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.
Hydroclimate extremes critically affect human and natural systems, but there remain many unanswered questions about their causes and how to interpret their dynamics in the past and in climate change projections. These uncertainties are due, in part, to the lack of long-term, spatially resolved hydroclimate reconstructions and information on the underlying physical drivers for many regions. Here we present the first global reconstructions of hydroclimate and associated climate dynamical variables over the past two thousand years. We use a data assimilation approach tailored to reconstruct hydroclimate that optimally combines 2,978 paleoclimate proxy-data time series with the physical constraints of an atmosphere-ocean climate model. The global reconstructions are annually or seasonally resolved and include two spatiotemporal drought indices, near-surface air temperature, an index of North Atlantic variability, the location of the intertropical convergence zone, and monthly Niño indices. This database, called the Paleo Hydrodynamics Data Assimilation product (PHYDA), will provide a critical new platform for investigating the causes of past climate variability and extremes, while informing interpretations of future hydroclimate projections.
Using a large ensemble of simulations from a state‐of‐the‐art Earth System Model, we show that 42% of global vegetated land areas are projected to have “greening” in the form of additional vegetation growth at the same time as “drying” in the form of reduced soil moisture in a business‐as‐usual world. Simultaneous greening and drying is curious and suggests that future ecosystems—which could demand more water due to warmer and longer growing seasons and CO2 fertilization—siphon water that historically would have become the runoff that fills rivers and streams, termed “blue water.” We show that warming and changes in plant growth from CO2 creates an explicit water trade‐off in which future vegetation directly diminishes runoff relatively or absolutely for nearly half of global land areas. Our results have important implications for future water availability, but also point to the crucial importance of resolving model uncertainties associated with terrestrial vegetation and its response to increasing CO2.
Climate models project significant twenty-first-century declines in water availability over the American West from anthropogenic warming. However, the physical mechanisms underpinning this response are poorly characterized, as are the uncertainties from vegetation’s modulation of evaporative losses. In this study we have used a 35-member single model ensemble is to examine the response of summer soil moisture and runoff to anthropogenic forcing to understand the drivers and uncertainties of future hydroclimate in the American West. We found that widespread dry season soil moisture declines across the region despite increases in total water-year precipitation and ubiquitous increases in plant water-use efficiency. These modeled soil moisture declines are initially forced by significant snowpack losses that directly diminish summer soil water, even in regions where water-year precipitation increases. When snowpack priming is coupled with a warming- and CO2-induced shift in phenology and increased primary production, widespread increases in leaf area further reduces summer soil moisture and runoff by outpacing decreased stomatal conductance from high CO2. The net effects lead to the co-occurrence of both a “greener” and “drier” future across the western United States. Because simulated vegetation exerts a large influence on predicted changes in water availability in the American West, these findings highlight the importance of reducing the substantial uncertainties in the ecological processes increasingly incorporated into numerical Earth system models. Journal of Climate