Abstract: By comparing the global carbon cycle of the IPCC assessment reports in 1990, 1996, 2001 and 2007, it is found that despite the estimated sizes of the main carbon reservoirs and the fluxes between them in four reports were similar, the estimated carbon fluxes between surface and intermediate-deep sea water were quite different. The δ¹³C budget was used to test the reasonable range of these fluxes. It shows that the IPCC assessment reports in 1996 and 2007 have overestimated, whereas those in 1990 and 2001 underestimated the fluxes between surface and intermediate-deep sea water.

Key words: IPCC, CO₂ budgets, δ¹³C, isotope fractionation