Previous studies have indicated that the regression slope between the interhemispheric difference (IHD) of CO2 mixing ratios and fossil fuel (FF) CO2 emissions was rather constant at about 0.5 ppm/Pg C yr(1 during 1957Á2003. In this study, we found that the average regression slopes between the IHD of CO2 mixing ratios and IHD of FF emissions for 16 sites in the Northern Hemisphere (NH) decreased from 0.6990.12 ppm/Pg C yr(1 during 1982Á1991 to 0.3790.06 ppm/Pg C yr(1 during 1996Á2008 (IHD of CO2 defined as the differences between each site and the South Pole, SPO). The largest difference was found in summer and autumn. The change in the spatial distribution of FF emissions driven by fast increasing Asian emissions may explain the slope change at three sites located north of 608N but not at the other sites. A 30-yr SF6 simulation with time-varying meteorology and constant emissions suggests no significant difference in the decadal average and seasonal variation of interhemispheric exchange time (tex) between the two periods. Based on the hemispheric net carbon fluxes derived from a two-box model, we attributed 75% of the regression slope decrease at NH sites south of 608N to the acceleration of net carbon sink increase in the NH and 25% to the weakening of net carbon sink increase in the SH during 1996Á2008. The growth rate of net carbon sink in the NH has increased by a factor of about three from 0.02890.023 [mean92s] Pg C yr(2 during 1982Á1991 to 0.09390.033 Pg C yr(2 during 1996Á2008, exceeding the percentage increase in the growth rate of IHD of FF emissions between the two periods (45%). The growth rate of net carbon sink in the SH has reduced 62% from 0.05890.018 Pg C yr(2 during 1982Á1991 to 0.02290.012 Pg C yr(2 during 1996Á2008.