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Global warming means that US farms are likely to increase production of soybeans, cotton, sorghum and oranges during the coming

admin2011-01-02  300
问题    Global warming means that US farms are likely to increase production of soybeans, cotton, sorghum and oranges during the coming decades, according to a new US government report.
   Farmers will also need less irrigation water and more pesticides because of slightly warmer temperature expected across the continental United States from carbon emissions, the Environmental Protection Agency report said.
   In the document, the Bush administration endorsed for the first time the widely held view of scientists that US greenhouse gas emissions will rise significantly over the next two decades due mostly to human activities.
   The White House had previously said there was not enough scientific evidence to blame oil refining, power plants and automobile emissions for global warming. The administration has rejected participating in the international Kyoto treaty to reduce greenhouse gases, saying a less costly and effective approach is to encourage voluntary emission cuts by industry.
   Although climate change could threaten US barrier islands and mountain meadows, the EPA report said most American farmers had little to worry about.
   The US has more than 1 billion acres of farmland, pastures and grazing land.
   The rising concentration of carbon dioxide in the soil is beneficial to many crops and should help continue decades of rising crop yields, the report said.
   Higher concentrations of carbon dioxide in the soil and slightly warmer temperature should benefit crops such as cotton, corn, soybeans, sorghum, barley, sugar beets and citrus fruits. "In general, northern areas such as the Midwest, West and Pacific Northwest are projected to show large gains in yields, while influences on crop yields in other regions vary more widely," the report said.
   For example, some scientific studies cited by the report estimated soybean crop yields could climb by 23 percent to 40 percent, depending on the specific scenario of rainfall and temperature. Orange crop yields could soar by 13 percent to 120 percent. However, corn yields would climb by a more modest 1 percent to 9 percent, the report said.
   More carbon also helps the soil hold water and plants grow faster, so that farmers will need less irrigation water, the EPA report said. However, climate change also means many growers will need to use more pesticides, the report said without elaboration.
   The EPA acknowledged that scientific analyses have not considered all of the consequences of climate change. on crop pests, diseases and extreme weather. "Agricultural technology is currently undergoing rapid change, the future production technologies and practices seem likely to be able to contain or reduce these impacts," the EPA report said.
   Farmers in the Southeast face more uncertainties linked to climate change.
   The EPA report said a change in planting dates and genetically altered crop varieties will be important for the Southeast in case summer rainfall declines.
What is the main theme of the following passage?
选项 A、The increasing impact of global warming on US crops.
B、The positive impact of global warming on US crops.
C、The negative impact of global warming on US crops.
D、The dual impact of global warming on US crops.
答案B
解析 本文第一段指出，根据美国一份新的政府报告，全球变暖会提高美国大豆、棉花、高梁和柑橘的产量。后面又说土壤中二氧化碳的增加有利于多种作物生长，将使作物持续增产几十年，并以豆类、橘类和玉米为例，说明农作物增产的速度。另外，碳的增加有助于土壤保持水分，能促进植物生长。虽然文章末尾提到气温升高会引起害虫等问题，但从全文来看，绝大部分内容是谈全球变暖对美国农业的积极作用。因此正确答案为B。
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