In the United States the science of climate change still remains a controversial issue. Pa

rt of the problem is that it is complex and hard to isolate its parts. We do know how vital it is to life. The atmosphere forms a gaseous, protective envelope around earth protecting the earth from the cold of space, from harmful ultraviolet light, and from all but the largest meteors. After traveling over 93 million miles, solar energy strikes the atmosphere and earth's surface, warming the planet and creating what is known as the "biosphere" which is the region of earth capable of sustaining life. Atmospheric circulation is one important reason that life on earth can exist at higher latitudes because equatorial heat is transported poleward, moderating the climate.

The atmosphere is not just a protector but also a great transporter. The transport of water vapor in the atmosphere is an important mechanism by which heat energy is redistributed poleward. Heat energy absorbed at the equator is deposited at the poles and the temperature gradient between these regions is reduced. The circulation of the atmosphere and the weather it generates is but one example of the many complex, interdependent events of nature. The web of life depends on the proper functioning of these natural mechanisms for its continued existence.

Some argue that climate change is too complicated to be attributed only to human activity. For example, fossil studies of radiolaria, a group of one-celled animals characterized by silicon-containing shells, have given scientists a fairly accurate account of climatic conditions in the distant past. Geologists found these 450, 000-year-old microorganisms preserved in cores of sediment taken beneath the floor of the Indian Ocean. They provide a record of cycles of climatic changes, determined by the alternating layers of species of warm and cold-preferring radiolaria. We now have confirmation that changes in the earth's orbital geometry caused the ice ages.

This does not let us off the hook because although we can hardly alter the basic forces at work that alter climate, even small contributions by humans can have dramatic effects. Given nature's interconnectedness, it is possible that the most serious threats have yet to be recognized.

The first paragraph of the text deals mainly with the earth atmosphere's ______.

A．sheltering effect

B．reviving effect

C．invigorating effect

D．cleansing effect