Holocene Synthesis Project

Science Center Objects

The Holocene Synthesis Project integrates a variety of information about past climate variability across the North American continent for the past ~12,000 years to characterize the spatial patterns of previous climate states, advance our understanding about mechanisms that drive natural climate variability, and improve the quality of climate models that are used to predict future climate patterns and trends. This project is a collaborative effort that combines USGS paleoclimate research expertise in simulating paleoclimate variability using computer models and proxy-based paleoclimate reconstructions. Members of the project team specialize in climate reconstruction from ocean, coastal, and continental environments using a diverse set of geochemical, biological, and geophysical climate proxies.

Project Co-Authors:

Lesleigh AndersonThomas M. Cronin,  Miriam C. JonesRobert S. ThompsonDavid B. Wahl,  Debra A. WillardJason A. AddisonJay R. AlderKatherine H. AndersonLysanna AndersonJohn A. Barron,  Christopher E. Bernhardt,  Steven W. Hostetler,  Natalie M. KehrwaldNicole S. KhanJulie N. RicheyScott W. Starratt,  Laura E. StricklandMichael R. Toomey,  Claire C. Treat,  G. Lynn Wingard


Holocene Synthesis project anomaly data

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. Click the image to learn more! (Credit: Jessica Rodysill, USGS. Public domain.)

Recent floods and droughts occurring across North America highlight the importance of accurately predicting future climate patterns and trends to improve disaster relief and mitigation planning.  Climate models predict that expansive regions of North America will become drier over the next century, while many regions of the continent are at risk of increased precipitation and flooding.  The accuracy of climate model prediction relies on the ability to test the model output with independent datasets, such as comparing recent climate trends computed in the model with instrumental climate data.  However, instrumental records are too short to capture natural climate variability at century and millennial timescales, so we rely on paleoclimate reconstructions in order to determine long histories of climate that capture these low frequency variations.

Paleoclimate records come from a range of biological, chemical, and physical data that are preserved in environmental archives such as lakes and cave speleothems.  For the North American continent, many paleoclimate records that capture century and millennial scale climate variations come from pollen, plant macrofossils, lake water elevation, stable isotope geochemistry of cave speleothems and lacustrine sediments, testate amoebae assemblages, diatom assemblages, elemental and mineralogical abundances in lake sediments, and sediment grain size.  In the Holocene Synthesis Project, we combined information from many of these archives to illuminate the spatial patterns of drought and wet periods across North America for the past 2000 years.  The past 2000 years is referred to as the Common Era (CE), where 0 CE = 0 AD.

We found that there were two widespread, prolonged droughts from 50 BCE (years before the Common Era) to 450 CE and from 800 to 1100 CE.  There were major reorganizations in wet and dry spatial patterns between regions of the continent occurring every few hundred years, and while there tended to be more dry areas at times when the average Northern Hemisphere temperature was higher, most of the wet and dry reorganizations occurred out of sync with Northern Hemisphere temperatures, with both wet and dry periods occurring during both warm and cool periods.  Shifts from wet to dry or dry to wet were time-transgressive within and between regions, indicating that climate variability was complex and spatially heterogeneous.

This study generated a paleoclimate dataset for testing climate model output data at the century timescale for the past 2000 years.  This project also identified a number of regions of the North American continent where paleoclimate data are too sparse to resolve the patterns of drought and wet periods and/or the patterns are too complex to understand the mechanisms controlling low frequency climate variability.  These data-poor regions, particularly the southeastern U.S., the Great Plains, and the Upper Midwest, will be the focus of future paleoclimate reconstructions as part of the Natural drought and flood histories from lacustrine archives project, which began in FY19.

Above: The time range on the top of each map is the century for which proxy values were averaged and anomalies were calculated.  Blue triangles pointed upward indicate a wet climate anomaly in the proxy data, and brown triangles pointed downward indicate a dry anomaly.  The size and shading of each symbol scale with the percent of age model possibilities that indicate an anomaly, with larger, darker symbols indicating a greater portion of age models that align a particular hydroclimate anomaly with that century. (Rodysill et al., 2018) https://doi.org/10.1016/j.gloplacha.2017.12.025.