Surface global warming hiatus blamed on Pacific winds

Stronger than normal trade winds in the central Pacific are the main cause of a 13-year halt in global surface temperatures increases, an Australian study reveals.

The authors reject the suggestion that the study gives impetus to climate change deniers and instead suggest that when the winds ease, global warming will accelerate rapidly.

The heat that is there can find its way [again] to the surface of the ocean, it's not like we are getting a correction for global warming.- Matthew England, lead researcher

The research, published inNature Climate Change, shows more than 20 years of strengthening trade winds has increased the circulation of water in the western Pacific Ocean.

This has led to more heat being drawn from the atmosphere and circulated down into the subsurface ocean between 100 to 300 metres.

At the same time cooler waters are brought to the ocean's surface leading to a corresponding drop in surface temperature.

Lead author Matthew England, a climate scientist and oceanographer at theUniversity of New South Wales, says since 2001 global surface temperatures have remained steady despite an increase in atmospheric greenhouse gases.

Scientists had long suspected this conundrum was linked to increased heat uptake by the oceans, but how this was occurring remained elusive, he says.

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To solve this mystery England and colleagues from Australia and the US compared climatic trends over the past 20 years.

This covered the transition from a period of global surface warming in the 1990s to the current post-2000 hiatus.

They found wind acceleration in the Pacific Ocean over the past two decades was at levels twice as strong as previous accelerations, England says.

"Basic physics of the ocean is if you blow those winds stronger along the Pacific that pushes a lot of warm water to the west Pacific and pushes water into the interior of the ocean," says England.

Rather than let the climate models "freely evolve", the team specified these wind trends into the models and then examined the impact.

"When you do that you do get a lot of heat taken up by the ocean and you also get a much cooler east Pacific and that also drives further cooling in other regions," he says.

Not unique

England says the current hiatus is not unique in global warming history with another period of plateauing temperatures in the 30 years after World War II.

"A lot of people thought [the previous hiatus] was because we started emitting a lot of aerosols which reflects incoming solar radiation," says England.

However he says this study shows a corresponding acceleration in wind trends in the Pacific at that time suggesting the "mid-20th century hiatus also a case of strengthening trade winds".

England says it would be wrong for governments to use the respite in increasing surface temperatures to argue against action.

"All this talk of a stall in global warming refuting the science isn't actually the case," he says.

"There has been no hiatus at all in my opinion in terms of global warming because the fact is the ocean is just taking up a lot more heat in the last 10 years or so than it did previously."

"Even though there is this hiatus in this surface average temperature, we're still getting record heat waves, we're still getting harsh bush fires it shows we shouldn't take any comfort from this plateau in global average temperatures."

England says the study also shows oceans "don't take this heat up without some impact".

The study finds sea levels in the West Pacific have risen at a rate faster than the rest of the world.

'Isn't buried for good'

Importantly for ongoing warming trends the heat "isn't buried for good", adds England.

"It actually will resurface in time. The heat that is there can find its way [again] to the surface of the ocean, it's not like we are getting a correction for global warming."

He expects trade wind strength will return to normal by about 2016 or after a series of El Nio weather events.

The research suggests when this happens heat will quickly accumulate in the atmosphere.

"So global temperatures look set to rise rapidly out of the hiatus, returning to the levels projected within as little as a decade," he says.

England says while the acceleration of the winds is driven half by natural variability, the next area of research is to find out what other factors are involved.

Critically, England says, further modelling shows increasing greenhouse gas emissions will affect any future temporary cessation in warming.

Eventually warming caused by greenhouse gas emission will "overwhelm" the cooling effect of increased heat uptake by the oceans.

"We show by 2030 these hiatus will be a thing of the past if we keep increasing greenhouse gases and in the future any hiatus will be shorter lived and much less effective at halting global warming," says England.