Abstract
The northern US Rocky Mountains are experiencing rapid warming. Combined analysis of Ground Temperature (GT) measurements at two high-fidelity boreholes with Surface Air Temperature (SAT) measurements near Helena Montana spanning the past 40 years indicate the northern US Rockies have warmed on average 0.12°C - 0.32°C/decade since 1975, at least a factor of ~5 higher than the predicted 500-year-average. Warming appears to be accelerating, with warming rates since 2013 4 - 7 times higher than the 40 year average. Though uncertainty exists, the most significant GT warming appears to occur at higher elevation. Warming estimates are consistent with modelling predictions, snowpack observations, and stream temperature studies, all suggesting rapid surface temperature change in this region during the past ~40 years. The analysis indicates GT warming measured at remote borehole sites is slightly lower than regional SAT measurements collected near urban environments. We associate the discrepancy between GT/SAT measurements to both anthropogenic effects (urban development) that increase warming at the nearest SAT measurement station and a 14-year period of anomalously low snowfall that reduces surface insulation and GT warming. Using a derived average forty-year surface warming rate of 0.22°C/ decade and regional temperature-elevation trends, we calculate that the elevation of the winter freeze line during the three coldest months of the year (December, January, and February) in the northern US Rocky Mountains is retreating upward, on average, 33 m/decade. This implies a 21% reduction in freeze-line area since 1974. If this trend continues, we estimate that within the next 40 years (by 2060), the total area where ground freeze occurs during the three coldest months of the year will be ~60% of 1974 values. Since GT measurements indicate accelerated warming, this may be an underestimate. The analysis has important implications for the snowpack-water budget for Montana and the northern US Rocky Mountains.
Highlights
Global Surface Air Temperature (SAT) measurements indicate increasing surface temperatures on Earth since the mid-20th century, with average SAT increasing at a rate of ~2 ̊C during the past century [1]-[4]
We analyzed the temperature-depth profile at both wells for all depths greater than 20 m, below which no significant temperature changes associated with daily or seasonal temperature fluctuations exist [36]
At depths of ~20 m below the surface, measured 2014 temperatures at the Fort Harrison and compare temperature-depth measurements from 1973 (CD9) wells are approximately 0.25 ̊C warmer, than 1975 measurements (Figure 2(a) inset)
Summary
Global Surface Air Temperature (SAT) measurements indicate increasing surface temperatures on Earth since the mid-20th century, with average SAT increasing at a rate of ~2 ̊C during the past century [1]-[4]. Warming in the Rocky Mountains may have a crippling effect on snowpack and water supply for the United States, as recently demonstrated with the California drought [9] [10]. Multiple studies suggest population centers in the western United States where water supplies depend on snowmelt are more vulnerable to the effects of increased temperatures over time, with areas at higher elevation and higher latitudes generally experience the most significant impact [7] [11]-[13]. In areas that rely heavily on snowpack to maintain their water supply, accurate assessment of long-term surface temperature change is critical for predicting future water resources [6] [7] [14]
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