Unit 9   

    There's a famous saying that goes: "Everybody talks about the weather, but no one ever does anything about it!" Humans do, of course, affect the climate, as we know from scientists' warnings about everything from the greenhouse effect to nuclear winter. Conscious of our power to harm the environment, we can sometimes forget that as a species we are still helpless to control -- or even predict with any accuracy -- many of the powerful surprises that nature can spring on us. The first two texts in this unit examine the causes and effects of one of the world's most trouble-making natural environmental phenomena -- El Niño -- while Text C speculates about what factors are making our weather so wild, and what we might be able to do about it.

Text B     Study Finds Good Effects of El Niño

  The climatic phenomenon that is being blamed for floods, hurricanes and early snowstorms also deserves credit for encouraging plant growth and helping to control the pollutant linked to global warming, a new study shows.

  El Niño -- the periodic warming of eastern Pacific Ocean waters -- causes a burst of plant growth throughout the world, and this removes carbon dioxide from the atmosphere, researchers have found.

   The new study shows that natural weather events, such as the brief warming caused by El Niño, have a much more dramatic effect than previously believed on how much carbon dioxide is absorbed by plants and how much of the gas is expelled by the soil.

   Atmospheric carbon dioxide, or CO₂, has been increasing steadily for decades. This is thought to be caused by an expanded use of fossil fuels and by toppling of tropical forests. Scientists have linked the CO₂ rise to global warming, a phenomenon known as the greenhouse effect. Alarmed, nations of the world now are drawing up new conservation policies to reduce fossil-fuel burning, in hopes of reducing greenhouse gases in the atmosphere.

   But David Schimel of the National Center for Atmospheric Research, a co-author of the new study, says that before determining how much to reduce fossil-fuel burning we should consider the effects of natural climate variations on the ability of plants to absorb CO₂.

   Schimel said satellite measurements of CO₂, plant growth and temperature show that natural warming events such as El Niño at first cause more CO₂ to be released into the atmosphere, probably as the result of accelerated decay of dead plant matter in the soil. But later, within two years, there is an explosion of growth in forests and grasslands, which means plants suck more carbon dioxide out of the atmosphere.

  "We think that there is a delayed response in vegetation and soil to the warming effects of such phenomena as El Niño, and this leads to increased plant growth," said Schimel.

   However, he said, it is not clear whether the warming by El Niño causes a net decrease in the build-up of CO₂ over the long haul. "We don't really know that yet," said Schimel. What the study does show, however, is that the rise and fall of CO₂ in the atmosphere is strongly influenced by natural changes in global temperature, said B. H. Braswell of the University of New Hampshire, another co-author of the study.

   Braswell said that in years when the global weather is cooler than normal, there is a decrease in both the decay of dead plants and in new plant growth. This causes an effect that is the opposite of El Niño warming: CO₂ atmosphere levels first decline and later increase.

   "I think we have demonstrated that the ecosystem has a lot more to do with climate change than was previously believed," said Braswell. "Focussing on the role of human activity in climate change is important, but manmade factors are not the only factors."

   The researchers used satellite measurements taken from 1980 to 1991. This period included a major El Niño in 1982-83 and another in 1986. Each of these events, said the authors, had a direct, but often delayed, effect on the CO₂ levels in the atmosphere.

研究发现“厄尔尼诺”现象有好的影响

   一项新的研究表明，被认为导致洪涝、飓风和暴风雪提前发生的那种气候现象也有值得称赞的一面，因为它促进植物的生长，并有助于控制与全球变暖有关的污染物质。

   研究人员已经发现，厄尔尼诺现象—即东太平洋水域的周期性升温—促使全球植物迅速生长，而这又可清除大气中的二氧化碳。

   该项新的研究表明，厄尔尼诺现象引起的短暂升温这类自然气候活动，对植物吸收多少二氧化碳，土壤中释放出多少二氧化碳的影响比我们先前认为的要大得多。

   几十年来，大气中的二氧化碳或CO₂一直在不断增加。人们认为这是广泛使用矿物燃料及砍伐热带森林所造成的结果。科学家们将CO₂的增加与被称为温室效应的全球变暖现象联系了起来。惊恐之余，世界各国目前正在制定新的保护政策以减少矿物燃料的燃烧，希望能减少大气中导致温室效应的各种气体的含量。

   但是，该项新研究的合作者、国家大气研究中心的戴维·希梅尔说，我们应该先考虑一下自然气候的变化对植物吸收CO₂能力的影响，然后再决定减少燃烧多少矿物燃料。

   希梅尔说，卫星对CO₂、植物生长以及气温的测量数据表明，厄尔尼诺这类自然变暖活动，一开始造成更多的CO₂被释放到大气中，这可能是土壤中死亡植物物质加速腐烂所产生的结果。但在以后的两年中，就有一个森林和草地的生长高峰，这意味着植物会从大气中吸收更多的二氧化碳。

   希梅尔说：“我们认为植被和土壤对厄尔尼诺这类现象的变暖效应有一种滞后反应，而这会导致植物加速生长。”

   然而，他又说，从长远来看，厄尔尼诺现象造成的气候变暖是否会引起CO₂积聚的绝对下降，这一点还不清楚。“对此我们确实还不清楚，”希梅尔说。然而，这项研究确实表明了一点，即大气中CO₂含量的增减，在很大程度上受到全球气温自然变化的影响，该项研究的另一位合作者，新罕布什尔大学的B.H.布拉斯韦尔说。

   布拉斯韦尔说，在全球气候比平时寒冷的年月里，已死植物的腐烂活动和新植物的生长活动都会减弱。其后果与厄尔尼诺气候变暖所造成的后果正好相反：大气中的CO₂含量先降后升。

   “我认为我们已经证明了这一点，即生态系统与气候变化之间的关系比我们以前认为的要密切得多，”布拉斯韦尔说。“重点研究人类活动在气候变化方面的作用是重要的，但人为因素并不是唯一的因素。”

   研究人员利用了1980年到1991年间所获得的卫星测量数据。这一时期包括了1982-83年发生的一次重大的厄尔尼诺现象和1986年发生的一次。研究报告的作者们说，这两次现象都对大气中CO₂含量有着直接的但常常是滞后的影响。