Where Did the Carbon Go?
Over the last several hundred thousand years, atmospheric carbon dioxide (CO2) levels decreased when Earth’s glacial ice sheets were large, and increased when the ice sheets were small. The CO2 and ice-sheet changes occurred in conjunction with one another. The ice sheets grew slowly over many tens of thousands of years, but melted in less than 10,000 years. CO2 concentrations gradually fell as the ice sheets grew, and rose quickly when the ice melted. When ice sheets were at their maximum during the cold glacial periods, the atmosphere held only about 420 billion tons of carbon (compared to 600 billion tons during interglaciations, the periods between glacial periods), so about 180 billion tons of carbon must have been transferred somewhere else. But where?
We know that this carbon did not go into the glacial-age forests,because the northern ice sheets covered vast areas of Canada and Scandinavia where forests exist today. Elsewhere, glacial-age forests were generally smaller in extent and the vegetation was less dense than today, because the climate was colder and drier,and because lower CO2 values in the atmosphere reduced photosynthesis rates and the amount of vegetation. Also, lower vegetation cover naturally led to lower carbon storage in the underlying soils of most regions. The combined effect of reduced forest vegetation and reduced soil carbon adds roughly another530 billion tons of carbon to the total that must have been stored somewhere else.
We also know that the carbon did not move into the surface ocean. Because the upper ocean layers exchange CO2 with the overlying atmosphere over years to decades, the average amount of carbon in the global surface ocean does not stray far from the concentration in the atmosphere. Given this close link, the carbon concentration in the surface ocean during the last time when ice sheets were large must also have been lower than it was during interglaciations, by about 300 billion tons. Combined with the deficits in the atmosphere (180 billion tons) and land vegetation(530 billion tons), the amount of “missing carbon” totals just over1.000 billion tons.
So where on Earth did 1.000 billion tons of carbon go? The major reservoir left-the deep ocean, a layer below and distinct from the surface layer-holds more than 60 times the amount of carbon in the atmosphere. Several kinds of evidence suggest that the extra carbon that had been held in the atmosphere, the vegetation, and the surface ocean during warm interglacial climates moved into the deep ocean as glaciations developed, and then returned to the surface reservoirs once climate warmed and ice sheets melted.
Scientists are exploring several mechanisms by which carbon could move into and out of the deep ocean during glacial cycles,and two ideas seem the most promising. One possibility is that phytoplankton, small organisms with shells that live in ocean surface waters, were sending more carbon down to the deep ocean during glacial times. Like all forms of life, these floating organisms carry organic carbon in the soft tissue of their bodies.When they die and their shells sink, some of this organic carbon is carried down into the deep ocean. The strength of this process can vary through glacial cycles, because changes in climate produce different wind patterns that drive different water circulation patterns in the upper ocean layers, such as an increase in upwelling (rising)of nutrient-rich water from below, a change that promotes phytoplankton population growth. Also, windy glacial climates may have delivered more iron-rich dust from the land to the sea, which can act as a vitamin supplement” that fertilizes surface waters and enhances phytoplankton growth, leading to increased amounts of sinking phytoplankton.
Another possible mechanism for varying the amount of carbon stored in the deep ocean is a change in deep-ocean circulation especially in the Atlantic Ocean. Deep water forms in polar or near-polar regions, where cold winds make surface waters colder and saltier, causing them to sink. This process carries carbon down into the deep ocean, and the rate of formation of deep water in polar regions varies in response to changes in climate (primarily the temperature and salinity of ocean water). As a result, the amount of carbon carried down can increase during colder glacial climates, as compared to warm interglacial climates.
1
Over the last several hundred thousand years, atmospheric carbon dioxide (CO2) levels decreased when Earth’s glacial ice sheets were large, and increased when the ice sheets were small. The CO2 and ice-sheet changes occurred in conjunction with one another. The ice sheets grew slowly over many tens of thousands of years, but melted in less than 10,000 years. CO2 concentrations gradually fell as the ice sheets grew, and rose quickly when the ice melted. When ice sheets were at their maximum during the cold glacial periods, the atmosphere held only about 420 billion tons of carbon (compared to 600 billion tons during interglaciations, the periods between glacial periods), so about 180 billion tons of carbon must have been transferred somewhere else. But where?
Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
Factual Information Questions事实信息题
AAbout 180 billion tons of carbon must have been released into the atmosphere to account for the temperature difference between the cold glacial maximums and interglaciations.
BAbout 180 billion tons of carbon were held in ice sheets during cold glacial maximums, so when the carbon was released during interglaciations, there was a total of 600 billion tons of carbon in the atmosphere.
CBecause about 180 billion tons of carbon were transferred somewhere else during the cold glacial periods, ice sheets decreased from their maximums
DBecause the atmosphere held about 180 billion fewer tons of carbon during glacial maximums than it did during interglaciations, 180 billion tons of carbon must have gone somewhere else.
