The Arctic Bellweather Is Ringing!: Paleoclimate Data Quantify the Magnitude of Past Arctic Amplification

Paleoclimate estimates of Arctic summer temperature anomalies may be compared to Northern Hemisphere or global summer temperature anomalies to constrain the magnitude of positive feedbacks that amplify Arctic summer temperature change. Four past time intervals have sufficient paleoclimate data to make these comparisons with realistic estimates of their uncertainties. These are the Holocene thermal maximum (HTM; ~8 ka ago), the last glacial maximum (LGM; ~20 ka ago), the last interglaciation (LIG; 130-125 ka ago), and the middle Pliocene (~3.5-3.0 Ma ago). For intervals colder and warmer than present, the Arctic summer temperatures exhibit substantially larger shifts than global averages. For all but the Pliocene, part of the explanation is related to orbital forcing that influences the summer energy balance across the entire Northern Hemisphere, but the insolation forcing is in the opposite direction for the Southern Hemisphere. Consequently, we compare Arctic anomalies with Northern Hemisphere, rather than global anomalies. This comparison results in a linear relation for which the trend line suggests that summer temperature changes are amplified 3 to 4 times in the Arctic over the Northern Hemisphere average. We suggest that the strongest feedbacks are related to changes in Arctic Ocean sea ice and the seasonal distribution of snow cover, with the Atlantic sector also strongly influenced by changes in the strength of the meriodonal overturning circulation in the North Atlantic Ocean. These previous warm intervals speak to the vulnerability of the Arctic to change ( e.g., sea ice duration, glaciers and ice sheets, and ecosystem shifts) decreasing the uncertainty of future consequences for modern society.