Papers on 1940s dip in atmospheric CO2

This is a list of papers on 1940s dip in atmospheric CO₂ concentration. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.

Update (February 1, 2016): Bastos et al. (2016) added. Thanks to Cathy Trudinger for pointing it out.
Update: (February 1, 2016) Rubino et al. (2013) added.

Re-evaluating the 1940s CO₂ plateau – Bastos et al. (2016) [Full text]
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 2.0PgCyr⁻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 0.5PgCyr⁻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.
Citation: Bastos, A., Ciais, P., Barichivich, J., Bopp, L., Brovkin, V., Gasser, T., Peng, S., Pongratz, J., Viovy, N., and Trudinger, C. M.: Re-evaluating the 1940s CO2 plateau, Biogeosciences, 13, 4877-4897, https://doi.org/10.5194/bg-13-4877-2016, 2016.

A revised 1000 year atmospheric δ¹³C-CO₂ record from Law Dome and South Pole, Antarctica – Rubino et al. (2013) “We present new measurements of δ¹³C of CO₂ extracted from a high-resolution ice core from Law Dome (East Antarctica), together with firn measurements performed at Law Dome and South Pole, covering the last 150 years. Our analysis is motivated by the need to better understand the role and feedback of the carbon (C) cycle in climate change, by advances in measurement methods, and by apparent anomalies when comparing ice core and firn air δ¹³C records from Law Dome and South Pole. We demonstrate improved consistency between Law Dome ice, South Pole firn, and the Cape Grim (Tasmania) atmospheric δ¹³C data, providing evidence that our new record reliably extends direct atmospheric measurements back in time. We also show a revised version of early δ¹³C measurements covering the last 1000 years, with a mean preindustrial level of −6.50‰. Finally, we use a Kalman Filter Double Deconvolution to infer net natural CO₂ fluxes between atmosphere, ocean, and land, which cause small δ¹³C deviations from the predominant anthropogenically induced δ¹³C decrease. The main features found from the previous δ¹³C record are confirmed, including the ocean as the dominant cause for the 1940 A.D. CO₂ leveling. Our new record provides a solid basis for future investigation of the causes of decadal to centennial variations of the preindustrial atmospheric CO₂ concentration. Those causes are of potential significance for predicting future CO₂ levels and when attempting atmospheric verification of recent and future global carbon emission mitigation measures through Coupled Climate Carbon Cycle Models.” Rubino, M., et al. (2013), A revised 1000 year atmospheric δ13C-CO2 record from Law Dome and South Pole, Antarctica, J. Geophys. Res. Atmos., 118, 8482–8499, doi:10.1002/jgrd.50668. [Full text]

Climate effects on atmospheric carbon dioxide over the last century – Rafelski et al. (2009) “The buildup of atmospheric CO₂ since 1958 is surprisingly well explained by the simple premise that 57% of the industrial emissions (fossil fuel burning and cement manufacture) has remained airborne. This premise accounts well for the rise both before and after 1980 despite a decrease in the growth rate of fossil fuel CO₂ emissions, which occurred at that time, and by itself should have caused the airborne fraction to decrease. In contrast, the buildup prior to 1958 was not simply proportional to cumulative fossil fuel emissions, and notably included a period during the 1940s when CO₂ growth stalled despite continued fossil fuel emissions. Here we show that the constancy of the airborne fraction since 1958 can be in part explained by decadal variations in global land air temperature, which caused a warming-induced release of CO₂ from the land biosphere to the atmosphere. We also show that the 1940s plateau may be related to these decadal temperature variations. Furthermore, we show that there is a close connection between the phenomenology producing CO₂ variability on multidecadal and El Niño timescales.” Lauren Elmegreen Rafelski, Stephen C. Piper, Ralph F. Keeling, Tellus B, Volume 61, Issue 5, pages 718–731, November 2009, DOI: 10.1111/j.1600-0889.2009.00439.x.

