About 15 years ago, the Earth's temperature was rising fast. Most climate models predicted that trend would continue, as humans continued to pump greenhouse gases into the atmosphere.

Instead, the Earth's surface temperature over the past 13 years has been mostly stable.

In recent years, scientists have worked to understand why this "pause" in warming has occurred and was not predicted by models (ClimateWire Nov. 1, 2013).

"The fact that [the pause] has lasted a full 13 years has really challenged the scientific community to explain this mismatch between models and observations," said Matthew England, a climate scientist with the Australian Research Council's Centre of Excellence for Climate System Science.

It's not that researchers expected warming to be completely linear.

They'd seen slowdowns in the past, often associated with natural cycles in the Earth's climate -- England pointed to periods when the Earth has taken a break from warming, such as from 1945 to the late 1970s.

Nonetheless, the recent pause has left many scientists scratching their heads, he continued, especially given how much more heat-trapping carbon dioxide has entered the atmosphere in the past decades.

Winds driving heat storage in ocean
England thinks he has a persuasive explanation for the warming hiatus, which he and a number of co-authors published yesterday in the journal Nature Climate Change.

The findings outlined in the paper bolster the idea that much of the warmth that would otherwise have heated the Earth's surface has gone into the Pacific Ocean. This hypothesis, put forth a few years ago, has been receiving continued support in the published scientific literature (ClimateWire, April 8, 2013).

This study adds a reason for this heat storage in the Pacific Ocean: increases in trade winds blowing east to west.

This trade wind strengthening, which occurs during a the negative phase of a phenomenon called the Interdecadal Pacific Oscillation (also known as the Pacific Decadal Oscillation), pushes warm water westward and and changes Pacific Ocean circulation.

That wind-driven circulation change leads to cooler ocean temperatures on the surface of the eastern Pacific, and more heat being mixed in and stored in the western Pacific down to about 300 meters (984 feet) deep, said England.

Gerald Meehl, a climate scientist at the National Center for Atmospheric Research who was also an author on the paper, said this research expanded on past work, including his own research, that pointed to the Interdecadal Pacific Oscillation as a factor in a warming slowdown by finding a mechanism behind how the Pacific Ocean was able to store enough heat to produce a pause in surface warming.

"This paper makes the case that, though other factors could contribute somewhat to the early-2000s hiatus, the Pacific is a major driving force in producing naturally-occurring climate variability that can overwhelm the warming from ever-increasing greenhouse gases to produce the hiatus," Meehl wrote in an email.

The researchers were able to test their hypothesis that stronger winds were driving the ocean heat uptake by putting the observations of wind behavior into climate models.

When ocean cycle shifts, globe is likely to warm up
When climate models were run that included the stronger winds, they were able to reproduce the slowdown in surface temperatures.

"This wind acceleration in the Pacific Ocean is of a magnitude that can fully account for the hiatus, and it certainly accounts for the mismatch between models and observations," said England.

While it is still possible that other factors, such as heat storage in other oceans or an increase in aerosols, have led to cooling at the Earth's surface, this research is yet another piece of evidence that strongly points to the Pacific Ocean as the reason behind a slowdown in warming.

"The paper makes a convincing case for the importance of Pacific processes that can make major contributions to hiatus periods," said NCAR's Meehl.

Like all climate cycles, the Interdecadal Pacific Oscillation will, at some point, flip from negative to positive again. When this happened in the late 1970s, the Earth's surface temperatures shot up.

Although scientists are unable to predict when the oscillation will switch modes, when it does, the heat, since it is not stored very deep in the ocean, can "readily resurface," said England.

"When that occurs, it's highly likely that the air temperature change over the planet will be one of relatively rapid warming, probably exceeding the warming rate of the '80s and '90s actually, because greenhouse gases are much, much higher today than they were even just 30 years ago," he said.
