ABSTRACT
The water-rich boundary region at the interface of Earth’s surface with the atmosphere, a paper-thin skin compared to the dimensions of Earth or its atmosphere, is the biosphere where life exists. The biosphere includes soil on which plants grow, a small bit of the atmosphere into which trees extend and in which birds fl y, the oceans, and various other bodies of water. Although the numbers and kinds of organisms decrease very rapidly with distance above Earth’s surface, the atmosphere as a whole, extending many kilometers upward, is essential for life as a source of oxygen, medium for water transport, blanket to retain heat by absorbing outgoing infrared radiation, and protective fi lter for high-energy ultraviolet radiation. Indeed, were it not for the ultraviolet-absorbing layer of ozone in the stratosphere, life on Earth could not exist in its present form.
