Already nearing the halfway point of meteorological summer and shocker: It’s a hot one. Again. For the third consecutive year, we are likely to register one of the 15 hottest summers in Twin Cities history.
Last summer was the hottest recorded for the Twin Cities, beating 1988 and any summer in the 'Dust Bowl' decade. If the computer models are correct, we could be headed toward one of the ten hottest Julys on record. This comes after the 11th hottest June, which followed the 2021 June that was the second hottest on record.
When it comes to climate change, June and September are our fastest warming months in the warm season. July and August, the peak of our summer heat, are warming more slowly and subtly. Basically, summers are getting longer rather than more extreme in Minnesota. It gets hot quicker and earlier, and the heat lasts longer into September. That’s why some of the most dramatic changes we are noticing are in early summer and prolonged, warm autumns.
This is PRECISELY what you’d expect in a Greenhouse Effect world. Elevated fossil fuel emissions result in our atmosphere trapping more heat, not necessarily magnifying it or creating more of it. This is why you’d expect summers to get longer, winters shorter, and nights to be warmer. This is also the reason why the Arctic is warming fastest in winter despite being almost totally in the dark.
Below is a rough illustration of what’s happening:
Understanding the uneven warming of summer months helps put into context summer extremes, like the Twin Cities hitting 101 degrees on June 20. One-hundred degrees is not unheard of in Minnesota, but it is infrequent. What's much more interesting is that it happened in June. On the rare occasion we hit 100 degrees (on average about once every five years), 71% of the time it’s in July.
June average temperatures have warmed about 1.5 degrees F, which may not sound like a lot, but that’s half of a standard deviation (a measure of the normal range from average). That warming is enough to increase the probability of hitting 101+ degrees in June by 3 to 4 times the historical average. I’ve discussed before how shifting the average makes extremes exponentially more (or less on the cold side) likely. Here’s a graphic reminder:
What about scorching heat during the Dust Bowl?
A keen weather enthusiast knows that the Dust Bowl summers of the 1930s saw many 100-degree days – more than in any other decade. The 1930s however are an anomaly that we need to separate from the century and a half record (in fact when we apply trend lines they usually disappear).
There are studies now that show fingerprints of human-caused climate change we previously didn’t expect. Watch for an upcoming article on some of the interesting science on the 1930s and how it compares to the recent decade.
The basic takeaway is that despite all the extreme hot days in the 1930s, the recent decade is hotter than the 1930s. In fact, if we look at the 15 hottest summers, No. 1 was last summer. Yes, 2021 beat all of the 1930s summers. Three of the 15 hottest summers are from the 1930s (1933, 1936, 1937). Three of the hottest are from the past ten years: 2021, 2012, 2020. Even the average of the three most recent summers is hotter than the three hottest 1930s summers.
The sure sign that our hot summers now ARE a result of human-caused climate change is that the heat is consistent and persistent. Sure, July 1936 hit 108 – the hottest recorded temperature in Twin Cities history – and many 100+ days, but June of that year was chilly and August that summer was just normal.
While July 1936 was very hot, the overall summer climate behaved more like normal, with ups and downs. Last summer was warmer overall by being consistently hot, just like this summer so far. The cool breaks, if they happen, aren't as cool as they used to be.
If we take out the anomaly of the 1930s (again this will be discussed in a future post) and look at the first 50 years of our 149 years record-keeping compared to the most recent 50 years, you see a VERY clear trend: There are 2.5 times more 100+ days in the most recent five decades than the first five, and four times more 101+ days. Even just looking at the hottest day of each summer and plotting a graph, the trend is upward, even if you include the 1930s.
The most relevant measure to show our warming summers is average temperature, and the average now is absolutely, undeniably much warmer than ever before.
One of the big shifts in our average temperatures are a result of very warm summer nights, even separate from the urban heat island effect we see in the Twin Cities.
