GMAT机经是指GMAT考生对GMAT考试的总结和试题的回忆。可以说GMAT机经是GMAT考试真题的再现。很多考生都将机经作为备考资料之一，作用毋庸置疑，当月机经最为重要，7月份的机经也是千呼万唤始出来，等待了许久的同学们一定迫不及待了。下面，楷维留学小编为GMAT考生分享GMAT阅读机经:关于大气CO2与地球温度。

　　1、文章主旨：

　　提出现象：40million years的温度较之前有所下降，why?

　　观点1、海洋面积增多，吸热也多。But!作者不同意，觉得不够说服力(为什么不同意是考点)

　　观点2、CO2和雨水反应→落到海里一系列循环反应，于是CO2就沉在海底了。虽然CO2也会回到大气中，但CO2沉的速度比从海底释放出来的速度快，所以atmosphere的CO2总体是减少了。

　　观点3：地形变化，高原抬升导致更多石头裸露出来，于是CO2会腐蚀石头，也消耗了大气中的CO2

　　2.段落大意

　　(!!!狗主原话：这篇基本就是的 第一种观点表达得没有这么复杂 被精简了很多，大家仔细研读熟悉一下词汇吧)

　　During the past 40 million years, and particularly during the past 15 million years, this warm, wet climate largely disappeared. Colder climates and much greater regional extremes of precipitation have developed. What caused this cooling and diversification of climate and vegetation into a complex mosaic of many regionally distinctive types?

　　One school of thought focuses on the changing positions of the earth’s continents and oceans. The Atlantic Ocean has expanded at the expense of the Pacific Ocean, whereas an ancient equatorial sea that extended across much of Eurasia (called the Tethys Sea) has shrunk to become the modern, much smaller Mediterranean Sea

　　. In addition, the fraction of continents flooded by shallow inland seas has slowly decreased, exposing large amounts of land and creating climates less moderated by the temperature-stabilizing effects of oceans. Computer model simulations show that changes in the arrangement of the continents and the size of inland seas can have important effects on global climate over very long intervals of geologic time. But they are significantly less convincing as sole explanations for the dramatic changes of the past 40 million years.

　　Another possibility is a long-term decline in the concentration of carbon dioxide in the atmosphere, which would lessen the amount of heat trapped by the atmosphere and lead to “greenhouse cooling.” The amount of carbon dioxide in the earth’s atmosphere over million-year timescales is controlled by two major processes. Chemical weathering of continental rocks removes carbon dioxide from the atmosphere and carries it in dissolved chemical from to the ocean, where it is taken in by marine biota and deposited in sediments on the seafloor.Tectonic activity eventually frees this trapped carbon dioxide, in the earth’s lithospheric plates transports the seafloor to ocean trenches, where subduction carries old crust and sediments down toward the earth’s hot interior. At great depths, the sediments melt, releasing carbon dioxide, which emerges from the volcanic islands that overlie the buried curst and rejoins the atmosphere, completing the cycle.

　　If the pace of seafloor spreading (and hence of subduction) slowed significantly, less carbon dioxide would be vented to the atmosphere, the atmosphere would become relatively depleted of carbon dioxide and temperatures would fall. In fact, globally averaged seafloor spreading rates slow little or no net change in the past 40 million years. Subduction and volcanism eventually return the carbon dioxide to the atmosphere, but this process requires a long time (tens to hundreds of millions of years) to complete.

　　Plateau uplift may alter climate by increasing chemical weathering of rocks, thereby reducing atmospheric carbon dioxide concentrations. Carbon dioxide combines with rainwater and ground water to form carbonic acid, which reacts with silicate minerals in rocks during weathering. The resulting bicarbonate ions drain into the oceans, where they are taken up by marine animals such as plankton and corals and eventually deposited on the seafloor. The net effect is that chemical weathering removes carbon dioxide from the atmosphere and locks it away at the bottom of the oceans.

　　Maureen Raymo proposed that uplift of plateaus and mountain ranges has increased the rate of chemical erosion of continental rock on the globally averaged basis. Uplift could enhance chemical weathering in several ways. Heavy monsoons, which develop at the margins of plateaus, unleash particularly intense rainfall. In these regions, uplift-related faulting and folding also expose fresh rock to the weathering process. Moreover, the steeper slopes created by plateau uplift cause faster runoff, which removes erosion products and intensifies the chemical attack on the rock. Raymo suggests that long-term uplift in Tibet and other regions may have increased the rate at which carbon dioxide is removed from the atmosphere. In this way, concentrations would have fallen even though the amount of carbon dioxide exhaled by volcanoes (as inferred from seafloor spreading rates) remained nearly constant. Falling carbon dioxide levels would reduce the ability of the atmosphere to retain heat, thereby amplifying the global cooling

　　3、考点：

　　Q1.在第一段，问现在环境跟原来环境有什么区别，选择较Warm, Wet(此题也许是取非题，问现在的环境怎么样，那以前的来取非)

　　Q2.第二段，这个过程(二氧化碳溶解沉入海底再回到大气这个过程)需要hundreds of millions of years(隐含40百万年对它来说是相对短的过程，此处有题)

　　Q3.有一个Except题

　　Q4. 哪个最能cast doubt on 那个关于weathering的理论?/哪项weaken了R的观点?(削弱题)

　　40 miles years前的什么什么变化并不比40 miles years前来的大。

　　定位于二段后半部分，注意R的观点有两个部分，这题削弱的是第二个部分。Q5.第二段高亮

　　Q6. 根据第二段的理论下面除了哪个都是正确的? ---记不清了

　　选项有：

　　1、化学风化(chemical weathering)需要水

　　2、地势抬高(uplift)是吸收的二氧化碳的主要因素

　　Q7.什么情况下新的观点才会真正有效?

　　我选的是CO2沉积的速度大于CO2释放的速度。