The strange divide in how Americans experience summer temperatures

CNN

2 days ago

Chris Mooney is a Pulitzer Prize-winning journalist and a CNN Climate contributor. He is currently a professor of practice in the Environmental Institute at the University of Virginia.
The contiguous United States has endured another searing summer. June was unusually warm, and a major heatwave afflicted nearly a third of the population late in the month, and July offered little relief.
This is hardly a surprise: Summers in the Lower 48 are now 1.6 degrees Fahrenheit warmer on average than they were in 1896, according to the Environmental Protection Agency.
Summer is the season in which the effects of climate change are arguably most apparent: It's getting hotter, longer, more humid and more dangerous. Yet averages elide a complex reality: The country's experience of hotter summers — and thus one of the most visceral aspects of climate change itself — is fractured along geographic lines.
Summer is behaving very erratically as the country warms, with large changes in some regions, especially the West, and very muted ones in the central and southeast US.
Comparing summers of the past 30 years with a broad period between 1901 and 1960, the limited warming and even slight cooling in some locations becomes strikingly apparent.
It's dubbed the summertime 'warming hole.'
Scientists have acknowledged the warming hole's existence for some time, and sought to explain what is causing it. They are not at all convinced it will continue — frankly, many suspect it won't. But for now, it stands out enough that it requires an explanation, which many research papers have attempted to do.
'There are not too many places on the planet that are showing this, honestly,' said Joseph Barsugli, a climate researcher at the University of Colorado at Boulder who contributed to a recent study on the topic. 'It's pretty unique, I think.'
The pattern is so widely recognized among scientists who study warming trends in the US that it was prominently featured in the most recent installment of the US National Climate Assessment, which came out in 2023. Under the Trump administration, that document no longer resides at what was once its main government website, but it is still available here. It states that in summer, 'seasonal temperatures in some regions east of the Rockies have decreased,' although it adds that in the Southeast, a trend of cooling temperatures had 'recently reversed.'
This is an oddity amid a warming climate, as the general pattern is that the world's land areas are warming up more quickly than the oceans. Europe, for instance, is one of the fastest warming land areas on the planet.
But where you are on Earth determines the kind of climate change you get, and there's an enormous amount of variability.
Scientists are still trying to understand the reasons behind the bifurcated pattern — which they sometimes call a 'dipole' — in US summer warming rates between the? West and East.
It has been attributed to anything from the cooling effects of reforestation in the Southeast to 'corn sweat' in the Midwest tied to more productive agriculture. The 'sweating' refers to how corn crops transpire and put more water into the air, which then can fall as cooling rain.
'I think a piece of it is the land use change, and basically, the increase in agricultural intensification, which just kind of dumps water into the atmosphere,' said Jonathan Winter, a professor at Dartmouth University who has found that the warming 'hole' has actually been good for Midwest corn yields.
There's also reason to think that in the Southeast, as people abandoned less profitable farms during the 20th century and forests regrew, that too muted warming. Especially in the summer, forests draw up water through tree roots and transpire it into the air, making it more humid. In a recent paper, Mallory Barnes of Indiana University and colleagues find that this had a significant cooling effect.
Reforestation 'cools off big areas, not just small areas,' said Barnes.
On top of all that, some of the phenomenon has to do with the extreme temperatures associated with the Dust Bowl of the 1930s. That human-induced phenomenon skews the data extra-warm in that era for some parts of the US, making it hard to find a warming temperature trend today. Very warm summer temperatures in the 1930s are noticeable, for instance, in the temperature history of Tulsa County, Oklahoma.
The summer warming hole is also sensitive to how it is defined, and in recent decades, there are signs that it may be lessening, somewhat.
In southeastern states like Alabama, Georgia, Kentucky and Tennessee, summer temperatures were very cool in the 1960s and 1970s but have warmed considerably since then. It's just that they haven't necessarily risen much beyond where they were earlier in the 20th century – yet. In some places, like Tuscaloosa County, Alabama, the overall change remains in the direction of cooling:
Finally, the dearth of warming in summer doesn't necessarily mean regions are not warming over the whole year — summer only comprises a quarter of that and gets averaged in with everything else. Still, it mutes the overall change and is an anomaly that needs explaining.
It's also significant because summer is when warm temperatures are potentially the most dangerous. It's also the season we most feel and remember the experience of heat.
One thing the different hypotheses tend to have in common is an emphasis on the cooling effects of rainfall.
For instance, in one striking 2023 paper in the Journal of Climate, a group of leading researchers sought to understand why this 'warming hole' pattern has 'not disappeared,' as might have been expected, and why many climate models don't produce it.
One of their key findings was that an increase in rainfall is the explanation. The rain comes in during summer afternoons and cools temperatures down, keeping a lid on them.
'More rain or cloudier conditions have limited daytime temperatures from rising across a large part of the region,' said Zachary Labe, a climate scientist with Climate Central who was involved with the research.
Indeed, the study shows that nights in the region have been warming as expected. It's the days that have not really done so.
Ultimately, beyond local factors, some researchers are tracking the source of the changes farther afield. They believe a rainier pattern over the eastern US may be rooted in the behavior of the Pacific Ocean.
It's complex to decipher what may be happening, but generally, scientists believe ocean patterns can affect weather and climate in very distant regions. And that includes, perhaps, why one part of the US might be getting more rain while another gets drier.
The atmospheric alignment is similar to what has been called the positive phase of the 'Pacific North America' pattern, with a cool eastern US and a warm West, said Labe.
If ocean surface temperatures are warmer than normal 'in the central equatorial Pacific near the Dateline, no matter what time period you use, it appears you'll get a warming hole somewhere in the eastern US in summer due to increased clouds and precipitation,' said Gerald Meehl, a climate scientist at the National Center for Atmospheric Research who has studied the warming hole and its connection to the Pacific.
The relative lack of warming has been highlighted by some critics of various aspects of climate science. That includes John Christy, the state climatologist of Alabama and a professor at the University of Alabama in Huntsville. Christy recently co-authored an Energy Department report that did not deny the Earth is warming in general, but criticized 'exaggerated projections of future warming.' That document has mobilized a large number of climate researchers seeking to refute it.
'One can immediately see how determining the warming effect on Alabama of the extra [greenhouse gases] is a problem as the temperatures of the recent decades (which should be responding to the warming influence of extra [greenhouse gases]) have actually been cooler than earlier decades when this influence was essentially absent,' writes Christy.
And it's true Alabama shows up in the summer warming hole.
But because the lid on temperatures is so closely tied to rainfall, Barsugli warns that there's no guarantee it'll continue. The warming can leap back without something to suppress it.
'If you do have a very dry year, it probably means you'll break records in maximum temperatures,' he said. 'It's sort of a latent threat.'
It's also hard not to notice the summer warming hole primarily affects central and southern states that tend to vote Republican.
It's unlikely that temperatures themselves are significantly affecting people's ideology, however, said James Druckman, a political scientist at the University of Rochester who has studied the factors underlying beliefs about climate change.
'I don't think they're having a substantial impact on what people think. They might at the margin,' Druckman said. 'I think the politics have become too polarized or entrenched on this issue.'
Ultimately the question is whether the hole will finally be overcome by broader trends. Experts say it might, though they're not sure when.
'It's this tug of war between the increase in rainfall and the increase in temperature,' said Winter. 'At some point, I do expect that it will dissipate. It will still be cool relative to the rest of the United States. But relative to historical temperatures, I think you will eventually see climate change.'