Tracing long-term changes in Rocky Mountain climate, water, and ecosystems

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This article is part of the Fall 2015 issue of the Earth Science Matters Newsletter. 

Annual runoff from Rocky Mountain snowpack replenishes the essential water resources that supply the semi-arid regions of the west, many of which are faced with increasing water demand.  An understanding of the climate controls on seasonal precipitation facilitates effective planning for future water availability.  Since observations began in the early 20th century, Rocky Mountain precipitation has been dominated by winter snow. The amount of snowpack varies from year to year, and these variations are related to Pacific ocean-atmosphere processes such as El Niño/Southern Oscillation (ENSO).  However, the century-long instrumental records are not sufficiently long to characterize natural snowpack variability.

Examining Rocky Mountain precipitation over the last 10,000 years can show how snowpack has changed in response to natural variations in the global energy balance and whether recent trends are attributable to natural variability.  New research by USGS scientists is yielding insights on snow and water variability by examining geochemical information contained within snowpack and sediments from lakes located within the Rocky Mountain snowpack accumulation zone.  These data provide a detailed record of natural snowpack variability, including long-term averages and extremes. They also provide a context to evaluate recent trends and help understand how the region may respond to future climate change.

scientists collect sediment cores from frozen lake

Scientist works on a frozen lake in early spring to collect sediment cores for regional climate and snowpack reconstructions.

(Credit: Lesleigh Anderson, USGS. Public domain.)

map of sampling sites

Maps showing locations where snowpack has been collected (inset) for isotope analyses. The colored symbols indicate average oxygen istope composition from locations that have more than 10 years of measurements since 1993. IRMS stands for Isotopes in Rocky Mountain Snopack. (From Figure 1 in Anderson et al., 2016)

(Credit: (Anderson et al., 2016). Public domain.)

In a study carried out over the entire U.S. Rocky Mountain region, scientists from the USGS and the University of Illinois Chicago examined oxygen and hydrogen isotope tracers within the snowpack from ~60 locations over the 21 year period since 1993. The isotopic tracers provide quantitative information about precipitation and temperature in the region at the time of deposition. Results of this research indicated that snowpack isotopes in some locations are strongly correlated with regional climate.  The scientists then applied these relationships to an ~10,000 year lake sediment isotope record from northwest Colorado to make estimates of past climate conditions. This effort showed that regional 20th century snowpack trends are within the range of natural variability.

A related study involving scientists from the USGS and the University of Utah evaluated the relative importance of recent human land use and natural climate change on an alpine landscape. Scientists examined biological and chemical information from a lake sediment record to distinguish changes in vegetation distribution, fire and other natural disturbances. The combined results indicate that vegetation and fire regimes changed dramatically in response to climate and hydrologic change of the last two thousand years.  Although post-settlement (circa last 150 years) disturbances were significant, they had relatively less impact suggesting that alpine forests are resilient to the combined impacts of recent human and natural agents of change.

The papers are published in Quaternary Science Reviews http://authors.elsevier.com/sd/article/S0277379115001304 and The Holocene https://doi.org/10.1177/0959683615574583.

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