The Faint Young Sun Paradox
Paragraph 1：When the Sun first formed 4.6 billion years ago, it was about 30 percent less luminous than it is today. The Sun’s luminosity increased slowly at first and then more rapidly as the buildup of helium in its core continued. At present, the Sun is thought to be brightening by about one percent every hundred million years. By the time the Sun ends its lifetime as a normal star, about five billion years from now, it is expected to have brightened by a factor of two to three as compared to today.
1. Which of the following can be inferred from paragraph 1 about the early Sun？
O It contained less helium than it does today.
O It was larger in size than it is today.
O Its luminosity increased more rapidly than it does today.
O It was brighter than it will be near the end of its lifetime.
Paragraph 2：How would reduced solar luminosity have affected early Earth? If all other factors had remained constant, early Earth should have been colder than it is today. Indeed, calculations show that the entire ocean should have been ice-covered prior to two billion years ago. We know, however, that liquid water has existed on Earth’s surface for at least the last 3.8 billion years because sedimentary rocks (which form from sediments or deposits in liquid water) have been forming since that time. And organisms, which require liquid water to survive, have been around for at least 3.5 billion years. Early Earth could not have been a global ice ball, at least not during the time for which a geologic record is available.
2. Select the TWO answer choices that according to paragraph 2 are indications that the early Earth was NOT colder than it is today.
To receive credit, you must select TWO answers.
O The presence of organisms on early Earth
O The absence of sedimentary rocks on early Earth
O The reduction in solar luminosity after Earth formed
O The existence of water on early Earth’s surface
Paragraph 3：This apparent discrepancy is called the faint young Sun paradox. This paradox (seeming contradiction) can be understood only by considering the Earth system as a whole. The most likely explanation has to do with greenhouse gases, meaning gases such as water vapor, carbon dioxide, and chlorofluorocarbons (CFCs) that allow sunlight to enter Earth’s atmosphere but trap heat in the atmosphere when sunlight is reflected back from Earth. Evidently, the level of greenhouse gases in Earth’s primitive atmosphere was significantly higher than it is today. But why should this have been true, and why would greenhouse gas concentrations have declined as the Sun grew brighter? Does Earth’s climate system have some built-in stability mechanism that has kept the mean surface temperature within survivable limits?
3. Which of the following statements best defines the faint young Sun paradox described in the third paragraph？
O Lower solar energy did not result in colder temperatures on early Earth.
O The Sun was not as bright when it was first formed as it is today.
O More sunlight was reflected than absorbed by the atmosphere of early Earth.
O Higher solar luminosity resulted in lower levels of greenhouse gases.
4. 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.
O The most likely explanation of the faint young Sun paradox has to do with the reflection of the Sun’s heat by greenhouse gases such as water vapor, carbon dioxide, and CFCs.
O The faint young Sun paradox is most likely explained by the presence of greenhouse gases that prevent heat from the Sun from leaving Earth’s atmosphere.
O The faint young Sun paradox most likely has to do with gases such as water vapor, carbon dioxide, and CFCs that are trapped within Earth’s atmosphere.
O The most likely explanation of the faint young Sun paradox is that greenhouse gases in Earth’s atmosphere trap the heat from incoming sunlight before it can reach Earth.
5. Why does the author ask But why should this have been true, and why would greenhouse gas concentrations have declined as the Sun grew brighter?
O To cast doubt on the view that greenhouse gas levels in primitive Earth’s atmosphere were significantly higher than today
O To raise an objection to the view that the Sun’s brightness was related to the level of greenhouse gases
O To introduce the idea that Earth might have a built-in mechanism to keep its climate stable
O To suggest areas for research that could solve the faint young Sun paradox
Paragraph 4：James Lovelock, a British biochemist, and Lynn Margulis, an American biologist, have argued that life itself has been responsible for maintaining the stability of Earth’s climate. In the process of photosynthesis, organisms such as green plants use sunlight, carbon dioxide, and water to produce organic matter and oxygen. (Organic matter is the carbon-rich material of which organisms are composed.) Through photosynthesis, followed by carbon burial in sediments, Earth’s biota (plant and animal life) may have lowered atmospheric carbon dioxide levels at just the right rate to counteract the gradual increase in solar luminosity. Alternatively, the biota may have affected the rate at which atmospheric carbon dioxide is sequestered in carbonate rocks. Carbonate rocks form from reactions of carbon dioxide with elements (primarily calcium and magnesium) derived from other types of rocks. Lovelock and Margulis suggest that Earth has remained habitable precisely because it is in some sense alive.
