• The ocean maintained a relatively steady temperature throughout most of the 20th century before rising steeply, new research suggests.
  • 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.
  • Ocean warming is a concern, as it can cause circulation changes, reduced ability to absorb carbon, and fuel more intense storms.
  • It can also cause sea level rise and harm undersea life environments.

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.

Fig. 1: Estimates of global ocean heat content.
Estimates of global ocean heat content.
Image: University of California

There are two main possibilities for why the effects of global warming took so long to reach the ocean, De Vries says.

“One is that anthropogenic warming might have been weaker than previously thought during the 20th century, perhaps due to the cooling effects of aerosol pollution,” he says. The other is that the deep ocean may still be exhibiting the effects of climate events long past.

“It can take centuries for climate signals to propagate from the surface to the interior,” he says. Thus, the effects of a cooling event such as the Little Ice Age might be deep history to us on the surface, but the echoes of the event may have continued to resonate in the deep ocean into the 20th century, providing a buffer to the warming Earth.

What's the World Economic Forum doing about the ocean?

Our ocean covers 70% of the world’s surface and accounts for 80% of the planet’s biodiversity. We can't have a healthy future without a healthy ocean - but it's more vulnerable than ever because of climate change and pollution.

Tackling the grave threats to our ocean means working with leaders across sectors, from business to government to academia.

The World Economic Forum, in collaboration with the World Resources Institute, convenes the Friends of Ocean Action, a coalition of leaders working together to protect the seas. From a programme with the Indonesian government to cut plastic waste entering the sea to a global plan to track illegal fishing, the Friends are pushing for new solutions.

Climate change is an inextricable part of the threat to our oceans, with rising temperatures and acidification disrupting fragile ecosystems. The Forum runs a number of initiatives to support the shift to a low-carbon economy, including hosting the Alliance of CEO Climate Leaders, who have cut emissions in their companies by 9%.

Is your organization interested in working with the World Economic Forum? Find out more here.

The delayed cooling effect ended in 1990, after which ocean temperatures, according to the study, have been accelerating upward.

“The lag is catching up and the ocean is warming more strongly now,” Bagnell says. The Atlantic Ocean and Southern Ocean are currently where most of the warming is, with the Pacific Ocean and Indian Ocean not far behind.

Ocean warming is a concern on many levels, as it can cause changes in circulation, reduce its ability to absorb carbon, and fuel more intense storms, in addition to causing sea level rise and creating inhospitable environments for undersea life.

If the trend continues, the effects might last centuries, thanks to the same lag that kept the oceans cool until the last 30 years.

“The ocean remembers,” DeVries says.