Climate impact assessments allow researchers to understand and anticipate the effects of climate change on the environment. However, being able to conduct accurate assessments requires understanding how and why climate changes have occurred. To do this, researchers need to be able to differentiate natural climate variability from human - induced climate changes.
“Deciphering patterns of natural hydroclimate variability from underlying anthropogenic climate change is only possible by understanding the climate of the recent geologic past,” explains Steven Bacon, the principal investigator of the project.
“The goal of this study is to evaluate the sensitivity of the longest tree-ring record in North America, the Methuselah Walk bristlecone pine chronology from the White Mountains, to different components of the hydrologic system using a coupled watershed runoff and lake surface evaporation model for the Owens River-Lake system in California.”
This study is also part of Bacon’s Ph.D. research and includes collaboration with his Ph.D. advisor Dr. Rina Schumer of DRI, as well as Dr. Adam Csank of the University of Nevada, Reno. The researchers will evaluate the sensitivity of the Methuselah Walk chronology by comparing historical ring widths to a variety of hydrologic outputs from the coupled water balance and lake evaporation model. The hope is to make confident correlations in order to develop an 8,000-year record of the southern Sierra Nevada hydroclimate, which will include the precipitation and temperature fluctuations in the region over that time period.
“LaMarche (1974) demonstrated that the growth patterns of some annual tree-ring chronologies from moisture- and temperature-sensitive species show strong correlations with historical hydroclimate variability at watershed to regional scales,” Bacon says.
“Depending on the type of tree species analyzed, tree-ring chronologies are commonly compared to a wide range of instrumental records of the watershed’s hydroclimatic system, such as precipitation, temperature, snow water equivalent, soil moisture, streamflow, lake water level, and drought indices. Ultimately, the goal of these analyses is to develop a hydroclimatic proxy dataset that could be used to infer paleoclimatic change over the length of the tree-ring chronology.”
Combining the precipitation- and temperature-sensitive tree-ring chronologies from the White Mountains will potentially allow the researchers to reconstruct the hydrologic system of the Sierra Nevada region.
“The coupled watershed runoff and lake surface evaporation model we will use has three primary model parameters: precipitation, temperature, and solar insolation. If we can demonstrate strong correlations between the White Mountain tree-ring chronologies and the historical modeled components of the hydrologic system in the watershed, then we could in turn use the associated precipitation and temperature reconstructions as input parameters in the coupled water balance model,” Bacon says.
“The accuracy of the modeled lake levels based on the tree-ring hydroclimatic reconstructions will be assessed by comparing them with the Holocene shoreline record of Owens Lake in combination with the paleotemperature depression record developed in the Sierra Nevada from glacial deposits.”
The research for this study will also incorporate some novel techniques. This project will combine precipitation- and temperature-sensitive tree-ring chronologies to reconstruct a hydrologic system. The resultant chronology will be approximately 6,000 years longer than similar records in the western United States and potentially have the longest tree-ring-based reconstruction of runoff and lake-level fluctuations in North America.
“Tree-ring and other high-resolution paleoproxies that are resolved to annual time scales allow the range of precipitation and temperature scenarios used in climate impact assessments to be extended,” Bacon explains, “which could provide an extended perspective on the range in magnitude and duration of potential hydroclimatic variability that could be experienced under future climate change in eastern California and western Nevada.”
Additionally, this project will use variable estimates of solar insolation values over time in the evaporation component of the water balance model.
“The total energy received by the Earth from the Sun changes with time, so the change in the magnitude of paleo-solar insolation needs to be included in a paleoclimate water balance model to accurately estimate evaporation from the water surface and evapotranspiration from the land surface,” Bacon says. “Fortunately, previous work has computed the positions of Earth’s orbit and rotation for the last 10 million years for paleoclimatic research. The resolution of this dataset is monthly insolation values for intervals of 10 degrees of latitude at 1,000-year time steps.”
Although the project is still in its early stages, it has already produced some interesting results.
“Our preliminary analysis and model results have confirmed that traditional statistical techniques yield problematic correlations to observed precipitation and temperature, which have also been found in previous dendrochronologic research of the White Mountain bristlecone pine tree-ring chronologies conducted by Hughes and Graumlich (1996) and Salzer et al. (2014),” Bacon explains. “These results are likely related to the bristlecone’s harsh, upper montane to alpine mountainous environment and its short growing season and complex physiology. As a result, our ongoing research has primarily focused on finding which component of the hydrologic system and at what extent within the watershed the tree rings are recording.”
The modeling techniques that the researchers are developing for this study could also benefit future watershed studies.
“Although the watershed-scale modeling approach we are using is relatively similar in principle to the study of Saito et al. (2015) in the Sierra Nevada,” Bacon says, “this project also includes lake surface evaporation and perennial snow/glacier accumulation elements that are calibrated to the shoreline record of Owens Lake and the glacial record in the watershed.”
If the researchers observe confident correlations between the observed precipitation and temperature and the other hydrologic system components, then they will be able to estimate precipitation and temperature in the southern Sierra Nevada region for up to 8,000 years. The model could then be used to estimate the paleohydrologic surface conditions in closed Nevada basins.
“With additional research, the results could possibly provide Holocene groundwater recharge estimates for these basins,” he adds, “which could then be used to provide a paleohydrologic context of modern groundwater recharge rates used for water resources management.”
Hughes, M.K., and L.J. Graumlich, 1996.
Multimillennial dendroclimatic studies from the western United States. In (eds.) R.S. Bradley, P.D. Jones, and J. Jouzel. Climatic Variations and Forcing Mechanisms of the Last 2000 Years. Berlin: Springer Verlag, p. 109–124.
LaMarche, V.C., 1974. Paleoclimatic inferences from long tree-ring records. Science 183(4129), 1,043-1,048.
Saito, L., F. Biondi, R. Devkota, J. Vittori, and J.D. Salas, 2015. A water balance approach for reconstructing streamflow using tree-ring proxy records. Journal of Hydrology 529, 535-547.
Salzer, M.W., A.G. Bunn, N.E. Graham, and M.K. Hughes, 2014. Five millennia of paleotemperatures from tree-rings in the Great Basin, USA. Climate Dynamics 42, 1,517-1,526.