Statewide, nights are getting warmer (summer and winter). While the 101-degree day of June has definite climate change fingerprints, it’s not as large as the fingerprints on the two warm nights that surrounded that scorcher. We had three overnight lows in a row in the 70s; the low on the June 20 was almost 80 degrees. The climate shift index from Climate Central illustrates this impact well.
The climate shift index statistically extracts the component of human-caused climate change from the data to give a measure of climate change’s impact on a specific temperature reading for a region.
For example, the 101-degree day had a climate change impact, but to a lesser degree:
With still the majority of July to get through, looking at all the models combined with the data already in the books, the Twin Cities has a 93% chance of being warmer than normal. As I mentioned at the start, the model data suggests this could even be one of the ten hottest Julys of all time.
If you’re looking for a break in August, good luck.
If the long-range forecasts for the remainder of July and August are realized, this could be a VERY hot summer again. It would likely make it the fifth-hottest summer, hotter than 1936 or 1937 but just behind 2012 and 1933. There’s still a little more than half of summer left and Mother Nature may have other plans, but it’s difficult to see how we reverse the hotter-than-average trend any time soon.
Using July 12 as an example
To understand statistics as they apply to temperatures (normalized data, standard deviation, averages, etc.), let’s look at July 12. We can plot every high temperature at MSP Airport on July 12 from 1873 through 2021.
The Greek letter after each number on the very bottom is ‘sigma.’ It equates to one standard deviation, or how far the temperature deviates from the average.
If data is normalized (i.e. the median and the average are the same), it will follow a bell curve. The more data you have, the better chance of this happening. This is why we can’t tell much from five years of data, but ten, 30 or even 100 years tells us a lot. In this particular case, we have high temperatures for every July 12 for 149 years. The data lines up nicely in a "normal" way.
We can calculate the average (mean) and the standard deviation (sigma) from the data.
In this case, the average high temperature is 84. The standard deviation is 6.6 degrees F. The area under the curve represents probabilities. So we can say that the average high is 84 and your +/- 1 standard deviation range is 77 to 91 degrees F. This represents the darkest blue part of the bell curve in the center. It equates to 68%, or 68% of the time the high temperature on July 12 will be between 77 and 91.
If we want to further expand our range to two standard deviations (2 sigma), we take the 6.6 degrees F and double it +/- from the average. The two standard deviation range, which represents the darkest, and middle-darkest blue areas, is about +/- 13 degrees F, or a range of 71 to 97. That equates to 95%, or 95% of the time the temperature falling between 71 and 97 on July 12. Anything outside of this range (a day in the 100s, or a day in the 60s) is pretty rare.
To give an example of how a seemingly small shift can affect everything, I’ve put the probability of each range being warmer or colder than each standard deviation value.
Let’s say the average for July 12 were to warm 3 degrees F, or about half of a standard deviation (0.5 sigma). The average would now be 87 instead of 84, not a seemingly big shift. But the extreme end of 104 degrees F would change. As of now, 104 degrees F on July 12 is nearly impossible (0.2% chance): a 1 in 500 year occurrence.
But if we shift this data 3F as Minnesota's average temperature warms, an average temperature of 87 would make the chances of hitting 104F much more likely.
At an average of 84F, hitting 104F would represent 3 standard deviations. But at 87F it becomes 2.5 standard deviations. The probability of hitting that temperature goes from just 0.2% to 1.3%. 104F would still be infrequent, but it’s now six times more likely to happen: 1 in 77 years versus once in half of a millennium.
On the cool side of things, let’s say we take the 2 sigma value of 71 degrees F, which has a 2.3% chance. Well, now it has only a 1.3% chance, meaning it has become almost twice as unlikely as it used to be.
BMTN Note: Weather events in isolation can't always be pinned on climate change, but the broader trend of increasingly severe weather and record-breaking extremes seen in Minnesota and across the globe can be attributed directly to the rapidly warming climate caused by human activity. The IPCC has warned that Earth is "firmly on track toward an unlivable world," and says greenhouse gas emissions must be halved by 2030 in order to limit warming to 1.5C, which would prevent the most catastrophic effects on humankind. You can read more here.