Holocene Synthesis Project

This figure qualitatively highlights the trends in the number of wet, dry, and neutral climate states through the Common Era as an approximation of changes in moisture over a large portion of the continent, as represented by the 73 proxy reconstructions in the NAHS, for comparison with multicentury NH and NA temperature anomalies. The top panel is the decadal mean extra-tropical Northern Hemisphere temperature anomaly (red line) relative to the 1961–1990 mean and the two standard deviation error bars (light red shading; Ljungqvist, 2010). The middle panel are the 30-year mean North American temperature anomalies from pollen (green) and trees (brown) relative to the 1961–1990 mean (PAGES 2k Consortium, 2013). The bottom panel illustrates the number of proxy data locations in North America that indicate wet, dry, and neutral century-scale hydroclimate states represented as the difference between the total number of wet and dry sites in the NAHS for 100-year window midpoints from 50 BCE to 1850 CE. Note that each data point represents the difference between the number of wet and dry anomalies calculated from 100-year averages of the proxy data and the time step between bars is 1 year, thus adjacent data points contain proxy information from overlapping time windows and every 100 data points are non-overlapping. The thin black line represents the difference between the numbers of wet and dry sites at any given time. For the thick gray line, at least

50% of the age model possibilities indicated each site's anomaly, thus the confidence in the timing of the anomaly is higher. The Roman Warm Period (RWP) and Medieval Climate Anomaly (MCA) discussed by Ljungqvist et al. (2016) and in this manuscript are outlined in solid red lines, and the Dark Ages Cold Period (DACP) and Little Ice Age (LIA) are outlined in solid blue lines. Alternative time intervals for the MCA and LIA discussed in this manuscript are outlined in dashed lines. Blue shading above “0” on the y-axis highlights the value range where more sites in the NAHS exhibited a wetter climate than the number of sites that exhibited a relatively drier climate, and brown shading below “0” in the y-axis highlights the value range where more sites exhibited a drier climate than a wetter climate. The hatched areas indicate the values that exceed one standard deviation from the mean of the thick gray line, and the cross-hatched areas indicate values that exceed one standard deviation from the means of both the thick gray line and the thin black line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).