While a lively debate in political circles and the press questions whether human activity is significantly warming the Earth, scientific evidence has been accumulating steadily in support of the idea. Much of this evidence, unobscured by special economic interests which sometimes cloud popular debate, is not at all ambiguous. With the exception of a minority (see Chapter 3), the human role in a rapid warming of the Earth has become nearly incontrovertible.
Kathy Maskell and Irving M. Mintzer, writing in the British medical journal Lancet, describe the carbon cycle’s natural balancing act:
Over the past 10,000 years, the concentration of CO2 and other greenhouse gases has remained fairly constant, and this represents a remarkable balancing act of nature. Every year natural processes on the land and in the oceans release to and remove from the atmosphere huge amounts of carbon, about 200 billion tons (gigatons) in each direction. Since the atmosphere contains about 700 gigatons of carbon, small changes in natural fluxes could easily produce large swings in atmospheric concentrations of CO2 and CH4 [methane]. Yet for ten millennia natural fluxes have remained in remarkably close balance. (Maskell and Mintzer 1993, 1027)
Human activities are disturbing this long-standing balance in a fundamental manner, adding to concentrations of several greenhouse gases, including, according to Maskell and Mintzer, “Water vapor, the principal greenhouse gas… [which] is expected to increase in response to higher temperatures that would further enhance the greenhouse effect” (Maskell and Mintzer 1993, 1027).
Scientists who study the future potential of human-induced warming also point to several other natural mechanisms which could cause the pace of change to accelerate, through “biotic feedbacks,” such as release of carbon dioxide and methane from permafrost and continental shelves in the oceans. The possibility of a “runaway” greenhouse effect by the year 2050 is raised in the literature,