• Please note: Nature published an update on the uncertainties in these results on 19th November.

The seas are getting hotter – and researchers have thought again about just how much faster ocean warming is happening. They believe that in the last 25 years the oceans have absorbed at least 60% more heat than previous global estimates by the UN’s Intergovernmental Panel on Climate Change (IPCC) had considered.

And they calculate this heat as the equivalent to 150 times the annual human electricity generation in any one year.

“Imagine if the ocean was only 30 feet (10 m) deep,” said Laure Resplandy, a researcher at the Princeton Environment Institute in the US. “Our data show that it would have warmed by 6.5 °C every decade since 1991. In comparison, the estimate of the last IPCC assessment report would correspond to a warming of only 4 °C every decade.”

The oceans cover 70% of the Blue Planet, but take up about 90% of all the excess energy produced as the Earth warms. If scientists can put a precise figure to this energy, then they can make more precise guesses about the surface warming to come, as humans continue to burn fossil fuels, release greenhouse gases such as carbon dioxide into the atmosphere, and drive up the planetary thermometer.

At the academic level, this is the search for a factor known to climate researchers as climate sensitivity: the way the world responds to ever-increasing ratios of greenhouse gas in the atmosphere.

At the human level, this plays out as ever-greater extremes of heat, drought and rainfall, with ever-higher risks of catastrophic storm or flood, or harvest failure, and ever-higher tallies of human suffering.

Comprehensive global measurements of ocean temperature date only from 2007 and the network of robot sensors that deliver continuous data about the top half of the ocean basins.

Resplandy and her colleagues report in the journal Nature that they used a sophisticated approach based on very high-precision measurements of levels of oxygen and carbon dioxide in the air.

Gases released

Both gases are soluble, and the oceans are becoming more acidic as the seas absorb ever-greater levels of carbon dioxide. But as seas warm, they also become less able to hold their dissolved gases, and release them into the atmosphere.

This simple consequence of atmospheric physics meant that the researchers could use what they call “atmospheric potential oxygen” to arrive at a new way of measuring the heat the oceans must have absorbed over time.

They used the standard unit of energy: the joule. Their new budget for heat absorbed each year between 1991 and 2016 is 13 zettajoules. That is a digit followed by 21 zeroes, the kind of magnitude astronomers tend to use.

That the oceans are warming is no surprise: this has been obvious from the crudest comparison of old naval data with modern surface checks, and for years some researchers argued that ever-higher ocean temperatures could account for the so-called slowdown in global warming in the first dozen years of this century.

The new finding counts first as an academic achievement: there is now a more precise thermometer reading, and new calculations can begin.

One of the researchers, Ralph Keeling of the Scripps Institution of Oceanography, said: “The result significantly increases the confidence we can place in estimates of ocean warming and therefore help reduce uncertainty in the climate sensitivity, particularly closing off the possibility of very low climate sensitivity.”

But the result also suggests that internationally agreed attempts to hold planetary warming to a maximum of just 2 °C – and the world has already warmed by around 1 °C in the last century – become more challenging.

It means that there will have to be an even more drastic shutdown of fossil fuel investment and an even faster switch to renewable sources of energy such as sun and wind power.

• This article first appeared at Climate News Network.