Re-evaluating the 1940s CO 2 plateau

• Published in: Biogeosciences
• Main Authors: A. Bastos, P. Ciais, J. Barichivich, L. Bopp, V. Brovkin, T. Gasser, S. Peng, J. Pongratz, N. Viovy, C. M. Trudinger
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2016
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Law Dome; ice core
• Online Access: https://doi.org/10.5194/bg-13-4877-2016; https://doaj.org/article/9a2146ec20394fbcbcc46df2e9e33079

The high-resolution CO 2 record from Law Dome ice core reveals that atmospheric CO 2 concentration stalled during the 1940s (so-called CO 2 plateau). Since the fossil-fuel emissions did not decrease during the period, this stalling implies the persistence of a strong sink, perhaps sustained for as long as a decade or more. Double-deconvolution analyses have attributed this sink to the ocean, conceivably as a response to the very strong El Niño event in 1940–1942. However, this explanation is questionable, as recent ocean CO 2 data indicate that the range of variability in the ocean sink has been rather modest in recent decades, and El Niño events have generally led to higher growth rates of atmospheric CO 2 due to the offsetting terrestrial response. Here, we use the most up-to-date information on the different terms of the carbon budget: fossil-fuel emissions, four estimates of land-use change (LUC) emissions, ocean uptake from two different reconstructions, and the terrestrial sink modelled by the TRENDY project to identify the most likely causes of the 1940s plateau. We find that they greatly overestimate atmospheric CO 2 growth rate during the plateau period, as well as in the 1960s, in spite of giving a plausible explanation for most of the 20th century carbon budget, especially from 1970 onwards. The mismatch between reconstructions and observations during the CO 2 plateau epoch of 1940–1950 ranges between 0.9 and 2.0 Pg C yr −1 , depending on the LUC dataset considered. This mismatch may be explained by (i) decadal variability in the ocean carbon sink not accounted for in the reconstructions we used, (ii) a further terrestrial sink currently missing in the estimates by land-surface models, or (iii) LUC processes not included in the current datasets. Ocean carbon models from CMIP5 indicate that natural variability in the ocean carbon sink could explain an additional 0.5 Pg C yr −1 uptake, but it is unlikely to be higher. The impact of the 1940–1942 El Niño on the observed stabilization of atmospheric CO 2 ...