Has global warming paused?

Winds in the Pacific Ocean may hold the answer as to why average air temperatures around the world have risen less than scientists predicted 10 years ago. By James Mitchell Crow.

Stronger winds are driving ocean currents that drag warm surface waters deep into the ocean in the western Pacific. – Roger Ressmeyer/CORBIS

Global warming has many climate scientists pretty hot under the collar right now. But it’s not because of rising temperatures, quite the opposite: in the last decade global temperatures have barely budged.

From the late 1970s to the late 1990s, average air temperatures around the world rose by around half a degree. But since then, the warming seems to have paused. Global air temperatures today are barely any higher than they were in 2000, almost a decade and a half ago.

Computer models did not predict this. Bumps in the road were expected, but overall the climate was predicted to keep on warming at roughly 0.2° per decade. It has started to become a bit of an embarrassment to the field.

It wasn’t so bad at the beginning of the pause. “When people asked what was going on we would say it is just natural variation,” says Matthew England, a climate researcher at the University of New South Wales. Some years are simply hotter than others, and the pause would soon end. “But then the hiatus got longer and longer,” he says. Today, average air temperatures are cooler than the very coolest scenarios predicted by the models. According to US record keepers the National Oceanic and Atmospheric Administration, 2013 was equal-fourth hottest year on record – the same as 2003.

But don’t go rushing to buy oceanfront real estate in Kiribati. Unfortunately for the low-lying Pacific island state – and the rest of us – climate scientists are more convinced than ever that global warming is real. As the latest IPCC report showed, the evidence is all around us, from disappearing glaciers and Arctic sea ice to sea levels that have risen 20 centimetres on average in the last century. So the climate models must be missing something. Now some researchers think they have finally found it.

An unprecedented acceleration of the winds blowing across the Pacific, which cannot yet be explained, is whipping up the ocean.

The answer – as Bob Dylan once crooned – is blowin’ in the wind. The Pacific trade winds to be precise. England’s team has found that an unprecedented acceleration of the winds blowing across the Pacific, which cannot yet be explained, is whipping up the ocean. And that in turn is churning more hot air into the cold deeper water below, effectively cooling the atmosphere like a global air-conditioner. The heat loss can fully explain the global warming hiatus, England says.

The idea that the ocean is absorbing more heat than usual is not new. We know that 93% of the extra heat trapped on Greenhouse Earth this last century has ended up in the ocean. So researchers suspected that the ocean was a part of the puzzle. What was missing was the mechanism to explain it.

To find it, England looked at the pronounced change in wind patterns blowing across the Pacific. Starting in the mid-1990s, weather observations recorded a dramatic acceleration in the easterly winds blowing from South America toward Indonesia. For the past two decades, the wind has been 30% stronger than the long-term average.

“The amount the winds have accelerated has never been seen before – and none of the climate models get close to capturing a trend like it,” says England. “We started to look at what would those winds do to the climate system.”

When the team manually entered these winds into climate models, they saw a major increase in the amount of heat being drawn down into the ocean. Almost two decades of strong winds have dramatically wound up the conveyor belt-like ocean currents that drag warm surface waters deep into the ocean in the western Pacific.

Although the degree of wind acceleration is unprecedented, a cycle of increasing and then diminishing wind speed is part of the well-known pattern called the Interdecadal Pacific Oscillation (IPO). From the late 70s to the late 90s –corresponding exactly to the period of intense atmospheric warming –the IPO was in a “positive phase”, characterised by weak winds, warm tropical waters, and El Nino type weather patterns – hot and dry in Australia, and wet in the eastern Pacific. At the end of the 90s, the trend flipped toward La Nina-like conditions bringing generally wetter weather to Australia. Predictably the wind started to strengthen – but there was nothing predictable about how strong it actually got. The 30% increase observed in wind strength is close to double what has been seen in previous negative phases of the IPO.

Late last year, Kevin Trenberth at the National Center for Atmospheric Research in Colorado, US, published his own research linking the global warming hiatus with the IPO phase.

“England’s paper shows quite convincingly that there are changes in the mixing of the ocean that do carry some of the heat down into deeper parts of the Pacific where it might not otherwise have gone,” he says. But Trenberth points out that changes in Pacific Ocean circulation patterns certainly affect the world’s other oceans as well, potentially increasing the heat uptake elsewhere too.

Regardless of where exactly each calorie of extra heat goes, what is certain is that the air-conditioning system is set to turn down a notch. Sooner or later – almost certainly within the decade - the strong Pacific winds will slow as the IPO pendulums back to ‘positive’ conditions. And when they do, the air conditioner will go into reverse cycle: heat sunk into the ocean will radiate out again putting global warming right back on track. Within a decade or so of the winds returning to normal, it will be like the hiatus never happened.

“You can get this hiatus lasting for much of the present decade if the winds stay strong,” England says. “But when these winds eventually do go back to normal, heat from the ocean gets back into the atmosphere very quickly.”

England is currently examining what caused the Pacific winds to accelerate as they did. Understanding the process will help climate scientists incorporate it into their models, increasing their forecasting accuracy as the IPO swings back and forth over coming decades.

“Palaeoclimatologists have been saying for a while that they see much greater decadal wind variability than the models capture,” says England. “So it is a big question to resolve.”

And for those of us who hoped the warming hiatus would either buy us time, or better still, meant climate change was just a bad dream. Sorry. Climate change is here to stay.

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