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Anthropogenic and Natural Influences

The Science of Global Climate Change

The photographs from the Apollo missions show earth glowing in the stillness of space like a blue-white opal on black velvet. Cool and beautiful, it hurries along in the Sun’s gravitational embrace. The earth is our home, our whole wide world.

Our enfolding blanket of air, our atmosphere, is both the physical condition for human community and its most compelling symbol. We all breathe the same air. Guarding the integrity of the atmosphere—without which complex life could not have evolved on this planet—seems like common sense. Yet a broad consensus of modern science is that human activity is beginning to alter the earth’s atmospheric characteristics in serious, perhaps profound ways. For the past century, researchers have been gathering and verifying data that reveal an increase in the global average temperature. Until recently, scientists could not say with great confidence whether or not this phenomenon was in any way the result of human activity or entirely the result of natural changes over time.

To deal with the difficulty of making precise measurements and arriving at definite conclusions, the World Meteorological Organization and the United Nations Environment Programme established the Intergovernmental Panel on Climate Change (IPCC) to seek a clear explanation of the causes and possible impacts of this global climate change. 14Because of the large number of scientists involved in the IPCC and its process of consultation, its reports are considered widely as offering the most authoritative scientific perspectives on the issue. IPCC’s findings have met with general—but because of remaining uncertainties, not complete—agreement within the wider scientific community.

In 1996, the IPCC issued its Second Assessment Reports, which summarized the current state of knowledge. The first of these reports concluded that ” the balance of evidence suggests that there is a discernible human influence on global climate.” 15 The Third Assessment Reports, approved in early 2001, found even stronger evidence and concluded, ” most of the observed warming over the last 50 years is likely to have been due to the [human-induced] increase in greenhouse gas concentrations” (italics added). 16

The IPCC offers convincing evidence that there exists if not a clear and present danger then a clear and future one, and that coming changes will affect all aspects of the environment and societal well-being. Based on measurements taken over both land and sea, the global average surface-air temperature has increased by about one degree Fahrenheit since 1860, building up as the Industrial Revolution was hitting full stride. While this is hardly a frightening increase for a particular geographic location, the temperature change is global in extent, so one must read it against the background of the earth’s average temperature during historic times. According to IPCC, the rate and duration of warming in the twentieth century appears to be the largest in the last one thousand years. The twentieth century also experienced precipitation increases in mid- and high-northern latitudes; drier conditions in the subtropics; decreases in snow cover, mountain glaciers, and Arctic sea ice; and a rise of four to eight inches in mean sea level