Carbon dioxide (CO2) is the principal man-made greenhouse gas and the primary atmospheric component of the global carbon cycle. Precise ground-based measurements of CO2 made since the late 1950s indicate that the atmospheric CO2 concentration has increased from ~310 to over 380 parts per million (ppm) over this period [1]. Interestingly, comparisons of these data with CO2 emission rates from fossil fuel combustion, biomass burning, and other human activities indicate that only about half of the CO2 that has been emitted into the atmosphere during this period has remained there.