Investigating century-scale extremes across North America

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This article is part of the Spring 2019 issue of the Earth Science Matters Newsletter.

Instrumental records of North American precipitation and temperature exist only for the last century or so, leaving significant uncertainty on natural patterns of variability over decadal and longer time scales. To improve our understanding of how North American water availability has varied over local to regional scales, USGS researchers have established a network of paleoclimate research sites to provide historical baselines for comparison to recent and future conditions. Members of this working group recently published the North American Hydroclimate Synthesis (NAHS), which summarizes the hydroclimate history of the last 2,000 years from 69 locations across the continent (Figure 1).

map of sample site locations

Figure 1: Map displaying synthesis site locations and proxy types.  Blue circles are oxygen (δ18O) and carbon (δ13C) stable isotope data, yellow stars are lake sediment chemistry data (% carbonate, Mg/Ca, gypsum abundance, loss-on-ignition-based bulk geochemistry), purple triangles are lake elevation (tracking shoreline migrations in sediment cores, dating of highstand and lowstand benchmarks), light green trapezoids are peat indicator data (plant macrofossils, n-alkanes), dark green X’s are pollen-based vegetation data, gray squares are productivity indicators (diatom assemblages, moss production), red diamonds are terrigenous input indicators (grain size, % sand, Ti, Al2O3, mineral mass accumulation rate), and orange crosses are testate amoebae-inferred water depth. (Figure 2 in Rodysill et al., 2018)

(Credit: Figure 2 in Rodysill et al., 2018. Public domain.)

This initial study investigated the histories of wetter- and drier-than-average conditions (i.e. hydroclimate) at 69 locations to identify the natural ranges of hydroclimate variability at the century timescale. The study also determined whether there have been broad regional or continent-scale spatial patterns in hydroclimate during the past 2000 years. The collaborative effort by 22 USGS researchers is advancing our understanding of broad-scale patterns of past climate change to improve climate model capabilities and guide future paleoclimate studies through identification of critical spatial and temporal data gaps.

The NAHS is a multi-proxy atlas of 69 continental climate reconstructions based on previously published and peer-reviewed research. Working group members compiled paleoclimate information derived from geochemical, geophysical, and biological climate proxy data from lake sediments, caves, wetlands, and coastal marine environments. Based on these proxies, wetter- and drier-than-average periods were identified from each record. A uniform method to determine ages of samples was applied to each site, ultimately allowing comparison of different sites and proxies with statistical rigor.

By comparing NAHS with similar reconstructions of atmospheric temperature, the study indicated that many localities became drier during warmer centuries, particularly between 50 BCE (years before the Common Era, where 0 CE = 0 AD) and 450 CE and between 800 CE and 1100 CE (Figure 2). However, there was substantial variability among sites, and the patterns of drought differed between the two multi-century periods.

graph of temperature and precipitation

Figure 2: Top: 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). Bottom: 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. 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 cross-hatched areas indicate values that exceed one standard deviation from the means of both the thick gray line and the thin black line. The warm Roman Warm Period (RWP) and Medieval Climate Anomaly (MCA) are outlined in solid red lines, and the cool Dark Ages Cold Period (DACP) and Little Ice Age (LIA) are outlined in solid blue lines. (Figure 11 in Rodysill et al., 2018)

(Credit: Figure 11 in Rodysill et al., 2018. Public domain.)

In general, the reconstruction revealed that large areas of the continent became dry for centuries at a time, while other areas tended to become wetter. Not only did many of these shifts persist for hundreds of years, but there were numerous fluctuations between wet, dry, and average conditions over the course of the last 2000 years in any given region. These findings are important, because they imply that widespread natural variations in hydroclimate at the centennial timescale have been a common trait of North American landscape history.

Interestingly, significant hydroclimate pattern transitions occurred out of sync with reconstructed mean temperature changes. Thus, while temperature may play a role in when and where century-scale shifts toward drier conditions occur, other factors are clearly involved. More research that incorporates additional paleoclimate proxy data and computer model simulations is needed in order to better understand the mechanisms that drove century-scale hydroclimate changes in the past, which is crucial information for predicting future trends and patterns of drought.

The paper, “A North American hydroclimate synthesis (NAHS) of the Common Era” was published in Global and Planetary Change and is available here: https://doi.org/10.1016/j.gloplacha.2017.12.025

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