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  3. The Greenhouse Effect Earth’s climate has been changing constantly over its 5-billion-year history. Sometimes, the clima

The Greenhouse Effect Earth’s climate has been changing constantly over its 5-billion-year history. Sometimes, the clima

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问题                              The Greenhouse Effect
    Earth’s climate has been changing constantly over its 5-billion-year history.
    Sometimes, the climate has warmed so that the oceans have risen and covered much of the Earth. Each of the changes may seem extreme, but they usually occurred slowly over many thousands of years.
Ancient Climate History
    The first people arrived in America between 151 000 and 30, 000 years ago. During that time, much of North America was covered by great ice sheets. Some 14, 000 years ago, the last ice sheet began to melt very quickly. By 7, 000 years ago, the ice was gone.
    This end to the ice ages caused big changes on the Earth. The changes caused many kinds of plants and animals to die. For example, mastodon-elephant-like animal-and other large mammals that preferred cold climates may not have been able to live in the warmer, drier conditions.
The Little Ice Age
    Starting in the 14th century, Europeans lived through what is known as the "Little Ice Age." The Little Ice Age lasted for several hundred years. During the Little Ice Age, the advance of glaciers along with hard winters and famines caused some people to starve and others to leave their homes.
Recent Climate History
    The Earth has warmed about 1°F in the last 100 years. And the four warmest years of the 20th century all happened in the 1990s. Periods of increased heat from the sun may have helped make the Earth warmer. But many of the world’s leading climatologists think that the greenhouse gases people produce are making the Earth warmer, too.
    Scientists think the sea has risen partly because of melting glaciers and sea ice. When some glaciers melt, they release water into the sea and make it higher than it was before. Scientists also think that warmer temperatures in the sea make it rise even more. Heat makes water expand. When the ocean expands, it takes up more space.
What Might Happen?
    Scientists are not fortune-tellers. They don’t know exactly what will happen in the future. But they can use special computer programs to find out how the climate may change in the years ahead. And the computer programs tell us that the Earth may continue to get warmer.
    Together, the melting glaciers, rising seas, and computer models provide some good clues. They tell us that the Earth’s temperature will probably continue to rise as long as we continue increasing the amount of greenhouse gases in the atmosphere.
    Scientists have to think like detectives. They look for clues to help them understand how the world works. Then they investigate the clues to find evidence -- real facts that can give them a better idea of what is going on. Here are some of the ways that scientists gather evidence about climate, both past and present:
Weather Stations
    Weather stations help us find out the temperature on the surface of the Earth. Weather stations use special thermometers that tell us the temperature. They can be set up almost anywhere on land. Weather stations also can tell us how fast the wind is moving and how much rain falls on the ground during a storm.
Weather Balloons
    Almost everyone likes balloons -- including scientists! Weather balloons are released to float high up into the atmosphere. They carry special instruments that send all kinds of information about the weather back to people on the ground.
Ocean Buoys
    A buoy is an object that floats on water, and is often used to warn boats away from dangerous places in the ocean or on a river. But some buoys have special instruments on them. These buoys can tell us the temperature and other things about the conditions of the atmosphere.
Weather Satellites
    Humans send satellites into space to travel around the Earth. The satellites send back information to scientists on the ground. Some of the information they give us is about the weather and the Earth’s temperature.
Ice Cores
    Some scientists who want to find out more about climate study ice for clues. Not just any ice - they are studying the ice from glaciers that have been around for a very long time. They cut pieces of ice and look for air bubbles that were trapped in the ice hundreds or even thousands of years ago. The air bubbles help them discover what the climate used to be like on Earth. The evidence they uncover is creating a historical record of regional temperatures and greenhouse gas concentrations dating back 160, 000 years.
Sediment Analyses
    Sediment is the earth and rock that has built up in layers over time. Scientists are learning a great deal about past climate from studying these layers. Sediment layering pro vides information about where glaciers have been in the past. Ocean sediments provide a map of how ocean currents have flowed in the past. And fossilized pollen found in sediment layers tells us about where different plants have grown in the past.
Tree Rings
    You can tell how old a tree is by counting its rings because it grows a new ring every year. Tree rings also can tell us how much precipitation fell each year in the place where the tree lives.
What Does All of This Mean?
    Weather stations, balloons, ocean buoys, and satellites tell us the Earth’s temperature today. Ice cores, sediment layers, and tree rings tell us about what the Earth’s climate has been like in the past. With this evidence, scientists are learning how climate changes over time.
What Are Scientists Still Unsure About?
    How do clouds respond to changes in temperature and precipitation? How do oceans transport heat? How do climate and intense weather events like hurricanes affect each other? As scientists try to answer these and other questions, they will discover many more clues about how the Earth’s climate system works.
    It may seem hard to believe that people can actually change the Earth’s climate. But scientists think that the things people do that send greenhouse gases into the air are making our planet warmer.
    Once, all climate changes occurred naturally. However, during the Industrial Revolution, we began altering our climate and environment through agricultural and industrial practices. The Industrial Revolution was a time when people began using machines to make life easier. It started more than 200 years ago and changed the way humans live. Before the Industrial Revolution, human activity released very few gases into the atmosphere, but now through population growth, fossil fuel burning, and deforestation, we are affecting the mixture of gases in the atmosphere.
    Since the Industrial Revolution, the need for energy to run machines has steadily in creased. Some energy, like the energy .you need to do your homework, comes from the food you eat. But other energy, like the energy that makes cars run and much of the energy used to light and heat our homes, comes from fuels like coal and oil-fossil fuels. Burning these fuels releases greenhouse gases.
Scientists can figure out where glaciers have been in the past through ____________.
选项
答案sediment layering
解析 Scientists can figure out where glaciers have been in the past through ______.词汇线索是where，glaciers，past，定位到小标题sediment analysis段落次句，则答案应是 sediment layering。
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