Earthquakes reveal how rapidly the ocean is warming

As greenhouse gases build up in Earth’s atmosphere, the planet retains heat that can dissipate into space. The ocean absorbs most of this extra warmth, and researchers have now turned to an unlikely data source – earthquakes – to study how quickly seawater is warming. Their measurements reveal that the Indian Ocean is heating at about 0.044 Kelvin per decade. The authors report that this is much faster than the rate measured by a group of independent floaters.

Climatic pacemaker

“The ocean plays an important role in the climate system.” More than 90% of the energy trapped by greenhouse gases ends up warming the ocean. As a result, the global ocean is somewhat of a climate regulator to regulate pacemakers, said Jorn Calles, an oceanographer at the California Institute of Technology in Pasadena and one of the authors of the new study. “The ocean plays an important role in the climate system.”

But dealing with how quickly the ocean is warming was, until very recently, kind of absurd. “You were going to have an instrument down on its side [of a ship] Calise said. This arduous process resulted in sporadic and biased coverage because sailors had little interest in sailing the well-known Southern Ocean, for example.

However, that all changed in 2000 when the first standalone Argo buoy was published. Each of these roughly a meter long machine tools take repeated conductivity, temperature and pressure readings of the ocean as it moves up and down the water column. Approximately 4,000 Argo buoys now roam the ocean of the world, drifting with currents.

Go deeper than Argo

But one of the limitations of Argo floats is that they are only 2,000 meters deep. (A program known as the Deep Argo, which is currently being piloted, will send buoys to a depth of 6,000 meters.) To get a better idea of ​​how the ocean – including deep sewers – is warming, Callies and colleagues ignored a suggested idea more than 40 years ago: measuring changes At times when sound waves travel through water.

“We are using earthquakes to generate sound.” Physics dictates that sound waves travel faster through warm water than in cold water. But researchers in the 1990s discovered that generating sound waves using underwater loudspeakers, for example, came with its own difficulties, said Bruce Cornwell, a physicist oceanographer at the Scripps Institute of Oceanography in La Jolla, California, who is not involved in the research. “Creating the sound sources is expensive, and there were a lot of questions about what it could do for a marine mammal.”

Callies and his colleagues chose to switch to a natural source of sound waves. “We are using earthquakes to generate sound,” said Calise. When seismic waves shake the sea floor, they generate sound waves in the ocean. These waves travel at 1.5 kilometers per second – much slower than the primary (P) and secondary (S) seismic waves – and turn back into seismic waves when they hit the sea floor again.

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Kalez and his collaborators focused on the area near the Indonesian island of Sumatra, one of the most earthquake-prone sites in the world. This area is special, said Wenbo Wu, a seismologist at the California Institute of Technology and lead author of the new study. “Sublimation is sliding there all the time.” Because the same section of the fault slides repeatedly over time, it generates earthquakes with nearly identical wave shapes. These “recurring earthquakes” can be separated in time by hours, days, or years.

Using the seismological catalog compiled by the International Seismological Center, scientists isolated 2,047 pairs of repeated earthquakes that occurred near Sumatra from 2004 to 2016. Finding these duplicates made this research possible, Calise said. He said that seismic waves of repeated earthquakes originate in the same location, so any changes in travel time can be linked to changes in ocean temperature. “If we had not used repeaters, the uncertainty in the earthquake site would have overwhelmed any peripheral signals.”

For each pair of repeated earthquakes, the researchers calculated the difference in wave travel times between their origin in Sumatra and the seismic receiver in Diego Garcia, an atoll about 3,000 km away and part of the British Indian Ocean Territory.

The team found that travel time differences – typically a few tenths of a second – tend to increase over time. This is the signature of warm ocean.

A clear trend of warming

On average, the researchers calculated that the temperature of the Indian Ocean rose by about 0.044 K per decade between 2004 and 2016, they reported last month in the journal Science. This is about 70% more than the Argo-derived rate of 0.026 K per decade. The discrepancy is somewhat expected, Calise said, because the two data sets are looking at different parts of the water column. (Reassuringly, both groups show fluctuations in water temperature with the same periods – 6 and 12 months – which are likely driven seasonally, the team showed.)

The results are interesting, Cornwell of Scripps said, but that there is more to investigate. For example, it is important to understand the amount of travel time differences caused by real changes in temperature versus small shifts in earthquake sites.

In the future, Calise said, it would be beneficial to use underwater receivers – water devices – to directly detect the sound waves traveling, rather than relying on converting them back into seismic waves and capturing them by seismic sensors on the ground. This will make it possible to collect measurements from the smaller – and therefore more numerous – earthquakes.

There is also the option to look at sound waves of different frequencies, Calles said, to study temperature changes at different water depths. “The higher the frequency, the more sensitive you are to different parts of the water column.” This investigation focused on the frequencies between 1.5 and 2.5 hertz, but anything between 1.0 and 10.0 hertz the researchers suggest. “These are things from the future,” said Calise.

—Katherine Kornei (KatherineKornei), science writer

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