Introduction

Abstract

Human-induced global warming is superimposed upon naturally occurring patterns of multi-decadal variability of the unforced climate system, of which there are at least two: ENSO-related variability centered in the Pacific sector, commonly referred to as the Pacific Decadal Oscillation (PDO) and the so-called Atlantic Multidecadal Oscillation (AMO). The PDO is a coupled atmosphere-ocean phenomenon involving wind-driven oceanic upwelling whereas Atlantic multidecadal variability involves fluctuations in the strength of the oceanic thermohaline circulation (THC). It has been suggested that human-induced global warming might shift the polarity of the PDO and AMO in a preferred direction, e.g., forcing the AMO by slowing down the THC. Naturally occurring, unforced multidecadal variability, in turn, regulates the rate of global warming; for example, during the 1980s and 1990s the rate of global warming may have been enhanced by an apparent speeding up of the thermohaline circulation, as evidenced by a pronounced warming of the subpolar North Atlantic. In much of the literature on climate sensitivity and global warming, the multidecadal variability in the climate record tends to be attributed to temporal variations in the forcing by industrial and volcanic aerosols and the sun’s emission, without careful consideration of the role of this unforced variability.