6. In paragraph 4, the author mentions all of the following as processes that help to maintain the stability of Earth’s climate EXCEPT
O the use of carbon dioxide and water in photosynthesis
O the burying of carbon in sediments
O the formation of carbonate rocks
O the changing luminosity of the Sun
Paragraph 5：This theory of long-term climate stabilization is part of what Lovelock and Margulis called the Gaia hypothesis. In ancient Greek mythology, Gaia was the mother goddess of Earth. In its most basic form, the Gaia hypothesis states that Earth is a self-regulating system in which the biota play an integral role. Some proponents of this hypothesis further suggest that the biota manipulate their environment for their own benefit or even, by optimizing the conditions for life, for the benefit of all living things. Such assertions are difficult to justify. Lovelock himself is quick to point out that the biota cannot be expected to cope with all possible disturbances. As an example, we cannot assume that we can safely emit CFCs into the atmosphere because Gaia will somehow protect the stratospheric ozone layer. But it is clear that the Gaia hypothesis is correct at some level: organisms do play an important role in the overall functioning of the Earth system.
7. Why does the author mention ancient Greek mythology in the passage?
O To indicate the source of the name given by scientists to their hypothesis
O To emphasize the progress that scientists have made in understanding climate stability
O To recognize the importance of Greek myths in understanding long-term climate stabilization
O To identify a historical basis for believing that the Gaia hypothesis is correct
8. According to paragraph 5, some supporters of the Gaia hypothesis believe that
O the stratospheric ozone layer should be considered part of the Earth system
O organisms influence the environment for their own benefit
O organisms have only a minor impact on the Earth system
O CFCs are not as harmful to the atmosphere as many scientists believe
Paragraph 6：Some form of self-regulation must exist in order for Earth’s climate to remain stable over long time scales. Higher greenhouse gas concentrations in the past are the most likely solution to the faint young Sun paradox. But whether the biota are essential to the control mechanism remains controversial.
Paragraph 4：■James Lovelock, a British biochemist, and Lynn Margulis, an American biologist, have argued that life itself has been responsible for maintaining the stability of Earth’s climate. ■In the process of photosynthesis, organisms such as green plants use sunlight, carbon dioxide, and water to produce organic matter and oxygen. ■(Organic matter is the carbon-rich material of which organisms are composed.) Through photosynthesis, followed by carbon burial in sediments, Earth’s biota (plant and animal life) may have lowered atmospheric carbon dioxide levels at just the right rate to counteract the gradual increase in solar luminosity. ■Alternatively, the biota may have affected the rate at which atmospheric carbon dioxide is sequestered in carbonate rocks. Carbonate rocks form from reactions of carbon dioxide with elements (primarily calcium and magnesium) derived from other types of rocks. Lovelock and Margulis suggest that Earth has remained habitable precisely because it is in some sense alive.
9. Look at the four squares that indicate where the following sentence could be added to the passage.
They have proposed an explanation as to how this life-sustaining regulation of climate might occur.
Where would the sentence best fit？ Click on a square to add the sentence to the passage.
10. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selected THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points. Drag your choices to the spaces where they belong. To review the passage, click on View Text.
Answer Choices
O Organisms first arose on Earth about 2 billion years ago, when the ice-covered ocean was heated by the Sun and became liquid water.
O Early Earth should have been colder than it is today, but high levels of greenhouse gases trapped the Sun’s heat within the Earth’s atmosphere.
O As the Sun’s luminosity has increased, the level of greenhouse gases may have decreased, resulting in a stable climate that supports life.
O The Gaia hypothesis holds that Earth’s living organisms cause its climate to remain stable, although there is disagreement over the extent and nature of their role.
O Long-term climate stability on Earth will require an increase in the biota living on the planet, especially those organisms that engage in photosynthesis.
O Although certain effects of greenhouse gases, such as their impact on the stratospheric ozone layer, are unknown, it is clear that organisms are able to withstand such disturbances.
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