Fire emissions from C3 and C4 vegetation and their influence on interannual variability of atmospheric CO2 and δ13CO2 -: art. no. GB2019

[ 1] Measurements of atmospheric trace gases provide evidence that fire emissions increased during the 1997/1998 El Nino event and these emissions contributed substantially to global CO2, CO, CH4, and delta(13) CO2 anomalies. Interpretation and effective use of these atmospheric observations to assess changes in the global carbon cycle requires an understanding of the amount of biomass consumed during fires, the molar ratios of emitted trace gases, and the carbon isotope ratio of emissions. Here we used satellite data of burned area, a map of C-4 canopy cover, and a global biogeochemical model to quantitatively estimate contributions of C-3 and C-4 vegetation to fire emissions during 1997 - 2001. We found that although C-4 grasses contributed to 31% of global mean emissions over this period, they accounted for only 24% of the interannual emissions anomalies. Much of the drought and increase in fire emissions during the 1997/1998 El Nino occurred in tropical regions dominated by C-3 vegetation. As a result, the delta(13) CO2 of the global fire emissions anomaly was depleted (-23.9 parts per thousand), and explained approximately 27% of the observed atmospheric decrease in delta(13) CO2 between mid-1997 and the end of 1998 ( and 61% of the observed variance in delta(13) CO2 during 1997 - 2001). Using fire emissions that were optimized in an atmospheric CO inversion, fires explained approximately 57% of the observed atmospheric delta(13) CO2 decrease between mid-1997 and the end of 1998 ( and 72% of the variance in delta(13) CO2 during 1997 - 2001). The severe drought in tropical forests during the 1997/ 1998 El Nino appeared to allow humans to ignite fires in forested areas that were normally too moist to burn. Adjacent C-4 grasses ( in woodlands and moist savannas) also burned, but emissions were limited, in part, by aboveground biomass levels that were 2 orders of magnitude smaller than C-3 biomass levels. Reduced fuel availability in some C-4 ecosystems may have led to a negative feedback on emissions.