Recent results of a study finds that that sensor changes have significantly biased temperature observations from the Snowpack Telemetry (SNOTEL) station network.
More than 700 SNOTEL sites monitor temperature and snowpack across the mountainous western U.S. SNOTEL provides critical data for water supply forecasts. Researchers often use SNOTEL data to study mountain climate trends and impacts to mountain hydrology and ecology. 

University of Montana and Montana Climate Office researcher Jared Oyler and colleagues applied statistical techniques to account for biases introduced when equipment was switched at SNOTEL sites in the mid-1990s to mid-2000s.

The revised datasets reduced the biases to reveal that high-elevation minimum temperatures were warming only slightly more than minimum temperatures at lower elevations. The western U.S. has warmed, just not as much as claimed. 

"Observations from other station networks clearly show that the western U.S. has experienced regional warming," Oyler said, "but to assess current and future climate change impacts to snowpack and important mountain ecosystem processes, we need accurate observations from the high elevation areas only covered by the SNOTEL network. The SNOTEL bias has likely compromised our ability to understand the unique drivers and impacts of climate change in western U.S. mountains."

Citation: Jared W. Oyler, Solomon Z. Dobrowski, Ashley P. Ballantyne, Anna E. Klene, Steven W. Running, 'Artificial Amplification of Warming Trends Across the Mountains of the Western United States', Geophysical Research Letters 9 January 2015 DOI: 10.1002/2014GL062803. Source: The University of Montana