Guest commentary from Ralph Keeling (UCSD)

I, with the other co-authors of Resplandy et al (2018), want to address two problems that came to our attention since publication of our paper in Nature last week. These problems do not invalidate the methodology or the new insights into ocean biogeochemistry on which it is based, but they do influence the mean rate of warming we infer, and more importantly, the uncertainties of that calculation.

We would like to thank Nicholas Lewis for first bringing an apparent anomaly in the trend calculation to our attention. We quickly realized that our calculations incorrectly treated systematic errors in the O 2 measurements as if they were random errors in the error propagation. This led to under-reporting of the overall uncertainty and also caused the ocean heat uptake to be shifted high through the application of a weighted least squares fit. In addition, we realized that the uncertainties in the assumption of a constant land O 2 :C exchange ratio of 1.1 in the calculation of the “atmospheric potential oxygen” (APO) trend had not been propagated through to the final trend.

As the researcher in charge of the O 2 measurements, I accept responsibility for these oversights, because it was my role to ensure that details of the measurements were correctly understood and taken up by coauthors.

We have now reworked our calculations and have submitted a correction to the journal.

Details

In our definition ΔAPO, we used a default value of 1.1 for O 2 :C oxidative ratio (OR) of land carbon. However, a lower ratio is probably more appropriate. Specifically, Randerson et al. (2006) argued for a ratio of around 1.05, based on the composition of stems and wood, given that woody biomass dominates long­term carbon sources and sinks on land. Other recent studies have suggested similar ratios e.g. Clay and Worrall (2015). Our previous calculations did, in fact, allow for a range from 1.05 ± 0.05, consistent with above estimates and typical uncertainty ranges. However, we applied this range only for the ΔAPO Climate­ to­ ΔOHC ratio but neglected the impact on the APO budget itself, which used a fixed ratio of 1.1. If the actual OR were lower than 1.1, the observed APO decrease (ΔAPO OBS ) would include a contribution from the global land carbon sink, because the ΔO2 term then imperfectly cancels the 1.1 ΔCO2 term.

In the updated calculations we now also allow apply the OR range (1.05 ± 0.05) to the APO calculation which by itself increases the APO Climate trend by 0.15 ± 0.15 per meg/y­r relative to an estimate using 1.1.

Bottom Line

We recomputed the ΔAPO Climate trend and its uncertainty based on the distribution of the unweighted least square fits to each of the 106 ensemble realizations of ΔAPO Climate generated by combining all sources of uncertainty, with correlated errors now treated as systematic contributions to the trend. The resulting trend in ΔAPO Climate is 1.05 ± 0.62 per meg/y­r (previously 1.16 ± 0.18 per meg/yr) which yields a ΔOHC trend of 1.21 ± 0.72 x 1022 J/yr (previously 1.33 ± 0.20 x 1022 J/yr), as summarized in the updated Figure 1:





The revised uncertainties preclude drawing any strong conclusions with respect to climate sensitivity or carbon budgets based on the APO method alone, but they still lend support for the implications of the recent upwards revisions in OHC relative to IPCC AR5 based on hydrographic and Argo measurements.

Update (9/26/2019): The Nature paper has been retracted with the following text:

Shortly after publication, arising from comments from Nicholas Lewis, we realized that our reported uncertainties were underestimated owing to our treatment of certain systematic errors as random errors. In addition, we became aware of several smaller issues in our analysis of uncertainty. Although correcting these issues did not substantially change the central estimate of ocean warming, it led to a roughly fourfold increase in uncertainties, significantly weakening implications for an upward revision of ocean warming and climate sensitivity. Because of these weaker implications, the Nature editors asked for a Retraction, which we accept. Despite the revised uncertainties, our method remains valid and provides an estimate of ocean warming that is independent of the ocean data underpinning other approaches. The revised paper, with corrected uncertainties, will be submitted to another journal. The Retraction will contain a link to the new publication, if and when it is published.

Update: (3/6/2020): Without much fanfare, a corrected version of the paper has been published in Scientific Reports. The final numbers are slightly different (1.29 ± 0.79 × 1022 J yr−1) from what was discussed above:

References