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Based on evidence from tree rings, pollen samples and other records, scientists have for a long time assumed that interglacials-warm interludes between ice ages-were as mild and uniform as the Holocene, the present interglacial, has been for all of its 8,000 to 10,000 years. But new research in Greenland has put this assumption into question.

Researchers on two teams, the Greenland Ice-Core Project (GRIP) and the Greenland Ice Sheet Project 2 (GISP2), have analyzed two different cylinders of ice, each about two miles in depth, pulled up from the Greenland ice sheet. Such ice cores trap gases, bits of dust, and other chemicals that were present in the snow that fell over Greenland for thousands of years and then became compressed into ice. By studying these components, scientists have obtained a detailed archive of many aspects of climate, including air temperatures, snowfall, and concentrations of greenhouses gases in the atmosphere.

Findings from the upper sections of the cores have confirmed what scientists already knew climate during the last ice age fluctuated rapidly. But scientists were astonished by findings from the lower sections of the GRIP core, which provided a close look at an interglacial period other than our own, the Eemian interglacial, a period that lasted from 135,000 to 115,000 years ago. Data from GRIP seem to indicate that the Eemian climate swung at least as wildly as the climate of ice age periods.

Researchers clues to the Eemian climate come from measurements of the ratios of two slightly different types of oxygen, isotopes oxygen-16 and oxygen-18, preserved in the GRIP core. These ratios register the fluctuations of air temperatures over the seasons and years. When the air was warm, vapor containing the heavier isotope, oxygen-18, condensed and formed precipitation, in the form of snow, more readily than did vapor containing oxygen-16. Thus, snow that fell during warmer periods contains proportionally more oxygen-18 than snow deposited during cold spells. Evidence of rapid climate shifts was also drawn from other sources, such as measurements of amounts of dust and calcium ions in the ice layers during cold periods: winds were strong, causing calcium-rich dust from loess deposits, which are composed of loose surface sediment, to blow across the ice sheet. Thus, differing amounts of dust in the layers also indicate changing climatic conditions.

However, finds from the lower section of GISP 2 do not confirm those of GRIP. The wild climate swings shown by GRIP in the last interglacial are not seen in the GISP2 core. According to a GISP 2 scientist, the weight of flowing glacial ice above has stressed the lower sections of both cores. This may have deformed the lower ice, disrupting its annual layers and thereby causing the discrepancy between the records. Still, some climatologists believe GRIPs record may be the more reliable of the two. It was drilled closer to a location called the ice divide, where stresses would have been lower, they say.
The passage is primarily concerned with
Which of the following describes research that is most clearly analogous to the testing done by GRIP scientists?
According to the passage, which of the following is the most accurate statement of what scientists believed, prior to the GRIP findings, about Earths climate?
The passage suggests that which of the following is most likely to have been true of the oxygen-16 and oxygen-18 isotopes found in the lower sections of the GRIP core?
One reason researchers have long believed that Mars never enjoyed an extensive period of warm and wet climate is that much of the surface not covered by wind-borne dust appear to be composed of unweathered material. If water flowed for an extended period, researchers reasoned, it should have altered and weathered the volcanic minerals, creating clays or other oxidized, hydrated phases (minerals that incorporate water molecules in their crystal structure) .

It turns out, though, that the scientists were not looking closely enough. New high-resolution mapping data and close-up surface studies have revealed clays and other hydrated minerals in many regions. The clay deposits are scattered all over, in ancient volcanic surfaces and heavily cratered highland regions, some of which have apparently been exposed by erosion only recently.
According to the passage, scientists are able to discover weathered material on the Martian surface because they have benefited from which of the following?
Scientific consensus is that humans first began to have a warming effect on Earths climate within the past century, after coal-burning factories, power plants, and motor vehicles began releasing carbon dioxide (CO2) and other greenhouse gases in significant quantities into the air.However, evidence suggests that human agricultural activities may have had such an effect much earlier:concentrations of CO2 started rising about 8000 years ago, even though natural trends indicate they should have been dropping; methane levels rose similarly about 3,000 years later. Without these rises, however, current temperatures in northern parts of North America and Europe would be cooler by three to four degrees Celsius-enough to inhibit agriculture-and an ice age would probably have begun several thousand years ago in northeastern Canada.
Which of the following best describes the function of the highlighted sentence?
The author mentions "natural trends" most likely in order to
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