Estimating global ocean heat uptake from the atmospheric Ar/N2 ratio

93% of all additional heat trapped on Earth due to the atmospheric rise of greenhouse gases will eventually accumulate in the ocean (IPCC, 2013). However, temporally and spatially sparse sampling of the oceans and known biases in the instrumental record have limited our ability to quantify historical ocean warming. Here, we present a recently developed, globally integrated approach to measure and reconstruct ocean heat uptake over the last two decades through observations of the atmospheric argon-to-nitrogen ratio (Ar/N₂).

Water temperature increases drive a degassing of the chemically inert noble gas Ar and N₂ from the oceans. A warming ocean releases proportionally more Ar than N₂ to the atmosphere (Ar is twice as soluble as N₂), where the gases are rapidly mixed on a time-scale of about one year. Atmospheric mixing thus intrinsically integrates the ocean warming signal globally and makes discrete local observations a novel measure of global ocean heat uptake.

Atmospheric records of Ar/N₂ from the Scripps O2 flask network, suggest stronger ocean warming over the last 15 years than other observational estimates based on traditional in-situ temperature readings. However, stratospheric observations of Ar/N₂ demonstrate a significant gravitational fractionation in the middle atmosphere and suggest that any tropospheric Ar/N₂ record will need to be corrected for variability in stratospheric-tropospheric mass exchange that can bias the global ocean warming estimate. Furthermore, maintaining a long-term high-fidelity time series of Ar/N₂ is challenging due to slow drift in standard gases. We explore these complications, assess the current uncertainty, and discuss future steps.