The rate of the ocean’s warming has more than quadrupled over the past four decades, according to researchers.
While ocean temperatures were rising at about 0.06 degrees Celsius per decade in the late 1980s, they are now increasing at 0.27 degrees Celsius per decade, scientists said Tuesday.
“As you know, the last two years have been very warm years and that was partly due to the El Niño and that was expected that they should be record-breaking years. But, they were warmer than people had anticipated even in that context,” Christopher Merchant, a professor of ocean and Earth observation at England’s University of Reading, told The Independent. “So, this gives rise to a question: Has something changed or why were our expectations not including what actually transpired in the last couple of years in terms of how big the records were broken by?”
Merchant is the lead author of the findings published in the journal Environmental Research Letters.
To help explain why 2023 and 2024 — the hottest years on record — saw unprecedented ocean temperatures, the study’s authors examined years-long sea surface temperature data from satellite observations dating back to 1980. They compared it with NASA mission data regarding the balance of Earth’s energy, measuring the subtle difference between energy coming into the Earth and the energy that is emitted to space.
They found that accelerated ocean warming is driven by the Earth’s energy imbalance: taking in more energy from the sun than is escaping into space. The imbalance has roughly doubled since 2010, in large part due to increasing concentrations of greenhouse gas emissions and because Earth now reflects less sunlight to space than it did before.
The Earth can reflect some sunlight back towards space through reflective surfaces like ice, water, and snow, as well as clouds. This ability is decreasing as snow and ice melts and with a decline in lower altitude clouds. Loss of sea ice increases heat in the Arctic, where temperatures have risen about four times the global average, according to NASA.
From 2023 to early 2024, global ocean temperatures hit record days for 450 consecutive days. By comparing the recent El Niño warming event — the warm phase of the El Niño Southern Oscillation (ENSO) — in the Pacific Ocean to a similar event from 2015 to 2016, they found that the rest of the record warmth was explained by the sea surface warming up faster over the past 10 years than in previous decades. Nearly 45 percent of the record warmth was due to the oceans absorbing heat at an accelerating rate.
“And, we found that part of the explanation for the recent two warm years was that, over the course of the decades, there’s steadily been a bit of an acceleration in global warming underlying all the year-to-year fluctuations that sort of hide it,” he explained. “But, you can extract that, if you look carefully at the data and correlate it with this other data set.”
“It’s not surprising. In terms of the physics, it’s sort of what you expect. But, we’ve extracted that relationship for the first time, I think,” said Merchant.
The findings show that the overall rate of global ocean warming seen in recent decades is not an accurate guide to what happens next. It is possible that the increase observed over the past 40 years will be exceeded over the course of the next 20 years.
“An important thing that’s emerged is that the expectations for the rates of change that we see when we do that in the next couple of decades are really at the high end of what we might have been expecting from climate models. It’s maybe a bit faster than the expectations that are based on climate models,” said Merchant.
That’s partially because, in the last 15 years, the Earth’s energy imbalance has gone up more than people would have expected, he says.
What does this mean for the planet?
The oceans set the pace of global warming. Millions of people subsist on fishing that rely on the fish that live in reefs declining in warmer waters. As the oceans warm, the atmosphere warms, bringing more flooding, wildfires, and other deadly natural consequences. Accelerating warming underscores the urgency, the researchers said, of reducing fossil fuel burning.
“If nature continues to behave in the next couple of decades as it has in the last 15 years or so, all those impacts that people are talking about are actually a little bit closer in the future than they might have previously expected,” Merchant said.
There is some uncertainty. Nature could do something different than what scientists might assume.
“But, generally, how things have behaved in the last 15 years is a good starting point for thinking about how they will behave in the next 15 years,” he added.
