Quantification of ocean heat uptake from changes in atmospheric O 2 and CO 2 composition

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 policyrelevant measurements of the Earth response to climate change, such as climate sensitivity to greenhouse gases7 and the thermal component of sea-level rise8. As shown in Fig. 1, recent temperature-based hydrographic estimates of ocean warming9–12 show good agreement for the years 2007– 2016 (1.09 ± 0.10 × 1022 to 1.16 ± 0.20 × 1022 J yr−1), but a larger spread when extending back to include the sparser data of the 1990s (0.90 ± 0.09× 1022 to 1.36 ± 0.10 × 1022 J yr−1 for 1993–2015). The spread is mostly caused by gap-filling methods and systematic errors5,9, which together introduce uncertainties of up to 25%–50% in warming trends4. Because temperature-based estimates also use the same upperocean observations and linear warming trend for depths below 2,000 m (ref. 11), they may share additional unknown systematic errors12. An alternative method based on the top of the atmosphere energy balance13 is also not truly independent, because it is subject to large systematic errors when estimating long-term trends and therefore depends on the same hydrographic measurements for calibration13–15. Here we introduce a third method, based on changes in the abundances of gases in the atmosphere, which respond to whole-ocean warming through the temperature dependence of gas solubility in sea water. This method is not limited by data sparseness, because fast mixing in the atmosphere efficiently integrates the global ocean signal. Changes in ocean heat content on seasonal16 and glacial–interglacial17 timescales have been reconstructed using measurements of noble gases in modern or ancient air. Our method is similar, but instead of relying on noble gases (for example, ratios of argon to nitrogen), which lack sufficient accuracy as yet16, we rely on measurements of atmospheric O2 and CO2, which can be summed to yield a tracer ‘atmospheric potential oxygen’ (APO) that responds to warming similarly to a noble gas18. When the ocean warms, the solubility of O2 and CO2 drops, and the amount of gas lost by the ocean can be quantified with the complementary change observed in the atmosphere. Precise atmospheric O2 measurements began in 1991 (CO2 in 1958), enabling APO-based reconstructions of ocean heat content that span nearly three decades6. APO (O2 + 1.1 × CO2) is computed using observed atmospheric O2/N2 molar ratios and CO2 molar fractions (see Methods)6,19. By design, APO is insensitive to exchanges with land ecosystems, which produce changes in O2 and CO2 that largely cancel in APO owing to their approximate 1.1 O2/C oxidative ratio. Time-series measurements at remote sites show a global long-term decline in APO, with ΔAPOOBS being −243.70 ± 10.10 per meg (units defined in the Methods) between 1991 and 2016. ΔAPOOBS is driven by four primary contributors, illustrated in Fig. 2: