Decades of research shows that Earth was once entirely or almost entirely frozen. The episode is known as Snowball Earth, and though its occurrence is widely accepted, many of its details remain hypothetical. Snowball Earth took place in Earth’s Cryogenian Period, which spanned from about 720–635 million years ago during the Neoproterozoic Era.

The Cryogenian had two major global ice ages that are commonly referred to as Snowball Earth. One is the Sturtian glaciation and the other is the Marinoan glaciation. Researchers have puzzled over these events. Research shows that the Sturtian lasted 56 million years, but climate models have struggled to explain how it could’ve endured for so long.

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have a new explanation for the Sturtian glaciation’s length. They’re presenting it in new research published in the Proceedings of the National Academy of Sciences. It’s titled “Repeated snowball–hothouse cycles within the Neoproterozoic Sturtian glaciation,” and the lead author is Charlotte Minsky from the SEAS.

There’s ample evidence of the Sturtian glaciation in rocks all around the world. These sedimenatry rocks are a stratigraphic record of the advance and retreat of glaciers. But though the evidence is solid, existing climate models are at odds with it.

In their research the authors point out that arriving at a coherent explanation for the Sturtian glaciation is a longstanding problem. How life survived it is difficult to explain, as are aspects of the geologic record. The main challenge is explaining how it lasted for 56 million years, “far longer than can be accommodated by standard models of climate evolution,” the authors write. A global glaciation that long would’ve had severe consequences for life.

“Global glaciations near the dawn of animal life—the so-called Neoproterozoic Snowball Earth events—are among the most extreme climatic perturbations in Earth’s history and likely exerted a strong influence on biological evolution,” the authors explain. “Nonetheless, the cause(s), severity, and environmental/biological effects of these glaciations are still vigorously debated.”

The team used simulations to try to unerstand what happened during the Sturtian. They’re centered on a large volcanic region in Canada called the Franklin Large Igneous Province that erupted more than 700 million years ago and persisted for about 2 million years. It was one of the largest magmatic episodes in Earth’s history and had a powerful effect on the planet’s climate.

The authors say there’s was a sort of tug-of-war between the carbon released by volcanoes and the weathering of newly exposed basalt. When volcanoes (and other processes) built carbon up in Earth’s atmosphere, the climate warmed. That caused the ice to retreat, which exposed more basalt. That increased the weathering of basalt, which slowly removed carbon from the atmosphere. As carbon was removed, the climate cooled.

The researchers found that rather than one single glaciation event, the Sturtian actually consisted of multiple events. “Thus, instead of a single, continuous Snowball, the climate repeatedly flipped between short, self-terminating Snowball glaciations and similarly short warm, largely ice-free interglacial climates,” the researchers write in their paper.

*This figure from the research shows how Earth alternated between Snowball episodes and warm episoded during the Sturtian glaciation. Image Credit: Minsky et al. 2026 PNAS*

Their model explains some of the disagreement between the geologic evidence and existing climate models.

One of those sticking points concerns atmospheric oxygen and the persistence of life. “In the traditional Snowball scenarios, maintaining an oxygenated atmosphere requires the biosphere to be productive enough to continually replace oxygen lost via chemical reaction with volcanic reduced gases,” the authors explain. In those scenarios, the lack of oxygen due to extreme cold and volcanic output is a severe barrier to life if the Sturtian did last for 56 million years. So are “a lack of light in the surface ocean, and a severely limited nutrient supply.”

But if there were instead repeated cycles of glaciation and warming, those barriers, though still in existence, are not as severe.

“In the limit cycle scenario, the biosphere would have only had to persist through shorter—million year timescale—Snowballs. These Snowballs are short enough that even in the absence of photosynthetic oxygen production, the atmospheric O2 reservoir could have persisted without being fully depleted,” the authors explain.

*This figure from the research shows the “Evolution of the atmospheric oxygen inventory during the Snowball limit cycle,” as explained by the authors. p02 describes the abundance of oxygen in the atmosphere, and PAL means Present Atmospheric Level, which for oxygen is 21%. The figure shows how enough oxygen could’ve persisted in the atmosphere if the authors’ limit cycle model is correct. Image Credit: Minsky et al. 2026 PNAS*

If Snowball Earth was repeatedly interrupted by warmer periods, the atmosphere wouldn’t have become depleted of oxygen. “This could help explain how aerobic life persisted through such an extreme interval,” lead author Minsky said in a press release.

If Earth went through global glaciations interspersed with warmer periods, so could other exoplanets. So these results, which go a long way to explaining how life survived these frigid episodes, could also help scientists understand the growing number of exoplanets.

