The Interhemispheric Difference Of Atmospheric CO₂: Implications For Carbon Sources And Sinks

During the past50years, over90%of fossil fuel （FF） CO₂emissions havebeen emitted into the Northern Hemisphere （NH） and then redistributedworldwide through interhemispheric mixing on a timescale of1-1.5years. Inresponse, the atmospheric CO₂mixing ratio in the NH is greater than that in theSouthern Hemisphere （SH）, and has increased faster than that in the SH. As theFF emissions increased, the interhemispheric difference of CO₂mixing ratios(IHD_CO2) also increased. Previous studies indicated that the regression slopebetween the IHD_CO2and FF CO₂emissions was rather constant at about0.5ppm/Pg C yr^-1during1957-2003. In this study, we found that the averageregression slopes between the IHD_CO2（defined as the differences between eachsite and SPO） and interhemispheric difference of FF emissions （IHDE） for16sites in the NH decreased from0.69±0.12ppm/Pg C yr^-1during1982-1991to0.37±0.06ppm/Pg C yr^-1during1996-2008. The larger difference is found insummer and autumn.We further examined three possible reasons which can cause the decreaseof regression slope:1) the change in the spatial distribution of FF emissions,2)the change in interhemispheric exchange time (τ_ex),3) the change in hemisphericnet carbon sinks. The change in the spatial distribution of FF emissions drivenby fast increasing Asian emissions during1996-2008may explain the slopechange for the three sites located north of60oN. A30-yr SF₆simulation withtime-varying meteorology and constant emissions suggests no significantdifference in the decadal average and seasonal variation of interhemisphericexchange time between the two periods. Based on the hemispheric net carbonfluxes derived from a two-box model, we attributed75%of the regression slopedecrease for sites located at0-45oN to the acceleration of net carbon sinkincrease in the NH and25%to the weakening of net carbon sink increase in theSH during1996-2008. The growth rate of net carbon sink in the NH hasincreased by about a factor of three from0.028±0.023[mean±2σ] Pg C/yr²during1982-1991to0.093±0.033Pg C/yr²during1996-2008, exceeding the percentage increase in the growth rate of IHDEbetween the two periods （45%）.The growth rate of net carbon sink in the SH has reduced62%from0.058±0.018Pg C/yr²during1982-1991to0.022±0.012Pg C/yr²during1996-2008. Theincreasing trend of net carbon sink in the NH during1996-2008in this study isconsistent with the studies on regional land and ocean sinks, but a magnitude aslarge as0.093±0.033Pg C/yr²needs further investigation. The increasing trendof net carbon sink in the SH during1996-2008in this study （0.022±0.012PgC/yr²） contradicts with some studies on both land and ocean sinks whichestimated a decline. Therefore, further analysis on the locations of the changingsinks is clearly needed.