The PaleoDynamics Lab at the Lamont-Doherty Earth Observatory is led by Prof. Jason E. Smerdon, with strong connections to colleagues in the Division of Ocean and Climate Physics, the Lamont Tree-Ring Laboratory and the NASA Goddard Institute for Space Studies.  The broad objective of our group is to characterize and understand climate variability and change on decadal to centennial timescales.  Research on these timescales is limited by the brevity of instrumental records, which are not widely available for more than about 100-150 years.  To circumvent this limitation, we combine modern instrumental records with climatic proxy records and climate model simulations.  We are particularly interested in how multiple climate proxies can be combined to yield hemispheric and global maps of climate variables that span the Common Era (the last two thousand years), how proxies and climate models represent climatic variability and change over this time period, and how to use proxy-model comparisons to inform future climate projections.  Several of our recent efforts are highlighted below and all of our publications are publicly available at our Publications page.

Development of the most severe 19-year droughts since 800 CE

Severe and persistent 21st-century drought in southwestern North America (SWNA) motivates comparisons to medieval megadroughts and questions about the role of anthropogenic climate change. We used hydrological modeling and new 1200-year tree-ring reconstructions of summer soil moisture to demonstrate that the 2000–2018 SWNA drought was the second driest 19-year period since 800 CE, exceeded only by a late-1500s megadrought. The megadrought-like trajectory of 2000–2018 soil moisture was driven by natural variability superimposed on drying due to anthropogenic warming. Anthropogenic trends in temperature, relative humidity, and precipitation estimated from 31 climate models account for 47% (model interquartiles of 35 to 105%) of the 2000–2018 drought severity, pushing an otherwise moderate drought onto a trajectory comparable to the worst SWNA megadroughts since 800 CE.  Read More

SOM analysis of the change in the incidence of modes of SST variability and consequent hydrological change

Multidecadal “megadroughts” were a notable feature of the climate of the American Southwest over the Common era, yet we still lack a comprehensive theory for what caused these megadroughts and why they curiously only occurred before about 1600 CE. We used the Paleo Hydrodynamics Data Assimilation product, in conjunction with radiative forcing estimates, to demonstrate that megadroughts in the American Southwest were driven by unusually frequent and cold central tropical Pacific sea surface temperature (SST) excursions in conjunction with anomalously warm Atlantic SSTs and a locally positive radiative forcing. This assessment of past megadroughts provides the first comprehensive theory for the causes of megadroughts and their clustering particularly during the Medieval era. This work also provides the first paleoclimatic support for the prediction that the risk of American Southwest megadroughts will markedly increase with global warming. Read More

Relative changes in the hydrological budget at the land surface as measured by the Blue Water Metric

Plants are expected to generate more global-scale runoff under increasing atmospheric carbon dioxide concentrations through their influence on surface resistance to evapotranspiration. Recent studies using Earth System Models from phase 5 of the Coupled Model Intercomparison Project ostensibly reaffirm this result, further suggesting that plants will ameliorate the dire reductions in water availability projected by other studies that use aridity metrics. We complicated this narrative by analyzing the change in precipitation partitioning to plants, runoff and storage in multiple Earth system models under both high carbon dioxide concentrations and warming. We showed that projected plant responses directly reduce future runoff across vast swaths of North America, Europe, and Asia because bulk canopy water demands increase with additional vegetation growth and longer and warmer growing seasons. These runoff declines occur despite increased surface resistance to evapotranspiration and vegetation total water use efficiency, even in regions with increasing or unchanging precipitation. We demonstrated that constraining the large uncertainty in the multimodel ensemble with regional-scale observations of evapotranspiration partitioning strengthens these results. We concluded that terrestrial vegetation plays a large and unresolved role in shaping future regional freshwater availability, one that will not ubiquitously ameliorate future warming-driven surface drying.  Read More