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July 1936 was the hottest July ever recorded in the Twin Cities and across Minnesota. During an intense heat wave July 10-14 that year, an estimated 900 Minnesotans died, including 100 in the Twin Cities in an 18-hour period. 

We’ve not quite seen anything like it since. The Twin Cities' hottest ever recorded temperature of 108 was set on the final day of that heat wave, July 14, 1936. 

I recall my grandfather telling me that everyone slept outside that week. It was the middle of a severe drought so there were no insects to bother them and there was no air conditioning to provide relief.

Understanding what caused the excessive heat of the 1930s isn’t just a curiosity, it can provide important clues to understanding our warming world today and in the future. As our planet warms, we’re seeing desertification increase. This means areas of the planet are becoming more arid, which exacerbates warming.

Recent research has shown that while there was an extraordinary confluence of patterns that led to a series of hotter and drier summers, our alteration of the landscape of the Plains amplified these conditions to a tipping point, to a new, record level.

We know that blocking patterns on a semi-regular basis can create the conditions that lead to drought and hotter than normal patterns. A lot of energy has gone into deciphering what elements play a role in this. Several studies show that anomalous ocean temperatures – such as a ‘cool phase’ PDO (Pacific Decadal Oscillation) and a ‘warm phase’ AMO (Atlantic Multidecadal Oscillation) – impact spring and summer patterns in the central U.S. 

The PDO is a somewhat regular back and forth of colder- and warmer-than-normal sea surface temperatures (SSTs) in the Pacific that influence large scale, often blocking patterns. Similarly, the Atlantic Multidecadal Oscillation (AMO) is a sea surface temperature-driven event. When the PDO is in a cool phase, the eastern tropical Pacific is cooler than normal (much like current La Nina conditions), while the north Pacific is warmer than normal. In a warm phase AMO, the north Atlantic is warmer than normal.

In fact, this year and the last couple years, as well as 2010-2012, saw the confluence of similar patterns. In both periods, we saw hot summers and drought. This chart, from the Climate Impact Company, compares the PDO & AMO for 2020-2022 and 2010-2012.

16jun22_pdoamo

It’s worth noting these patterns only partially explain the heat of recent years. We must distinguish between weather and climate. In a warmer world, just as in the past, there will be ups and downs. Especially in a volatile place like Minnesota. The temperature trends are undoubtedly upward in recent decades but these patterns help explain the ups. 

Last summer (2021) was the hottest ever recorded for the Twin Cities, made possible by the combination of human caused climate change and these semi-regular weather patterns.

Some fascinating research published in the science journal Nature attempts to model conditions to recreate the 1930s heat waves. Atmospheric models have been able to create unusually warm conditions when several factors come into alignment on rare occasions, but not to the magnitude of the Dust Bowl era. It turns out THE deciding factor was adjusting the ground behavior, or land forcing. 

In the decades leading up to the 1930s there was a rapid expansion of agriculture throughout the Plains. Native prairie (and the bison that helped cultivate it) diminished greatly, or in some areas entirely. While farming appeared to temporarily be successful in these new lands prior to the 1930s, when disaster struck it was made worse by the altered landscape. Those promoting settlement in the Plains even sold the notion that if the Plains were farmed and therefore tamed, the rains would somehow increase. Newly developed railways aimed to increase business too, so promoting expansion west suited the interests of railroaders.

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We now know that native grasses and plants of the prairies are much better suited to the boom and bust cycles of Plains weather (similarly to Minnesota) than traditional agriculture. Floods, droughts, heat waves and cold snaps are commonplace in the central U.S.. When droughts did strike, failed crops left exposed soils. It turns out this huge increase in exposed soils made a bad situation much worse.

The researchers in the Nature publication found that when they increased the exposed soils from a reference of 15% to 30%, 50% and even 80% – at the expense of the native grasses – they were able to accurately create the heat waves of the magnitude of the record-breaking 1930s.

Key to all of this was the addition of very dry springs in the Central and Southern Plains, which essentially pre-conditioned the summer scenario. It appears that particularly dry Aprils and Mays are significant to the formation of a heat dome that can envelope the central U.S. by summer.

The reason bare soils make a region hotter is the same reason a dusty or sandy desert does. When there’s less vegetation there’s less moisture (both through evapotranspiration and in the soil). Moisture helps keep soils and an air mass cooler through evaporation (think of sweat on your skin, or coming out of the lake or pool on a dry, windy day). A particularly hot, dry pattern can lead to less vegetation, which can in turn make it hotter in a sort of feedback loop. It’s for this related reason that deforestation in places like the Amazon (and other rain forests) is leading to those areas becoming much more arid and hotter. It turns out you need not just rain to make a rain forest, but a forest. Likewise you can’t have a healthy Great Plains without its grasslands.

By increasing the amount of land made vulnerable to drought through the removal, or tilling over of native grasslands and replacing them with cropland – which can quickly become bare soil given spring drought conditions –  we made a bad situation MUCH worse. Native grasses and plants of the plains are also much more drought tolerant. Based on numerous model simulations, it appears highly unlikely we would have had quite the record breaking temperatures of the 1930s without a human role.

As I’ve discussed previously, some recent research has even shown that the 1930s had the first fingerprints of human-caused climate change through greenhouse gas emissions, though more subtly compared to modern times where greenhouse gas emissions are significantly greater.

Likelihood climate chg contribution

Understanding how desertification impacts heat waves is crucial. Recent heat waves in Europe and the U.S. are examples of these naturally occurring phenomena becoming more frequent and more severe in our warming world.

Coming back full circle, I’ve discussed previously that as hot as the 1930s summers were, our modern summers are actually hotter. That’s because the average temperatures are warmer. If you take the three hottest summers of the last decade and compare them to the three hottest summers of the 1930s, the recent summers beat them. It is appearing increasingly likely that 2022 will finish as one of the 15 hottest summers ever. 

Red arrows point to 2011 to 2021 summers, brown arrows to 1930s

Red arrows point to 2011 to 2021 summers, brown arrows to 1930s

The 1930s had hotter extremes, no doubt. The heat wave of July 10-14, 1936 saw each of those 5 days between 105 and 108 degrees. But, it was a cool June and a normal August in 1936. While extreme heat waves blew in Minnesota’s direction occasionally through the 1930s, there were breaks from the heat because that extreme heat was very regional. Modern summers are warmer consistently, a further sign that the climate is warmer overall.

Here's a great example. While the Plains (and Minnesota) were hot many summers of the 1930s, the globe was not.

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Compare that to 2018, which wasn’t a particularly hot summer for Minnesota, but note the different looking big picture.

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Where the 1930s heat required a perfect storm to come together, including the added manipulation of the landscape from humans, modern heat is very easily produced because there’s more heat in the whole system.

Summed up, there may be no better way to decipher the Dust Bowl heat than to quote FarmDocDaily's "Lessons Written in the Dust."

“The Dust Bowl presents a visceral historical analogue. It should counsel humility about the ability of humans to perpetually push natural resources for their benefit; written in the dust of the thirties are warnings aplenty.” 

Or, one could simply read Dr. Seuss’s "The Lorax."

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