“Finally, the mechanism discussed here has potentially observable implications for understanding more generally when repeated Snowball episodes might occur on Earth-like exoplanets,” the authors conclude.

In the early weeks of the COVID-19 pandemic, health officials struggled to impress upon the public the grave risks associated with the disease, as well as how easily it could spread.

Now, six years later, public fears have surrounded another type of virus that has killed and sickened passengers on a Dutch-flagged cruise ship, including four Californians who were exposed to the virus and recently returned to the United States. This time however, officials are taking a very different approach to messaging surrounding the deadly Andes virus — a type of hantavirus.

While officials and infectious disease experts have been quick to note the seriousness of the rodent-borne disease, they have also stressed key differences between hantavirus and COVID-19. Namely, that this virus is far less transmissible.

Public alarm over the illness began to grow following reports that three passengers died aboard the stricken vessel, MV Hondius. Worries grew further over the weekend when officials announced that 18 U.S. cruise passengers had disembarked and were returning home.

On Monday the California Department of Public Health said during a media briefing that four Californians had been exposed to the virus, but none had contracted it. Three of them were cruise ship passengers, while the fourth was a Sacramento resident who was on a plane with an infected person in South Africa.

As of now, all four individuals lack symptoms and appear healthy, according to Dr. Erica Pan, director of the California Department of Public Health.

One passenger, a Santa Clara resident, disembarked the cruise before the outbreak was recognized and returned to California, she said.

“This person was reported to our department last week and is being monitored by the local public health department where they live,” Pan said. “The other two passengers disembarked over the weekend in the Canary Islands and have been flown” to a bio-containment facility at the University of Nebraska Medical Center.

The individuals in Nebraska are undergoing a health assessment, and federal authorities will determine when they can return to California.

Of the total U.S. cruise ship passengers, sixteen boarded a medical repatriation flight arranged by the U.S. government to Nebraska and have remained there as of Monday, including one person who tested “mildly” positive for hantavirus — that person is staying in biocontainment at the University of Nebraska Medical Center.

Two other passengers, one of whom is showing symptoms of the virus, traveled to Atlanta and are staying in a biocontainment facility at Emory University.

That brings the total number of cases of hantavirus to nine, seven laboratory confirmed and two probable cause, including three deaths.

It’s reasonable for people to be concerned about this latest outbreak, said Dr. Nicole Iovine, chief medical epidemiologist and an infectious disease expert at the University of Florida Shands Hospital. Photographs of healthcare personnel in full personal protective equipment assisting cruise passengers are likely to spark recollections of the pandemic.

Even though this is not an easily transmitted disease, it is transmissible and has a high mortality rate, Iovine said. U.S. Centers for Disease Control and Prevention officials said 38% of people who develop respiratory symptoms from hantavirus may die from the disease.

“So it’s reasonable for the medical personnel to take maximal precautions so that they don’t contract it,” Iovine said. “It’s not a reflection of [the virus] being extremely contagious.”

In the U.S., hantavirus cases occur year-round and are transmitted via the urine, feces and saliva of wild rodents.

The Andes virus, a strain of the disease that’s endemic to Argentina, similarly passes from the exposure of wild rodent particles. Infected humans can transmit the virus to other people.

Unlike other infectious respiratory illnesses, hantavirus “infects cells very deep in the lungs, so it’s not as easily transmitted then when someone is speaking or coughing,” Iovine said.

COVID-19 transmission occurred when an infected person breathed out droplets and very small particles that contained the virus. Other humans could then inhale the particles or come into contact with them on the surface of objects.

“That’s one of the reasons that makes it much more difficult to transmit person-to-person, and is the reason why this is just not going to turn into a pandemic,” she said.

Experts say person-to-person transmission of the virus occurs only with close and prolonged contact. The hantavirus outbreak is rare but it’s not unusual for a viral outbreak to occur in a cruise ship, where people are packed in and close to each other, said Dr. Afif El-Hasan, member of the American Lung Assn.’s national board of directors.

“From an infectious disease standpoint, that is one of the most difficult and challenging situations and one where it’s more easy to catch something versus other situations,” El-Hasan said.

Experts including Scott Pegan, professor of biomedical sciences at the University of Riverside, say the average American’s risk of contracting the disease — if they aren’t within close proximity of an infected individual for a prolonged period — is really low.

Pegan acknowledged it’s confusing to the public when a health incident like this occurs because “they hear ‘this is a really bad disease.’”

“At certain levels, we should worry about it because we don’t want to be interfacing with this virus,” he said.