Regional climate change in northern North America during the Holocene

Paleoclimate studies document long-term climate variability and their impacts on freshwater and terrestrial ecosystems. However, these impacts are difficult to identify by using a single paleoclimate time series or a single proxy-climate record. One way to more clearly detect Holocene climate variability is to use a multi- proxy network of sites to better identify climate impacts from local non-climatic events. We present regional temperature and precipitation reconstructions for key regions of North America during the Holocene and last glacial period. For example, in Eastern Beringia, paleoclimate time series constructed from a network of pollen records show low- and high frequency climate variability during the past 25,000 years, with temperatures 2-4oC colder during the full glacial. Reconstructions further show differences in temperature and precipitations patterns across Beringia at different time periods of the Holocene, suggesting atmospheric circulation changes at multiple time and space scales. The magnitude of millennial-scale climate variability in this region was greater during the last glacial and late glacial periods than during the past 8,000 years, but millennial-scale climate variations occur at the same time as those identified in other regions. Time series from other regions of North America also reveal similar variability during the Holocene with large impacts on the aquatic and terrestrial communities. Although the magnitude or even direction of the change may vary regionally, major climate transitions occur simultaneously. Across the Arctic, major transitions in diatom communities and pollen assemblages occur at comparable times and can be correlated with changes in ice core and marine records. Vegetation response can vary locally, but some vegetation changes are large- scale. For example, taxa such as Populus (poplar) responded almost synchronously across all of North America to abrupt climatic events such as the Younger Dryas. All of this evidence suggests that the vegetation and freshwater ecosystems have responded rapidly to climate changes of the past.