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.