The ocean is the main source of thermal inertia in the climate system1. During recent decades, ocean heat uptake has been quantified by using hydrographic temperature measurements and data from the Argo float program, which expanded its coverage after 20072,3. However, these estimates all use the same imperfect ocean dataset and share additional uncertainties resulting from sparse coverage, especially before 20074,5. Here we provide an independent estimate by using measurements of atmospheric oxygen (O2) and carbon dioxide (CO2)—levels of which increase as the ocean warms and releases gases—as a whole-ocean thermometer. We show that the ocean gained 1.33 ± 0.20 × 1022 joules of heat per year between 1991 and 2016, equivalent to a planetary energy imbalance of 0.83 ± 0.11 watts per square metre of Earth’s surface. We also find that the ocean-warming effect that led to the outgassing of O2 and CO2 can be isolated from the direct effects of anthropogenic emissions and CO2 sinks. Our result—which relies on high-precision O2 measurements dating back to 19916—suggests that ocean warming is at the high end of previous estimates, with implications for policy-relevant measurements of the Earth response to climate change, such as climate sensitivity to greenhouse gases7 and the thermal component of sea-level rise8.
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We thank M. Winton for useful suggestions. L.R. acknowledges support from the Climate Program Office of the National Oceanic and Atmospheric Administration (NOAA), grant NA13OAR4310219, and from the Princeton Environmental Institute. The National Center for Atmospheric Research (NCAR) is sponsored by the National Science Foundation (NSF). We also thank the people who maintain the APO time series at Scripps and the groups developing the models CESM, GFDL, IPSL and UViC, used in this study. The Scripps O2 program has been supported by a series of grants from the US NSF and the NOAA, most recently 1304270 and NA15OAR4320071. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF and NOAA. We thank the staff at the Cape Grim Baseline Air Pollution Station of the Canadian Greenhouse Gas program for collection of air samples.
Nature thanks L. Cheng, F. Primeau and the other anonymous reviewer(s) for their contribution to the peer review of this work.
The authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
a, Anomaly, relative to 1850 levels, in deposition of atmospheric anthropogenic aerosols (N, P and Fe) at the air–sea interface between 1960 and 2007, derived from model simulations with and without aerosols22. b, Impact of aerosol eutrophication on atmospheric O2 (solid lines) and CO2 (dashed lines) for all aerosols (black lines) and for each aerosol taken individually (coloured lines). c, Overall impact of aerosol eutrophication on ΔAPOClimate referenced to the first year that has observations (1991).
a, Ocean observations of O2*, O2sat, Cpi* and Cpisat as a function of potential temperature in the Glodapv2 database32. b, OPOsat ( = O2sat + 1.1Cpisat, in grey) and the expected effects on APO owing to the combined effects of OPOsat and the thermal exchanges of N2 ( = O2sat + 1.1Cpisat – XO2 / XN2 [N2 – mean(N2)], in red). For clarity only 16 × 103 points randomly picked out of the 78,456 data points available are shown for each variable. Note that very low values of O2* (around 450 μmol kg−1) at low temperature (less than 10 °C) correspond to data collected in the Arctic Ocean, where phosphate concentrations (used for O2* calculation) are comparatively lower than in other cold ocean regions. Low O2* values in the Arctic explain the relatively low values of OPO shown in Extended Data Fig. 3a at temperatures below 10 °C.
a, c–f, OPO concentrations (yellow) and OPO concentrations at saturation based on O2 and CO2 solubility (OPOsat, grey) as a function of ocean temperature in the GLODAPv2 database32 (a) and four Earth-system models (IPSL, GFDL, CESM and UVic; c–f). Slopes give the OPO-to-temperature ratios in nmol J−1. b, The link between ΔAPOClimate and changes in ocean heat content (that is, ΔAPOClimate-to-ΔOHC ratio) in the four models is tied to their OPO-to-temperature ratios and can be constrained using the observed OPO-to-temperature of 4.45 nmol J−1 (vertical dashed lines). To avoid visual saturation, only 16,000 points, picked randomly, are shown for OPO.
Extended Data Fig. 4 Changes in APOClimate (ΔAPOClimate) and ocean heat content (ΔOHC) in four Earth-system models.
a, Simulated ΔAPOClimate (black outlines) are decomposed into the contributions (percentage of total) from changes in ocean thermal saturation (light blue) and biologically driven changes (dark blue), the latter including changes in photosynthesis/respiration and changes in ocean circulation that transport and mix gradients of biological origin. For each model, ΔAPOClimate is further decomposed into its O2, CO2 and N2 components—that is, how much of ΔAPOClimate is explained by changes in O2, CO2 and N2 air–sea fluxes due to ocean saturation changes and biologically driven changes. b, Model ΔAPOClimate-to-ΔOHC ratios over the 180 years of simulation (referenced to year 1991) in per meg per 1022 J units are: 0.85 ± 0.01 (CESM), 0.83 ± 0.01 (GFDL), 0.89 ± 0.03 (IPSL) and 0.99 ± 0.02 (UVic).
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Resplandy, L., Keeling, R.F., Eddebbar, Y. et al. Quantification of ocean heat uptake from changes in atmospheric O2 and CO2 composition. Nature 563, 105–108 (2018). https://doi.org/10.1038/s41586-018-0651-8
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