New research explores future limits of survival and livability in extreme heat conditions

Commonly associated with longer days and slower paces, this summer's record-smashing heat in Arizona demonstrated a concerning future for the planet's warmest season. From power outages endangering entire neighborhoods and heat-related deaths rising among some of the state's most vulnerable populations, the city of Phoenix found itself in national headlines. As national attention grew, one question became clear: How does anyone live there?

The consequences of extreme heat do not affect Arizona residents alone.

Extreme heat made worldwide news this year, including in November when a 23-year-old woman died of cardiorespiratory arrest at a Taylor Swift concert in Brazil where heat indexes that day exceeded 120 degrees.

Jennifer Vanos, associate professor in the School of Sustainability at Arizona State University, studies extreme heat and its health impacts.

She is the lead author of a new paper published Nov. 29 in Nature Communications. Titled "A physiological approach for assessing human survivability and liveability to heat in a changing climate," the paper explores temperatures at which humans can survive.

The research demonstrates that the current estimated upper temperature and humidity limits used for human survivability may not paint an accurate picture of the impacts of a warming planet on human health.

"For the past decade or so we have been using what we call a 'wet bulb temperature' of 35 degrees Celsius, or 95 degrees Fahrenheit, as the limit for human survivability," said Vanos, also a Senior Global Futures Scientist in the Julie Ann Wrigley Global Futures Laboratory.

The wet-bulb temperature limit for human survival indicates the maximum combinations of temperature and humidity that humans can tolerate without suffering inevitable heat stroke over a fixed duration of exposure.

"The idea is that you could survive for up to six hours at that level of heat exposure," Vanos said.

"That number really oversimplifies what happens physiologically in the body when your body is exposed to that temperature, and it doesn't account for important variables like age or other vulnerability factors."

Vanos said the commonly-used wet-bulb temperature for human survivability assumes the person is indoors or shaded, unclothed, completely sedentary, fully heat acclimatized and of an "average size." These assumptions do not align, in most cases, with how humanity navigates the summer season.

The paper models scenarios that adjust for factors such as humidity, age, activity level and sun exposure, and provides a range of safe temperatures based on a series of characteristics.

"We didn't only want to better understand the conditions that people could survive in," Vanos said.

"We wanted to understand the conditions that allowed people to live their lives. If the only safe way to live in an area is to be completely sedentary, people won't want to live there. Being able to spend time outdoors and live your life without seeing a sustained rise in core temperature is a really important metric to understand today and as we move into the future."

Vanos said Gisel Guzman Echavarria, an ASU student, was instrumental in creating the figures used throughout the paper to demonstrate the research findings.

The research, funded by the National Science Foundation, was conducted by a combination of climate scientists and physiologists, a collaboration that Vanos said was crucial in understanding the intertwined nature of heat and human health.

Ollie Jay, professor and director of the Heat and Health Research Incubator at the University of Sydney, said the combined perspectives allow for a cohesive understanding of exactly how climate outcomes can impact people on the physiological and biophysical level.

"The existing wet-bulb temperature estimate of 35 degrees Celsius is used very commonly, with one example being the Intergovernmental Panel on Climate Change report," said Jay, senior author of the paper.

"These kinds of reports can shape policy efforts, but they are using a model for heat that is a very conservative estimate of what the impacts are going to be on humans. If we start using a more realistic, human-based model, the impacts are going to be more severe. They're going to be more widespread and they're going to happen sooner than we are projecting."

Vanos and Jay agree that the survivability ranges provided in the paper can give an important glimpse into the future: one that includes an increased need for cooling infrastructure, a personalized approach to heat protection and possible heat-driven migration.

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Survival of the newest: the mammals that survive mass extinctions aren't as 'boring' as scientists thought

When an asteroid hit the Earth 66 million years ago, it set off a devastating mass extinction. The dinosaurs (except for a few birds) all died out, along with lots of the mammals. But some small mammals survived, laying the groundwork for all the mammals alive today. For decades, scientists have assumed that mammals and their relatives that survived challenging times (like those during mass extinctions) made it because they were generalists that were able to eat just about anything and adapt to whatever life threw at them. A new study into the mammal family tree through multiple mass extinctions revealed that the species that survived aren't as generic as scientists had thought: instead, having new and different traits can be the key to succeeding in the aftermath of a catastrophe.

"The idea of the 'survival of the unspecialized' goes back to the 1800s, and the conventional wisdom is that generalized animals are the least likely to go extinct. But we found that the ones that survived more often only seemed generalized in hindsight, when compared with their later descendents. They were actually pretty advanced animals for their time, with new traits that might have helped them survive and provided evolutionary flexibility," says Ken Angielczyk, the MacArthur Curator of Paleomammalogy at the Field Museum and senior author of the study in Nature Ecology and Evolution.

