Tracking long-term changes in Rocky Mountain snowpack
Changes in Rocky Mountain snow is a leading stressor on alpine ecosystems and the communities that depend on the water resources that snowpack provides. As the region is faced with increasing water demands from a growing population, effective future planning will be assisted by a better understanding of how future climate change, based on long-term projections, will affect future snowpack. Scientists are conducting research to provide new insights on how snowpack may evolve in the future, using observations of the present and long-term records of the past.
Estimates of past snow and rain precipitation amounts can be made from the geochemical signatures within sediments deposited in lakes during the past 11,000 years. One signature, isotope ratios, compares the abundance of two oxygen isotopes (18O and 16O). In precipitation, the temperature, humidity and history of water vapor movement in the atmosphere control relative isotope abundances. The oxygen isotope ratios at a given time are preserved in geologic materials that span long periods of time including, ice cores, tree rings, corals, soils and cave deposits, such as speleothems and can be measured in incremental samples to generate continuous records of past oxygen isotope ratios. Interpretations of oxygen isotope ratios from geologic archives are largely based upon comparisons between modern depositional environments and simultaneously observed meteorology and climatology.
For environments where snowfall accounts for the majority of annual precipitation, snowmelt, rather than precipitation, is likely to have the strongest influence on isotopic values contained within geologic materials. Although climate patterns affect snowpack amount and duration, scientists have lacked the modern measurements of snowpack isotope ratios to compare with climate variables that are needed to interpret past climate from geologic materials. To address this uncertainty, USGS scientists and academic colleagues made ~1300 oxygen isotope measurements of snowpack from a network of ~60 sites in the Rocky Mountains for a 20 year period beginning in 1993. The network, known as the Isotopes in Rocky Mountain Snowpack (IRMS) network, provides the most extensive documentation of snowpack isotope values outside of Polar Regions.
The results of this study provide new understanding of the processes that influence snowpack oxygen isotope ratios. Isotopic values in snowpack are rarely controlled by temperature alone but also reflect additional atmospheric processes that alter storm track orientations and moisture sources. Other processes that occur after snow accumulation such as evaporation and sublimation also cause a great deal of spatial and temporal differences. To address these complexities, scientists identified regionally representative locations with the most consistent climate-isotope patterns to improve their abilities to use oxygen isotope ratios to define past climate estimates and their uncertainty.
Knowledge of Rocky Mountain snowpack responses to climate variability is critical for understanding long-term patterns of snowfall, water availability and quality, as well as their response to different climate and environmental stressors. Improved capabilities, such as those that the IRMS network can provide, to accurately document pre-historic snowpack fluctuations provides past analogues for potential future climate scenarios to better inform water resource planning.
The paper, "Isotopes in North American Rocky Mountain Snowpack 1993-2014," was published in Quaternary Science Reviews.