Abstract

Abstract. The high-resolution CO2 record from Law Dome ice core reveals that atmospheric CO2 concentration stalled during the 1940s (so-called CO2 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 CO2 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 CO2 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 CO2 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 CO2 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 CO2 cannot be confirmed nor discarded, as TRENDY models do not reproduce the expected concurrent strong decrease in terrestrial uptake. Nevertheless, this would further increase the mismatch between observed and modelled CO2 growth rate during the CO2 plateau epoch. Tests performed using the OSCAR (v2.2) model indicate that changes in land use not correctly accounted for during the period (coinciding with drastic socioeconomic changes during the Second World War) could contribute to the additional sink required. Thus, the previously proposed ocean hypothesis for the 1940s plateau cannot be confirmed by independent data. Further efforts are required to reduce uncertainty in the different terms of the carbon budget during the first half of the 20th century and to better understand the long-term variability of the ocean and terrestrial CO2 sinks.

Highlights

  • Study of the long-term variability in atmospheric composition from air trapped in polar ice has improved our understanding of processes and feedbacks between climate and the carbon cycle on decadal to millennial scales and allows us to evaluate the magnitude of human impact on the Earth’s atmosphere

  • Double-deconvolution analyses have attributed this sink to the ocean, conceivably as a response to the very strong El Niño event in 1940– 1942. This explanation is questionable, as recent ocean CO2 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 CO2 due to the offsetting terrestrial response

  • Given that the estimates of ocean and biospheric fluxes from Joos et al (1999), OJ and BJ, were calculated using a previous version of the CO2 record used here, they are expected to reproduce the observed variations in CO2, as given by the general agreement between observations (AGR) and reconstructed (AGRJ) shown in Fig. 2

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Summary

Introduction

Study of the long-term variability in atmospheric composition from air trapped in polar ice has improved our understanding of processes and feedbacks between climate and the carbon cycle on decadal to millennial scales and allows us to evaluate the magnitude of human impact on the Earth’s atmosphere. Direct measurements of the difference between the partial pressure gradient of CO2 between sea water and the overlying air ( pCO2) have been available since the early 1970s (Takahashi et al, 2009). These measurements enabled the study of variability in sinks and sources of CO2 at seasonal to interannual timescales. Most of these time series are still short, hampering the study of variability on scales longer than a few decades. As direct high-precision CO2 measurements are only available for the later decades of the 20th century, ice-core records remain a valuable source of information about atmospheric CO2 variability and trends during earlier periods

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