Now, researchers at Columbia University say they have finally uncovered the mechanism responsible. Their new study explains how carbon dioxide (CO₂) interacts with different wavelengths of light in ways that cool the upper atmosphere while warming the planet below.

“It explains a phenomenon that’s a fingerprint of climate change, has been known to occur for decades, and has not been understood,” says Robert Pincus, a research professor of ocean and climate physics at Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School, and co-author of the study published in Nature Geoscience.

Why CO₂ Cools the Stratosphere

Near Earth’s surface, CO₂ traps heat that would otherwise escape into space, contributing to global warming. But conditions are very different higher up in the atmosphere.

In the stratosphere, the atmospheric layer stretching from about 11km to 50 km above Earth’s surface, CO₂ behaves more like a cooling system. The molecules absorb infrared energy rising from below and then release part of that energy back into space. As atmospheric CO₂ levels increase, the stratosphere becomes even more effective at shedding heat, causing temperatures there to drop.

Scientists first predicted this effect in the 1960s through climate models developed by climatologist Syukuro Manabe, whose work later earned a Nobel Prize. Since the mid-1980s, the stratosphere has cooled by about 2 degrees Celsius. Researchers estimate that this cooling is more than 10 times greater than it would have been without human generated CO₂ emissions.

Although scientists understood the broad idea behind stratospheric cooling, many of the detailed processes remained unresolved.

“The existing theory was incredibly insightful, but at the moment we lack a quantitative theory for CO₂-induced stratospheric cooling,” says Sean Cohen, a postdoctoral research scientist at Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School, and the study’s lead author.

The “Goldilocks Zone” of Infrared Light

To solve the puzzle, Cohen worked with Pincus and Lorenzo Polvani, a geophysicist in Columbia Engineering’s Department of Applied Physics and Applied Mathematics. The team built mathematical models that identified the major processes driving stratospheric cooling. They repeatedly compared their calculations with climate simulations and observational data, refining the equations over several months until the models aligned with reality.

Their research pointed to a key factor: the way CO₂ molecules interact with infrared light, also known as longwave radiation.

Not all infrared wavelengths behave the same way in the atmosphere. The researchers found that some wavelengths are especially effective at promoting cooling. They described this highly efficient range as a “Goldilocks zone.” As CO₂ concentrations rise, this zone widens, increasing the atmosphere’s cooling efficiency.

“It’s those changes in efficiency that are going to ultimately be what’s driving stratospheric cooling,” says Cohen.

The researchers also examined the effects of ozone and water vapor. While both can influence heating and cooling processes in the atmosphere, their impact on stratospheric cooling turned out to be relatively small compared with CO₂.

How Stratospheric Cooling Strengthens Warming Below

The team’s equations successfully reproduced several known features of the atmosphere. They matched observations showing that cooling becomes stronger with altitude, with the greatest cooling occurring near the top of the stratosphere. The calculations also confirmed that every doubling of CO₂ leads to about 8 degrees Celsius of cooling at the stratopause, the upper boundary of the stratosphere.

The study also highlights an important climate feedback. Although increased CO₂ helps the stratosphere radiate heat more effectively, the resulting cooler temperatures mean the Earth system ultimately releases less infrared energy into space overall. That strengthens heat retention closer to the surface, intensifying warming in the lower atmosphere.

“Here’s this process that we’ve known about for 50-plus years, and we had a pretty decent qualitative understanding of how it worked. However, we didn’t understand the details of what actually drove that process mechanistically,” says Cohen.

According to Cohen and Pincus, the research is less about proving climate change exists and more about improving scientific understanding of how the atmosphere works.

“This is really telling us what is essential,” says Pincus.

The findings could also have applications beyond Earth. Researchers say the same principles may help scientists better understand the atmospheres of other planets and distant exoplanets.

“Maybe we can better understand what’s going on in the stratospheres of other planets in our solar system or exoplanets,” says Cohen.

Some of these eruptions produce solar proton events (SPEs), during which charged particles race toward Earth at speeds reaching 90% of the speed of light. In 1972, several SPEs erupted between the Apollo 16 and Apollo 17 Moon missions. If astronauts had been exposed during a lunar mission, they could have faced lethal radiation levels. As space agencies prepare for future Moon exploration, scientists are working to better understand these unpredictable solar events.

