Reconstruction and Attribution of Multidecadal Temperature Variability over the Common Era

Multidecadal surface temperature changes may be forced by natural as well as human factors, or arise unforced from the climate system. Distinguishing these factors is essential for estimating sensitivity to multiple forcings, and the amplitude of the unforced variability. Here we present 2000-year long global mean temperature reconstructions using seven different statistical methods that draw from a new global collection of temperature-sensitive paleoclimate records. Our new reconstructions display synchronous multi-decadal temperature fluctuations which are coherent with one another and with realistically forced CMIP5 millennial model simulations across the Common Era. The most significant attribution of pre-industrial variability at multi-decadal timescales is with aerosol forcing associated with explosive volcanic activity. The largest and most consistent warming trends at time-scales of 20 years and longer occur during the second half of the 20^th century, highlighting the exceptional character of the warming in recent decades within the context of the past one to two millennia. Reconstructions and simulations agree on the magnitude of unforced global mean temperature multidecedal variability (0.67°C [0.27, 1.64]), thereby increasing confidence in the ability of these models to project future decadal-scale climate change and conservative baseline estimates against which forced variations may be detected.