It's 1997. The United States Senate has called a hearing about global warming. Some expert witnesses point out that past periods in Earth’s history were warmer than the 20th century. Because such variations existed long before humans, they claim the current trend is also the result of natural variation. Can you spot the problem with this argument?

现在是1997年，美国参议院召开了一次关于全球变暖的听证会。一些专家证人指出，地球历史上过去的时期比20世纪更温暖。因为这种变化在人类出现之前就已经存在了 他们声称目前的趋势也是自然变化的结果，你能发现这个论点的问题吗？你能发现这种说法的问题吗？