| Summary: | Rapid changes in the Arctic climate system that occurred in the relatively recent past can be compared with the output of climate models to improve the understanding of the processes responsible for nonlinear system change. In particular, this study focuses on the transition between the Holocene thermal maximum (HTM) and the onset of Neoglaciation, and on the step-like changes that occurred subsequently during the late Holocene. The millennial-scale cooling trend that followed the HTM coincides with the decrease in Northern Hemisphere summer insolation driven by slow changes in Earth's orbit. Despite the nearly linear forcing, the transition from the HTM to the Little Ice Age (1500-1900 AD) was neither gradual nor uniform. To understand how feedbacks and perturbations result in rapid changes, a geographically distributed network of proxy climate records was examined to study the spatial and temporal patterns of change, and to quantify the magnitude of change during these transitions. During the HTM, summer sea-ice cover over the Arctic Ocean was likely the smallest of the present interglacial period; certainly it was less extensive than at any time in the past 100 years, and therefore affords an opportunity to investigate a period of warmth similar to what is projected during the coming century. This study focuses on lakes because lakes are the most widely distributed sources of proxy climate records that consistently extend through the post-glacial interval. Because climate change is amplified in the Arctic, the climate signal preserved in Arctic lake sediments should be stronger than elsewhere. Confidence in the paleoclimate reconstructions was bolstered by a multi-proxy approach, and by replicate lake records to distinguish basin-scale thresholds from regional-scale climate shifts. The results we provide are part of a collaborative project (with ~10 other PIs) who all used lacustrine sediments to produce new high-resolution proxy climate records of the past 8000 years. The study sites we focused on were in southeastern Greenland, where four lakes were studied. Sediment cores were recovered, dated and analyzed for organic and inorganic content. Each data set was then interpreted in terms of past environmental changes that the sediments recorded. This project contributes to an understanding of climatic variability, a key challenge facing society. Specifically, it provides insights into the feedback processes that cause the climate system in the Arctic to change faster than any other region on Earth. The spatial network of paleoclimate reconstructions serves as key benchmarks for validating climate models and for improving their ability to accurately simulate nonlinear change, including changes in sea-ice cover. Sponsor: University of Massachusetts Amherst, Research Administration Building, Amherst, MA NSF Program: ARCSS
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