Slowdown of global warming fleeting

UNIVERSITY PARK, Pa. The recent slowdown in the warming rate of the Northern Hemisphere may be a result of internal variability of the Atlantic Multidecadal Oscillation - a natural phenomenon related to sea surface temperatures, according to Penn State researchers. "Some researchers have in the past attributed a portion of Northern Hemispheric warming to a warm phase of the AMO," said Michael E. Mann , Distinguished Professor of Meteorology. "The true AMO signal, instead, appears likely to have been in a cooling phase in recent decades, offsetting some of the anthropogenic warming temporarily." According to Mann, the problem with the earlier estimates stems from having defined the AMO as the low frequency component that is left after statistically accounting for the long-term temperature trends, referred to as detrending. "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." Mann and his colleagues took a different approach in defining the AMO, which they report online in a special "Frontier" paper in Geophysical Research Letters.