Deeper regions of the Pacific Ocean are warming 15 times faster now compared to previous warming phases over the past 10,000 years, a study has found.
The findings lend further weight to the idea that the recent “pause” in global surface temperatures may be due to large amounts of heat in the atmosphere being absorbed by the deep ocean, scientists said.
The study used indirect, “proxy” temperature readings estimated from the chemical makeup of the shells of tiny marine creatures which had been washed from the middle depths of the Pacific into seabed sediments that had built up off Indonesia.
These showed a gradual long-term cooling of the Pacific Ocean over thousands of years at depths of between 1,500 and 3,000 feet, until they started to rise slightly at the start of the Medieval Warm Period in northern Europe around 1100AD.
Temperatures then fell again with the rate of cooling increasing during the so-called Little Ice Age of the 17 and 18 Centuries, when “frost fairs” were held regularly on the frozen River Thames, the study found.
However, the temperature of the deeper Pacific Ocean over the past 60 years of direct thermometer readings has risen 15 times faster than they did during the warming cycles of the past 10,000 years, based on proxy measurements, said Braddock Linsley, a climate scientist at Columbia University’s Lamont-Doherty Earth Observatory in New York.
“Our work showed that intermediate waters in the Pacific had been cooling steadily from about 10,000 years ago. This places the recent warming of the Pacific intermediate waters in temporal context. The trend has now reversed in a big way and the deep ocean is warming,” Dr Linsley said.
“We’re experimenting by putting all this heat in the ocean without quite knowing how it’s going to come back out and affect the climate. It’s not so much the magnitude of the change, but the rate of change,” he said.
The study, published in the journal Science, is the latest to suggest that huge amounts of heat are being absorbed by the deep ocean. A previous study for instance found that changes to the cold Pacific current, called La Nina, may have resulted in the absorption of excess heat from the atmosphere.
Although global surface temperatures from land-based stations show that the world is warmer now than for thousands of years, the rate of increase has levelled off over the past 15 years or so, leading climate sceptics to question the link between global warming and carbon dioxide emissions, which have continued to increase during the same period.
The oceans and atmosphere are intimately related to one another, exchanging gases as well as heat, and heat energy can be transported to deep layers which can store vast amounts of heat for long periods of time.
One recent estimate for instance suggested that the heat being absorbed by the deep ocean is equivalent to the power generated by 150 billion electric kettles.
Yair Rosenthal of Rutgers University in New Jersey, who led the latest study, said that the findings indicate that the deep ocean may be storing far bigger quantities of heat than previous estimates had suggested.
“We may have underestimated the efficiency of the oceans as a storehouse for heat and energy. It may buy us some time, but how much time, I don’t really know, to come to terms with climate change. But it’s not going to stop climate change,” Professor Rosenthal said.
The temperatures of the Pacific over the past 10,000 were estimated from levels of magnesium and calcium in the shells of Hyalinea balthica, a one-celled organism that gets buried in the seabed sediments off Indonesia as water flows from the middle layers of the Pacific Ocean.
Kevin Trenberth, a climate scientist with the US National Centre for Atmospheric Research in Boulder, Colorado, said that global surface temperatures are unlikely to go down for any length of time and are more likely to start to rise again.
“With global warming you don’t see a gradual warming form one year to the next. It’s more like a staircase. You trot along with nothing much happening for 10 years and then suddenly you have a jump and things never go back to the previous level again,” Dr Trenberth said.
Drew Shindell, a climate scientist at Columbia University, said: “Surface temperature is only one indicator of climate change. Looking at the total energy stored by the climate system or multiple indicators – glacier melting, water vapour in the atmosphere, snow cover and so on – may be more useful than looking at surface temperature alone.”