. Lean and other scientists have used such data to deduce that solar radiation decreased by a quarter of a percent during that period.
If a decrease of 0.25 percent in solar output can cause such drastic effects on our climate, what would an equivalent increase do? According to climate simulations conducted by Lean and David Rind of the Goddard Institute for Space Studies in New York, the news is not good. "When you increase total solar radiation by 0.25 percent, the model says you get a nine-tenths of a degree Farenheit or half a degree Celsius increase in surface temperature," says Lean. But divining exactly how climate would respond is trickythat's why there's a global-warming debate.
"Temperature changes over time are not linear," says Rind, a climate modeler. The nonlinearity is caused by various feedback mechanisms that set the planet's temperature. As Earth warms, for example, sea ice melts. That means more sunlight is absorbed, and the atmosphere warms even more. But that effect only lasts until the sea ice is goneat some point greater than a 4-percent increase in solar radiation.
WATER VAPOR'S ROLE
Less well understood is the role of water vapor, a powerful greenhouse gas. "As temperature increases, the concentration of water in the air increases exponentially," says Rind. "There's a possibility of runaway greenhouse warming."
The sun's sunspot pattern could have a profound effect. Sunspots are composed of darkened, cooler regionsthe spots themselvesand the surrounding faculae, areas that are brighter and hotter. "Younger stars are dominated more by the spots," says Lean. "In our sun, faculae dominate. We expect that as the sun evolves faculae will become even more dominant."
Because faculae emit an order of magnitude more UV radiation than visible-light radiation, a facula-dominated sun is a stronger UV source. And UV creates ozone in the stratosphere. That ozone, in turn, can either increase or decrease the rate of warming depending on whether it forms in the upper or lower stratosphere.
The further away from present conditions, the harder it is to predict how climate will respond. When Rind runs his model with a 2-percent increase in solar radiation, the global temperature increases about 7° F (4° C). Climatologists don't usually think a billion years out, so Rind hasn't run that simulation, but he speculates that "a ten-percent increase in the solar luminosity would probably give you in the ball park of twenty-one degrees Farenheit or twelve degrees Celsius warming." And the results of that? "Pretty catastrophic." A 3.6° F (2° C) increase in global temperature is expected to cause the sea level to rise by 15.6 inches (40 cm), due to melting ice. Even a 12-inch (30-cm) rise in the sea level would inundate an estimated 3,000 square miles of Louisiana's biologically rich coastline and wetlands, one-third of Florida's Everglades, and fresh-water sources for major coastal cities. A 2-foot rise would swamp an estimated 10,000 square miles of U.S. coastal land.
Aktywność słoneczna zwiększyła się mocna od wczesnego 1997 (po lewej) do późnego 1998 (po prawej). Linie śledzą 1.2 milionowo stopniową koronę gazów.NASA, SOHO, EIT Consortium
"Twenty-one degrees [F] would be beyond the pale." says Rind. We're talking melting polar ice caps, severely disrupted climate, more frequent and more violent storms, general misery, and mayhem. Still, we'll probably have advanced enough to make it through those coming changes more or less in stride, or at least without going extinct. But hot weather, it turns out, is going to be the least of our problems.
FAILING FEEDBACK MECHANISMS
Earth's climatic system contains a beautifully balanced thermostat and is no stranger to major changes in solar output. As the sun has become 30 percent more luminous over the past 4.6 billion years, the feedback mechanisms have provided Earth with liquid water. Ironically, one of the most powerful control mechanisms may eventually do us in. Cloud cover and ice sheets can be effective regulators in the short term, but the main thermostat lies in the relationship between atmospheric carbon dioxide and global surface temperatures. Most of the carbon in the world is locked up as carbonate in rocks. Warmer temperatures mean increased rates of evaporation, rainfall, and stronger winds, all of which contribute to erosion. "So if it's warmer, you get faster weathering of silicate rocks," says James Kasting, a geoscientist at Pennsylvania State University. That kicks off a series of reactions: calcium eroded from the silicate rocks makes its way into the oceans, where it combines with carbonate to make shells and corals and so on, which sink to the sea floor. The ocean's carbonate supply is replenished by carbon dioxide from the atmosphere. Carbon dioxide that was once in the air ends up at the bottom of the ocean as carbonate rock.
The process has helped keep global warming under control through the sun's evolution so far, says Kasting. "This is the process that some people think has stabilized temperature over the last four billion years." But there are limits even to this powerful regulator.
In climate-model simulations, says Kasting, who studies the physical factors that define the "habitable zone" around stars, "the levels got down to about one hundred forty parts per million (ppm) carbon dioxide in about five hundred million years." And most plants, termed C3 plants after the type of photosynthesis they use, require about 150 ppm to photosynthesize. "C3 plants make up about ninety-five percent of what's out there," says Kasting, including most of Earth's substantial array of food crops.
Five hundred million years is a long time, long enough for the C4 plants, which require less carbon dioxide, to replace the vanishing C3s, and long enough for plants to evolve new carbon-dioxide-harvesting mechanisms. But once initiated, the carbon-dioxide loss rolls along relentlessly. "By 900 million years, the C4 plants start having problems," says Kasting. No carbon, no plants, no us.
APOCALYPSE
It's hard to imagine surviving those crises. Maybe we will ranch the sulfur-based ecosystems of deep-ocean vents. Tube-worm and giant-clam brochette, anyone? But that's just a prelude to full planetary extinction. "In about 1.1 billion years the stratosphere gets wet," says Kasting. That hardly sounds ominous, unless you know a little photochemistry. "The water," he says, "is dissociated by UV light, and the liberated hydrogen is lost forever." The oceans evaporate and all remaining forms of life are extinguished.
So in just over a billion years, long before the sun exhausts its supply of hydrogen, the show will come to a harsh end. Kasting's simulations ignore the effects of increasing cloud cover with hotter temperature, which are notoriously difficult to model. "Cloud feedbacks will probably slow the rate of warming," he says, "so our scenario is the most pessimistic." At best, clouds offer temporary reliefthere's only so much sky to be filled before that feedback mechanism is saturated. Whether in 300 or 800 million years, life on Earth will be scarce. Microbes may flourish longer underground.
What of the planet we leave behind? "It's frustrating that we won't be around to see how it turns out," says Lean. It's also a bit of a relief. Until recently astronomers believed the sun would lose so much mass on its way to becoming a white dwarf that Earth's orbit would expand enough to avoid being obliterated by the growing sun. Lee Anne Willson, an astrophysicist at Iowa State University in Ames, has spent 25 years working out the details of the final stages of solar evolution, as the sun transforms from a red giant into the dormant white-dwarf stage. "It turns out that the fate of Earth is very sensitive to how that process goes," she says.
It all depends on how much mass the sun loses, and at which point during its evolution that happens. If mass is lost early on, the dry, sterile Earth of the future could escape being vaporized by its dying star as the planet's orbit would expand in response to the sun's lower mass. That would let Earth orbit somewhere near Mars's current position as a silent memorial to the life that once coated its surface. If the mass loss occurs late in the process, however, Earth would be scorched to a cinder and possibly pulled into the growing sun's fiery maw. Willson, who's finishing her calculations, will reveal the ending at a February conference on the sun's future, but she says, "Earth probably doesn't get away."
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