Washington: Scientists say the world's oceans are warming far more quickly than previously thought, a finding with dire implications for climate change because almost all the excess heat absorbed by the planet ends up stored in their waters.
A new analysis, published on Thursday in the journal Science, found that the oceans are heating up 40 per cent faster on average than a UN panel estimated five years ago. The researchers also concluded that ocean temperatures have broken records for several straight years.
"2018 is going to be the warmest year on record for the Earth's oceans," said Zeke Hausfather, an energy systems analyst at independent climate research group Berkeley Earth and an author of the study. "As 2017 was the warmest year, and 2016 was the warmest year."
As the planet has warmed, the oceans have provided a critical buffer. They have slowed the effects of climate change by absorbing 93 per cent of the heat trapped by the greenhouse gases humans pump into the atmosphere.
But the rising water temperatures are already killing off marine ecosystems, raising sea levels and making hurricanes more destructive.
As the oceans continue to heat up, those effects will become more catastrophic, scientists say.
Coral reefs, whose fish provide key sources of protein to millions of people, will come under increasing stress; a fifth of them have already died in the last three years.
Rainier, more powerful storms like Hurricane Harvey in 2017 and Hurricane Florence in 2018 will become more common, and coastlines around the world will flood more frequently.
The report comes as studies show sections of the Great Ocean Road are at risk of being washed away in part because of a rising sea level along Victoria's coast.
Because they play such a critical role in global warming, oceans are one of the most important areas of research for climate scientists. Average ocean temperatures are also a consistent way to track the effects of greenhouse gas emissions because they are not influenced much by short-term weather patterns, Hausfather said.
"Oceans are really the best thermometer we have for changes in the Earth," he said.
But, historically, understanding ocean temperatures has also been difficult. An authoritative UN report, issued in 2014 by the Intergovernmental Panel on Climate Change, presented five different estimates of ocean heat, but they all showed less warming than the levels projected by computer climate models — suggesting that either the ocean heat measurements or the climate models were inaccurate.
Since the early 2000s, scientists have measured ocean heat using a network of drifting floats called Argo, named after Jason's ship in Greek mythology. The floats measure the temperature and saltiness of the upper 6500 feet of the ocean and upload the data via satellites.
But before Argo, researchers relied on temperature sensors that ships lowered into the ocean with a copper wire. The wire transferred data from the sensor to the ship for recording until the wire broke and the sensor drifted away.
That method was subject to uncertainties, especially around measurement depth, that hamper today's scientists as they stitch together 20th-century temperature data into a global historical record.
In the new analysis, Hausfather and his colleagues assessed three recent studies that better accounted for the older instrument biases. The results converged at an estimate of ocean warming that was higher than the IPCC predicted and more in line with the climate models.
The researchers also reviewed a fourth study that had used a novel method to estimate ocean temperatures over time and had also found that the world's oceans were heating faster than the IPCC prediction.
But that study contained an error that caused its authors to revise their estimates downward, suggesting that ocean warming was less of a problem than they originally reported.
As it turned out, the downward revision brought that study's estimates much closer to the new consensus.
"The correction made it agree a lot better with the other new observational records," Hausfather said. "Previously it showed significantly more warming than anyone, and that was potentially worrisome because it meant our observational estimates might be problematic. Now their best estimate is pretty much dead-on with the other three recent studies."
The scientists who published the four studies were not trying to make their results align, Hausfather said.
"The groups who were working on ocean heat observations, they're not climate modelers," he said.
"They're not particularly concerned with whether or not their observations agree or disagree with climate models."
Laure Zanna, an associate professor of climate physics at the University of Oxford who was not involved in the study, said the new research was "a very nice summary of what we know of the ocean and how far the new estimates have come together."
Zanna was an author of a recent study that used existing data to estimate ocean temperatures dating back to 1871. The goal was to figure out places where sea level rise might happen even faster than expected because of the way ocean currents redistribute heat, allowing regions that are especially at risk to better plan for those changes.
As the oceans warm, sea levels rise because warmer water takes up more space than colder water. In fact, most of the sea level rise observed to date is because of this warming effect, not melting ice caps.
"We are warming the planet but the ocean is not warming evenly, so different places warm more than others," said Zanna. "And so the first consequence will be that sea level will be different in different places depending on the warming."
Though the new findings provide a grim forecast for the future of the oceans, Hausfather said that efforts to mitigate global warming, including the 2015 Paris climate agreement, would help.
"I think there's some reason for confidence that we'll avoid the worst-case outcomes," he said, "even if we're not on track for the outcomes we want."
The New York Times