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Rivers in some of the world's most populated regions are losing water, many because of climate change, researchers reported on Tuesday.
Affected rivers include the Ganges in India, the Niger in West Africa, and the Colorado in the southwestern United States.
When added to the effects from damming, irrigation and other water use, these changes could add up to a threat to future supplies of food and water, the researchers reported in the American Meteorological Society's Journal of Climate.
"Reduced runoff is increasing the pressure on freshwater resources in much of the world, especially with more demand for water as population increases," Aiguo Dai of the National Center for Atmospheric Research in Boulder, Colorado, who led the study, said in a statement.
"Freshwater being a vital resource, the downward trends are a great concern."
Researchers looked at records of river flow in 925 big rivers from 1948 to 2004, finding significant changes in about a third of the world's largest rivers.
For instance, annual freshwater discharge into the Pacific Ocean fell by about 6 percent, or 526 cubic kilometers -- about the equivalent volume of water that flows out of the Mississippi River each year.
The Columbia River in the US Northwest lost about 14 percent of its volume from 1948 to 2004, largely because of reduced precipitation and higher water usage in the West.
Annual discharge from melting ice into the Arctic Ocean rose about 10 percent, or 460 cubic kilometers.
"Also, there is evidence that the rapid warming since the 1970s has caused an earlier onset of spring that induces earlier snowmelt and associated peak streamflow in the western United States and New England and earlier breakup of river-ice in Russian Arctic rivers and many Canadian rivers," the researchers wrote.
"As climate change inevitably continues in coming decades, we are likely to see greater impacts on many rivers and water resources that society has come to rely on," said NCAR scientist Kevin Trenberth, who worked on the study.
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(Agencies)
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研究人員于本周二報告稱，世界上人口最密集的一些地區的河流出現了流量下降的情況，而這大多是由氣候變化所致。
受到影響的河流包括印度的恒河、西非的尼日爾河以及美國西南部的科羅拉多河等等。
研究人員在報告中稱，如果再加上筑壩、灌溉、以及其它水資源利用的影響，這些變化會對這些地區未來的食物和水源供應構成威脅。該報告在美國氣象學會的《氣候》雜志上發表。
研究負責人、科羅拉多州波爾得美國國家大氣研究中心的戴愛國（音）在一份聲明中稱：“河流的徑流量減少正增大世界大部分地區淡水資源的壓力，尤其是隨著人口增長，淡水資源需求更大的地區。”
“淡水是一種重要的資源，所以大河流量的減少是一個十分令人擔憂的問題。”
研究人員分析了全球925條大河在1948年至2004年間的流量記錄，發現其中約三分之一的主要河流的流量都發生了顯著變化。
例如，每年流入太平洋的淡水總量下降了約6%（合526立方千米），相當于于每年從密西西比河流出的淡水總量。
在1948年至2004年間，位于美國西北部地區的哥倫比亞河的流量減少了約14%，這主要是由于降水量減少以及美國西部用水量增加。
冰川融化流入北冰洋的水年度流量增加了約10%，合460立方千米。
研究人員在報告中寫道：“也有證據表明，自20世紀70年代以來，全球變暖的加劇導致春天提前到來，這造成冰雪消融期提前，美國西部和新英格蘭地區出現洪峰，以及俄羅斯北極地區和加拿大很多河流的河冰提早消融。
研究小組成員、美國國家大氣研究中心的科學家凱文?特倫伯斯說：“氣候變化的趨勢在未來幾十年內不可逆轉，我們可能會看到全球變暖對社會賴以生存的很多河流和水資源產生更大影響。”
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（實習生許雅寧 英語點津姍姍編輯）
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