Land use change and carbon exchange in the tropics: II. Estimates for the entire region: Comment

Estimated annual releases of carbon of 1.2 to 1.5 BMT in 1980 from tropical vegetation and soils due to land use change are presented by Detwiler and others ,(1985). These estimates are compared with the oceanic uptake of CO 2 released from forests and soils as derived from tree ring ~3C records. Detwiler and others (1985) reported that on the basis of new models of oceanic carbon cycle published by Peng and others (1983), the ocean could have accommodated at least 1.2 BMT of forest-soil carbon in 1980. They reached the conclusion that "the global carbon budget may be balanced if there is no significant release from nontropical ecosystems due to land use change and all mature ecosystems are in collective equilibrium with the atmosphere." They misinterpreted our article (Peng and others 1983) and, as a result, their conclusion concerning a balanced global carbon budget cannot be supported. In our article, a time history of the releases of forestsoil CO z, but not of oceanic uptake of this CO 2, is calculated for the period between 1800 and 1980. These release estimates are based on the tree ring xsC records summarized by Freyer and Belacy (1983). The estimated release of forest-soil carbon in 1980, derived from the model calculations involving oceanic uptake of atmospheric ~3C and the response of terrestrial ecosystems to the input of low 13C-CO2, is about 1.2 BMT C/year. The actual amount of forest-soil carbon taken up by the ocean during the whole period is not given in the article. In fact, how much of the forest-soil carbon released into the atmosphere will be taken up by the ocean has not been calculated. A comparison of the results from Detwiler and others (1985) and Peng and others (1983) can best be described as indicating a similar estimate of forest-soil carbon release for 1980. It has nothing to do with a balanced global carbon budget, and as Figure 13 of our article showed, the ocean cannot absorb the excess forest-soil CO z during the period of Mauna Loa record unless we unrealistically manipulate ocean model parameters. Developments since the publication of our article have warranted the downward revision of our estimate of forest-soil releases, weakening the correspondence with the Detwiler and others (1985) results and making their global balance calculations even more untenable. Since our estimate of the time history of forest-soil carbon release depends on tree-ring 13C records, the changes in tree ring tsC data will certainly change in estimates of carbon release history. Freyer (1986) has revised the global mean tree-ring 13C curve on the basis of additional new measurements published since his last summary in 1983 (Freyer and Belacy 1983). Deconvolution of the revised i~C record presents a very different time history of forest-soil carbon release (Peng 1985, Peng and Freyer 1986). Two maximum periods of carbon release were seen, one in 1860 and the other in 1910. For 1980, the estimate from this revised 13C data was about 0.5 BMT C/year. This value is less than half the estimate given by Detwiler and others (1985). A similar calculation of the transfer of biospheric carbon to the atmosphere has been performed by Stuiver and others (1984). Their deconvolution of the average 13C curve derived from six Pacific coastal trees indicates that the forest-soil reservoir became a carbon sink during the periods centered in 1910 and in 1960. The average forest-soil carbon release during the whole period between 1900 and 1975 is estimated to have been about 0.3 BMT C/year. In addition, Stuiver and others studied the relationships between xsC, ring area, and ring width and found a few trees showing significant correlation between ~3C and ring area. Many environmental factors could influence the isotopic composition of trees. Among those, variable isotope fractionation of the trees was the major cause of the variability in the 13C records. Use of the variation of 13C in tree-ring as an analogue of changes in atmospheric a3C has been considered to be questionable. A more reliable way of deconvolving the forest-soil carbon release history should come from an actual time history of atmospheric CO2 variations. Precise measurements of atmospheric CO 2 have been made continuously at Mauna Loa Observatory since 1958 (Bacastow and Keeling 1981). Also, the air bubbles trapped in polar ice offer the possibility of a way to reconstruct the history of atmospheric CO 2 variations before 1958. This can be done by extracting the air bubbles from the ice and measuring the CO 2 concentration in these bubbles, using very precise instruments. A recent article by Neftel and others (1985)