Intermittent multidecadal-to-centennial fluctuations dominate global temperature evolution over the last millennium

Observed climate time series covering several centuries are often characterized by fluctuations on multidecadal-to-centennial timescales. These are not homogeneously distributed in time: Instead, they appear within irregularly intermittent temporal intervals, whose irregular duration varies, in general, with the signal fluctuation frequency. A similar irregularly intermittent, frequency-dependent appearance of energetic fluctuations is found in long-term Earth system model integrations, consisting of a multi-millennia control experiment (i.e., an unforced simulation) and forced simulations covering the last millennium. Here, for the first time, we investigate the long-term relative importance of internal and externally-driven variability and their possible interferences on Global Surface Temperature (GST). Multidecadal GST fluctuations are mostly associated to internal variability. Externally-forced perturbations acting predominantly on centennial timescales tend to overwhelm such variability and to enhance O(∼200 years) GST fluctuations. Externally-forced perturbations tend also to correspond to major changes in the coherency among internal climate processes, and among them and GST. Copyright 2010 by the American Geophysical Union.