The Arctic Multi-Decadal AMO Oscillation (AMO), has a strong link to observed Global temperatures, and is far better correlated to observed global temperature trends then any single atmospheric parameter (including carbon-dioxide).
The AMO operates over a 20 to 40 year period and is poorly predicted in Global Climate Models (GCMs). When the AMO is in the warm phase, droughts are more prevalent and hurricane activity tends to be enhanced. The cause of a negative phase shifting in the AMO, is a direct result of a slight weakening in the Gulf Stream. The temperature fluctuations resulting from the AMO extend from the equator to the north-Atlantic basin, with temperatures up to +0.5'C between extremes.
The AMO is a major factor in determining the mild climate of the Northern-hemisphere and Arctic ice-sheet. The AMO makes attribution of carbon-dioxide to temperature rises very difficult. Since the 1990's the AMO has been in a strong negative phase, but recently there are signs of an abrubt cooling due to a weakening (not shut-down) of the Gulf stream flow.
The negative phase of the AMO is likely to lead to far lower temperatures in North and West Europe, and the entire Northern-Hemisphere.
http://www.cdc.noaa.gov/Timeseries/AMO/
