The last ice age peaked around 20,000 years ago and was marked by extensive glaciation and dramatic climate shifts that reshaped Earth’s oceans, landscapes and ecosystems. A study led by the University of Arizona suggests that Earth’s last ice age may provide crucial insights into future El Niño weather events. El Niño is one of the most influential climate patterns affecting global weather.

The study, published in Nature, combines data from ancient shells of marine organisms with advanced climate modeling to shed light on how El Niño patterns might change in a warming world.

El Niño is a climate phenomenon characterized by the irregular but periodic warming of sea surface temperatures in the central and eastern Pacific Ocean. This leads to disruption of global weather patterns and causes extreme events like droughts, floods and heat waves.

“El Niño is a formidable force of nature—it induces droughts, floods and wildfires, disrupting marine and terrestrial ecosystems across the planet, with pervasive societal impacts across numerous sectors, from agriculture to the aviation industry,” said Kaustubh Thirumalai, the study’s co-lead author and an assistant professor in the U of A Department of Geosciences.

El Niño events occur approximately every two to seven years, and anticipating how these events might change in the future is a major challenge for climate scientists.

“There are several state-of-the-art climate models out there, and they suggest different El Niño responses to ongoing and future human-caused warming,” Thirumalai said. “Some say El Niño variations will increase, others say it will decrease—it is a complex, multifaceted phenomenon. So, addressing what might happen to El Niño is a key priority for climate science.”

To address this uncertainty, the research team—which included collaborators from the U of A, University of Colorado Boulder, University of Texas, Middlebury College and Woods Hole Oceanographic Institution—turned to the past. They focused on the Last Glacial Maximum—a period about 20,000 years ago when there were ice sheets over much of North America and Europe.

The researchers used the Community Earth System Model—developed to simulate the Earth’s climate system and predict future climate scenarios—to simulate climate conditions from the Last Glacial Maximum to the present day. This model is a collaborative project primarily led by the National Center for Atmospheric Research, with contributions from numerous institutions. The modeling portion of the study was conducted by co-lead author Pedro DiNezo at the University of Colorado Boulder.

To validate this model, Thirumalai and his team compared the model’s results with data from the remains of tiny marine organisms called foraminifera. They are found in ocean samples extracted from the seabed that contain layers of sediments deposited over thousands to millions of years.

“These beautiful, microscopic creatures, which float in the upper ocean, build shells that lock in the ocean temperature when they were alive,” Thirumalai said.

As foraminifera grow, they secrete shells using materials from the surrounding seawater. The chemical composition of these shells changes based on the water temperature. This enables the preservation of a snapshot of ocean conditions at the time the shell formed.

When foraminifera die after a few weeks of life, their shells sink to the ocean floor and become part of the sediment. By analyzing shells from different layers of sediment, scientists can reconstruct ocean temperatures from thousands of years ago and compare them to the model simulations of past climates.

The team analyzed individual foraminiferal shells, allowing them to capture seasonal temperature variations that would otherwise be impossible to detect.

“We zoom in to a tiny section of the sediment core and analyze multiple individual shells from the same layer. This gives us a range of Pacific Ocean temperatures within a short time period, which we can compare between the ice age and today,” Thirumalai said.

The study found that El Niño variability was significantly lower during the Last Glacial Maximum compared to the present day, and that future extreme El Niño events could become more prevalent as the planet warms. This could lead to more intense and frequent weather disruptions worldwide.

Importantly, these findings suggest a common mechanism of extreme El Niño variations under both ice age and future conditions, allowing the researchers to validate the climate model’s prediction.

“This gives us more confidence in the model’s projections for the future,” Thirumalai said. “If it can accurately simulate past climate changes, it’s more likely to give us reliable predictions about future changes in the El Niño system.”

During her summer travels to her native Belgium, University of Arizona professor Valerie Trouet noticed something that turned casual curiosity into a major scientific discovery: when the sun hid behind an overcast sky and people around her put on sweaters instead of summer clothes, the weather tended to be warm and dry in Italy, Greece and the Balkans, popular summer escapes for tourists from the cooler climates of central and northern Europe.

