Ice core evidence of rapid air temperature increases since 1960 in alpine areas of the Wind River Range, Wyoming, United States

Abstract

Site-specific transfer functions relating delta oxygen 18 (δ 18 O) values in snow to the average air temperature ( T A ) during storms on Upper Fremont Glacier (UFG) were used in conjunction with δ 18 O records from UFG ice cores to reconstruct long-term trends in air temperature from alpine areas in the Wind River Range, Wyoming. Transfer functions were determined by using data collected from four seasonal snowpacks (1989–1990, 1997–1998, 1998–1999, and 1999–2000). The timing and amount of each storm was determined from an automated snowpack telemetry (SNOTEL) site, 22 km northeast of UFG, and ∼1060 m in elevation below UFG. Statistically significant and positive correlations between δ 18 O values in the snow and T A were consistently found in three of the four seasonal snowpacks. The snowpack with the poor correlation was deposited in 1997–1998 during the 1997–1998 El Niño Southern Oscillation (ENSO). An ultrasonic snow-depth sensor installed on UFG provided valuable insights into site-specific storms and postdepositional processes that occur on UFG. The timing of storms recorded at the UFG and Cold Springs SNOTEL sites were similar; however, selected storms did not correlate. Snow from storms occurring after mid-October and followed by high winds was most susceptible to redeposition of snow. This removal of lower temperature snowfall could potentially bias the δ 18 O values preserved in ice core records to environmental conditions reflecting higher air temperatures and lower wind speeds. Transfer functions derived from seasonal snow cover on UFG were used to reconstruct T A values from δ 18 O values determined from two ice cores collected from UFG. Reconstructed air temperatures from the ice core data indicate an increase in T A of ∼3.5°C from the mid-1960s to the early 1990s in the alpine areas of northwestern Wyoming. Reconstructed T A from the ice core records between the end of the Little Ice Age (LIA), mid-1800s, and the early 1990s indicate a T A increase of ∼5°C. The historically reconstructed T A values from the UFG were significantly higher than the global average observed during the 20th Century but were in agreement with T A increases observed at selected, high-altitude and high-latitude sites in other parts of the world. Additional research is required to determine if part of the observed trend toward heavier δ 18 O values in ice from the UFG since the LIA (and increased T A ) is due to an increased proportion of snowfall from southerly storm tracks and moisture sources, as seems to have been the situation in 1997–1998.