Climate forecasting requires insight into the predictability of forcing factors that influence Earth’s climate. External forcing factors run the gamut in terms of predictability. The orbital variations of obliquity, eccentricity, and precession follow patterns set by the law of gravity that should continue indefinitely. Sunspot numbers oscillate with an 11-year period. In contrast, external factors such as comets crashing into Earth seem to be exceedingly rare, random occurrences and therefore are often ignored in forecasts or treated as a very slight possibility.
Internal forcing factors also run the gamut. Albedo, the extent to which Earth’s surface reflects incoming solar electromagnetic radiation, declines as global warming melts snow or ice packs and raises the sea level. Albedo increases with deforestation and desertification and with greater cloud cover. GCMs incorporate the interactions between albedo and these changes. On the other hand, future geologic events— orogeny, epeirogeny, and volcanism—while always in the realm of possibilities, are highly uncertain or infrequent, and so most GCMs either do not include such factors or treat them as a small probability.
The future of greenhouse gas concentrations in the atmosphere proves difficult to anticipate because of its strong dependence on human activities and the unpredictability of such activities. The Intergovernmental Panel on Climate Change (IPCC), an organization established by the United Nations Environment Programme and the World Meteorological Organization in 1988, developed 40 scenarios about future concentrations of greenhouse gases. These scenarios fall into four different narrative storylines about possible demographic, social, economic, technological, and environmental circumstances that will drive greenhouse gas emissions.
One storyline (A1) assumes slowing human population growth but rapid economic development based on either fossil fuels (A1FI), alternative fuels (A1T), or a mix of both (A1B). Another storyline (A2) emphasizes regional autonomy (instead of international cooperation) and sustained human population growth, fragmented economic development, and slow technological adoption. A third storyline (B1) assumes a worldwide shift to a service and information economy with international agreements that promote general adoption of clean and resource-efficient technologies. A fourth storyline (B2) focuses on local solutions to economic, social, and environmental sustainability, with intermediate economic growth and diverse technological change. All of these scenarios depend upon an understanding of the global cycles for greenhouse gases with particular sources and sinks for each one.
The IPCC considers that any of these emission scenarios is equally likely. The scenarios are not predictions, but assumptions that GCMs require as input. They serve as “if–then” statements: If human activities follow scenario W (which includes factors such as amounts and types of economic activity, adoption of evolving technologies, and rates of human population growth), then a GCM predicts that the amount of greenhouse gas emissions will be X (in terms of amount of carbon equivalents per year), atmospheric concentrations of the greenhouse gases will be Y (in terms of carbon dioxide equivalents), and the climate will be Z (in terms of temperature, precipitation, storms, etc.).
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
©2010 Sinauer Associates and UC Regents