E1 Nino—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.
(47)The new study shows that natural weather events，such as the brief warming caused by E1 Nino, 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. (48)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 E1 Nino 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 E1 Nino, and this leads to increased plant growth, " said Schimel.
(49)However, he said, it is not clear whether the warming by E1 Nino causes a net decrease in the buildup 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 ply 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 E1 Nino warming: CO2 atmosphere levels first decline and later increase.
(50) " I think we have demonstrated that the ecosystem has a lot more to do with climate change than was previously believed," said Braswell, " Focusing on the role of human activity in climate change is important, but manmade factors are not the only factors. \
参考答案：[参考译文] 然而，从长远来看，厄尔尼诺造成的气候变暖是否最终会引起二氧化碳积聚量的下降，这一点还不清楚。
解析： 句子的框架是However, he said, it is not clear whether the warming…causes…。句中he said是插入语，it is not clear…是主句；it是形式主语，真正的主语是whether引导的主语从句，主语从句中的介词短语in the built-up of CO2中built-up的意思是“增长，积聚”；词组over the long haul是“从长远来看”之意。