阅读理解

　　Tuvalu, a tiny country in the Pacific Ocean, has asked for help as it fears it will be swallowed up by the sea．

　　Storms and huge waves are a constant threat and none of Tuvalu’s nine little islands is more than five meters above sea level．Salt water is already entering the country’s drinking water supply, as well as damaging plants(种植园) that produce fruit and vegetable．Withouturgent(紧急的) help, the country’s days are numbered．

　　But Tuvalu is not the first place to face sinking into the sea．Venice, a historic city in Italy best known for its canals, has sunk about 24 cm over the past 100 years．Experts say that it will have sunk another 20-50 cm by 2050．A century ago, St．Mark’ s Square, the lowest point in the city, flooded about nine times a year．Nowadays, it happens more than 100 times．

　　While Venice is slowly sinking into the mud on which it stands, Tuvalu’ s rising sea level is caused by global warming．

　　The average global temperature has increased by almost 0.5 centigrade degree over the past century．Scientists expect it to rise by an extra 1-3 degrees over the next 100 years．

　　Warmer weather makes glaciers (冰川) melt, adding more water to the ocean．The warmer temperatures also make water increase, so it takes up more space, causing the sea level to rise．

　　The sea level has risen about 10-25 cm in the last 100 years．

　　The main cause of global warming is human pollution．Through burning coal, oil and gas, people have been increasing the greenhouse gases in the atmosphere, such as CO₂．This adds to the power of the greenhouse effect, making the planet even warmer．

　　Many scientists believe that, if the warming is not stopped, there will be huge climate changes．The sea level could rise by one meter this century．

　　Should this come true, millions of homes will be swallowed up by the sea and the world will be flooded with "climate refugees (难民) looking for somewhere to live．

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