L. Hu, A.E. Andrews, K.W. Thoning, C. Sweeney, J.B. Miller, A.M. Michalak, E. Dlugokencky, P.P. Tans, Y.P. Shiga, M. Mountain, T. Nehrkorn, S.A. Montzka, K. McKain, J. Kofler, M. Trudeau, S.E. Michel, S.C. Biraud, M.L. Fischer, D.E.J. Worthy, B.H. Vaughn, J.W.C. White, V. Yadav, S. Basu, I.R. van der Velde
The terrestrial carbon sink plays an important role in regulating the amount of carbon dioxide in the atmosphere. Quantifying how regional carbon cycling responds to climatic events, such as El Niño, is important for understanding the overall effect of climate variability and climate change on an ecosystem's ability to take up carbon. In this study, we used atmospheric carbon dioxide observations to infer North American net ecosystem exchange, the amount of carbon exchanged between the atmosphere and terrestrial ecosystems. We found that there is more carbon uptake in North America during El Niño events than La Niña events because of increased water availability and more favorable temperatures. These findings enhance our understanding of the relationship between regional climate and carbon uptake, which can be a major source of uncertainty in future climate projections.
Figure: Variability of monthly anomalies in atmospheric carbon dioxide (B) and δ13CO2 observations (an indicator of carbon uptake by plants) (C) shows a broadly consistent response to El Niño events (A). Six-month running averages of monthly CO2 mole fraction and δ13CO2 anomalies (B and C) averaged across NOAA’s long-term flask air sampling sites over North America. The number of sites included to calculate the monthly average anomalies of CO2 and δ13CO2 is 7 to 12 for 1995–2003, 16 to 19 for 2004–2007, and 25 to 30 for 2008–2015. Gray shading indicates standard errors of the calculated 6-month running average anomalies. Light yellow shading indicates El Niño periods, whereas light blue indicates La Niña periods.
Long-term atmospheric CO2 mole fraction and δ13CO2 observations over North America document persistent responses to the El Niño–Southern Oscillation. We estimate these responses corresponded to 0.61 (0.45 to 0.79) PgC year−1 more North American carbon uptake during El Niño than during La Niña between 2007 and 2015, partially offsetting increases of net tropical biosphere-to-atmosphere carbon flux around El Niño. Anomalies in derived North American net ecosystem exchange (NEE) display strong but opposite correlations with surface air temperature between seasons, while their correlation with water availability was more constant throughout the year, such that water availability is the dominant control on annual NEE variability over North America. These results suggest that increased water availability and favorable temperature conditions (warmer spring and cooler summer) caused enhanced carbon uptake over North America near and during El Niño.
Hu, L., A.E. Andrews, K.W. Thoning, C. Sweeney, J.B. Miller, A.M. Michalak, E. Dlugokencky, P.P. Tans, Y.P. Shiga, M. Mountain, T. Nehrkorn, S.A. Montzka, K. McKain, J. Kofler, M. Trudeau, S.E. Michel, S.C. Biraud, M.L. Fischer, D.E.J. Worthy, B.H. Vaughn, J.W.C. White, V. Yadav, S. Basu, I.R. van der Velde (2019) "Enhanced North American carbon uptake associated with El Niño," Science Advances, 5 (6), eaaw0076, doi:10.1126/sciadv.aaw0076.