[A-CG32] Global Carbon Cycle Observation and Analysis

[ACG32-06] Ocean uptake and land use change emissions suppressed atmospheric CO₂ growth in the 2015/16 El Niño

*近藤 雅征¹、Landschützer Peter ²、Iida Yousuke ³、Houghton Richard ⁴、Sitch Stephen ⁵ (1.千葉大学　環境リモートセンシング研究センター、2.Max Planck Institute for Meteorology、3.Global Environment and Marine Department, Japan Meteorological Agency、4.Woods Hole Research Center、5.University of Exeter)

キーワード：Carbon Cycle、El Nino、Attribution analysis

Large variability in El Niño-Southern Oscillation (ENSO) induces anomalous CO₂ fluxes both in biosphere and hydrosphere. In the negative phase of ENSO (i.e., El Niño), dryer climate over lower latitudes decreases productivity of land vegetation and increases vulnerability to biomass burning. At the same time, suppressed upwelling of carbon-rich water near the Pacific South America reduces outgassing of oceanic CO₂ to atmosphere. In years of a strong El Niño, anomalous land and ocean CO₂ fluxes account for a large part of interannual variability of the total net CO₂ flux of the Earth surface. Thus, anomalously large atmospheric CO₂ growth rates co-occur with periods of El Niño events.

Biomass burning is believed to be the dominant component of variability in atmospheric CO₂ growth rate during El Niño. The 1997/98 El Niño induced the most significant anomalous CO₂ release on record, largely due to forest and peat fires in Southeast Asia. Likewise, a massive fire haze over Southeast Asia was found during the recent 2015/16 El Niño. Despite the seemingly large impact of biomass burning in the two events, the 2015/16 El Niño accompanied substantially less atmospheric CO₂ growth compared with that in the 1997/98 El Niño. It turns out that CO₂ emission from biomass burning in 2015 was substantially less than that in 1997 because precipitation before the peak of fire emission (Sept-Oct) shortened the drought length of 2015.

However, it is still arguable that the insignificant biomass burning is the dominate cause behind no obvious signs of large CO₂ release in the 2015/16 El Niño. In the recent decades, both biospheric and hydrospheric CO₂ fluxes has shown trends towards a net sink of CO₂, which suggest a possibility of that increasing CO₂ uptake has suppressed the atmospheric CO₂ growth in the 2015/16 El Niño. To evaluate this possibility, a synthesis of land-ocean CO₂ flux over the period of the two El Niño events is necessary. Here, we explore mechanisms behind differences in the atmospheric CO₂ growth in the 1997/98 and 2015/16 El Niño. The aim of this study is to fully characterize the causes for insignificant atmospheric CO₂ growth in 2015/16 through analyzing types (anomaly/trend) of relative contributions of land-ocean fluxes to atmospheric CO₂ growth between the two El Niño events. Through the analysis, we discuss about components that would likely continue to mitigate atmospheric CO₂ growth in future El Niño.