Understanding Century- to Millennial-Scale Droughts and Other Hydroclimate Variability over the past 15,000 years

Hydrologic variability poses significant challenges to society. Water is critical both for meeting the needs of population growth and economic development, and for sustaining energy production including fossil-fuel extraction, thermoelectric power generation, and hydropower. However, the potential for persistent hydrologic change remains poorly understood. Sedimentary patterns within small lake basins record past changes in water levels and can be used to track shifts in moisture availability over time. Tree-ring records and archeological evidence indicate that past centuries have contained multi-decadal “megadroughts” far more severe than any drought of the past 150 years, but we are using evidence from lake-level changes to document past dry periods that exceeded the severity of multi-decadal “megadroughts” and persisted for centuries to millennia.

 

In the Rocky Mountains

Wyoming and the other Rocky Mountain states have historically experienced extended periods of drought, which have had significant economic and social impacts. Our current work aims to better study the timing, extent and severity of much longer dry episodes in the critical high-elevation headwaters of the Colorado, Columbia, and Missouri River systems. Preliminary direct measurements of the prehistoric shoreline elevations of lakes in the North Platte watershed indicate that these natural reservoirs experienced prolonged draw downs of 30% or more of their volumes for periods of centuries to multiple millennia during the past 4500 years.

In the Northeast U.S.

Questions exist about the potential for dramatic hydrologic change in humid, heavily populated regions like the eastern U.S. This study offers important baseline data regarding the range of hydrologic variability there. Droughts may have been more frequent during a period when pollen, isotope, and macrofossil data indicate that winters were warmer than today. The creation, analysis, and synthesis of new, well-resolved sedimentary data, in combination with detailed fossil pollen data, are enabling us to evaluate the potential sub-millennial hydrologic variability and its controls.

Examining Ecosystem Responses to Changing Moisture Levels

Droughts have historically changed ecosystems at regional scales, and far broader changes have happened in the geologic past and are a likely outcome in the future. Large regions are expected to experience prolonged (>100 yr) moisture changes, and evaluating the potential responses of vegetation to such moisture-level changes is important because the responses can feedback to further affect climate, regional carbon storage, and the behavior of geomorphic and hydrologic systems such as rivers. The role of water, however, in shaping the ecological response to ongoing climate change is poorly understood, and interactions among moisture and other factors like disturbance are highly likely to generate surprising ecological patterns. Ongoing and proposed studies, based on comparisons of independently-documented climate and ecosystem histories, including disturbance history, focus on illustrative examples of biogeographic responses to past moisture variations (e.g., grassland-forest borders in Minnesota and Colorado, and vegetation gradients in New England).

Ecotones are ecological transition zones where one vegetation type (e.g., forest) gives away to another (e.g., grassland). Ecotones have been commonly thought to respond rapidly to climate, but the role of fire and other processes complicates the response to climate change. Steppe-forest ecotones in the Rockies and the Upper Midwest have been dynamic over time, and are ideal for examining the long-term interacting effects of climate and non-climate factors on regional vegetation pattern. Therefore, we are studying climate, fire, and vegetation history in both regions to examine the hypothesis that disturbance regimes, their interactions, and their sensitivity to climate alter the spatial and temporal responses of forest extent and composition to persistent drought.

The influences of abrupt and gradual climate changes on ecosystems in the Northeast U.S.

We are reconstructing and comparing vegetation, disturbance (including fire, wind, and human activity), and climate histories in the northeast U.S. to evaluating three prominent ecological changes during the Holocene: (1) regional expansion of American beech tree populations ~8200 years ago coincident with a rapid increase in moisture, a progressive rise in temperature, and a brief century-scale cold event; (2) the range-wide decline of Eastern hemlock tree populations ~5500 years ago coincident with warm, dry conditions and some evidence of extreme drought; (3) regional declines in beech and hemlock during the last ~2000 years during an interval of progressive cooling, short-term environmental changes, and European settlement.