Abstract
The Pacific decadal oscillation (PDO)-the leading pattern of climate variability driving changes over the North Pacific and surrounding continents-is now thought to be generated by processes internal to the climate system
. According to this paradigm, the characteristic, irregular oscillations of the PDO arise from a collection of mechanisms involving ocean and atmosphere interactions in the North and tropical Pacific
. Recent variations in the coupled ocean-atmosphere system, such as the 2015 El Niño, ought to have shifted the PDO into its positive phase
. Yet, the PDO has been locked in a consistent downward trend for more than three decades, remanding nearby regions to a steady set of climate impacts. Here we show that the main multidecadal variations in the PDO index during the twentieth century, including the ongoing, decades-long negative trend, were largely driven by human emissions of aerosols and greenhouse gases rather than internal processes. This anthropogenic influence was previously undetected because the current generation of climate models systematically underestimate the amplitude of forced climate variability. A new attribution technique that statistically corrects for this error suggests that observed PDO impacts-including the ongoing multidecadal drought in the western United States-can be largely attributed to human activity through externally forced changes in the PDO. These results indicate that we need to rethink the attribution and projection of multidecadal changes in regional climate.