[Sample] Tracking ocean heat for global climate assessments

More than 90% of the excess heat trapped by greenhouse gases ends up in the ocean. Measuring how much, and where, is one of the most direct ways of tracking the pace of climate change — and for two decades that measurement has rested on a fleet of robots nobody sees.

The Argo array keeps close to 4,000 autonomous floats drifting through the open ocean. Every ten days each float sinks to 2,000 metres, rises back to the surface recording temperature and salinity on the way, and transmits its profile by satellite before diving again. Together they return more than 100,000 profiles a year, from every ocean basin, in every season.

From profiles to a global heat budget

Individual profiles say little on their own. Their value appears when they are quality-controlled to common standards, shared openly within 24 hours, and combined with ship-based sections, moorings and satellite altimetry into a single estimate of ocean heat content. That integration is what GOOS coordinates: the networks agree on what to measure, how, and how to make the data usable by everyone.

The result is a record that climate assessments can rely on. The IPCC Sixth Assessment Report drew on ocean heat content series built largely from Argo and GO-SHIP data to conclude that ocean warming is unequivocal and that the upper ocean has warmed faster since the 1990s.

Without sustained, open ocean observations there would be no global heat budget to assess. Argo turned a scientific question into a monitoring system.

Amara Diallo, Australian Institute of Marine Science

What the record changed

• Ocean heat content is now reported yearly as a headline climate indicator, alongside surface temperature and sea level.
• Sea level budgets close: thermal expansion measured by Argo matches the altimetry record once ice loss is added.
• Seasonal forecasts and marine heatwave outlooks start from an observed, not modelled, subsurface ocean.

The observing system behind the number

Network	Contribution	Status
Argo	Upper-ocean temperature and salinity profiles, global, every 10 days	Mature
GO-SHIP	Full-depth reference sections for calibration and the deep ocean	Mature
OceanSITES	Fixed-point time series at key locations	Mature

Every float that reaches the end of its life has to be replaced. Sustaining the array is a decision countries make every year.

Tom Whitaker, Great Barrier Reef Marine Park Authority

The next step is depth. Deep Argo floats now profile to 6,000 metres, closing the last gap in the heat budget, while biogeochemical floats add oxygen and carbon to the same platform. The global heat record will keep improving as long as the array is sustained.