"What's been thought previously is that every time a new group of mammals evolves, you start out with a small generalist animal, since when disaster strikes, those are the guys that keep on trucking -- they can hide anywhere, they can eat whatever is around," says Spencer Hellert, an Assistant Professor at Columbia College Chicago, a research associate at the Field Museum, and co-lead author of the study. "The kind of mammal that survives a mass extinction won't be a specialist like a panda bear that can only eat bamboo."

David Grossnickle, an Assistant Professor at the Oregon Institute of Technology and co-lead author, published a study in 2019 that highlighted how small, insect-eating mammals are often the lineages that survive challenging times, including the extinction event that killed the dinosaurs, and serve as forerunners of major diversifications. He approached Hellert and Angielczyk to see if that trend held true for earlier mammals and their ancestors.

Hellert created a massive family tree of the synapsids, the group of animals of which mammals are the last surviving members. This family tree is one of the largest fossil trees ever produced, and it takes into account all the previous family trees made by scientists for this group. This method is a more formal, rigorous, and repeatable way to summarize information from lots of trees instead of just picking a few and sticking them together.

"We couldn't test this idea without a humongous family tree," says Angielczyk, "along with general information about the animals' diets and body sizes. Then we looked at what happened over time through the five major evolutionary radiations in synapsids," when a few species branched out into greater diversity. When a new disaster led the majority of those species to go extinct, the process repeated itself.

The researchers, including co-authors Graeme Lloyd and Christian Kammerer, found that the story of synapsid evolution wasn't one of "survival of the small and unspecialized." At some points, larger synapsids were the ones that survived, and the winners weren't just generalist insect-eaters.

"We were pretty surprised -- it's pretty well-established that those mammal radiations go from these small insectivores into the bigger taxa repeatedly, so I was kind of expecting to see that as we went back into synapsid history. And when we went back, that pattern starts to disappear," says Grossnickle.

While some of the survivors of mass extinctions at first appeared to be unspecialized, closer analysis revealed that they had newer, more novel characteristics. For instance, many mammals from the time of the dinosaurs had teeth that were good for cutting into prey. A few had tooth structures that acted like a mortar and pestle and were able to grind in addition to just cutting. This "fancier" tooth may have been an advantage in hard times with less food availability, because this more specialized tooth structure would have let them eat a wider variety of food.

These findings don't mean that hyper-specialized animals, like pandas that only eat bamboo, are less vulnerable to the threat of extinction than more generalist species, like raccoons that can eat a wider variety of foods. Instead, the study shows that the mammal relatives that made it through mass extinctions aren't as generic as previously assumed.

"Animals with novel traits like new tooth features, or jaws that work a little better at breaking down different food items, don't really take over ecologically until the incumbent, older lineages go extinct," says Grossnickle. "You often need an extinction event like the one that killed the dinosaurs to knock out some of those older groups, and then it allows those fancier animals to persist and diversify."

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A star's unexpected survival

Hundreds of millions of light-years away in a distant galaxy, a star orbiting a supermassive black hole is being violently ripped apart under the black hole's immense gravitational pull. As the star is shredded, its remnants are transformed into a stream of debris that rains back down onto the black hole to form a very hot, very bright disk of material swirling around the black hole, called an accretion disc. This phenomenon -- where a star is destroyed by a supermassive black hole and fuels a luminous accretion flare -- is known as a tidal disruption event (TDE), and it is predicted that TDEs occur roughly once every 10,000 to 100,000 years in a given galaxy.

With luminosities exceeding entire galaxies (i.e., billions of times brighter than our Sun) for brief periods of time (months to years), accretion events enable astrophysicists to study supermassive black holes (SMBHs) from cosmological distances, providing a window into the central regions of otherwise-quiescent -- or dormant -- galaxies. By probing these ``strong-gravity'' events, where Einstein's general theory of relativity is critical for determining how matter behaves, TDEs yield information about one of the most extreme environments in the universe: the event horizon -- the point of no return -- of a black hole.

TDEs are usually "once-and-done" because the extreme gravitational field of the SMBH destroys the star, meaning that the SMBH fades back into darkness following the accretion flare. In some instances, however, the high-density core of the star can survive the gravitational interaction with the SMBH, allowing it to orbit the black hole more than once. Researchers call this a repeating partial TDE.

A team of physicists, including lead author Thomas Wevers, Fellow of the European Southern Observatory, and co-authors Eric Coughlin, assistant professor of physics at Syracuse University, and Dheeraj R. "DJ" Pasham, research scientist at MIT's Kavli Institute for Astrophysics and Space Research, have proposed a model for a repeating partial TDE. Their findings, published in Astrophysical Journal Letters, describe the capture of the star by a SMBH, the stripping of the material each time the star comes close to the black hole, and the delay between when the material is stripped and when it feeds the black hole again. The team's work is the first to develop and use a detailed model of a repeating partial TDE to explain the observations, make predictions about the orbital properties of a star in a distant galaxy, and understand the partial tidal disruption process.

