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  3. EARTH’S ENERGY CYCLE (1) To understand most of the processes at work on Earth, it is useful to envisage interactions within

EARTH’S ENERGY CYCLE (1) To understand most of the processes at work on Earth, it is useful to envisage interactions within

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问题                                                 EARTH’S ENERGY CYCLE
    (1) To understand most of the processes at work on Earth, it is useful to envisage interactions within the Earth system as a series of interrelated cycles. One of these is the energy cycle, which encompasses the great "engines"—the external and internal energy sources—that drive the Earth system and all its cycles. We can think of Earth’s energy cycle as a "budget”: energy may be added to or subtracted from the budget and may be transferred from one storage place to another, but overall the additions and subtractions and transfers must balance each other. If a balance did not exist, Earth would either heat up or cool down until a balance was reached.
    (2) The total amount of energy flowing into Earth’s energy budget is more than 174,000 terawatts (or 174,000 x 1012 watts). This quantity completely dwarfs the 10 terawatts of energy that humans use per year. There are three main sources from which energy flows into the Earth system.
    (3) Incoming short-wavelength solar radiation overwhelmingly dominates the flow of energy in Earth’s energy budget, accounting for about 99.986 percent of the total. An estimated 174,000 terawatts of solar radiation is intercepted by Earth. Some of this vast influx powers the winds, rainfall, ocean currents, waves, and other processes in the hydrologic (or water) cycle. Some is used for photosynthesis and is temporarily stored in the biosphere in the form of plant and animal life. When plants die and are buried, some of the solar energy is stored in rocks, when we burn coal, oil, or natural gas, we release stored solar energy.
    (4) The second most powerful source of energy, at 23 terawatts or 0.013 percent of the total, is geothermal energy, Earth’s internal heat energy. Geothermal energy eventually finds its way to Earth’s surface, primarily via volcanic pathways. It drives the rock cycle and is therefore the source of the energy that uplifts mountains, causes earthquakes and volcanic eruptions, and generally shapes the face of the Earth.
    (5) The smallest source of energy for Earth is the kinetic (motion) energy of Earth’s rotation. The Moon’s gravitational pull lifts a tidal bulge in the ocean; as Earth rotates, the tidal bulge runs into the coastlines of continents and islands, causing high tides. The force of the tidal bulge piling up against landmasses acts as a very slow brake, actually causing Earth’s rate of rotation to decrease slightly. The transfer of tidal energy accounts for approximately 3 terawatts, or 0.002 percent of the tidal energy budget.
    (6) Earth loses energy from the cycle in two main ways: reflection, and degradation and reradiation. About 40 percent of incoming solar radiation is simply reflected, unchanged, back into space by the clouds, the sea, and other surfaces. [A] For any planetary body, the percentage of incoming radiation that is reflected is called the "albedo" . [B] Each different material has a characteristic reflectivity. [C] For example, ice is more reflectant than rocks or pavement; water is more highly reflectant than vegetation; and forested land reflects light differently than agricultural land. [D] Thus, if large expanses of land are converted from forest to plowed land, or from forest to city, the actual reflectivity of Earth’s surface, and hence its albedo, may be altered. Any change in albedo will, of course, have an effect on Earth’s energy budget.
    (7) The portion of incoming solar energy that is not reflected back into space, along with tidal and geothermal energy, is absorbed by materials at Earth’s surface, in particular the atmosphere and hydrosphere. This energy undergoes a series of irreversible degradations in which it is transferred from one reservoir to another and converted from one form to another. The energy that is absorbed, utilized, transferred, and degraded eventually ends up as heat, in which form it is reradiated back into space as long-wavelength (infrared) radiation. Weather patterns are a manifestation of energy transfer and degradation.
Why does the author include information about the ’energy that humans use per year" in the discussion?
选项 A、To call into question the idea that humans can use up all the energy available in Earth’s energy budget
B、To provide a comparison that establishes how huge the amount of energy flowing into Earth’s energy budget
C、To explain why there must be more than one source of energy for the Earth system
D、To argue that the use of energy by humans amounts to such a small part of Earth’s energy budget that it cannot have significant effects
答案B
解析 本题询问作者讨论“人类每年使用的能源量”的目的，属于修辞目的题。第2段第1句提到，流入地球能量循环的总能量超过174,000大瓦；接着第2句说，这个数量完全超过了人类每年对10大瓦能量的需求。可见，作者将这两个数字进行对比是为了反衬出流入地球能量循环的能量总数之多，故选B项“为了比较出流入地球能量循环的能量数目之多”。A项“为了质疑‘人类可以用尽地球能量循环中可获得的所有能量’这个观点”，作者在第2段并未提到这个观点。C项“为了解释地球系统中为什么一定要有一种以上的能量来源”，文中未解释这个原因。D项“人类对能源的使用只占地球能量循环的一小部分，不会产生重大影响”，作者在第2段中也没提到人类对能源的使用所产生的影响。
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