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Coastal cities around the globe are sinking by up to several centimeters per year, on average, satellite observations reveal. The one-two punch of sinking land and rising seas means that these coastal regions are at greater risk of flooding than previously thought, researchers report in the April 16 Geophysical Research Letters.

Matt Wei, an earth scientist at the University of Rhode Island in Narragansett, and colleagues studied 99 coastal cities on six continents. “We tried to balance population and geographic location,” he says. While subsidence (下沉) has been measured in cities previously, earlier research has tended to focus on just one city or region. This investigation is different. Wei says, “It’s one of the first to really use data with global coverage.” Wei and his team relied on observations made mostly from 2015 to 2020 by a pair of European satellites. Instruments onboard beam microwave signals toward Earth and then record the waves that bounce back. By measuring the timing and intensity of those reflected waves, the team determined the height of the ground with millimeter accuracy. And because each satellite flies over the same part of the planet every 12 days, the researchers were able to trace how the ground deformed over time.

The largest subsidence rates — up to five centimeters per year — are mostly in Asian cities like Tianjin, China; Karachi, Pakistan; and Manila, Philippines, the team found. What’s more, about one-third, or 34, of the analyzed cities are sinking in some places by more than a centimeter per year. That’s a worrying trend, says Dario Solano-Rojas, an earth scientist at the National Autonomous University of Mexico in Mexico City who was not involved in the research. These cities are being hit with a double whammy: At the same time that sea levels are rising due to climate change, the land is sinking. “Understanding that part of the problem is a big deal,” Solano-Rojas says.

Wei and his colleagues think that the subsidence is largely caused by people. When the researchers looked at Google Earth imagery of the regions within cities that were rapidly sinking, the team saw mostly residential or commercial areas. That’s a tip-off that the culprit (元凶) is groundwater extraction, the team concluded. Landscapes tend to settle as water is pumped out of aquifers (含水层). But there’s reason to be hopeful. In the past, Indonesia’s Jakarta, for example, was sinking by nearly 30 cm/yr, on average. But now subsidence there and in other places has slowed, possibly due to recent governmental regulations limiting groundwater extraction.

【小题1】How does Matt Wei and his colleagues’ research differ from previous ones?
A.It centers on observing European satellites.
B.It adopts comprehensive data on a global scale.
C.It demonstrates subsiding land and rising seas risk coastal regions.
D.It makes the connection between waves and the height of the ground.
【小题2】What can we know from the third paragraph?
A.Most Asian cities are sinking at the rate of five centimeters per year.
B.Dario Solano-Rojas is in charge of statistical analysis in the research.
C.Some cities are in a difficult situation with land sinking and sea rising.
D.Cities subsiding at largest rates account for one third of the analyzed cities.
【小题3】What is a cause of the subsidence according to the passage?
A.Launching satellites flying over the same part of the earth.B.Governments’ regulations on limiting water usage.
C.Rising seas flooding some coastal regions regularly.D.Pumping groundwater out for certain purposes.
【小题4】What is the author’s attitude toward slowing down subsidence?
A.Optimistic.B.Skeptical.C.Tolerant.D.Conservative.
2022·黑龙江哈尔滨·模拟预测
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