No abstract

We present two reconstructions of annual average temperature over temperate North America: a tree-ring based reconstruction at decadal resolution (1200-1980 CE) and a pollen-based reconstruction at 30 year resolution that extends back to 480 CE. We maximized reconstruction length by using long but low-resolution pollen records and applied a three-tier calibration scheme for this purpose. The tree-ring-based reconstruction was calibrated against instrumental annual average temperatures on annual and decadal scale, it was then reduced to a lower resolution, and was used as a calibration target for the pollen-based reconstruction. Before the late-19th to the early-21st century, there are three prominent low-frequency periods in our extended reconstruction starting at 480 CE, notably the Dark Ages cool period (about 500-700 CE) and Little Ice Age (about 1200-1900 CE), and the warmer medieval climate anomaly (MCA; about 750-1100 CE). The 9th and the 11th century are the warmest centuries and they constitute the core of the MCA in our reconstruction, a period characterized by centennial-scale aridity in the North American West. These two warm peaks are slightly warmer than the baseline period , but nevertheless much cooler than temperate North American temperatures during the early-21st century.

We present a 2000-year pollen-based reconstruction of total annual precipitation (ANNP) for North America. Application of Local Indices of Spatial Association (LISA) permitted us to distinguish regional large-scale hydroclimate patterns from local site-specific conditions across North America during this time period. Century-scale filtered pollen-based reconstructions are more spatially coherent than unfiltered reconstructions, allowing for better identification of past hydroclimate patterns across North America. Similar spatial patterns dominated during both the 'Medieval Warm Period' (MWP; 950-1250 CE) and Roman Warm Period (RWP; 50 BCE-150 CE), although the MWP was generally drier than the RWP. In contrast, hydroclimate changes during Dark Ages Cold Period (DAC; 350-650 CE) and the 'Little Ice Age' (LIA; 1550-1850 CE) show opposing patterns across the western and eastern boreal region of North America. During warm periods, eastern and western boreal regions had similar moisture condition, whereas during cold periods, the eastern boreal region was relatively wet and the west was dry.

Summary1 Species-level pollen identifications within the genera Acer , Fraxinus and Juglans have the potential to increase the detail available from regional-and landscape-scale studies of palaeo-forest dynamics. 2 Data from the North American Pollen Database (NAPD) and the North American Modern Pollen Database (NAMPD) for sites in eastern North America for which species-level identifications have been recorded enabled us to consider pollen taxonomy at high resolution. 3 Species within each of Acer , Fraxinus and Juglans have important differences in habitat, functional type and responses to climatic change. Analysis of the modern distribution of these taxa and their pollen rains confirms that species-level pollen identifications provide detailed ecological information, but the lack of distinction to the species level in many fossil and modern pollen studies renders palaeoenvironmental reconstructions incomplete. 4 Within each of three selected high-resolution sites, ordinations indicate that individual species of Acer , Fraxinus and Juglans follow different trajectories through the Holocene, showing that analysing only generic categories results in the loss of ecologically valuable information. For example, pollen of Acer rubrum increased in abundance in response to canopy openings in a southern Ontario forest around 500 years ago, while that of A. saccharum declined. Similarly, Juglans nigra pollen percentages increased while J. cinerea decreased at this time. 5 Regional-scale comparisons of pollen percentage curves indicate that, despite the uncertainties associated with the low temporal resolution and the chronologies, it is reasonable to conclude that individual species of Acer , Fraxinus and Juglans responded synchronously to palaeoclimatic changes within each region. 6 Taking analyses to the specific level shows that different species followed different tracks in their post-glacial history, a fact previously blurred by less resolved taxonomy. This information is meaningful in biogeographical terms, providing much more specific evidence of how the selected trees behaved in the past.

Abstract. Lake level fluctuations provide evidence about past variations in the global hydrological balance. The geostatistical approach is here used to more objectively identify global patterns using an ensemble of lake level databases by examining spatial autocorrelation between sites. The spatial structures of the lake level data are then modeled and grids produced for the last 12,000 years at 3000-year intervals using ordinary and indicator kriging techniques. The two gridding techniques produced almost identical estimated regional lake status patterns, thus suggesting a robust estimation. The resulting lake-status grids are in general agreement with previous paleoclimatic reconstructions using only site-by-site lake status point maps; however, the reduction of local fine-scale variability resulted in more coherent regional spatial patterns in areas of high local variability. The 6 ka lake-status grids were compared to simulations of four atmospheric general circulation models to illustrate their usefulness in validating broad-scale climate model outputs.