Law Dome CO₂, CH₄ and N₂O ice core records extended to 2000 years BP – MacFarling Meure et al. (2006) “New measurements of atmospheric greenhouse gas concentrations in ice from Law Dome, Antarctica reproduce published Law Dome CO₂ and CH₄ records, extend them back to 2000 years BP, and include N₂O. They have very high air age resolution, data density and measurement precision. Firn air measurements span the past 65 years and overlap with the ice core and direct atmospheric observations. Major increases in CO₂, CH₄ and N₂O concentrations during the past 200 years followed a period of relative stability beforehand. Decadal variations during the industrial period include the stabilization of CO₂ and slowing of CH₄ and N₂O growth in the 1940s and 1950s. Variations of up to 10 ppm CO₂, 40 ppb CH₄ and 10 ppb N₂O occurred throughout the preindustrial period. Methane concentrations grew by 100 ppb from AD 0 to 1800, possibly due to early anthropogenic emissions.” MacFarling Meure, C., D. Etheridge, C. Trudinger, P. Steele, R. Langenfelds, T. van Ommen, A. Smith, and J. Elkins (2006), Geophys. Res. Lett., 33, L14810, doi:10.1029/2006GL026152.

Role of land cover changes for atmospheric CO2 increase and climate change during the last 150 years – Brovkin et al. (2004) Not mentioned in the abstract, but in the article they say: “The period between 1938 and 1950 is interesting because of an almost constant atmospheric CO₂ recorded at Law Dome, despite substantial fossil fuel emissions (about 1.3 PgCyr^-1). … We conclude that the stalling of atmospheric CO₂ during the 1940s was unlikely to have been caused by changes in land cover, although this might be one of the contributing factors. Other factors, like internal variability of the climate system, may also be responsible (Joos et al., 1999). Better statistics for land cover changes during the 1940s would help to clarify the role of land cover changes in this period.” Victor Brovkin, Stephen Sitch, Werner Von Bloh, Martin Claussen, Eva Bauer, Wolfgang Cramer, Global Change Biology, Volume 10, Issue 8, pages 1253–1266, August 2004, DOI: 10.1111/j.1365-2486.2004.00812.x. [Full text]

Kalman filter analysis of ice core data 2. Double deconvolution of CO₂ and δ¹³C measurements – Trudinger et al. (2002) “A new method for deconvolving ice core CO₂ and δ¹³CO₂ measurements to estimate net CO₂ uptake by the terrestrial biosphere and the oceans has been developed. The method, which uses the Kalman filter, incorporates statistical analysis into the calculation. This allows a more rigorous analysis of CO₂ variability than the usual deconvolution method. The Kalman filter method estimates uncertainties on the deduced fluxes as part of the calculation. The deconvolution method is applied to the Law Dome CO₂ and δ¹³C ice core record. The calculation suggests that natural variability in CO₂ fluxes may be as large as 1 GtC yr⁻¹ (GtC is gigatonnes carbon, 1 Gt = 10¹⁵ g) on the timescale of just less than a decade. The Law Dome CO₂ measurements show a slight decrease in CO₂ around the 1940s. Analysis with the carbon cycle model and a numerical model of firn processes suggests that about 3 GtC yr⁻¹ uptake (mostly oceanic) is required in the 1940s to match the ice core measurements. The estimates of variation in the terrestrial biospheric flux between 1950 and 1980 from the double deconvolution calculation are in very good agreement with an independent estimate of the global terrestrial flux from a climate-driven ecosystem model.” Trudinger, C. M., I. G. Enting, P. J. Rayner, and R. J. Francey (2002), J. Geophys. Res., 107(D20), 4423, doi:10.1029/2001JD001112.

Biotic Feedbacks in the Warming of the Earth – Woodwell et al. (1998) Not mentioned in the abstract, but in the article they say: “Except for the brief period 1935–1945 heat-trapping gases continued to accumulate in the atmosphere (Etheridge et al., 1996). The coincidence between the cooling and the brief stabilization of concentration around 1940 is, perhaps, significant.” G. M. Woodwell, F. T. Mackenzie, R. A. Houghton, M. Apps, E. Gorham and E. Davidson, Climatic Change, Volume 40, Numbers 3-4, 495-518, DOI: 10.1023/A:1005345429236. [Full text]

Terrestrial carbon storage during the past 200 years: A Monte Carlo Analysis of CO² data from ice core and atmospheric measurements – Bruno & Joos (1997) Issue is mentioned here and there in the article. Bruno, M., and F. Joos (1997), Global Biogeochem. Cycles, 11(1), 111–124, doi:10.1029/96GB03611. [Full text]