A two-dimensional global atmospheric diffusion simulation model is used to establish zonal and monthly average net surface fluxes of carbon dioxide (CO2) which are consistent with the variations in CO,. concentration observed at six monitoring stations distributed globally. These fluxes represent the zonally averaged net ecosystem production of primarily terrestrial biosphere. Annually, the global net ecosystem production during the photosynthetically more active season removed 6.8 x 1012 kg of carbon from the atmosphere, returning it during the less active winter months. This turnover represents about 14% of the annual continental net primary production of carbon and 0.8% of the total terrestrial biomass. During the growing season, net ecosystem production appears to be relatively independent of latitude in the northern hemisphere (-6 x 10 -9 kg carbon m -•-s-X). The model indicates that time correlations between the CO2 concentration at different altitudes of the northern hemisphere with the interhemispheric advection of air results in a net interhemispheric CO•. flux which becomes zero when the South Pole COe concentration is on average 0.92 ppmv above that at Mauna Loa, Hawaii. The influence of both the net and the gross atmospheric biospheric exchange of carbon on the atmospheric stable carbon isotope ratio is modeled. On time scales of a year the gross turnover of carbon has an insignificant effect, and all isotopic effects can be related to the net exchanges. It is argued that as with concentration, long-term trends in isotopic composition will be best observed in the southern hemisphere.It was argued that the seasonality of CO,. concentration is well known, at least for a small number of background locations, and given an adequate model of the atmospheric mixing, it should be possible to generate a set of global surface exchange rates consistent with the observations. In this paper we report Copyright ¸ !980 by the American Geophysical Union.Paper number 80C0348. 0148-0227/80/080C.034850 ! .00 preliminary attempts at such a simulation. The surface exchange rates generated are used to calculate the net ecosystem production of latitude zones for comparison with the net primary production of those zones. The significance of simultaneous observations of the stable carbon isotopic composition of atmospheric CO,. is discussed. MODELThe model used in this study has been developed for use in a number of projects concerned with the global distribution of atmospheric constituents [e.g., Hyson et al., !980], and it will be described in detail elsewhere. Briefly, it is a two-dimensional zonally averaged multiple-box model in which vertical and horizontal transport is assumed to occur via Fickian diffusion and advection. While the number of boxes in the

A two‐dimensional global atmospheric transport model is used to relate estimated air to surface exchanges of CO2 to spatial and temporal variations of atmospheric concentration of CO2. This serves to illustrate the gross features of the carbon cycle and the measurement precision required of the burgeoning global observational network if the latter is to contribute useful information to the quantitative description of the cycle. Calculations are based on the atmospheric model coupled to a fixed, or variable depth, oceanic mixed layer. Zonally averaged air‐sea fluxes of CO2 are estimated from a relationship between flux, partial pressure, temperature, and wind stress. The result is a time‐dependent (pre‐industrial to 1980) estimate of the meridional rates of net exchange of CO2 with the oceans. For example, it is estimated that, at present, the equatorial oceans release a net total of 1.3 Gt (Gt = 1012 kg) of carbon into the atmosphere annually, while high latitude oceans take up a net total of 4.4 Gt. Associated with the increasing release of fossil‐fuel derived CO2 into the northern hemisphere atmosphere, the model suggests that the interhemispheric difference in concentration of CO2 (high latitude north‐high latitude south) has changed from ∼‐1 ppmv preindustrially to ∼+1 ppmv in 1960 and to 4–5 ppmv at present. The 1960 distribution agrees with the limited observational data available for that time. Present day observations, although provisional, are in good agreement with model estimates of the annual mean global distribution of CO2. The influence of this changing interhemispheric distribution on the concept of the airborne fraction of CO2 is discussed. The sensitivity of the mean global distribution of CO2 to additional surface exchanges is demonstrated by modeling the hypothetical cases of a 1, 2, or 5 Gt yr−1 equatorial source (such as tropical deforestation) with a similar uptake by the temperate northern hemisphere forests. Such changes bring about a 20%, 40%, or 100% reduction, respectively, in the simulated interhemispheric concentration difference. Alternatively, the 1, 2, or 5 Gt yr−1 equatorial release is taken up by the global oceans. In this case, the south pole to equatorial concentration difference increases from 3 ppmv on average to 3.5, 4.0, and 5.5 ppmv, respectively. Taken in conjunction with presently available observations, which must be regarded as provisional, these results place constraints on the magnitude of any actual equatorial deforestation source. It is unlikely that such a source, combined with the sink, could amount to more than about 2 Gt yr−1. The difficulty of establishing more precisely the magnitude of such regional exchanges is discussed in light of the model limitations and required precision of the observational network.

Records of the annual variation of the atmospheric carbon dioxide concentration at Mauna Loa, Point Barrow, and Weathership P are examined for secular changes. The amplitude of the annual variation appears to have increased in recent years with a best estimate of the rate of change, based on the Mauna Loa data, of 0.45±0.42% yr−1. This change is discussed in terms of changes in biospheric respiration and photosynthesis and the use of fossil fuels. The analysis does not allow for the separation of several possible causes of amplitude change. However, if the change is interpreted as reflecting enhanced biospheric growth, the effect is equivalent to a 8% change in the net summer uptake of carbon over the years 1959–1978 and to a growth of the northern hemisphere seasonal biosphere of 0.5×1012 kg of carbon per year. Such a conclusion is consistent with recent inventory studies, which indicate that temperate zone forests have acted as a net sink of about 1012 kg of carbon per year in recent decades.

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