Nord Stream methane leaks spread across 14% of Baltic waters.

A suspected 443-486 kt of methane escaped from the Nord Stream pipelines in September 2022 at four explosion sites across three pipelines. Much of this methane rapidly escaped to the atmosphere, while an unknown amount was dissolved. We use sustained high-resolution observations of methane concentrations from autonomous gliders and an instrumented ship of opportunity to reveal the timing and spread of dissolved methane across different Baltic regions and marine protected areas. Estimates of methane spread and concentrations are essential to understand the ecosystem response, and for establishing accurate priors for atmospheric outgassing and transport models. A numerical model, initialized by engineering estimates and our observations, enables us to constrain the mass of locally dissolved Nord Stream methane (9.5-14.7 kt). We show that dissolved methane decreased rapidly through outgassing, however initial concentrations were so high that 14% of the Baltic Sea still experienced concentrations 5 times greater than average natural levels.