The world's oceans have been soaking up far more excess heat in recent decades than scientists realised, suggesting that Earth could be set to warm even faster than predicted in the years ahead, according to new research published yesterday.

Over the past quarter century, the Earth's oceans have retained 60 per cent more heat each year than scientists previously had thought, said Laure Resplandy, a geoscientist at Princeton University who led the startling study published yesterday in the journal Nature.

The difference represents an enormous amount of additional energy, originating from the sun and trapped by the Earth's atmosphere — more than eight times the world's energy consumption, year after year.

In the scientific realm, the new findings help to resolve long-running doubts about the rate of the warming of the oceans prior to the year 2007, when reliable measurements from devices called Argo floats were put to use worldwide. Before that, different types of temperature records — and an overall lack of them — contributed to murkiness about how quickly the oceans were heating up.

Advertisement

The higher-than-expected amount of heat in the oceans means more heat is being retained within the Earth's climate system each year, rather than escaping into space. In essence, more heat in the oceans signals that global warming itself is more advanced than scientists thought.

"We thought that we got away with not a lot of warming in both the ocean and the atmosphere for the amount of CO2 that we emitted," said Resplandy, who published the work with experts from the Scripps Institution of Oceanography and several other institutions in the United States, China, France, and Germany.

"But we were wrong. The planet warmed more than we thought. It was hidden from us just because we didn't sample it right. But it was there. It was in the ocean already."

The study also could have important policy implications. If ocean temperatures are rising more rapidly than previously calculated, that could leave nations even less time to dramatically cut the world's emissions of carbon dioxide, in hopes of limiting global warming to the ambitious goal of 1.5C above pre-industrial levels.

The world already has warmed 1C since the late 19th century. Scientists backed by the United Nations reported last month that with warming projected to steadily increase, the world faces a daunting challenge in trying to limit that warming to only another half degree Celsius. The group found that it would take "unprecedented" action by leaders across the globe over the coming decade to even have a shot at that goal.

The new research underscores the potential consequences of global inaction. Faster warming oceans mean that seas will rise faster and that more heat will be delivered to critical locations that already are facing the effects of a warming climate, such as coral reefs in the tropics and the ice sheets of Greenland and Antarctica.

"In case the larger estimate of ocean heat uptake turns out to be true, adaptation to — and mitigation of — our changing climate would become more urgent," said Pieter Tans, leader of the Carbon Cycle Greenhouse Gases Group at NOAA, who was not involved in the study.

The oceans absorb more than 90 per cent of the excess energy trapped within the world's atmosphere.

The new research does not measure the ocean's temperature directly. Rather, it measures the volume of gases, specifically oxygen and carbon dioxide, that have escaped the ocean in recent decades and headed into the atmosphere as it heats up. The method offered scientists a reliable indicator of ocean temperature change because it reflects a fundamental behaviour of a liquid when heated.

"When the ocean warms, it loses some gas to the atmosphere," said Resplandy. "That's an analogy that I make all the time: If you leave your Coke in the sun, it will lose the gas."

This approach allowed researchers to recheck the contested history of ocean temperatures in a different and novel way. In doing so, they came up with a higher number for how much warming the oceans have experienced over time.

"I feel like this is a triumph of earth system science. That we could get confirmation from atmospheric gases of ocean heat content is extraordinary," said Joellen Russell, a professor and oceanographer at the University of Arizona. "You've got the A-team here on this paper."

But Russell said the findings themselves are hardly as uplifting, saying they "have implications for climate sensitivity, meaning how warm does a certain amount of CO2 make us". He said the world could have a smaller "carbon budget" than once thought.

That budget refers to the amount of carbon dioxide humans can emit while still being able to keep warming below dangerous levels.

The scientists calculated that because of the increased heat already stored in the ocean, the maximum emissions that the world can produce while still avoiding a warming of 2C would have to be reduced by 25 per cent. This represents a very significant shrinkage of an already very narrow carbon "budget".

The UN panel of climate scientists said recently that global carbon emissions must be cut in half by 2030 if the world hopes to remain beneath 1.5C of warming. But Resplandy said that the evidence of faster warming ocean "shifts the probability, making it harder to stay below the 1.5C temperature target.

Understanding what's happening with Earth's oceans is critical simply because they, far more than the atmosphere, are the mirror of ongoing climate change.

According to a major climate report released last year by the US Government, the world's oceans have absorbed about 93 per cent of the excess heat caused by greenhouse gases since the mid-20th century. Scientists have found that ocean heat has increased at all depths since the 1960s, while surface waters also have warmed.

The world's oceans also currently absorb more than a quarter of the carbon dioxide emitted annually from human activities — an effect making them more acidic and threatening fragile ecosystems, federal researchers say. "The rate of acidification is unparalleled in at least the past 66 million years," the government climate report stated.

Paul Durack, a research scientist at the Lawrence Livermore National Laboratory in California, said yesterday's study offers "a really interesting new insight" but is "quite alarming".

The warming found in the study is "more than twice the rates of long-term warming estimates from 1960s and 70s to the present", Durack said, adding that if these rates are validated by further studies, "it means the rate of warming and the sensitivity of the earth's system to greenhouse gases is at the upper end".

He said that if scientists have underestimated the amount of heat taken up by the oceans, "it will mean we need to go back to the drawing board" on the aggressiveness of mitigation actions the world needs to take to limit future warming.

WHAT THE REPORT SAYS

• The Earth's oceans have retained 60 per cent more heat each year for the past 25 years than scientists previously had thought.

• More heat in the oceans means that global warming itself is more advanced than scientists thought.

• That could leave nations less time to dramatically cut the world's emissions of carbon dioxide in hopes of limiting global warming to 1.5C above pre-industrial levels.

• To prevent temperatures rising above 2C, carbon emissions from human activities must be reduced by 25 per cent more than previously estimated.