Interannual growth rate variations of atmospheric CO₂ and its δ¹³C, H₂, CH₄, and CO between 1992 and 1999 linked to biomass burning

Abstract

High‐precision, multispecies measurements of flask air samples since 1992 from CSIRO's global sampling network reveal strong correlation among interannual growth rate variations of CO 2 and its δ 13 C, H 2 , CH 4 , and CO. We show that a major fraction of the variability is consistent with two emission pulses coinciding with large biomass burning events in 1994/1995 and 1997/1998 in tropical and boreal regions, and observations of unusually high levels of combustion products in the overlying troposphere at these times. Implied pulse strengths and multispecies emission ratios are not consistent with any other single process, but do not exclude possible contributions from covarying processes that are linked through climatic forcing. Comparison of CO 2 with its δ 13 C indicates that most of the CO 2 variation is from terrestrial exchange, but does not distinguish forcing by biomass burning from imbalance in photosynthesis/respiration of terrestrial ecosystems. Partitioning of terrestrial CO 2 fluxes is constrained by H 2 , CH 4 , and CO, all of which are products of biomass burning but which have no direct link to net respiration of CO 2 . While CO is a strong indicator of biomass burning, its short lifetime prevents it from usefully constraining the magnitude of CO 2 emissions. If the H 2 and CH 4 variations were dominated by biomass burning, they would imply associated carbon emissions in excess of mean annual levels of other years, of 0.6–3.5 and 0.8–3.7 Pg C for 1994/1995 and 1997/1998, respectively. The large range in emission estimates mainly reflects uncertainty in H 2 /CO 2 and CH 4 /CO 2 emission ratios of fires in these years.