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第二篇Faint young sun paradox，强烈推荐V2构筑那个长篇，基本和原文差不多，但第三段稍微有点区别
第三段开始说一个叫Michael的学者说我算了算二氧化碳的含量得出了一个数据，比它多地球太热比它少地球太冷，我算的很精确；其他学者反驳说他的因果不对，实际是温度和二氧化碳互相制衡达到稳定
有个问题问Michael没有考虑到什么？我选的好像是没考虑到互相调节还是啥的
还有一个问题是以前的地球和现在有何不同，我选的是大气层构成不同
是原文吗？
The Faint-Young-Sun Paradox
Our interest in the role of carbon dioxide in the evolution of the earth, Mars and Venus had its roots in another cosmological puzzle relating to the origin of the earth: the faint- young-sun paradox. Virtually every model of stellar evolution indicates that the sun was between 25 and 30 percent dimmer when the solar system formed some 4.6 billion years ago than it is today. Since then the solar luminosity, or intensity, has apparently increased approximately linearly with time.
The paradox arises, as Carl Sagan and George H. Mullen of Cornell University pointed out about 15 years ago, when one realizes that if the earth's early atmosphere was the same as it is now, a weak sun would have resulted in an ice-covered earth until about two billion years ago. Yet the planet did not freeze. In fact, evidence from sedimentary rocks indicates that the earth has had liquid oceans since at least 3.8 billion years ago, when the geological record begins. Moreover, life has been present for at least the past 3.5 billion years, demonstrating that the earth's surface has never been entirely frozen during that time. (Water can remain fluid as long as the temperature is between zero and 374 degrees c.; it boils and evaporates at 100 degrees C. at sea level today but will stay liquid at higher temperatures if the atmospheric pressure is increased.)
Sagan and Mullen realized that the paradox disappears if one assumes the earth's atmosphere has changed in the course of time. For instance, if the young planet had fewer clouds than it has today, less of the sunlight that impinged on the earth would have been reflected back into space, and the planet would have been correspondingly warmer. Some 30 percent of the sunlight that currently reaches the top of the atmosphere is returned to space, most of it by clouds. A chillier earth might well have had fewer clouds but the geological record suggests the early earth was actually warmer than today's. Parts of the planet are covered with glaciers now, but there is no evidence of similar glaciation before
about 2.7 billion years ago.
A more probable explanation is that the greenhouse effect was more pronounced in the distant past. Sagan and Mullen suggested that ammonia (NH3), an efficient absorber of infrared, could have warmed the climate if the gas represented just 100 out of every million molecules of the air. Subsequent studies have shown, however, that the sun would have rapidly converted ammonia into the non greenhouse gases nitrogen and hydrogen unless it was continually resupplied to the atmosphere from the planet's surface.
Other investigations have focused on carbon dioxide, which sunlight does not readily decompose. Carbon dioxide is certainly abundant here; the amount now stored in the planet in carbonate rocks would exert a pressure of about 60 bars if it were released into the atmosphere. (One bar is equal to 14.5 pounds per square inch, the pressure at sea level. Today the earth's atmosphere contains about .0003 bar of carbon dioxide.) If just a few tenths of a bar of the stored carbon dioxide was originally present as a gas, its additional greenhouse warming would have compensated for the reduced sunlight.
The notion that higher carbon dioxide levels could have protected the early earth from freezing soon gave rise to a related idea: if the carbon dioxide level declined at a rate that precisely counteracted the increase in solar luminosity with time, the decline might account for the fact that the earth's temperature has always remained within reasonable limits. One investigator, Michael H. Hart of NASA, undertook to calculate such a compensatory rate.
Hart managed to work out a solution in which the levels of the gas declined approximately logarithmical
ly with time, but his most interesting finding was that very few of his calculations succeeded. In other words, if the composition of the atmosphere had at any time changed at a rate different from his precise solution, the planet would have become unable to support life. If the carbon dioxide level had declined too slowly, the earth would have become a hothouse; if it had declined too quickly, the oceans would have frozen.
Hart did similar calculations for cases in which the distance between the earth and the sun was varied by small amounts. He found that if the earth had formed 5 percent closer to the sun, the atmosphere would have become so hot that the oceans would have evaporated, a condition known as a runaway greenhouse. Conversely, the planet would have encountered runaway glaciation if it had formed as little as 1 percent farther from the sun. Only in the relatively narrow range of orbits between .95 astronomical unit and 1.01 A. U. could one or the other of these climatic catastrophes be avoided. (One A.U. is the distance between the sun and the earth, or 149.6 million kilometers.) Hart termed this narrow band of orbital distances the continuously habitable zone (CHZ).
Hart's conclusions were unsettling because they suggested that the earth must have beaten extraordinary odds in avoiding the fate of Mars or Venus. Only within the past few years have investigators discovered the flaw in his hypothesis. A mathematical model developed by James C. G. Walker and Paul B. Hays of the University of Michigan and by one of us (Kasting) suggests that the changes in carbon dioxide concentration did not arise by sheer luck. Rather, carbon dioxide levels have probably fluctuated in response to changes in surface temperature. When the temperature goes up, atmospheric carbon dioxide levels decline, cooling the surface; when the surface cools, the abundance of atmospheric carbon dioxide increases and warms the surface. The existence of such a negative-feedback loop means that the earth probably has never been in danger of undergoing either the runaway greenhouse or the runaway glaciation postulated by Hart.
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发表于 2018-10-11 21:55:10
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