*In the IPCC report (Houghton et al.,1990), the present-day observed concentration of CO₂ is listed as 353 ppmv, and the present observed rate of change (increase) as 1.8 ppmv/yr. Dividing one by the other yields about 200 yrs. The lower limit of 50 yrs was probably introduced to reflect the very large uncertainty associated with the magnitude of the assumed biosphere sink for CO₂. Thus this atmospheric life-time value reflects atmospheric concentration divided by the net annual change in CO₂ concentration, rather than the total fluxes of CO₂ into or out of the atmosphere due to photosynthesis, respiration and oceanic gas exchange. It is thus based on quite different assumptions than used for most residence time calculations presented in this course.