GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 11, NO. 4, PAGES 507-533, DECEMBER 1997 Carbon 13 exchanges between the atmosphere and biosphere I. Fung, •': C. B. Field, 3 J. A. Berry, 3 M. V. Thompson, 3'4 J. T. Randerson, 3,4 C. M. Malmstr6m, 3'4 P.M. Vitousek, 4 G. James Collatz, • P. J. Sellers, 6 D. A. Randall, z A. S. Denning, z F. Badeck, 8 and J. John 9 Abstract. We present a detailed investigation of the gross •2C and 13C exchanges between the atmosphere and biosphere and their influence on the 6•3C variations in the atmosphere. The photosynthetic discrimination A against •3C is derived from a biophysical model coupled to a general circulation model [Sellers et al., 1996a], where stomatal conductance and carbon assimilation are determined simultaneously with the ambient climate. The 6•3C of the respired carbon is calculated by a biogeochemical model [Potter et al., 1993; Randerson et al., 1996] as the sum of the contributions from compartments with varying ages. The global flux-weighted mean photosynthetic discrimination is 12-16%0, which is lower than previous estimates. Factors that lower the discrimination are reduced stomatal conductance and C4 photosynthesis. The decreasing atmospheric 6•3C causes an isotopic disequilibrium between the outgoing and incoming fluxes; the disequilibrium is •0.33% o for 1988. The disequilibrium is higher than previous estimates because it accounts for the lifetime of trees and for the ages rather than turnover times of the biospheric pools. The atmospheric 6•3C signature resulting from the biospheric fluxes is investigated using a three-dimensional atmospheric tracer model. The isotopic disequilibrium alone produces a hemispheric difference of •0.02% o in atmospheric •13C, comparable to the signal from a hypothetical carbon sink of 0.5 Gt C yr -• into the midlatitude northern hemisphere biosphere. However, the rectifier effect, due to the seasonal covariation of CO2 fluxes and height of the atmospheric boundary layer, yields a background 6•3C gradient of the opposite sign. These effects nearly cancel thus favoring a stronger net biospheric uptake than without the background CO2 gradient. Our analysis of the globally averaged carbon budget for the decade of the 1980s indicates that the biospheric uptake of fossil fuel CO2 is likely to be greater than the oceanic uptake; the relative proportions of the sinks cannot be uniquely determined using •2C and •3C alone. The land-ocean sink partitioning requires, in addition, information about the land use source, isotopic disequilibrium associated with gross oceanic exchanges, as well as the fractions of C3 and C4 vegetation involved in the biospheric uptake. • NASA Goddard Institute for Space Studies, New York. 2School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia, Canada. 3Department of Plant Biology, Carnegie Institution of Washing- ton, Stanford, California. 4 Department of Biological Sciences, Stanford University, Stan- ford, California. s NASA Goddard Space Flight Center, Greenbelt, Maryland. 6NASA Johnson Space Flight Center, Houston, Texas. 7Department of Atmospheric Sciences, Colorado State Univer- sity, Fort Collins. 8Laboratoire d'Ecologie Vegetale, Universite de Parris Sud XI, Orsay, France. 9Department of Applied Physics, Columbia University, New York. Copyright 1997 by the American Geophysical Union. Paper number 97GB01751. 0886-6236/97/97GB-01751 $12.00 1. Introduction Since the beginning of the industrial era, the combustion of fossil fuels has released a total of •250 Gt (1 Gt -- 1012 kg) of carbon into the atmosphere. Over the same period, land use modification in the middle latitudes, and more re- cently in the tropics, has released another • 100 Gt C. The • 150 Gt C increase in CO2 in the atmosphere, from •280 parts per million by volume (ppmv) in 1800 to 351 ppmv in 1988, is equivalent to •60% of the fossil fuel release or •40% of the total anthropogenic CO2 source. Balancing the carbon budget requires both the oceans and terrestrial bio- sphere to have acted as repositories, or sinks, for the anthro- pogenic CO2. Direct measurements of CO2 reservoir sizes and of CO2 fluxes into and out of the oceans and terrestrial biosphere are sparse, and the methodologies for extrapolat- ing laboratory or site measurements to the globe are under considerable debate. Hence estimates of the global strengths 507508 FUNG ET AL.: CARBON 13 EXCHANGES BETWEEN THE ATMOSPHERE AND BIOSPHERE of the terrestrial and oceanic sinks have not been established with certainty. CO2 is nearly but not completely mixed in the atmo- sphere. The mixing time of CO2 and other inert trace gases in the atmosphere is about 3 months within a hemisphere and about 1 year between the hemispheres. Therefore in- formation about broad patterns of CO2 exchanges between the atmosphere and different carbon reservoirs can be ex- tracted from the geographic and temporal variations of car- bon dioxide in the atmosphere with the aid of atmospheric transport models [e.g., Keeling et al., 1989a; Tans et al., 1990; Enting et al., 1993, 1995; Ciais et al., 1995a,b]. Over 90% of the fossil fuel emission is from the northern hemi- sphere, and most of the emission from land use modifica- tion is from the tropics. If all the anthropogenic CO2 re- mained airborne, this would yield a hemispheric gradient that is larger than that observed (Figure 1). Fossil fuel CO2 (o) :::::::::::::::::::::::::: .......... 0 •'*--:" ' ........ , , • • • , • , ß 0 -0.5 0.0 0.5 1.0 sin(latitude) 0.10 -0.00 -0.10 -0.20 i i 1 i::?:......-:•g!ii!:ig:i'.:i::!:::i:i!:i::-• ....... •---• • 8 "':"2:' '::' "- - -/:'F......-'>:• - '• ":"' '"'-?':•"•' '""":"--"•- --":•7•:: o -0.30 .... • .... • .... • .... - .0 -0.5 0.0 0.5 .0 sin(latitude) Figure 1. Latitudinal profiles of (a) CO2 and (b) 6:3C, rel- ative to the values at the south pole. The asterisks represent observed values at the National Ocean and Atmospheric Ad- ministration (NOAA) monitoring sites. The diamonds are the modeled values at the same sites if all the fossil fuel and land use CO2 remained airborne. The solid curves are the polynomial curve fits to the observed data, and the dotted- dashed curves are the polynomial curve fits to the modeled fossil fuel and land use contributions. The differences