TOEFL Listening: ETS-TOEFL听力机经 - Q481B135921BC941Q$

Lecture: Abrupt climate change: The Younger Dryas: Narrator: Listen to part of a lecture in an Earth science class. Professor: The last major Ice Age ended about 1,600 years ago. After reforming period, average temperatures in the northern hemisphere began to fall again. There is another cold spell that lasted for about 1,200 years. We call this cold period, the Younger Dryas. Dryas is the name of a flower that grows well, that ... that thrives in cold weather. In fact, the presence of the Dryas is an indicator of glacial or near glacial conditions. The Younger Dryas is named after this flower, because we find Dryas pollen and samples from lake and pond sediment, the stuff that settled to the bottoms of these bodies of water from that time period. But the Antarctic cold reversal started about a thousand years before the Younger Dryas. As its name suggests, the cold reversal was a time when average temperature on Earth was rising. So why, when things were getting warmer, did they suddenly get colder Again? The coming and going of an Ice Age is usually a gradual event occurring over thousands of years, but the Younger Dryas happened quickly. In geological terms, average temperatures dropped drastically in less than a hundred years. This is an abrupt climate change, and it ended even more abruptly. In fact, it's estimated, based on a study of ice core samples, that the average mean temperature increase ten degrees celsius in just ten years. These climatic shifts go against most theories that claim that climate change requires thousands of years to occur. So the most widely accepted hypothesis is that the Younger Dryas occurred because the ocean current, known as the North Atlantic conveyor belt, shut down for a while. The conveyor belt is a current that moves warm water northward from the Indian Ocean, around Africa and up to the North Atlantic. If this current stopped flowing temporarily, it would get pretty cold up in the North Atlantic region. Female Student: What would cause the conveyor about to stop? Professor: Well, the most likely candidate is the introduction of a lot of fresh water. So then the next question is, what would cause an increase of fresh water in the ocean? Well, there have been several suggestions, like glacial melting. In particular, It's been hypothesized that an ice-dam holding back water from a gigantic lake in North America melted, sending huge amounts of fresh water into the Atlantic Ocean. This is commonly referred to as the ice-dam theory. And some geophysicists have proposed what they call the meteor-impact theory, that the meteorite hit the north eastern part of North America. Uh, remember most meteor burned up when they enter Earth's atmosphere, but if they make it through and hit Earth's surface, they're called meteorites. Anyway, the heat from such a meteorite impact would have melted a huge chunk of the North American ice sheet, the glacier sending fresh water into the Atlantic Ocean. And there is some evidence of a meteorite impact – elevated levels of the element iridium, an element associated with meters and asteroids. Iridium is spreading a layer throughout Earth's crust dating from near the onset of the Younger Dryas. Its presence could be explained by a dust cloud that would have formed after a meteorite impact and later settled in a layer on the crust. Either way, it seems that the movement of fresh water into the North Atlantic did happen. Female Student: So which is it? What do you think Cause the fresh water input that stops the conveyor about? Professor: Tell you the truth, we really don't know. For the ice-dam scenario, the land around the lake where it might have occurred does not appear to have experienced the changes associated with the type of flooding we're talking about. Plus the nearby ocean does not show decrease in the salinity, a decrease in the salt content from that time period. Uh, there's yet another problem with this hypothesis. It was found that a second wave of melt water, although smaller than the first one, occurred at the end of the Younger Dryas. So why didn't it also trigger a similar chain of consequences on the climate system. For the meteor-impact theory, some scientists believe they found evidence of an impact in core samples from that period in the form of iridium and other metals. But there are a lot of unanswered questions associated with that theory. Uh, for one thing, where's the impact crater? A crater created when the meteorite hit. However, the presence of iridium in the crust indicates that there was probably a dust cloud, uh, dust cloud that would have block sunlight contributing to the temperature decrease.