Are Multi‐Decadal Fluctuations in Arctic and Antarctic Surface Temperatures a Forced Response to Anthropogenic Emissions or Part of Internal Climate Variability?

In this study, we investigate the drivers of observed multi‐decadal fluctuations in Arctic and Antarctic surface temperatures using multiple large ensembles of climate simulations and single‐forcing ensembles. We find that the observed oscillation in Arctic surface temperature around a linear trend since 1920 is a forced response to emissions of anthropogenic aerosols and greenhouse gases. In contrast, we show that observed multi‐decadal Antarctic surface temperature fluctuations are partially related to Pacific decadal variability which influences the climate of West Antarctica. Lastly, we demonstrate that internally driven multi‐decadal fluctuations at the two poles are not systematically correlated in any climate model examined here, as had been previously suggested. We conclude by discussing the implications of these results for understanding projections of Arctic and Antarctic surface climate of the coming decades. Plain Language Summary In this study, we investigate whether long‐term variations in Arctic and Antarctic surface temperatures over the past century were caused by emissions due to human activity or internal climate variability. Using a number of climate model simulations, we find that the periods of Arctic cooling during 1930–1975 and enhanced Arctic warming during 1975–2019 can be explained as a response to industrial aerosol and greenhouse gas emissions. In contrast, we show that slow modes of Pacific climate variability are important for understanding observed variations in Antarctic surface temperatures. Lastly, we demonstrate that in all of the climate models examined here, the climates of the Arctic and Antarctic are not connected on multi‐decadal timescales due to climate variability, as had been previously suggested. We conclude by discussing the implications of these results for understanding projections of Arctic and Antarctic surface climate of the coming decades. Key Points Observed multi‐decadal fluctuations in Arctic surface temperature are a forced response to anthropogenic aerosols and greenhouse gases In contrast, Pacific decadal variability is important for understanding observed Antarctic surface temperature fluctuations No modeling evidence for the reported 20th century “bipolar seesaw” of Arctic and Antarctic surface temperatures