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Is the United States prepared for a hot future?

Is the United States prepared for a hot future?
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In the not-too-distant future, when recalling the year 2020, we may remember a time when how we worked, travelled or even socialized, changed as a result of COVID-19.

 Others may reflect on how the current global pandemic pushed the world towards the deepest recession since the 1930s. Yet, many will probably forget that amid the pandemic, the Northern hemisphere was running its own fever, with the hottest July on record. And while it was only the 11th warmest July ever for the United States — which is still concerning — regions in the southwest and northeast sweltered through a relentless July heat wave that brought record-breaking hot temperatures.

Summer heat waves are not that unusual across the U.S. Typically drier and warmer summer conditions over the southwest and central U.S. coincide with cooler-than-usual eastern Pacific Ocean temperatures that develop off the Peruvian coast and extend westwards. Called La Niña (Spanish for "The Little Girl"), this oceanic pattern is now starting to emerge, helping to drive the hotter conditions across the southwest U.S. Numerous La Niña events in the 1950s were partially responsible for a decade-long drought and exceptional heat wave activity across the central U.S. Concurrent record-breaking heat waves and drought (aka., the Dust Bowl) in the 1930s during the Great Depression inflicted devastation on the U.S., with nearly half of the country affected by dust storms in 1934. In 1936, countless record daily temperatures were set over the Great Plains and Midwest such that 1936 still remains the hottest summer for the continental U.S. 

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But what made the 1930s such a remarkably hot decade, to the point that warm extremes have seemingly declined over the U.S. since that time?

Our research team at the University of Edinburgh in the UK set out to answer this question by investigating the possible factors that triggered and exacerbated the Dust Bowl heat waves. To achieve this, we combined results from; station-based and gridded observations from the 1930s, reconstructed weather data, global atmospheric-only climate models and a high-resolution regional model focused on the southern U.S. What we found was a complex interplay between influencing factors that ultimately caused the 1930s heat extremes.

The first factor that helped trigger the Dust Bowl was a warm North Atlantic. We determined this by conducting atmosphere-only climate model simulations, separating the influence from the Pacific and Atlantic Oceans on the U.S. climate of the 1930s. Model simulations influenced by the Atlantic resulted in significantly drier springs over the central U.S. with less moisture transported north from the Gulf of Mexico, as was observed in the 1930s. These drought conditions helped drive up atmospheric heating in the summers and created greater heat wave potential. 

The next crucial factor was the land-cover changes, with about 30 percent of the Great Plains transformed from native perennial grasses to shallow rooted crops susceptible to drought. Previous research has shown that exposure of bare soil brought on by drought and dust storms amplified the Dust Bowl temperatures. To test this in our atmospheric-only model, we replaced the temperate and tropical grasslands over the Great Plains with varying levels of bare soil. This led to dramatic spikes in the amount of simulated atmospheric dust and the intensity and frequency of the simulated heat waves.

The final piece of the puzzle was the role of greenhouse gases. Research suggests that a discernible human influence can be detected in record-breaking temperatures and global droughts as early as the 1930s. To evaluate this in the context of the Dust Bowl, we conducted thousands of regional model simulations of the summers of 1936, and perhaps unsurprisingly, found only a small amplification in the heat waves across the northern Great Plains in 1936 due to greenhouse gases. This got us thinking — what would be the likelihood of a Dust Bowl summer under today's greenhouse gas levels? To answer this, we ran the same simulations of 1936 again, but with present-day greenhouse gases. The results were quite alarming, with the risk of a Dust Bowl heat wave occurring nowadays more than 2.5 times as likely than for the 1930s, entirely due to greenhouse gases! 

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Across the world, heat waves now occur more frequently and are longer than during the last century, including across the southern and eastern U.S. In fact, there's been a significant increase in the co-occurrence of meteorological drought and heat waves over the U.S. since the 1960s, particularly over the southern states. And it's highly likely that this warmer climate will drive an increase in record-breaking temperatures, like was likely observed just recently in Death Valley National Park with temperatures reaching 130 degrees Fahrenheit. 

A future with hotter and longer heat waves (and droughts) would likely reduce crop yields, with possible global ramifications. So, 2020 might be a year ruined by a global pandemic, but perhaps it will be the year that opens our eyes to what future U.S. summers might be like. 

Tim Cowan, is currently a research fellow at the University of Southern Queensland, and previously held a post-doctoral research fellowship at the University of Edinburgh in the UK. He has published 70 papers on topics covering regional climate variability and change, with a particular focus on extremes like drought and heatwave; he now spends his time developing new seasonal forecast products that will assist graziers in northern Australia improving their bottom line during the summer monsoon.