Quantification of ocean heat uptake from changes in atmospheric O₂ and CO₂ 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 2007^2,3. However, these estimates all use the same imperfect ocean dataset and share additional uncertainties resulting from sparse coverage, especially before 2007^4,5. 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.