One of the major science stories in the late 1990's was the growing concern that Earth's climate is getting warmer. From the mid-1800's to 2000, the average temperature of the Earth's surface increased from 15 °C (59 °F) to 15.6 °C (60 °F)—its highest level in recorded history. If this warming trend continues, researchers warned, it could drastically alter the face of the planet. For example, the rising temperatures could begin to melt the icecaps of Greenland and Antarctica, flooding coastal regions around the world.
Although researchers in 2000 were still unable to fully explain what was causing the temperature increase, most climatologists (scientists who study climate) placed much of the blame on human activities, such as the burning of fossil fuels (coal, oil, and natural gas) and the clearing of forests for farmland or housing. On the other hand, a smaller number of scientists argued that though human-related factors have indeed increased the amounts of heat-trapping “greenhouse gases” such as carbon dioxide in the atmosphere, the increase had made no measurable difference in the Earth's climate. Those scientists said that the warming trend may be part of a normal cycle of change in the climate system. They cited studies of past climates indicating that the type of change now occurring is actually nothing new to the planet. In fact, a growing body of evidence from the study of climates long ago suggested that a stable climate may be the exception rather than the rule. In June 1999, a group of scientists led by French researcher Jean-Robert Petit reported that what we consider Earth's “normal” climate has been the norm for only about the past 11,500 years.