REGIONAL— If this past September seemed a lot warmer than usual to you, you’re right. And, in a way, also wrong.
Our region’s rapidly warming climate is reshaping what is typical, and that’s most apparent during three of the four final months of the year, according to climate trend data maintained by the Department of Natural Resources’ Office of Climatology for each of Minnesota’s watersheds. In recent years, temperatures in early autumn and early winter are showing dramatic change here in the North Country, and the pace of that change appears to be quickening and could soon significantly change how we experience these seasons.
When viewed across the span of a century, Septembers in the Rainy, Vermilion, and Little Fork river watersheds have grown warmer by just under half a degree F per decade.
Back in 1925, Septembers were noticeably chillier in our region. From 1925 through 1950, the mean September temperature across those three North Country watersheds was 52.7 degrees, but that has increased steadily, particularly in recent years. Since 2000, the mean September temperature in our region has increased to 55.2 degrees F, and it’s jumped to 57.2 degrees F when measured since 2020.
This September’s average temperature in the region, at 58.1 degrees, would have set a new all-time record high for the 75 years of records prior to 2000. But since 2000, it only ranks sixth warmest out of the past 25 years and seems downright brisk compared to September 2024’s monthly average of 61.4 degrees F. It was also colder than September 2023, which averaged 59.4 degrees.
So, who is to say what’s “usual” anymore.
“September temperatures have really jumped up,” said state climatologist Kenny Blumenfeld. “The peaks really started increasing in the 1970s, and they’re still climbing,” he added.
September isn’t alone. Minnesota has also seen dramatic warming in November, December, and January, notes Blumenfeld, but that fits well with the predictions of climate scientists. As the level of greenhouse gases in the atmosphere increases, the Earth doesn’t cool as quickly as it used to and that has made for warmer overnight low temperatures, particularly in the winter months.
But not every month has followed that trend, notes Blumenfeld. While September has warmed significantly, August and October — which bookend September — have experienced little detectable change in temperatures during the same periods.
“September is definitely unusual,” said Blumenfeld. “It’s by far the fastest warming non-winter month.”
While the month is warming, Blumenfeld said it’s not because of a greater frequency of heat waves. Instead, the warming is being driven by more mild weather, like days with highs in the upper 60s to mid-70s, combined with warmer overnight lows.
“We’re getting warmer but not necessarily hotter,” Blumenfeld added.
That was clearly the trend that accounted for this past September’s warmth, which came despite a much cooler than average start to the month.
By contrast, October has shown no discernible temperature trend over the past 100 years, a fact for which climatologists don’t have a clear understanding.
But the warming trend picks up in earnest in November and shows a similar trend line as September, with an increasing pace of change since 2000. Novembers in our region averaged just over 23 degrees F from 1925-1950. But since 2000, Novembers here are much warmer, averaging more than 28 degrees F, and since 2020, Novembers have averaged almost 30 degrees F, or a nearly seven-degree increase over the 1925-50 period.
Decembers have jumped even more dramatically. From 1925-1950, Decembers in the North Country were downright cold, averaging just 8.9 degrees F. Since 2000, however, that’s up to nearly 14 degrees, and over the past ten years Decembers have averaged almost 17 degrees.
Given that extent of warming, the nature of December in northern Minnesota appears on the cusp of significant change. Numerous scientific studies have pointed to a distinct mean temperature threshold of 17 degrees F for maintaining snow cover. Studies have referred to that temperature as a kind of “snow-loss cliff,” above which snow remains only intermittently. Historically, northern Minnesota experienced several months of continuous snow cover, typically beginning in early-to-mid November. As winters have warmed, however, the region is experiencing shorter periods of snow cover. During the previous two winters, for example, the region was unable to retain snow cover throughout the month of December due to excessive melting.
That has vast ecological implications beyond the scope of this article, but it is likely to have economic impacts as well. Shorter periods of snow cover, if the recent trend continues or accelerates, could result in a major hit for winter recreation, including snowmobiling, snowshoeing, and cross-country skiing. Even downhill skiing could eventually be affected as maintaining snow, even with snow-making equipment, becomes too costly in the face of warming temperatures.