V3. 【疑似原文，待确认】

　　Most people know methane (often written as the chemical formula CH4) as natural gas. Approximately 600 million metric tons of it—both anthropogenic (from human activities) and natural—rise into the atmosphere every year. Wetlands such as swamps, marshes and rice paddies provide the greatest share. Cattle, sheep and termites also make methane, as a by-product of anaerobic microbial digestion in their gut. Forest and savanna fires release methane, as does the combustion of fossil fuels. Over the years, researchers have gained considerable knowledge about the global methane cycle, and the consensus of the Intergovernmental Panel on Climate Change (IPCC) in 2001 was that the major sources had probably been identified (although the proportion each source contributes was still uncertain).

　　In 2005 the scientists’ satellite measurements revealed “clouds” of methane over tropical forests. In light of our findings, however, their work made sense: green vegetation was the source of the methane clouds. Our findings would explain a trend that has puzzled climate scientists for years: fluctuations in methane levels in parallel with changes in global temperatures. Ice cores serve as natural archives that store information about atmospheric composition and climate variability going back almost a million years. Tiny bubbles of air trapped in the ice reveal the relative concentrations of atmospheric gases in the past. We see in the ice cores, for example, that variations of past carbon dioxide levels are closely linked to changes in global temperatures. During ice ages, carbon dioxide concentrations are low; during warm spells, levels increase. Another explanation that has been suggested involves the gas in a form known as methane hydrates. These develop at high pressure, such as that found on the ocean floor. An unknown but possibly very large quantity of methane is trapped in this form in ocean sediments. The sudden release of large volumes of methane from these sediments into the atmosphere has been suggested as a possible cause for rapid global warming events in the earth’s distant past. Yet recent results from polar ice core studies show that marine methane hydrates were stable at least over the past 40,000 years, indicating that they were not involved in the abrupt increases of atmospheric methane during the last glacial cycle. We know that terrestrial vegetation is very sensitive to environmental changes, and thus the total amount of vegetation on the planet varies as the climate cools down and warms up during glacial cycles. In light of our findings, such variations should now be seriously considered as a possible cause of declines in methane levels during glacial periods and rises during the interglacials.

　　Our discovery also led to intense speculation that methane emissions by plants could diminish or even outweigh the carbon storage effect of reforestation programs. But our calculations show that the climatic benefits gained by establishing new forests to absorb carbon dioxide would far exceed the relatively small negative effect of adding more methane to the atmosphere (which may reduce the overall carbon uptake of the trees by 4 percent at most). The potential for reducing global warming by planting trees is most definitely positive. A more legitimate concern is whether the methane produced by vegetation can have an impact on climate in the near future. Although plants are not responsible for the massive increase of methane in the atmosphere since preindustrial times, they do tend to grow faster. As we can expect methane emissions from vegetation to increase with temperature, this would lead to even more warming. This vicious cycle would be a natural phenomenon except for its speed, which is accelerated mainly by anthropogenic activities such as burning fossil fuels. The large plant feedback to global climate change that most likely happened in the past, however, is probably unlikely today because so many forests have been cut down. The problem is not the plants; it is the global large-scale burning of fossil fuels.

　　【段落大意】

　　P1：介绍甲烷，甲烷是导致温室效应的一种气体，甲烷产生的途径。

　　P2：[转折]科学家发现植物也会释放甲烷，并研究发现了雨林上方的大气中甲烷浓度较大。这个发现可能可以说明植物排放的甲烷与全球升温有关。

　　P3：对冰层进行研究，因为留在冰层中的微小气泡中的空气能揭示过去大气气体的相对浓度，而这又对甲烷与气候关系的研究提供帮助。

　　P4：虽然植物会释放甲烷，但是希望通过减少植物来阻止温室效应不可行，要减缓温室效应还是应从减少石化燃料的使用下手。

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