The ocean maintained a relatively steady temperature throughout most of the 20th century before rising steeply, new research suggests.

In estimations of ocean heat content—important when assessing and predicting the effects of climate change—calculations have often presented the rate of warming as a gradual rise from the mid-20th century to today.

The new findings, which could overturn that assumption, may have significant implications for what we might expect in the future.

“There wasn’t an onset of an imbalance until about 1990, which is later than most estimates,” says Timothy DeVries, an associate professor in the geography department at the University of California, Santa Barbara, and a coauthor of the new paper in Nature Communications.

According to the study, the period from 1950 to 1990 saw temperature fluctuations in the water column but no net warming. After 1990, the entire water column switched from cooling to warming.

These findings are the result of the addition of a largely underexplored factor in ocean heat content (OHC): deep ocean temperatures.

“Prior studies didn’t consider the deep ocean,” says Aaron Bagnell, a graduate scholar in DeVries’ laboratory and the paper’s lead author.

Because of the challenges involved in getting temperature measurements in the deep ocean (below 2,000 meters) that region has gone largely unaccounted for, and data has been sparse. “There is some existing data, from research cruises and autonomous floats,” Bagnell says.

The researchers used an autoregressive artificial neural network (ARANN) and machine learning methods to connect the dots between data points and “produce a single consistent estimate of the top-to-bottom OHC change for 1946 to 2019.” The result was a trend that delays warming by decades over previous models.

Read the full article about ocean temperatures by Sonia Fernandez at Futurity.