Ocean water has absorbed more than 90% of the excess heat and nearly 30% of the carbon dioxide generated by human consumption of fossil fuels

The world’s oceans are warming at a quickening rate, with the past 20 years accounting for half of the increase in ocean heat content that has occurred since pre-industrial times, a new study has found.

US scientists discovered that much of the extra heat in the ocean is buried deep underwater, with 35% of the additional warmth found at depths below 700 meters. This means far more heat is present in the far reaches of the ocean than 20 years ago, when it contained just 20% of the extra heat produced from the release of greenhouse gases since the industrial revolution.

The paper, published in Nature Climate Change, sheds further light on the vast quantities of heat being absorbed by the world’s oceans.

Ocean water, which has a much higher heat capacity than air, has absorbed more than 90% of the excess heat and nearly 30% of the carbon dioxide generated by human consumption of fossil fuels. The vast Southern Ocean sucked up 1.2bn tonnes of carbon in 2011 alone – which is roughly equivalent to the European Union’s annual carbon output.

Scientists at the Lawrence Livermore National Laboratory, working with National Oceanic and Atmospheric Administration, analyzed heat content changes in varying depths of the world’s oceans using data and models stretching back to 1865.

For the upper reaches of the ocean, above 700m, the data went back to the HMS Challenger expedition, a pioneering effort in oceanography which took place in the 1870s. The heat content of the upper ocean has increased by a factor of 32 since fossil fuel use became widespread.

More recent, and consistent, information has come from the network of 3,000 Argo floats that dot the globe’s oceans. New “Deep Argo” robots, capable of measuring heat at far greater depths than the standard surface-dwelling devices, gave scientists the best idea so far of how much warmth is buried deep underwater.

The deepwater heat content has increased by “several tenths of a degree” since the industrial revolution when averaged out across the globe, according to Peter Gleckler, lead author of the paper. Gleckler said that while this is less than the 0.5C warming averaged across the upper reaches of the ocean, it is still a “huge increase” and is gaining pace.

“When we discuss global warming, the most familiar way we do that is talk about temperature changes on the surface – but it’s clear that the oceans are doing the bulk of the work in terms of absorbing the heat in the system,” he said. “And if we want to really understand how much heat is being trapped, we can’t just look at the upper ocean anymore, we need to look deeper.

“The findings are concerning. It’s clear evidence that the oceans are taking the brunt of the greenhouse gases and are accumulating a lot of heat. As for the ecological implications, that’s hard to say. There is a lot of life in the deep oceans and there’s lots we don’t know about the impact upon that life.”

As the oceans warm, storm intensity increases and aquatic species are forced from their traditional ranges. Absorption of carbon dioxide has also made the oceans 30% more acidic, which is when the pH of the water drops, making it harder for creatures such as coral, oysters and mussels to form the shells and structures that sustain them.

Scientists have already declared that a third global coral bleaching event is currently underway, where corals whiten and die off due to extreme heat. An analysis of more than 620 studies last year found that the food chains of the world’s oceans are at risk of collapse due to climate change, overfishing and localized pollution.