Can you hear ocean temperature change? La Jolla scientists say yes

Last Updated on July 10, 2025

A new study finds marine heatwaves and economic crashes left distinct acoustic signatures after 15 years of research studying whale and human activity.

Scientists from Scripps Institution of Oceanography at UC San Diego were able to hear the impacts of a marine heatwave and economic slowdowns by analyzing 15 years of ocean sounds recorded off the coast of Southern California.

The findings, published June 5 in the Journal of the Acoustical Society of America, show that listening to the sea can be a tool for monitoring ocean ecosystems and even human economic activity.

“There was a lot of change in the ocean” during the 15-year research period, said Vanessa ZoBell, a Scripps postdoctoral researcher, part of the Scripps Marine Bioacoustics Research Collaborative and lead study author.

The fluke of a blue whale off the coast of California.
Credit: Katherine Whitaker, CalCOFI

The recordings, collected between 2008 and 2023, allowed ZoBell and her team to hear whales moving north in response to a marine heatwave that began in 2014 as well as the massive decrease in noise from container ships during the 2008 financial crisis.

Using multiple acoustic sensors in the water, ZoBell and colleagues discovered the number of whales in the Southern California Bight declined, she said.

“It took years for those whales to come back to their normal sound levels,” she added, which surprised her given the heat wave only lasted from 2014 to 2016.

Hearing the evidence of how ocean changes “are affecting these animals is something really important to monitor,” ZoBell said.

Another piece of evidence clearly heard: “Noise levels were really tied tightly to economics and big container ships that were transiting through the region,” she said.

The sensors, located as far north as Point Conception near Santa Barbara and south to far offshore San Diego, were meant to help the researchers study whales: what species were where, how many and when they’re in various locations.

A fin whale breaking the surface off the coast of California.
Credit: Katherine Whitaker, CalCOFI

The loud, low-frequency songs of giant whales can be heard up to 620 miles away; sound travels about four times faster in seawater than in air.

Human activities, such as global shipping, are also filling the oceans with noise that can make it harder for whales to communicate.

Sound and ocean temperature “definitely are related,” ZoBell said. Temperature, salinity and pressure or depth all affect the speed of sound in seawater.

As temperature increases, sound travels faster in seawater, she said. With a decrease in temperature, sound travels slower.

ZoBell’s team has created the longest acoustic timeseries on the California coast.

“We didn’t know what was going to happen to our animals off of Southern California when there was a severe marine heat wave,” ZoBell said.

The research “shows that with climate change, we’re having more frequent and more severe marine heat waves,” she said, which means “the animals may change their distributions,” moving regions “to stay in temperatures that they feel comfortable in because the south gets too hot.”

The study revealed whale calls decreased by up to 50 percent at southern sites while increasing at northern sites near the colder water of the California Current.

Ship noise levels crashed by over 40 percent during the 2008-2009 financial crisis and still hadn’t returned to pre-recession levels by 2023. The recordings even captured daily “rush hours” in the ocean, with ship noise peaking at 3 am and 2 pm as vessels arrived and departed the Ports of Los Angeles and Long Beach.

The scientists are now wondering if, with more frequent marine heat waves, whether these animals are going to leave the Southern California Bight, ZoBell said. “And if so, where are they going to go?”

The team’s next steps are going to involve habitat modeling for some of these species, she said, to see where they are and how they’re moving and how their populations are connected to different ocean conditions.

“You can learn so much just by listening,” ZoBell said. “Even though we might not see blue and fin whales as often as we see the dolphin species, just because we can’t see them doesn’t mean that we don’t need to protect them.”

“Sound is a way that we can monitor them without having to be offshore on a boat with binoculars 365 days of the year.”