2
Over the last several hundred thousand years, atmospheric carbon dioxide (CO2) levels decreased when Earth’s glacial ice sheets were large, and increased when the ice sheets were small. The CO2 and ice-sheet changes occurred in conjunction with one another. The ice sheets grew slowly over many tens of thousands of years, but melted in less than 10,000 years. CO2 concentrations gradually fell as the ice sheets grew, and rose quickly when the ice melted. When ice sheets were at their maximum during the cold glacial periods, the atmosphere held only about 420 billion tons of carbon (compared to 600 billion tons during interglaciations, the periods between glacial periods), so about 180 billion tons of carbon must have been transferred somewhere else. But where?
According to paragraph 1, over the last several hundred thousand years, atmospheric CO2 levels
Factual Information Questions事实信息题
Aincreased as ice sheets became larger
Bchanged much more quickly than ice sheets changed
Cdecreased more slowly than they increased
Dremained high for a period of less than 10,000 years
3
We know that this carbon did not go into the glacial-age forests,because the northern ice sheets covered vast areas of Canada and Scandinavia where forests exist today. Elsewhere, glacial-age forests were generally smaller in extent and the vegetation was less dense than today, because the climate was colder and drier,and because lower CO2 values in the atmosphere reduced photosynthesis rates and the amount of vegetation. Also, lower vegetation cover naturally led to lower carbon storage in the underlying soils of most regions. The combined effect of reduced forest vegetation and reduced soil carbon adds roughly another530 billion tons of carbon to the total that must have been stored somewhere else.
It can be inferred from paragraph 2 that, unlike during glacial times,today large amounts of carbon are stored in
Inference Questions推理题
Acold, dry areas
Bareas with large forests
Cice sheets that cover northern areas
Dareas where vegetation is less dense today than it was in the past
4
We also know that the carbon did not move into the surface ocean. Because the upper ocean layers exchange CO2 with the overlying atmosphere over years to decades, the average amount of carbon in the global surface ocean does not stray far from the concentration in the atmosphere. Given this close link, the carbon concentration in the surface ocean during the last time when ice sheets were large must also have been lower than it was during interglaciations, by about 300 billion tons. Combined with the deficits in the atmosphere (180 billion tons) and land vegetation(530 billion tons), the amount of “missing carbon” totals just over1.000 billion tons.
The word “deficits” in the passage is closest in meaning to
Vocabulary Questions词汇题
Aconcentrations
Bestimates
Cshortages
Dproportions
5
We also know that the carbon did not move into the surface ocean. Because the upper ocean layers exchange CO2 with the overlying atmosphere over years to decades, the average amount of carbon in the global surface ocean does not stray far from the concentration in the atmosphere. Given this close link, the carbon concentration in the surface ocean during the last time when ice sheets were large must also have been lower than it was during interglaciations, by about 300 billion tons. Combined with the deficits in the atmosphere (180 billion tons) and land vegetation(530 billion tons), the amount of “missing carbon” totals just over1.000 billion tons.
According to paragraph 3, why is it thought that carbon levels must have been lower in the surface ocean during ice ages than during interglaciations?
Factual Information Questions事实信息题
AIce sheets that extended over the ocean during glacial periods would have prevented the surface ocean from absorbing as much CO2 as it does during interglaciations.
BChanges in carbon levels in the surface ocean closely follow those in the atmosphere, and it is known that carbon levels were lower in the atmosphere.
CThe amount of carbon in the surface ocean decreases when theCO2 in the overlying atmosphere increases.
DDuring glacial periods, CO2 becomes concentrated in the overlying atmosphere as it transfers from the surface ocean to the atmosphere.
6
Scientists are exploring several mechanisms by which carbon could move into and out of the deep ocean during glacial cycles,and two ideas seem the most promising. One possibility is that phytoplankton, small organisms with shells that live in ocean surface waters, were sending more carbon down to the deep ocean during glacial times. Like all forms of life, these floating organisms carry organic carbon in the soft tissue of their bodies.When they die and their shells sink, some of this organic carbon is carried down into the deep ocean. The strength of this process can vary through glacial cycles, because changes in climate produce different wind patterns that drive different water circulation patterns in the upper ocean layers, such as an increase in upwelling (rising)of nutrient-rich water from below, a change that promotes phytoplankton population growth. Also, windy glacial climates may have delivered more iron-rich dust from the land to the sea, which can act as a vitamin supplement” that fertilizes surface waters and enhances phytoplankton growth, leading to increased amounts of sinking phytoplankton.
Why does the author include the information that “changes in climate produce different wind patterns that drive different water circulation patterns in the upper ocean layers”?
