TOEFL Listening: ETS-TOEFL听力机经 - OWP5991Q1XXB8OB8Q$

Lecture: Natural gas: Narrator: Listen to part of a lecture in an environmental geology class. Professor: OK. Let's continue our discussion of natural gas. In class yesterday, we said that it is now widely considered to be the fossil fuel of the future, or at least of the next 50 to 100 years or so. Anybody remembers why? Jane? Jane: Um ... it's probably because it's a lot more plentiful than oil, right? And ... it doesn't produce as much carbon when it's burned. Professor: Great! It actually emits very little carbon into the air. (So) it forms much less carbon dioxide in the atmosphere. And that's because it's a gas. You see, it efficiently converts its energy content to power ... um ... much more efficiently than your conventional fossil fuel because gas burns more completely. And also, it goes a lot further in terms of what it can be used for. It can be used to generate electricity, heat homes, run cars ... Also proven reserves of methane which is the most important ingredient in natural gas have increased tremendously for the past few years as the exploration goes on. We estimate that just ... just taking the reserves we know about ... it can last for about a hundred years, some say even longer. So, what do you think? Pretty promising, right? Urn ... clean burning, multi-purpose and plentiful supply. But, what's the drawback? Tom? Tom: Well, methane itself is a greenhouse gas, isn't it? So if it'd find its way into the atmosphere ... Professor: Right. It could be a contribution to the greenhouse effect in a big way. Urn ... what else? Jane: It's explosive. Tom: And it stinks! [students laughing] Professor: Hmm, well, I think maybe there're ways around that last one. Anyway, what I want us to think about today is ... well, as I mentioned earlier, the current reserves of natural gas should be enough to last us for a hundred years or more. (But) that's not all. You see, there are other newer sources that we've only started to ... to explore their potential, um ... sources that we've only recently begun tapping because of technological advances." sources like deep underground sites or ... or gas trapped in unusually hard rock. And then there's another kind of natural gas that we still don't have the technology to mine. (In fact), if we could find out a way to get to it ... ah ... to extract this unconventional natural gas, it might turn out to be the most abundant source ever found. We're talking about anywhere from 6 to 13 times the gas currently believed to exist in the world's proven reserves, maybe more ... So naturally the related industry is going crazy about this new source. And, what I'm talking about here is what's called 'methane hydrates.' Methane hydrates are a little crystals that form when methane combines with molecules of frozen water. Together, they form a kind of a ice-like substance that contains a good amount of methane. Methane hydrates were first discovered in 19th century in the Arctic regions. But now they are found all over the world's oceans. So it's no surprise the energy industry is starting to pay serious attention to them. But the problem is ... uh, we're still in the beginning stages of the research process. Little research was done on it until recently. So nobody really knows how to begin extracting the methane from methane hydrates, or how much methane we could get. We also don't know how the extraction process might affect the environmental consequences. I mean, these hydrates are stable now as long as they're kept cool and pressurized. But if we start messing them up, well, we really don't know yet just what might happen. The consequences might be something we'd never think of Careless pursuit of mining could precipitate the release of a great amount of methane into the atmosphere. And methane, as we already noted, is a powerful greenhouse gas. And this could easily happen when we drill into the ocean floor and thereby creating an underwater landslide. See, the sediments on the ocean floor are unstable because they're mixed in with methane hydrates. So, they're prevented from solidifying. And if there were an undersea landslide, the methane hydrates could mix with the ocean water, melt and release methane into the atmosphere. Tom? Tom: Well, you ... you said something about these hydrates being stable as long as they remain cool and under pressure, right? So ... I'm thinking, you know, with the global warming and all ... Professor: That's an excellent point! Tom. In fact, I happen to be one of those scholars who believe that about 250 million years ago, methane hydrates were released under similar circumstances to what we're experiencing with global warming and climate change today. And these methane hydrates actually were responsible for the largest mass extinction in Earth's history when more than 90 percent of all life on Earth died out. So to get back to Tom's point, with today's global warming causing ocean temperature to rise, and those methane hydrates ... well could it happen again?