Ocean acidity a key climate change predictor

New international climate modelling and geochemical data research led by Yale and the University of St Andrews confirms major changes to Pacific Ocean currents, including those that drive El Niño events, may occur with just a few degrees of global warming.

The new findings, published in the journal Nature (Wednesday 20 October 2021), reflect the increased potential of climate models for predicting complex environmental dynamics. The findings also reveal ocean acidity as an essential variable in validating climate models.

El Niño affects weather, food security, economic productivity, and public safety for much of the planet. There is ongoing debate as to how well El Niño dynamics are captured by climate models.

Lead author Madison Shankle, a former Yale researcher now at the University of St Andrews School of Earth and Environmental Sciences, said: “Accurately capturing ocean dynamics in the equatorial Pacific in global climate models is crucial for predicting regional climate in the warmer decades to come.”

Over the past decade, Yale climate scientist Alexey Fedorov has conducted ground-breaking research on ocean dynamics around the world, including El Niño events, the warm phase of the El Niño Southern Oscillation, featuring unusually warm water in the Pacific. Fedorov developed climate model simulations that look at ocean temperature proxies of the distant past, when global temperatures were several degrees warmer, as well as the present, to predict what might happen in a future, warmer world.

But some of Fedorov’s colleagues at Yale, including climate scientists Pincelli Hull, Noah Planavsky, and the late Mark Pagani, wondered whether ancient temperature data in his model and other climate models were accurately capturing the past climate state.

“We decided to test model predictions of major changes to the winds and currents driving El Niño by measuring something else, rather than temperature,” said Hull, Assistant Professor of Earth and Planetary Sciences in Yale’s Faculty of Arts and Sciences and principal investigator for the new study, adding: “We measured ocean acidity instead.”

Ocean acidity describes the amount of pH in the Earth’s oceans, based primarily on the amount of carbon dioxide that oceans absorb from the atmosphere. As carbon dioxide in the ocean increases, pH decreases.

A team led by Shankle, Hull, Planavsky, and Fedorov, as well as researchers from George Mason University, the University of California-Riverside, the University Corporation for Atmospheric Research, the University of St Andrews and Queen Mary University of London, used boron isotopes to infer ancient ocean acidity.

The researchers focused on the equatorial Pacific during the Pliocene Epoch, 2.6 to 5.3 million years ago. The Pliocene was a warm period of Earth’s past that climate scientists often use as an analogue for today’s warming planet.

The researchers discovered three things.

First, using geochemical proxies, they found a much more acidic eastern equatorial Pacific during the Pliocene, compared to today.

Second, the new results matched climate model predictions from co-author Natalie Burls, a former Yale researcher who is now at George Mason University, due to a water circulation system that acted like a conveyor belt, bringing up deeper, older, more acidic water.

“Rather than being a few decades old as is found today, the upwelled water in the warm Pliocene travels thousands of miles from the North Pacific at depths of about 1000 metres before finally upwelling in the eastern equatorial Pacific, making the water in the order of hundreds of years old,” Shankle said.

Third, the researchers found that the delivery of this older, acidic water required an “overturning circulation”, the ocean conveyor belt, that had previously been predicted by Burls and Fedorov.

“It was this remarkable confirmation of Natalie and Alexey’s model,” Hull said. “It means our current set of climate models are working pretty well. It gives us more confidence in the ability of models to predict large, regional changes in ocean and climate dynamics that really matter.”

The new information also suggests that ocean acidity can be a key measurement as climate models attempt to make projections for warmer conditions than those found today.

“This is a powerful way to test models and ideas about how the climate system works that is beyond our current technological capacity to assess on the basis of past temperature proxy estimates alone,” Planavsky said.