Seismic ocean thermometry

Most of the excess heat that causes global warming is absorbed by the oceans. Quantifying that heat increase is challenging because it requires many different temperature measurements over both the vertical and horizontal extent of the oceans. Wu et al. report success in this effort through the use of a different method: They inferred temperature changes from sound waves generated by repeating earthquakes (see the Perspective by Wunsch). The travel time of these earthquakes from source to receiver reflects changes in the average water temperature that they encounter. This technique should substantially enhance our ability to monitor ocean warming. Science , this issue p. 1510 ; see also p. 1433 Global seismic ocean thermometry using noise from earthquakes provides a distinct means of measuring ocean warming. More than 90% of the energy trapped on Earth by increasingly abundant greenhouse gases is absorbed by the ocean. Monitoring the resulting ocean warming remains a challenging sampling problem. To complement existing point measurements, we introduce a method that infers basin-scale deep-ocean temperature changes from the travel times of sound waves that are generated by repeating earthquakes. A first implementation of this seismic ocean thermometry constrains temperature anomalies averaged across a 3000-kilometer-long section in the equatorial East Indian Ocean with a standard error of 0.0060 kelvin. Between 2005 and 2016, we find temperature fluctuations on time scales of 12 months, 6 months, and ~10 days, and we infer a decadal warming trend that substantially exceeds previous estimates.