Two of the most important questions being asked by scientists studying global climate are: Is Earth's temperature rising unnaturally and, if so, what is causing this increase? The answers may lie in temperature data from as far back as three hundred thousand years ago. This prehistoric information is valuable because it allows scientists to compare recent observations to warming and cooling trends of the distant past. Such comparisons provide a basis from which to argue that an observed warming trend in the last century is in or out of synch with the natural rhythms of climatic change.

Climate studies focused on the last hundred years show that Earth's temperature has risen about one degree Fahrenheit since the early 1900s. Other studies reaching back much further in time suggest that global climates have seldom been stable for long. Ice samples collected as part of the North Greenland Ice Core Project, which provide a snapshot of temperatures from the last three hundred thousand years, tell a dramatic story. It seems Earth has been subject to recurring ten-thousand-year warm periods followed by ninety-thousand-year cold spells -- with most of the transitions between warm and cold periods being extremely abrupt.

The most difficult question and, not surprisingly, the one that is the most politically charged is whether this century's global temperature changes are consistent with this natural climatic flux. The answer to this question would indicate, indirectly at least, whether humans are responsible for global warming. Based on available data and computer-generated models, many scientists have concluded that the changes we're seeing now are indeed related to an increase in the industrial emissions of greenhouse gases like carbon dioxide. To what extent human actions may ultimately impact global climate remains the focus of intensive scientific investigation.