The team is studying a TDE known as AT2018fyk (AT stands for ``Astrophysical Transient''). The star was captured by a SMBH through an exchange process known as "Hills capture," where the star was originally part of a binary system (two stars that orbit one another under their mutual gravitational attraction) that was ripped apart by the gravitational field of the black hole. The other (non-captured) star was ejected from the center of the galaxy at speeds comparable to ~ 1000 km/s, which is known as a hypervelocity star.

Once bound to the SMBH, the star powering the emission from AT2018fyk has been repeatedly stripped of its outer envelope each time it passes through its point of closest approach with the black hole. The stripped outer layers of the star form the bright accretion disk, which researchers can study using X-Ray and Ultraviolet /Optical telescopes that observe light from distant galaxies.

According to Wevers, having the opportunity to study a partial TDE gives unprecedented insight into the existence of supermassive black holes and the orbital dynamics of stars in the centers of galaxies.

"Until now, the assumption has been that when we see the aftermath of a close encounter between a star and a supermassive black hole, the outcome will be fatal for the star, that is, the star is completely destroyed," he says. "But contrary to all other TDEs we know of, when we pointed our telescopes to the same location again several years later, we found that it had re-brightened again. This led us to propose that rather than being fatal, part of the star survived the initial encounter and returned to the same location to be stripped of material once more, explaining the re-brightening phase."

First detected in 2018, AT2018fyk was initially perceived as an ordinary TDE. For approximately 600 days the source stayed bright in the X-ray, but then abruptly went dark and was undetectable -- a result of the stellar remnant core returning to a black hole, explains MIT physicist Dheeraj R. Pasham.

"When the core returns to the black hole it essentially steals all the gas away from the black hole via gravity and as a result there is no matter to accrete and hence the system goes dark," Pasham says.

It wasn't immediately clear what caused the precipitous decline in the luminosity of AT2018fyk, because TDEs normally decay smoothly and gradually -- not abruptly -- in their emission. But around 600 days after the drop, the source was again found to be X-ray bright. This led the researchers to propose that the star survived its close encounter with the SMBH the first time and was in orbit about the black hole.

Using detailed modeling, the team's findings suggest that the orbital period of the star about the black hole is roughly 1,200 days, and it takes approximately 600 days for the material that is shed from the star to return to the black hole and start accreting. Their model also constrained the size of the captured star, which they believe was about the size of the sun. As for the original binary, the team believes the two stars were extremely close to one another before being ripped apart by the black hole, likely orbiting each other every few days.

So how could a star survive its brush with death? It all comes down to a matter of proximity and trajectory. If the star collided head-on with the black hole and passed the event horizon -- the threshold where the speed needed to escape the black hole surpasses the speed of light -- the star would be consumed by the black hole. If the star passed very close to the black hole and crossed the so-called "tidal radius" -- where the tidal force of the hole is stronger than the gravitational force that keeps the star together -- it would be destroyed. In the model they have proposed, the star's orbit reaches a point of closest approach that is just outside of the tidal radius, but doesn't cross it completely: some of the material at the stellar surface is stripped by the black hole, but the material at its center remains intact.

How, or if, the process of the star orbiting the SMBH can occur over many repeated passages is a theoretical question that the team plans to investigate with future simulations. Syracuse physicist Eric Coughlin explains that they estimate between 1 to 10% of the mass of the star is lost each time it passes the black hole, with the large range due to uncertainty in modeling the emission from the TDE.

"If the mass loss is only at the 1% level, then we expect the star to survive for many more encounters, whereas if it is closer to 10%, the star may have already been destroyed," notes Coughlin.

The team will keep their eyes to the sky in the coming years to test their predictions. Based on their model, they forecast that the source will abruptly disappear around March 2023 and brighten again when the freshly stripped material accretes onto the black hole in 2025.

The team says their study offers a new way forward for tracking and monitoring follow-up sources that have been detected in the past. The work also suggests a new paradigm for the origin of repeating flares from the centers of external galaxies.

"In the future, it is likely that more systems will be checked for late-time flares, especially now that this project puts forth a theoretical picture of the capture of the star through a dynamical exchange process and the ensuing repeated partial tidal disruption," says Coughlin. "We're hopeful this model can be used to infer the properties of distant supermassive black holes and gain an understanding of their "demographics," being the number of black holes within a given mass range, which is otherwise difficult to achieve directly."

The team says the model also makes several testable predictions about the tidal disruption process, and with more observations of systems like AT2018fyk, it should give insight into the physics of partial tidal disruption events and the extreme environments around supermassive black holes.

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