NOAA/WDS Paleoclimatology - Moose Lake, Alaska 6000 Year Midge Inferred July Temperature

Despite their importance for evaluating anthropogenic climatic change, quantitative temperature reconstructions of the Holocene remain scarce from northern high latitude regions. We conducted high-resolution midge analysis on the sediments of the past 6000 years from a lake in south-central Alaska. Results were used to estimate summer temperature (TJuly) variations on the basis of a midge temperature transfer function. The TJuly estimates from the near-surface samples are broadly consistent with instrumental and treering-based temperature data. Together with previous studies, these results suggest that midge assemblages are more sensitive to small shifts in summer temperature (~0.5°C) than indicated by the typical error range of midge temperature transfer functions (~1.5°C). A piecewise linear regression analysis identifies a significant change point at ca. 4000 years before present (cal. BP) in our TJuly record, with a decreasing trend after this point. Episodic TJuly peaks (~14.5°C) between 5500 and 4200 cal. BP and the subsequent climatic cooling may have resulted from decreasing summer insolation associated with the precessional cycle. Centennial-scale climatic cooling of up to 1°C occurred around 4000, 3300, 1800-1300, 600, and 250 cal. BP. These cooling events were more pronounced and lasted longer during the last two millennia than between 2000-4000 cal. BP. Some of these events have counterparts in climatic records from elsewhere in Alaska and other regions of the Northern Hemisphere, including several roughly synchronous with known grand minima in solar irradiance. Over the past 2000 years, our TJuly record displays patterns similar to those inferred from a wide variety of temperature proxy indicators at other sites in Alaska, including climatic fluctuations coeval with the Little Ice Age (~245 cal. BP), the Medieval Climate Anomaly (~1000 cal. BP), and the First Millennial Cooling (~1400 cal. BP). To our knowledge, this study offers the first high-resolution, quantitative record of summer-temperature variation that spans longer than the past 2000 years from the high-latitude regionsaround the North Pacific.