Rhetorical Purpose Questions修辞目的题
ATo explain where the carbon carried in the soft tissues of phytoplankton comes from
BTo partly explain why differences occur in the rate at which organic carbon is delivered to the deep ocean
CTo argue that changes in wind patterns affect phytoplankton growth more than fertilizers do
DTo support the claim that glacial climates were windy
7
Scientists are exploring several mechanisms by which carbon could move into and out of the deep ocean during glacial cycles,and two ideas seem the most promising. One possibility is that phytoplankton, small organisms with shells that live in ocean surface waters, were sending more carbon down to the deep ocean during glacial times. Like all forms of life, these floating organisms carry organic carbon in the soft tissue of their bodies.When they die and their shells sink, some of this organic carbon is carried down into the deep ocean. The strength of this process can vary through glacial cycles, because changes in climate produce different wind patterns that drive different water circulation patterns in the upper ocean layers, such as an increase in upwelling (rising)of nutrient-rich water from below, a change that promotes phytoplankton population growth. Also, windy glacial climates may have delivered more iron-rich dust from the land to the sea, which can act as a vitamin supplement” that fertilizes surface waters and enhances phytoplankton growth, leading to increased amounts of sinking phytoplankton.
Based on paragraph 5, which of the following correctly describes the process by which phytoplankton transfer carbon to the deep ocean?
Factual Information Questions事实信息题
ACirculation changes in ocean waters carry the carbon in the bodies of dead phytoplankton into the deep ocean.
BDuring certain points in glacial cycles, phytoplankton survive by migrating into deep water, where their bodies release carbon.
CThe shells of dead phytoplankton drop down into the deep ocean, carrying carbon with them
DSurface winds force phytoplankton into the deep ocean, where the carbon in their bodies is released when they die.
8
Scientists are exploring several mechanisms by which carbon could move into and out of the deep ocean during glacial cycles,and two ideas seem the most promising. One possibility is that phytoplankton, small organisms with shells that live in ocean surface waters, were sending more carbon down to the deep ocean during glacial times. Like all forms of life, these floating organisms carry organic carbon in the soft tissue of their bodies.When they die and their shells sink, some of this organic carbon is carried down into the deep ocean. The strength of this process can vary through glacial cycles, because changes in climate produce different wind patterns that drive different water circulation patterns in the upper ocean layers, such as an increase in upwelling (rising)of nutrient-rich water from below, a change that promotes phytoplankton population growth. Also, windy glacial climates may have delivered more iron-rich dust from the land to the sea, which can act as a vitamin supplement” that fertilizes surface waters and enhances phytoplankton growth, leading to increased amounts of sinking phytoplankton.
According to paragraph 5, which TWO of the following are ways that wind can lead to an increase in phytoplankton during glacial times?
Factual Information Questions事实信息题
Select 2 answers
AWind can cause more water from the lower ocean to rise to the surface.
BWind can decrease harmful particles in surface waters by creating new water circulation patterns.
CWind can provide a vitamin supplement to the deep ocean by causing nutrients to sink.
DWind can carry iron from the land to the ocean’s surface water.
9
We know that this carbon did not go into the glacial-age forests,because the northern ice sheets covered vast areas of Canada and Scandinavia where forests exist today. [■] Elsewhere, glacial-age forests were generally smaller in extent and the vegetation was less dense than today, because the climate was colder and drier,and because lower CO2 values in the atmosphere reduced photosynthesis rates and the amount of vegetation. [■] Also, lower vegetation cover naturally led to lower carbon storage in the underlying soils of most regions. [■] The combined effect of reduced forest vegetation and reduced soil carbon adds roughly another530 billion tons of carbon to the total that must have been stored somewhere else. [■]
We also know that the carbon did not move into the surface ocean. Because the upper ocean layers exchange CO2 with the overlying atmosphere over years to decades, the average amount of carbon in the global surface ocean does not stray far from the concentration in the atmosphere. Given this close link, the carbon concentration in the surface ocean during the last time when ice sheets were large must also have been lower than it was during interglaciations, by about 300 billion tons. Combined with the deficits in the atmosphere (180 billion tons) and land vegetation(530 billion tons), the amount of “missing carbon” totals just over1.000 billion tons.
Look at the four squares [■] that indicate where the following sentence could be added to the passage
This is because the growth and death of vegetation is the mechanism by which carbon comes to be stored in soil.Insert Text Questions句子插入题
Where would the sentence best fit？Click on a square sentence to the passage.
10
During the last several hundred thousand years, atmospheric co2 levels and the size of ice sheets have varied periodically.
Prose Summary Questions概要小结题
Select 3 answers
AChanges in the size of Earth’s glacial ice sheets occurred very slowly over the last 100,000 years, but the levels of carbon in the atmosphere rapidly decreased after the ice sheets melted
BThe atmosphere, glacial-age forests, and surface ocean all held considerably less carbon during glacial periods than they did during interglaciations.
CCO2 is exchanged between the surface ocean and the deep ocean during upwellings, which occur primarily in polar regions.
DDuring glacial periods, the cold, dry climate killed off much vegetation, resulting in more carbon being stored in soil than in vegetation.
EA significant amount of carbon that had been stored in other reservoirs during warm periods seems to have been stored in the deep ocean during glacial periods
FAir and water circulation patterns during glacial periods may have increased levels of carbon in the deep ocean by encouraging phytoplankton growth and affecting the production of deep water.
答案：
