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麻烦楼主帮我看看气候变冷是不是下面这篇? http://funnel.sfsu.edu/courses/gmo405/articles/SciAm1991.PlateauUpliftClimChng.pdf
哪些段?During the past 40 million years, andparticularly during the past 15 million years, this warm, wet climate largelydisappeared. Colder climates and much greater regional extremes ofprecipitation have developed. What caused this cooling and diversification ofclimate and vegetation into a complex mosaic of many regionally distinctivetypes?
One school of though focuses onthe changing positions of the earth’s continents and oceans. The Atlantic Oceanhas expanded at the expense of the Pacific Ocean, whereas an ancient equatorialsea that extended across much of Eurasia (called the Tethys Sea) has shrunk tobecome the modern, much smaller Mediterranean Sea. In addition, the fraction ofcontinents flooded by shallow inland seas has slowly decreased, exposing largeamounts of land and creating climates less moderated by thetemperature-stabilizing effects of oceans. Computer model simulations show thatchanges in the arrangement of the continents and the size of inland seas canhave important effects on global climate over very long internvals of geologictime. But they are significantly less convincing as sole explanations for thedramatic changes of the past 40 million years. Another possiblity is a long-term decline inthe concentration of carbon dioxide in the atmosphere, which would lessen theamount of heat trapped by the atmosphere and lead to “greenhouse cooling.” Theamount of carbon dioxide in the earth’s atmosphere over million-year timescales is controlled by two major processes. Chemical weathering of continentalrocks removes carbon dioxide from the atmosphere and carries it in dissovledchemical from to the ocean, where it is taken in by marine biota and depositedin sediments on the seafloor. Tectonic activity eventually frees this trappedcarbon dioxide, in the earth’s lithospheric plates transports the seafloor toocean trenches, where subduction carries old crust and sediments down towardthe earth’s hot interior. At great depths, the sediments melt, releasing carbondioxide, which emerges from the volcanic islands that overlie the buried curstand rejoins the atmosphere, completing the cycle. If the pace of seafloor speading (and hence ofsubduction) slowed significantly, less carbon dioxide would be vented to theatmosphere, the atmosphere would become relatively depleted of carbon dioxideand temperatures would fall. In fact, globally averaged seafloor spreadingrates slow little or no net change in the past 40 million years. Subduction and volcanism eventually return the carbondioxide to the atmosphere, but this process requires a long time (tens tohundreds of millions of years) to complete. Plateau uplift may alter climateby increasing chemical weathering of rocks, thereby recuding atmospheric carbondioxide concentrations. Carbon dioxide combines with rainwater and ground waterto form carbonic acid, which reacts with silicate minerals in rocks duringweathering. The resulting bicarbonate ions drain into the oceans, where theyare taken up by marine animals such as plankton and corals and eventuallydeposited on the seafloor. The net effect is that chemical weathering removescarbon dioxide from the atmosphere and locks it away at the bottom of theoceans. Maureen Raymo proposed thatuplift of plateaus and mountain ranges has increased the rate of chemicalerosion of continental rock on the globally averaged basis. Uplift couldenhance chemical weathering in several ways. Heavy monsoons, which develop atthe margins of platueaus, unleash particularly intense rainfall. In theseregions, uplift-related faulting and folding also expose fresh rock to theweathering process. Moreover, the steeper slopes created by plateau uplift causefaster runoff, which removes erosion products and intensifies the chemicalattack on the rock. Raymo suggests that long-term uplift in Tibet and otherregions may have increased the rate at which carbon dioxide is removed from theatmosphere. In this way, concentrations would have fallen even though theamount of carbon dioxide exhaled by volcanoes (as inferred from seafloorspeading rates) remained nearly constant. Falling carbon dioxide levels wouldreduce the ability of the atmosphere to retain heat, thereby amplifying theglobal cooling. |
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