Fire emissions from C₃ and C₄ vegetation and their influence on interannual variability of atmospheric CO₂ and δ¹³CO₂

Abstract

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 CO 2 , CO, CH 4 , and δ 13 CO 2 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 Niño occurred in tropical regions dominated by C 3 vegetation. As a result, the δ 13 CO 2 of the global fire emissions anomaly was depleted (−23.9‰), and explained approximately 27% of the observed atmospheric decrease in δ 13 CO 2 between mid‐1997 and the end of 1998 (and 61% of the observed variance in δ 13 CO 2 during 1997–2001). Using fire emissions that were optimized in an atmospheric CO inversion, fires explained approximately 57% of the observed atmospheric δ 13 CO 2 decrease between mid‐1997 and the end of 1998 (and 72% of the variance in δ 13 CO 2 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 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 C4 ecosystems may have led to a negative feedback on emissions.

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Affiliated Institutions

• University of California, Irvine US
• University of Maryland, College Park US
• University of California, Santa Barbara US
• Cooperative Institute for Research in Environmental Sciences US
• Vrije Universiteit Amsterdam NL
• Science Systems and Applications (United States) US
• University of Colorado Boulder US
• Duke University US
• Goddard Space Flight Center US
• Institute of Arctic and Alpine Research US
• Earth System Science Interdisciplinary Center US

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