Measurements of atmospheric trace gases provide evidence that fire emissions increased during the 1997/1998 El Niño event and these emissions contributed substantially to global CO2, CO, CH4, and δ13CO2 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 C4 canopy cover, and a global biogeochemical model to quantitatively estimate contributions of C3 and C4 vegetation to fire emissions during 1997–2001. We found that although C4 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 Niño occurred in tropical regions dominated by C3 vegetation. As a result, the δ13CO2 of the global fire emissions anomaly was depleted (−23.9‰), and explained approximately 27% of the observed atmospheric decrease in δ13CO2 between mid-1997 and the end of 1998 (and 61% of the observed variance in δ13CO2 during 1997–2001). Using fire emissions that were optimized in an atmospheric CO inversion, fires explained approximately 57% of the observed atmospheric δ13CO2 decrease between mid-1997 and the end of 1998 (and 72% of the variance in δ13CO2 during 1997–2001). The severe drought in tropical forests during the 1997/1998 El Niño appeared to allow humans to ignite fires in forested areas that were normally too moist to burn. Adjacent C4 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 C3 biomass levels. Reduced fuel availability in some C4 ecosystems may have led to a negative feedback on emissions.