With widespread attention and national debate focused on the physical, biological and societal implications of increasing atmospheric concentrations of CO2, robust assessments of the response of Earth's climate to fluctuations in atmospheric CO2 are in high demand. Information about the climatic effects of past fluctuations in atmospheric CO2 concentrations can provide an important perspective on the likely climate consequences of current increases in CO2. Calcium carbonate, widely used for estimating atmospheric CO2, forms in soil and its carbon isotope composition (the relative abundance of 12CO2 and 13CO2) reflects that of the CO2 in soil air spaces, which is a mixture of atmospheric CO2 and CO2 released by respiring soil organisms. Previous reconstructions of atmospheric CO2 using carbon isotope data from soil carbonates assumed that carbonate formation occurred during relatively high CO2 concentrations during the growing season yet these estimates (panel A - black line) differed from all other methods (panel A - green line).
Recently, scientists from the University of New Mexico working at the Sevilleta LTER and other sites have refined our understanding of the prevailing soil CO2 concentrations during carbonate formation (Breecker et al, 2010). This work revealed that soil CO2 concentrations are substantially lower than previously assumed. This advance has enabled a reevaluation of carbonate carbon isotope composition as a proxy for past atmospheric CO2 concentrations, producing a result that agrees much more closely with the other proxies (panel B).
This research is important because the re-interpreted carbonate record, in close agreement with other proxies of atmospheric CO2, shows that CO2 concentrations during past greenhouse climates were on the order of those expected to occur within the coming century. Previous analyses had suggested that these greenhouse climate conditions occurred under much higher, and more variable, atmospheric CO2 concentrations than those supported by this work.