Scripps oceanographer has evidence for 135 years of ocean warming

By Claire Discenza

Last week, Scripps physical oceanographer Dean Roemmich presented what he considers to be “very powerful evidence that, in fact, the warming of the oceans didn’t begin 50 years ago, but rather in a time scale of about 100 years ago.”

Roemmich delivered this month’s Perspectives on Ocean Science lecture at the Birch Aquarium. In his talk, “135 years of global ocean warming between the Challenger Expedition and the Argo Program,” Roemmich explained why it is so important to look toward the seas for climate clues.

“Measuring the temperature of the ocean is a very fundamental index for the climate system as a whole,” Roemmich said. This is because the oceans absorb a small, but constant, amount of the sun’s radiation — about 0.9 watts per square meter.

Roemmich and others were able to calculate worldwide temperature change by comparing measurements taken aboard the 19th-century research vessel “Challenger” to those collected today.

“The Challenger Expedition, it was a real epic voyage that kicked off the science of oceanography,” he said. Between 1872 and 1876, the British naval ship covered 69,000 miles and took deep-sea temperature measurements at 300 locations.

Despite the enormity of the undertaking, the technologies available at the time were limited. The Challenger’s crew recorded temperatures by tying a thermometer to a rope, dropping it in the water, and waiting for it to fully sink.

However, because of water currents, the team had great difficulty keeping the rope completely vertical. As a result, scientists often inadvertently recorded from shallower waters than intended — an error that resulted in a small but persistent warm-bias in the data.

After the Challenger, the next global sampling project didn’t happen until the 1990s. WOCE, the World Ocean Circulation Experiment, took advantage of the electronic Conductivity Temperature Depth recorder, or CTD. Unlike a thermometer on a rope, a CTD can collect data continuously as it is lowered through the water. Using CTDs, WOCE was able to gather information from 8,000 profiles over seven years.

As revolutionary as WOCE was, it is dwarfed by today’s sampling technologies. In the 1950s, scientists began designing floats that would sink to a pre-disposed depth, drift for a while, and then return to the surface. It was not long before researchers started attaching CTDs to take measurements over longer distances and time periods.

“As soon as this float was designed, a number of colleagues and I looked at this thing and said, ‘we really need to do this globally,’ ” Roemmich recalled. “Let’s put out a global array.’”

Today this array of floats, the Argo Network, has more than 50 international research teams collaborating to deploy 3,000 floats to amass recordings from 10,000 profiles a month.

“No single institution — no single nation — could have done this. It’s a 30-nation collaborative program,” said Roemmich. “I think Argo has changed global oceanography.”

Putting it all together, when researchers subtracted Challenger temperatures from Argo temperatures, they saw significant warming throughout the world’s oceans. “There’s around a 0.55-degree temperature difference between the Challenger and Argo, and that’s a lower bound in temperature difference,” Roemmich reported.

Next Lecture

— The Leopard Sharks of La Jolla Shores, 6:30-8 p.m. Monday, July 9, Birch Aquarium. Andy Nosal, Ph.D. student, will discuss what gadgets he uses to track the sharks, what makes La Jolla the animals’ preferred hang out, why this shark population is particularly vulnerable and how the local no-take marine reserve protects it. Admission: $5-$8. https://aquarium.ucsd.edu/Education/Public_Programs/Adult_Programs/Lectures/