One day, at the beginning of the geologic epoch called the Pleistocene, Earth's skies turned an ominous gray. The north wind rose and began churning the frigid air into keening gusts. Soon the first snowflakes descended, signaling the onset of what the nature writer Loren Eiseley termed "the angry winter." It barely stopped snowing for the next 2 million years, during which the planet was held in thrall by massive ice sheets that covered all of Antarctica, most of Europe, large expanses of North and South America, and lesser parts of Asia. The only sounds were the thunder of great avalanches and the gnashing of the advancing glaciers. The most recent of the great ice ages had arrived, triggered perhaps by the completion of the Milky Way's latest rotation, which occurs only once every 300 million years.
Scientists tell us that we remain citizens of the Ice Age. And many of them believe that it is only a matter of time before Earth's surface disappears once more beneath the blinding snows and mile-thick glaciers. When this will come to pass, no one can say for certain.
At present, climatologists are preoccupied by a more immediate concern than the next revolution of the galactic wheel. For much of the last century, Earth and its atmosphere have been heating up, a process that most, though not all, scientists believe is due to the massive consumption of fossil fuels -- coal, oil, natural gas -- triggered by the industrial revolution. What is more, global warming is accelerating. The 1970s were warmer than the 1960s; the 1980s were warmer than the 1970s; and the 1990s have been warmer still.
Global warming is not a newly discovered phenomenon.