Researchers at the Okinawa Institute of Science and Technology (OIST) have now developed a new way to uncover evidence of past SPEs. The team combined medieval historical records with ultra precise carbon 14 measurements taken from buried asunaro trees in northern Japan. Using this method, they identified a solar proton event that likely occurred sometime between the winter of 1200 CE and the spring of 1201 CE, a period marked by unusually intense solar activity. The findings were published in the Proceedings of the Japan Academy, Series B.

Professor Hiroko Miyahara from the OIST Solar-Terrestrial Environment and Climate Unit explained: “Previous studies on historical SPEs have focused on rare, extremely powerful events. Our paper provides a basis for detecting sub-extreme SPEs — events that occur more frequently and are around 10-30% of the size of the most extreme cases, but still hazardous. Sub-extreme SPEs are more challenging to detect, but our method now allows us to efficiently identify them and better understand the conditions under which they are more likely to occur.”

Ancient Trees Preserve Clues About Solar Storms

Earth’s magnetic field blocks most high energy particles released during SPEs. Near the poles, however, magnetic field lines open into space, allowing some particles to enter the atmosphere. During especially powerful events, these particles collide with atmospheric gases and create carbon 14 compounds that spread around the globe and become trapped inside living organisms.

By analyzing carbon 14 levels in preserved organic material such as ancient buried trees, scientists can track changes in solar activity stretching back thousands of years. The OIST team used an ultra precise measurement technique they spent more than a decade refining. This method can detect much smaller carbon 14 fluctuations than conventional techniques, making it possible to identify weaker “sub-extreme” solar proton events that were previously invisible.

Because the carbon 14 analysis is extremely time intensive, the researchers first needed clues about when unusual solar activity may have occurred.

Medieval Japanese Diary Revealed “Red Lights” in the Sky

One of the key clues came from Meigetsuki, the diary of the Japanese poet and courtier Fujiwara no Teika (1162-1241). In February 1204 CE, he described seeing “red lights in the northern sky over Kyoto.”

Solar proton events do not directly create auroras, but they are often linked with the same kinds of solar disturbances that do. That historical observation gave researchers a timeframe to investigate more closely.

The scientists then measured carbon 14 levels in buried asunaro wood recovered from Aomori Prefecture in northern Japan. They discovered spikes in carbon 14 that pointed to a sub-extreme solar proton event. By combining those measurements with dendroclimatic studies — that is, a dating method based on comparing patterns of tree-ring growth associated with regional climate — the researchers determined that the event likely occurred sometime between the winter of 1200 CE and the spring of 1201 CE. Historical records from China also described a red aurora visible at unusually low latitudes during that same period.

Evidence of an Exceptionally Active Sun

“The high-precision data not only allowed us to accurately date sub-extreme solar proton events, but it also lets us clearly reconstruct the solar cycles of the period,” said Miyahara. “Today, the Sun’s activity fluctuates over eleven-year-long cycles, but we’ve found that the cycle was just seven to eight years long back then, indicating a very active Sun. The SPE we have dated occurred at the peak of one of these cycles.”

The research helps fill important gaps in the history of solar activity and improves scientists’ understanding of dangerous space weather events. According to Miyahara, carbon 14 analysis alone is not enough. Historical records and other scientific methods are also essential for reconstructing past solar behavior.

“Historical literature provides a candidate time window, and dendroclimatology enables direct intercomparison between detected SPE and reports of sunspots and auroras recorded in literature. Integrated approaches like these are necessary to accurately reconstruct past solar activity, helping us better understand the characteristics of extreme space weather,” concluded Miyahara. “For example, while the SPE we found occurred near the peak of the solar cycle, some of the prolonged low-latitude aurora recorded in the literature seems to fall near the minimum of our reconstructed solar cycle. This is unexpected, and we’re excited to look further into what solar conditions could cause this.”

But a major genetic analysis from researchers at RIKEN’s Center for Integrative Medical Sciences suggests the picture is far more complicated.

Using whole-genome sequencing on more than 3,200 people from across Japan, the team found evidence supporting a third ancestral group tied to northeastern Asia and possibly linked to the ancient Emishi people. The findings, published in Science Advances, add powerful support to the increasingly discussed “tripartite origins” theory of Japanese ancestry.

The results also revealed something else surprising: Japan’s population is genetically more diverse than many researchers once assumed.

