The "pause" in global warming since 2001 can be explained by the discovery of unusually strong winds in the Pacific, climatologists have found.
Global surface air temperatures have more or less flatlined since the turn of the century, prompting some observers to claim that the planetary warming trend has stopped. But the new research, published in Nature Climate Change, shows how stronger winds have driven the excess heat down into the ocean.
Researchers led by Matthew England, a professor of climatology at the University of New South Wales, began by looking at the differences between the 1990s, when Earth's surface was strongly warming, and the 2000s, after the hiatus began.
Previous research has already shown that cooler temperatures over the eastern Pacific are linked to a slowdown in worldwide warming, but researchers wanted to know why.
Data from ships and weather buoys revealed an unusual strengthening of the Pacific trade winds, which blow east-to-west near the Equator. In turn, that speeds up the "overturning circulation" of Pacific waters, driving warmer water down to depths of up to 300 metres in the western Pacific, while cooler water wells up in the ocean's east.
When plugged into climate models, the effect of the strengthened winds is equivalent to 0.1-0.2C of surface cooling - which accounts for almost all of the observed slowdown in global surface temperatures.
Previous climate models missed this effect because the strong winds had never been observed before, Professor England said.
That doesn't mean that climate models are "wrong", he insisted, although they are still better at predicting what might happen by the end of the century than at the end of the decade.
"The models are improving all the time, but in this case they missed the dramatic observed wind trend. Suddenly something comes along like this wind acceleration that goes beyond what you've ever seen before, so maybe it's no surprise that the models don't capture it when it is such an extreme event," he said.
Watching the wind
It is unclear exactly what has driven the upswing in wind intensity, the researchers said. It could be natural variation, or driven by another climate factor such as the warming of the adjoining Indian Ocean.
Professor England said it will be difficult to predict when the strengthened winds will ease. But when they do, the world is likely to see a resumption of warming air temperatures.
"That's a very important thing to keep track of and we need to keep making measurements of the oceans, to be sure to detect the first signs of when this change occurs."
Richard Allan, a professor of climate science at the University of Reading in the UK said: "We don't know what is causing these unprecedented changes, but the implications could be substantial. It would be surprising if these large changes in atmospheric and ocean circulation over the last two decades, including also a potentially long-term decline in the Atlantic ocean circulation, have not already disrupted our weather patterns."
Steve Rintoul, a researcher with the CSIRO's Marine and Atmospheric Research, said: "More than 93% of the warming of the planet since 1970 is found in the ocean, according to the IPCC report released last week. If we want to understand and track the evolution of climate change, we therefore need to look in the oceans. The oceans have continued to warm unabated, even during the recent "hiatus" in warming of surface temperature."
Professor England said his results fit with the observation that global surface warming can be held in check for a decade or two by the Pacific Ocean entering a "negative phase" of what is known as the Interdecadal Pacific Oscillation. As is occurring now, this state is characterised by cooler surface temperatures in the eastern Pacific, and has been associated with previous historical periods when planetary warming has stalled, such as the 1940s to 1970s.
"Climate scientists have long understood that global average temperatures don't rise in a continual upward trajectory, instead warming in a series of abrupt steps in between periods with more-or-less steady temperatures. Our work helps explain how this occurs," he said.
In contrast, previous positive phases of the Interdecadal Pacific Oscillation, when eastern Pacific surface waters are warmer than average, have underpinned strong warming episodes such as that seen during the 1980s to 1990s.
"We should be very clear: the current hiatus offers no comfort - we are just seeing another pause in warming before the next inevitable rise in global temperatures," Professor England said.