A climate researcher at the University of Montana has found that high-elevation warming trends over the past two decades in the Western United States are likely less severe than previously reported. 

Jared Oyler of the Montana Climate Office blamed inaccurate temperature sensors installed in more than 700 weather stations in the Snowpack Telemetry station network, also known as SNOTEL. The network monitors a range of data used to measure snowpack and generate models and forecasts for water resource management.

Oyler, who published the study last week in Geophysical Research Letters, said the new sensors were installed beginning in the mid-1990s as part of upgrades to the network, which has been run by the U.S. Department of Agriculture for nearly 40 years.

“What happened was the new sensor was biased relative to the old one, causing an apparent increase in temperatures across the whole network,” Oyler said Wednesday. “It was a really big error … [that] showed about a degree Celsius rise per decade, which is very large concerning trends.”

His study was able to account for the bias and the revised data set for high-elevation temperatures showed increases only slightly above those measured at ground level.

“We performed a first step that fixes a lot of the issue, but there’s definitely more work that needs to be done to repair the biases,” Oyler said.

He said the problem was unique to the Snowpack Telemetry network and the newer sensors appeared to be inaccurate rather than reflecting a problem with the old sensors.

“It looks like the new sensor they put in is truly biased; in other words, when it’s freezing the sensor is actually reading about 1.5 degrees Celsius,” he said. “That’s a large amount [of error] when you’re looking at understanding snowpack melt dynamics.”

While the temperature bias will require forecasts to be revised, it does not affect the measurement of snowpack levels, which rely on separate data.

He said the corrected data over the years in question did not show a statistically significant warming trend, but noted that long-term trends over the 20th century still showed significant increases in temperature consistent with global climate change.

However, the error could affect the accuracy of outside studies and models that used the inflated temperature record.

“Studies that look at higher-elevation warming will probably have to downgrade their results,” said Oyler. “But if you look at longer-term trends in the Western U.S., there is still significant warming, just not that extreme warming.”

Reporter Samuel Wilson may be reached at 758-4407 or by email at swilson@dailyinterlake.com