| 23 Jun 2026
Historical Nansen Cast Temperature Profiles and the Ocean Heat Content Change since 19th Century
Abstract. The estimation of the ocean heat content (OHC) is based on the subsurface temperature observations which abundance, quality and accuracy changed over time following the development of oceanographic techniques and methods. The majority of the available global OHC time series begins in 1950s when the International Geophysical Year 1957–1958 (IGY) observational campaign resulted in a rapid improvement of the observational basis. However, a significant amount of subsurface temperature observations was accumulated during the period before the IGY. We perform a three-step quality control (QC) of the Nansen cast temperature profiles with the focus on the time period before 1970. The QC includes automated and manual control of temperature observations along with the quality control of temperature anomalies. The World Ocean Circulation Experiment/Argo Global Hydrographic climatology based on the high-quality temperature observations between 1990 and 2025 is used as the reference. To interpolate temperatures vertically we use a combination of the parabolic Reiniger and Ross (1966) method and the temperature anomaly method by Frankignoul (1981). The method is shown to result in a smaller interpolation error. Correction for depth bias in Nansen cast profiles are also suggested. The oldest temperature profiles from 1870s are treated separately, with the data from “Challenger” expedition used as reference. Finally, we estimate the OHC changes between 1870s and 2020s for six selected multi-year time periods. We find that for the layer 0–2000 m the OHC increase between 1920–1944 and 1945–1969 amounts to 30 % of the total OHC increase between 1920–1944 and 2010–2025.
General Assessment
This manuscript describes the construction, quality control, and application of a global archive of Nansen cast temperature profiles spanning 1873–2025, and uses it to extend estimates of 0–2000 m ocean heat content (OHC) change back to the 1870s. This is a significant and welcome contribution. The pre-IGY hydrographic record is underutilized in the OHC literature, and the authors—who are among the most experienced groups worldwide in historical hydrographic data rescue and bias correction—have produced a dataset that will be of lasting value to the climate community. The methodology is sound, the treatment of uncertainty is generally honest, and the central result (that roughly 30% of the post-1920 OHC increase occurred before 1970) is important and well supported. I recommend acceptance after the authors address the items below. The most substantive concerns are internal numerical inconsistencies and citation/reference mismatches; the remainder are language and presentation issues that require careful copy-editing but no new work.
Strengths of the Manuscript
Items to Be Addressed
This is a valuable data paper on an important and under-examined segment of the ocean observing record, and I congratulate the authors on the substantial effort behind it. None of the items above challenges the methodology or the conclusions; they concern internal consistency, referencing, figure/text agreement, and language. Once these are addressed, I fully support publication in Earth System Science Data.