TOEFL Listening: ETS-TOEFL听力机经 - WA03S48VZVJ6B1Z5Q$

Lecture: Interglacial periods: Milankovitch theory: Narrator: Listen to part of a lecture in a climatology class. Professor: Over the past few million years, there have been several ice ages, cold periods when glaciers covered much of the planet. These ice ages have been separated by long intervals of relatively mild climates which we call interglacial periods. In interglacial periods, the glaciers recede to the most Polar Regions. The most recent ice age ended around ten thousand years ago and we're in an interglacial period right now. But it will probably end one day and there will be another ice age. Student: But there's global warming and the ice caps are melting, probably because we're burning fossil fuels and emitting greenhouse gases. How can we be so sure we're living in between two ice ages? Professor: Good question. To answer though, we must consider evidence that ice ages have occurred with striking regularity. Ice sheets have advanced and retreated about every one hundred thousand years over the past few million years. We know this from geological research and particularly by examining ancient coral reefs. See, ocean gets shallower during ice ages because so much water gets locked up in the ice sheets. As a result, coral reefs that normally grow in shallow waters in an interglacial period would end up high on dry land during an ice age and stop growing and then start growing a new layer when the glaciers receded and the water levels came back up. When we dated, determined how old the different coral layers are, we found the periods of coral growth appeared in regular cycles, suggesting that ice ages occurred in regular cycles. Student: So why do ice ages come and go? Professor: There are several theories about that like plate tectonics, the movement of continental plates that created mountain ranges far above sea level. This would have affected ocean currents and wind patterns in ways that could trigger ice ages. Student: But doesn't that theory predict that ice ages happen randomly? Because if they are random, there's no reason to believe that another ice age will happen, especially with global warming, right? Professor: It may explain global climate change to some degree. Climate is very sensitive to a variety of variables, but yeah, we need a theory to explain the regular intervals of ice ages and there's a theory that seems to do this. It's called the astronomical theory of climate change, or the Milankovitch theory. Milutin Milankovitch was an astrophysicist who developed a mathematical theory showing that regular ice age cycles are caused by irregularities, small fluctuations, in Earth's movement through space. In the 1920s, Milankovitch calculated these fluctuations and theorized that changes in the Earth's location and orbit were responsible for the ebb and flow of ice ages. For instance, he looked at changes at the angle of the Earth's tilt on its axis. Milankovitch determined that the angle of the tilt is not stable, that the Earth wobbles on its axis. The tilt changes by about three degrees on a cycle of about forty-one thousand years and this slight fluctuation in the tilt has an impact. More tilt causes summers to be warmer and winters to be colder. Less tilt results in the opposite. Cooler summers and milder winters. Another thing Milankovitch considered was the Earth's orbit around the Sun is not a perfect circle, but an ellipse. The Earth is slightly closer to the Sun during certain times of the year versus other times of the year. Today, the closest approach of the Earth to the Sun is January, making winters in the Northern Hemisphere milder. But eleven thousand years ago, the closest approach to the Sun occurred in July, making winters colder and the summers hotter. Well, according to Milankovitch, because they affect summer and winter temperatures, the tilt cycle and this uneven orbital cycle, combine to control the flow and retreat of ice sheets. In the Northern Hemisphere, where most of Earth's land mass is, hundreds or thousands of consecutive cool summers would allow snow and ice to persist to the next winter. As a result, ice sheets would grow and grow. Conversely, a period of warmer summers would shrink ice sheets by melting more ice than accumulates during the winter. It's a compelling theory, but there's still other things that might affect climate like cloud cover and variations in solar energy. So, while the Milankovitch theory is widely accepted, we shouldn't assume it's comprehensive.