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System of Units
Our discussion of magnitudes of carbon dioxide emissions on a global
scale requires a system of units that we're not accustomed to using for
everyday
purposes. Figure 6 defines the prefixes, such as kilo- and
mega- that we normally use, and other terms that may be unfamiliar. You
may find it useful to return to this table to refresh your memory on the meaning
of tera-, peta-, or exa- which will be used extensively in future units. I have given a couple of conversions to help
conceptualize the magnitudes of these numbers. One gigaton is the equivalent
of one petagram, and one gigaton of water is the equivalent of about one cubic
kilometer.
Trace gas concentrations will be given in parts per million by volume (ppmv), parts per billion by volume (ppbv), or parts per trillion by volume (pptv). One ppmv means one molecule out of a million. Occasionally, concentrations are reported in parts per million by mass, but such usage will be avoided in this course to retain uniformity.
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