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