The mean state of the tropical Pacific Ocean differed between the Medieval Warm Period and the Industrial Era

Changes in the El Niño Southern Oscillation over the last few decades have been linked to anthropogenic greenhouse gas emissions; however, the sign, magnitude, and drivers of these variations during the Common Era are not yet well constrained. Here, we present an 1800-year reconstruction of the El Niño Southern Oscillation mean-state derived from precipitation reconstructions based on lake sediments in East Asia and examine the long-term changes in the mean state and variance of the tropical Pacific. We find that the Medieval Warm Period was characterized by La Niña-like states with low variance, whereas El Niño-like states prevailed during the Little Ice Age and the Current Warm Period with high variance. Temperature changes caused by effective radiative forcing were probably the drivers of this response during the pre-industrial period, whereas greenhouse gas forcing likely accounts for the variations in the tropical Pacific mean-state during the industrial period.