“The Japanese population isn’t as genetically homogenous as everyone thinks,” said Chikashi Terao, who led the study at RIKEN. “Our analysis revealed Japan’s subpopulation structure on a fine scale, which is very beautifully classified according to geographical locations in the country.”

A Massive DNA Map of Japan

To investigate Japan’s deep genetic history, researchers analyzed DNA samples collected from seven regions stretching from Hokkaido in the north to Okinawa in the south. The project became one of the largest whole-genome studies ever conducted on a non-European population.

Instead of relying on older DNA microarray methods, the team used whole-genome sequencing, which reads nearly all three billion DNA base pairs in a person’s genome. According to the researchers, this provides roughly 3,000 times more information than traditional techniques.

“Whole-genome sequencing gives us the chance to look at more data, which helps us find more interesting things,” Terao explained.

The scientists then combined the genetic information with medical histories, disease diagnoses, family histories, and clinical test results to build a large database known as the Japanese Encyclopedia of Whole-Genome/Exome Sequencing Library (JEWEL).

One especially important focus involved rare genetic variants. These uncommon DNA changes can sometimes preserve clues about ancient migration patterns and long-lost ancestral populations.

“We reasoned that rare variants can sometimes be traced back to specific ancestral populations, and could be informative in revealing fine-scale migration patterns within Japan,” Terao said.

The Hidden Third Ancestor

The analysis uncovered striking regional differences across Japan.

Jomon ancestry appeared strongest in Okinawa, where it was found in 28.5% of samples, while western Japan showed much lower levels at 13.4%. Researchers found that people in western Japan had stronger genetic connections to Han Chinese populations, likely reflecting major migration waves from continental East Asia between 250 and 794 CE. Those migrations also coincided with the spread of Chinese-style government systems, writing, and education throughout Japan.

The newly identified Emishi-related ancestry was concentrated in northeastern Japan and became less common farther west.

The findings build on earlier ancient DNA studies published in 2021 that first proposed the idea that modern Japanese people descend from three major ancestral sources instead of two. Those studies suggested that a third migration connected to the Kofun period played a major role in shaping modern Japan.

Recent follow-up studies have continued strengthening that idea. Researchers analyzing ancient genomes and skeletal remains have found increasing evidence that multiple migration waves entered Japan over centuries, creating a much more layered population history than previously believed.

Ancient Neanderthal and Denisovan DNA Still Affects People Today

The study also explored genetic material inherited from Neanderthals and Denisovans, two ancient human groups that interbred with Homo sapiens tens of thousands of years ago.

Scientists have become increasingly interested in why some of these ancient DNA fragments survived in modern humans while others disappeared. In many cases, the inherited genes appear linked to health, adaptation, or disease risk.

For example, earlier studies showed that Tibetans inherited a Denisovan-related version of the EPAS1 gene that may have helped humans survive in high-altitude environments. Researchers also previously identified Neanderthal-derived DNA associated with severe Covid-19 complications in some populations.

The Japanese genome study identified 44 archaic DNA regions still present in modern Japanese populations, many of them unique to East Asians. One Denisovan-derived region inside the NKX6-1 gene was associated with type 2 diabetes and may influence how some patients respond to semaglutide treatments.

Researchers also found 11 Neanderthal-derived genetic segments connected to conditions including coronary artery disease, prostate cancer, and rheumatoid arthritis.

Toward Personalized Medicine

Beyond tracing ancestry, the researchers believe the work could eventually improve healthcare.

The team identified potentially harmful variants in the PTPRD gene that may be linked to hypertension, kidney failure, and myocardial infarction. They also found common loss-of-function variants in the GJB2 and ABCC2 genes, which are associated with hearing loss and chronic liver disease.

“What we’ve tried to do is to find and catalog loss-of-function gene variants that are very specific to Japanese people, and to understand why they are more likely to have some specific traits and diseases,” Terao said. “We’d like to connect population differences with differences in genetics.”

The study reflects a broader shift happening in genetics research. For years, most large genomic databases heavily focused on people of European ancestry, limiting scientists’ understanding of disease risk in other populations.

Terao hopes expanding JEWEL with more Asian genomic data will help change that.

“It’s quite important to expand this to the Asian population so that in the long run, the results can benefit us too,” he said.