As we approach the end of the growing season for many come Tuesday night, it’s worth taking a step back and looking at all the ways to measure just how long the warm season was this year and the warming trend we're witnessing.
We know temperatures are rising. The extremes and average temperature rankings get lots of attention, and should, but just as important is the duration of warmth.
Twenty-twenty-two went down as a year with a hot summer in the Twin Cities, though statewide it was a mixed map. Northeast Minnesota saw more or less a near normal – that’s the modern 30-year average – average summer temperature while southwest Minnesota up through the Twin Cities had a well above normal averaged summer temperature.
It was not our hottest summer on record like last year. That was last year. The Summer of 2022 was about the 17th warmest for the Twin Cities, which is in the top 11% in the record-keeping era. You could, however, make a good argument for it being the longest summer on record.
While there’s not an official definition for length of summer, and it would vary by where in the world you are, let’s say we measure it by the number of consecutive days at 70°+ high temperatures.
This year the Twin Cities had 119 consecutive days of high temperatures at 70°+, from May 26 to Sept. 22, the most ever on record.
The average number of consecutive 70s in the Twin Cities since 2000 is about 64 days, up from the historical average of just 48. So this year saw almost DOUBLE than even the more modern averages. Even the lower modern average of 64 days is up 31% from the historical average. That’s an extra 2+ weeks of warm weather.
The funny thing is, almost no matter what figure we look at you come up with a similar number of about two extra weeks. Even if we just look at the total number of 70-degree days in an average year, that number too has increased by about two weeks worth.
Of course, the latest number of relevance this week, and in the coming weeks is the growing season, i.e. consecutive days above freezing. No surprise, the growing season is getting longer – and by quite a bit. We’ve added up to several weeks to the growing season in the last 50 years.
The other stat that caught my eye recently was the unusually long stretch without a chilly night, what I deem to be in the 40s or colder. We had 118 consecutive nights where the overnight low stayed at 50 or warmer in the Twin Cities. The only summers on record with more are 2016, 2018 and 1881.
Considering the impacts of human-caused climate change in 1881 was minimal or non-existent, that was quite the season. Eight of the top 10 are just since 2000, so before someone tries to yell that it happened once before, long ago, it simply doesn’t match the trends and full context of data.
When we look outside the Twin Cities, where the urban heat island (UHI) doesn’t have any impact, we see similar trends but obviously shorter seasons. Saint Cloud for example gets cooler than the Twin Cities most nights due to it being a bit farther north and outside of the large sprawl of the urban Twin Cities metro. There you see a big increase in the length of the warm night season. In fact, it’s a staggering triple the value of 50 years ago.
In other words, 50 years ago you could basically count on just the warmest few weeks of summer staying above 50 at night. But now? That season has expanded to include all of July and into mid-August on average.
Going back to the growing season data, we can see the similar pattern in St. Cloud. While the growing season of central Minnesota is shorter than the Twin Cities area, it’s still increased by nearly three weeks since 1970.
Add to all of this another warm month this September, including that record high of 92 last week, and you start to see the big, complicated picture of climate change in Minnesota.