At U of A’s Laboratory of Tree-Ring Research, Trouet studies tree-rings to gather clues about what past climates were like, reading wavy, wooden lines like a linguist might decipher an ancient text. What if, she mused, the key to understanding the capricious summers in Europe could be hidden in trees, silent witnesses to centuries of warm and cold, sunshine, rain and snow?

Trouet assembled an international collaboration to collect tree-ring samples across Europe. The team published its results—the first reconstruction of the jet stream over the past 700 years—in the journal Nature.

The jet stream and the Black Death

Jet streams are concentrated bands of wind in the upper atmosphere that travel around the globe in the northern and southern hemispheres. Their exact locations are not fixed; in response to changes in the position and intensity of high- and low-pressure weather systems, they may shift north or south or change their course, resembling a swiftly running stream at some times, and a slow, meandering river at others.

The jet stream, it turns out, largely determines the summer climate in Europe, and it does so in a seesaw-type pattern that climate researchers call a “dipole.”

“When the jet stream is in an extreme northern position, we get cooler and wetter conditions over the British Isles and warmer and drier conditions over the Mediterranean and the Balkans,” explained study co-author Ellie Broadman, a former postdoctoral research fellow at Laboratory of Tree-Ring Research who is now a biologist at the Sequoia-Kings Canyon Field Station of the U.S. Geological Survey.

“This is related to the climate conditions we are witnessing right now, such as catastrophic flooding in central Europe.”

Hotter conditions over the Balkans cause more moisture than normal to evaporate from the Mediterranean Sea and rain down further north. Conversely, when the jet stream migrates further south, it drags warmer and drier air over the British Isles and pushes cooler temperatures and more moisture toward southeastern Europe.

Measurements of the jet stream have only been around since the late 1940s, Trouet said. By using tree-ring samples from across Europe as proxies for temperature, the research team was able to reconstruct jet stream variation over the past 700 years.

Each year, trees add a ring consisting of less dense wood in the spring and denser wood in the summer. By analyzing tree rings under the microscope, dendrochronologists can compile an archive of past climates.

“We link tiny, subcellular cell wall features in the wood to atmospheric winds that weave through the atmosphere many miles above the Earth, which is fascinating,” Trouet said.

Remarkably, the team found past patterns of the jet stream reflected on a societal level, recorded in historical documents.

“Europe has a long history of writing things down,” Trouet said. “For example, there were monks in Ireland who started recording storms that happened in the 600s, the early Middle Ages, and you have centuries-long records of grape harvests, grain prices and epidemics.”

By comparing historical records to the jet stream reconstruction, Trouet’s team discovered that the climate dipole created by the jet stream has influenced European society for the past 700 years and likely much longer.

“Epidemics happened more frequently in the British Isles when the jet stream was further north,” Trouet said. “Because summers were wet and cold, people stayed indoors, and the conditions were more conducive to spreading diseases.”

From 1348 to 1350, the plague, known as the Black Death, raged in Ireland. At that time, the jet stream was in an extreme, far-north position over Europe.

The findings provide critical data to improve climate models that researchers rely on to predict future climate, Broadman said. Much research has focused on how the jet stream is affected as a result of global warming.

“It’s hard to do that if you only have 60 years’ worth of data, which is why a reconstruction going back 700 years is very useful,” she said. “It allows you to actually compare the past to what’s been happening since we started putting greenhouse gases into the atmosphere.”

Harvest failures, wildfires and extreme weather

Scientists have observed a trend showing the jet stream is gradually shifting northward, independent of its seasonal or more short-term variations.

“When you combine our reconstruction with harvest failures, you see that this trend likely leads to issues with major cereal crops and other types of weather extremes,” Trouet said. “It gives you a preview of the kinds of extreme events and societal outcomes we could expect if that trajectory continues.”

The findings also set a precedent for a future trajectory of jet stream variation and extreme weather events, such as wildfires, Trouet said.

“We showed that wildfires in the Balkans historically happened substantially more when the jet stream was in that northern position that creates dry and hot conditions,” she said. “And that is exactly what we’re seeing this summer. The results that we’re seeing in our reconstruction act out in real life.”

“When you look at how the jet stream’s natural variability alone has impacted societies, you can get an idea of what might happen if you add more heat in the atmosphere and more variability,” Broadman added. “Being able to say, ‘OK, maybe we need to watch out for this or that particular jet stream configuration’ can be very helpful for predictions of climate-related extremes.”