RESERVOIR TIMESCALES FOR ANTHROPOGENIC CO2 IN THE ATMOSPHERE

Non-steady state timescales are complicated and their application to s pecific geophysical systems requires a common theoretical foundation. We first extend reservoir theory by quantifying the difference between turnover time and transit time (or residence time) for time-dependent systems under any mixing conditions. We explicitly demonstrate the er rors which result from assuming these timescales are equal, which is o nly true at steady state. We also derive a new response function which allows the calculation of age distributions and timescales for well-m ixed reservoirs away from steady state, and differentiate between time scales based on gross and net fluxes. These theoretical results are pa rticularly important to tracer-calibrated ''box models'' currently use d to study the carbon cycle, which usually approximate reservoirs as w ell-mixed. We then apply the results to the important case of anthropo genic CO2 in the atmosphere, since timescales describing its behavior are commonly used but ambiguously defined. All relevant timescales, in cluding lifetime, transit time, and adjustment time, are precisely def ined and calculated from data and models. Apparent discrepancies betwe en the current, empirically determined turnover time of 30-60 years an d longer model-derived estimates of expected lifetime and adjustment t ime are explained within this theoretical framework. We also discuss t he results in light of policy issues related to global warming, in par ticular since any comparisons of the ''lifetimes'' of different greenh ouse gases (CO2, CH4, N2O, CFC's etc.) must use a consistent definitio n to be meaningful.