题目内容：
In the last few decades, scientists have reached consensus and reported that human beings sare causing changes in the Earth's climate--something previously seen as beyond our control.They claim that in the last 50 years, gases pumped into the Earth's atmosphere by the factories and vehicles have been speeding up the process of global wanning and affecting our climate.Looking ahead, scientists believe that global warming could be one of the biggest environmental problems facing the 21st century.
参考答案：
答案解析：