单选题 The planet is getting lusher, and we are responsible. Carbon dioxide generated by human activity is stimulating photosynthesis (光合作用) and causing a beneficial greening of the Earth's surface. For the first time, researchers claim to have shown that the increase in plant cover is due to this 'CO₂ fertilization effect' rather than other causes. However, it remains unclear whether the effect can counter any negative consequences of global warming, such as the spread of deserts.
Recent satellite studies have shown that the planet is harboring more vegetation overall, but pinning down the cause has been difficult. Factors such as higher temperatures, extra rainfall, and an increase in atmospheric CO₂—which helps plants use water more efficiently—could all be boosting vegetation. To home in on the effect of CO₂, Randall Donohue of Australia's national research institute, the CSIRO in Canberra, monitored vegetation at the edges of deserts in Australia, southern Africa, the US Southwest, North Africa, the Middle East and central Asia. These are regions where there is ample warmth and sunlight, but only just enough rainfall for vegetation to grow, so any change in plant cover must be the result of a change in rainfall patterns or CO₂ levels, or both.
If CO₂ levels were constant, then the amount of vegetation per unit of rainfall ought to be constant, too. However, the team found that this figure rose by 11 per cent in these areas between 1982 and 2010, mirroring the rise in CO₂ (Geophysical Research Letters, doi. org/mqx). Donohue says this lends 'strong support' to the idea that CO₂ fertilization drove the greening.
Climate change studies have predicted that many dry areas will get drier and that some deserts will expand. Donohue's findings make this less certain. However, the greening effect may not apply to the world's driest regions. Beth Newingham of the University of Idaho, Moscow, recently published the result of a 10-year experiment involving a greenhouse set up in the Mojave Desert of Nevada. She found 'no sustained increase in biomass' when extra CO₂ was pumped into the greenhouse. 'You cannot assume that all these deserts respond the same,' she says. 'Enough water needs to be present for the plants to respond at all.'

单选题 Why do researchers claim that the increase in plant cover is due to this 'CO₂ fertilization effect'?______

【正确答案】 A

【答案解析】[考点] 原因根据题干可以将该题定位至文章第一段。第一段第二句Carbon dioxide generated by human activity is stimulating photosynthesis and causing a beneficial greening of the Earth's surface，人类活动所产生的二氧化碳刺激了光合作用，对地球表面的绿化有利。文章接着指出，研究人员第一次声称植被覆盖率的增加是因为“二氧化碳施肥效应”，而非其他原因。因此可以根据前文选出A选项，因为二氧化碳刺激了光合作用。其他选项都不是真正的原因所在。

单选题 Which of the following is the explanation of 'Recent satellite studies have shown that the planet is harboring more vegetation overall, but pinning down the cause has been difficult' (Para. 2)?______

【正确答案】 D

【答案解析】[考点] 文章细节根据题干可以将该题定位至文章第二段。题目要求是对句子Recent satellite studies have shown that the planet is harboring more vegetation overall, but pinning down the cause has been difficult的解释。根据选项，主要是要判断harboring和pinning down的意思。harbor的本意是“港湾”，其主要功能是让船能躲避风浪，在这里意为“隐匿，隐藏”；pinning down意为“确定”。因此答案为D选项。

单选题 Which of the following will cause changes in plant cover according to the second paragraph?______

【正确答案】 C

【答案解析】[考点] 原因根据题干可以把该题定位至文章第二段。文章第二段最后一句These are regions where there is ample warmth and sunlight, but only just enough rainfall for vegetation to grow, so any change in plant cover must be the result of a change in rainfall patterns or CO2 levels，or both指出，植被覆盖的变化是由于降雨格局的变化或者二氧化碳水平的变化，或者是由两者共同引起的。只有C符合此意。

单选题 Which of the following can be inferred from the passage?______

【正确答案】 B

【答案解析】[考点] 文章细节该题需要通读全文，A选项可以定位至文章第一段末句However, it remains unclear whether the effect can counter any negative consequences of global warming, such as the spread of deserts，二氧化碳施肥效应能否反驳全球变暖带来的不利影响还未得到确定，所以A选项排除。B选项可以定位至文章最后一段Climate change studies have predicted that many dry areas will get drier and that some deserts will expand. Donohue's findings make this less certain，该句指出Donohue的发现使许多干旱地区将会变得更加干燥，沙漠的面积也将进一步扩大的预测受到质疑，因此B选项符合，in line with意为“与……相符”。C选项也需要定位至文章最后一段，You cannot assume that all these deserts respond the same也就是说这种绿化效应不是对所有沙漠地区都适用，所以C是错误的。D选项语气比较强烈，文章在第二段指出higher temperatures，extra rainfall也有利于植被生长，所以D也不对。