#### Abstract

We explore the effects of a changing terrestrial biosphere on the atmospheric residence time of CO_{2} using three simple ocean carbon cycle models and a model of global terrestrial carbon cycling. We find differences in model behavior associated with the assumption of an active terrestrial biosphere (forest regrowth) and significant differences if we assume a donor-dependent flux from the atmosphere to the terrestrial component (e.g., a hypothetical terrestrial fertilization flux). To avoid numerical difficulties associated with treating the atmospheric CO_{2} decay (relaxation) curve as being well approximated by a weighted sum of exponential functions, we define the single half-life as the time it takes for a model atmosphere to relax from its present-day value half way to its equilibrium pCO_{2} value. This scenario-based approach also avoids the use of unit pulse (Dirac Delta) functions which can prove troublesome or unrealistic in the context of a terrestrial fertilization assumption. We also discuss some of the numerical problems associated with a conventional lifetime calculation which is based on an exponential model. We connect our analysis of the residence time of CO_{2} and the concept of single half-life to the residence time calculations which are based on using weighted sums of exponentials. We note that the single half-life concept focuses upon the early decline of CO_{2}under a cutoff/decay scenario. If one assumes a terrestrial biosphere with a fertilization flux, then our best estimate is that the single half-life for excess CO_{2} lies within the range of 19 to 49 years, with a reasonable average being 31 years. If we assume only regrowth, then the average value for the single half-life for excess CO_{2} increases to 72 years, and if we remove the terrestrial component completely, then it increases further to 92 years.

#### Publication Date

3-1994

#### Journal Title

Global Biogeochemical Cycles

#### Publisher

Wiley

#### Digital Object Identifier (DOI)

10.1029/93GB03392

#### Document Type

Article

#### Recommended Citation

Moore III, B., and B. H. Braswell (1994), The lifetime of excess atmospheric carbon dioxide, Global Biogeochem. Cycles, 8(1), 23–38, doi:10.1029/93GB03392.

#### Rights

Copyright 1994 by the American Geophysical Union.