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With the recognition that climate changes are inevitable as a result of the observed increase in greenhouse gas concentrations in the atmosphere, there is a great need for better observations and analyses to document the past and current climate and how it has changed with time. The need is for both regional and global evidence, to the extent that it exists, for many climatological variables.

While there are many uncertainties in any future projections, there are also some certainties. The levels of several greenhouse gases are increasing and will continue to do so. Carbon dioxide is the best known greenhouse gas. For example, observations at Mauna Loa show that since 1958 the carbon dioxide concentration has increased from 315 to approximately 350 ppm (Figure 5.1), and bubbles of air in ice cores reveal that the preindustrial carbon dioxide levels of the last century were around 280 ppm. So concentrations have already increased by 25 percent, largely because of man's burning of fossil fuels and clearing of forests. The other greenhouse gases of note are methane, nitrous oxide, and the chlorofluorocarbons (CFCs); the concentrations of all these gases

FIGURE 5.1 Monthly average carbon dioxide from Mauna Loa, Hawaii in parts per million by volume.

SOURCE: Reprinted, by permission, from Keeling et al. (1989).  Copyright © 1989 by American Geophysical Union.

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