A cause for pause? Scientists offer reasons for global warming 'hiatus'

Climate scientists say Earth's recent slowdown in warming is the result of decades-long heating and cooling cycles in the Atlantic and Pacific oceans. (NASA/Barcroft)

It's been dubbed both a "pause" and a "faux pause," and it's ignited debate among climate scientists and their critics.

After a period of rapid global warming throughout most of the 20th century, the pace of global temperature rise has slowed greatly over the last 10 to 15 years.

This unexpected slowdown has raised questions about the accuracy of climate change forecasts, and sent scientists searching for an explanation.
In a paper published Thursday in the journal Science, climate researchers argue that this slowdown is the result of natural and decades-long variations in sea water temperatures in the Pacific and Atlantic oceans.

These decadal oscillations, or swings, in sea temperature are tied to changes in ocean currents and wind patterns, according to researchers, and will play out for years before reversing themselves. Currently, the Pacific Ocean is in a cooling state and therefore masking the effects of human-caused global warming, according to the study authors.

It is not a condition that will last, they argue.

"In the next decade we will likely begin to see the flip side — instead of slowing global warming, this internal oscillation will likely add to global warming," said Michael Mann, a study coauthor and climatologist at Penn State.

"If so, we are in for a rude awakening," Mann said. "The false pause ... may have lulled us into a false complacency, a dangerous false complacency in fact."

Mann and his colleagues -- Byron Steinman, an environmental scientist at the University of Minnesota Duluth, and Sonya Miller, a meteorologist and climate modeler at Penn State -- based their conclusions on a new "semi-empirical" method that involves observational data and many climate model simulations.

The Atlantic and Pacific multidecadal oscillations occur on a scale of 50 to 70 years. The authors wrote that the Atlantic Ocean oscillation was more dominant during the middle of the last century, whereas the current cool Pacific Ocean oscillation has played a more dominant role in recent decades.

Mann said this Pacific Ocean trend was close to "bottoming out."
In an Insights article that accompanied the paper, climate scientist Ben Booth, of the United Kingdom's national weather service, or Met Office, said the research adds insight into the effects of natural ocean trends, but reflected limits in our ability to model climate correctly.

Booth said the effects of aerosols and clouds, which can reflect or trap heat, are often missing from many climate models. These effects include "a cluster of tropospheric volcanic eruptions in the past 10 years," which is not accounted for in the Science paper.

Nonetheless, Booth wrote that Mann and his colleagues had provided "much-needed longer-term context for the role that natural ocean-driven variations have played in past climate change."

Mann was among the team of scientists who established the well-known "hockey stick graph," which reconstructed 1,000 years of global temperature trends and illustrated rapid warming since the industrial age. He has frequently clashed with, and been vilified by, those who insist human activity and fossil fuel emissions have no clear impact on global climate.

On Thursday, Mann said he hoped the Science paper was "one more nail in the coffin of the oft-heard 'global warming has stopped' claim."