Shaky way to check on ocean warming

The rumble of earthquakes far below the surface of the ocean are far more interesting than just a way of understanding the Earth’s geology, say US and Chinese researchers. They’ve developed a way to use those seismic waves to measure ocean warming around the world.

The technique, called seismic ocean thermometry, could provide wide-ranging measurements of temperatures at depths of over 2000 metres, says a team led by Wenbo Wu from the California Institute of Technology, US. And that will assist in measuring the rates and patterns of ocean warming and its effects on climate change.

The team tested their technique in the East Indian Ocean, where they uncovered a 10-year warming trend that exceeds previous estimates. Their results are published in the journal Science.

Seismic ocean thermometry isn’t a new idea. The basic principles were first proposed about 40 years ago but only progressed slowly. Ocean acoustic work was largely the preserve of militaries, and what work was done largely involved artificial sound sources with a known source and timing.

More recently, there has been work into the idea of using background noise. Wu and his team have extended this idea, investigating using seafloor earthquakes as a low-frequency sound source. They examined seismological data between 2004 and 2016, analysing 2047 pairs of “repeating” earthquakes, whose source and timing can be correlated from other data sources.

The concept relies on the different speed of sound in water of different temperatures. Sound waves from a source, in this case an earthquake, cycle between different depths on their way to the receiver. As water temperatures differ depending on depth, the soundwaves have a different travel time by the time they reach the receiver.

Working backwards, the researchers can then reconstruct the temperature profile of the water. From the data, they were able to compile a marine-basin-wide temperature profile spanning 3000 kilometres.

Until now, it has been a challenge to measure changes accurately enough over a large-enough area to be able to pick apart the more subtle large-scale and long-term trends from the small-scale, short-term fluctuations. And while ocean temperature monitoring has improved recently, there is still little data from the depths. The new technique, suggest the researchers, could alleviate both concerns.

Monitoring and understanding the ocean’s heat uptake is critical to better understanding global climate drivers, say the researchers, given the ocean is the largest heat sink in the climate system.

Using this technique also means the researchers can start looking through historical data to build a profile of ocean temperatures from the times that global seismic data became widely available.

“Historic earthquake records might be used to infer ocean temperatures long before direct measurements became available,” writes Harvard University’s Carl Wunsch in an accompanying commentary. Wunsch, an oceanographer and climate expert, was not involved in the research.

Earthquakes may not be the only appropriate sound source, he says. “With enough receivers, the potential exists for blue whales or other marine mammals to be used as tomographic sources.

“This approach may not only benefit our understanding of the fluid ocean but also help us to understand biological systems in the ocean.”