The world relies on a modest number of countries to keep watch over the ocean. That arrangement is starting to fail. Europe and Asia must now decide whether to let the system unravel, or to take it up together.

Right now, in every ocean basin on Earth, a global network of instruments measures the state of the sea.

Research ships steam along oceanographic transects from surface to seafloor. Anchored buoys watch the tropical oceans for the first signs of El Niño or tropical cyclones and take the pulse of the thermohaline circulation. Some four thousand autonomous floats sink every ten days to two thousand metres before rising to transmit temperature and salinity to ground stations via satellite. Underwater gliders patrol continental margins, and drifting buoys ride the surface in the most remote waters. Hundreds of elephant seals carry miniaturised sensors beneath the polar sea ice…

Together, this network produces invaluable information that allows societies to anticipate and respond to a changing ocean and weather conditions, and protect the ocean in return.

It is also far more fragile than most people, and most governments realise. A new study published in Nature Climate Change has measured for the first time just how fragile the ocean watch network is.

The result is alarming. If observations from a single major contributor, the United States, were withdrawn from the Global Ocean Observing System (GOOS), the errors in our estimate of how fast the ocean is warming would jump by 163 percent. That is worse than randomly losing 80 percent of all global ocean data. The reason is geographical: US instruments cover every ocean basin and plug critical gaps that no other nation currently fills.

This is not a theoretical concern. Proposed cuts to National Oceanic and Atmospheric Administration (NOAA) and the National Science Foundation in the United States now threaten exactly this contribution. And the situation is barely better on the other side of the Atlantic.

The pressures are not confined to one side of the Atlantic, nor to the West. In China, scientists and policymakers are working to build a more resilient national contribution to ocean observation, but without the resources the moment requires. The marine monitoring system the world relies on is under strain almost everywhere.

An observing system, not a programme

Public conversations about ocean observations often focus on Argo floats.

Each Argo is essentially a sealed cylinder of pressurised electronics with a clever buoyancy chamber: it floods with seawater to sink and is evacuated to rise again. These autonomous robots have........

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