2021年5月22日托福阅读真题+题目+答案：The Faint Young Sun Paradox

2021年5月22日托福阅读真题+题目+答案：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.

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