Ocean Temperature Analysis Suggests Recent Slowdown in Warming is Fleeting

Climate researchers taking a new perspective on the available data suggest the recent "slowdown" in the warming rate of the Northern Hemisphere is likely a fleeting reality.

Writing in the journal Geophysical Research Letters, a team of researchers from Penn State University report that internal variability of a natural temperature phenomenon known as the Atlantic multidecadal oscillation (AMO) may be the cause for the apparent slowdown of warming in the Northern Hemisphere.

"Some researchers have in the past attributed a portion of Northern Hemispheric warming to a warm phase of the AMO," said Penn State meteorology professor Michael Mann. "The true AMO signal, instead, appears likely to have been in a cooling phase in recent decades, offsetting some of the anthropogenic warming temporarily."

Mann and his colleagues suggest that prior research did not adequately account for AMO and variability caused by external forces.

"Initial investigations into the multidecadal climate oscillation in the North Atlantic were hampered by the short length of the instrumental climate record which was only about a century long," said Mann. "And some of the calculations were contaminated by long-term climate trends driven or forced by human factors such as greenhouse gases as well as pollutants known as sulfate aerosols. These trends masqueraded as an apparent oscillation."

In their analysis, the researchers account for externalities such as greenhouse gases and aerosols. When factoring these variants into climate calculations - what they call the "differenced-AMO" - the calculations reveal that the most recent decade of climate variability "fall within expected multidecadal variability."

"We conclude that the AMO played at least a modest role in the apparent slowing of warming during the past decade," Mann said in a statement. "As the AMO is an oscillation, this cooling effect is likely fleeting, and when it reverses, the rate of warming increases."