Harvests, wildfires, epidemics: How the jet stream has shaped extreme weather in Europe for centuries

Tree-ring data reveal that periodic shifts in strong winds high above the Earth's surface have driven opposite climates in different parts of Europe for the past 700 years and likely much longer

University of Arizona

 

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Members of the research team collected tree ring samples at various locations in Europe, including the Balkan region. 

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Credit: Courtesy of Valerie Trouet

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 – on Tuesday 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 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 historic 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 gasses 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."

 

Wood samples from the Balkan region were among those analyzed for this study.

In Scotland, where virtually no living trees going back many hundreds of years are left, the team collected subfossil wood from lake bottoms for dendrochronological analysis in the lab.

Credit

Courtesy of Valerie Trouet

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Journal

Nature

DOI

10.1038/s41586-024-07985-x 

Method of Research

Data/statistical analysis

Subject of Research

Not applicable

Article Title

Jet stream controls on European climate and agriculture since 1300 CE

Article Publication Date

25-Sep-2024

 

Encoding human experience: Study reveals how brain cells compute the flow of time

UCLA researchers say findings could have implications for improving memory, cognitive functions and artificial intelligence

University of California - Los Angeles Health Sciences

A landmark study led by UCLA Health has begun to unravel one of the fundamental mysteries in neuroscience – how the human brain encodes and makes sense of the flow of time and experiences.

The study, published in the journal Nature, directly recorded the activity of individual neurons in humans and found specific types of brain cells fired in a way that mostly mirrored the order and structure of a person’s experience. They found the brain retains these unique firing patterns after the experience is concluded and can rapidly replay them while at rest. Furthermore, the brain is also able to utilize these learned patterns to ready itself for future stimuli following that experience. These findings provide the first empirical evidence regarding how specific brain cells integrate “what” and “when” information to extract and retain representations of experiences through time.

The study’s senior author, Dr. Itzhak Fried, said the results could serve in the development of neuro-prosthetic devices to enhance memory and other cognitive functions as well as have implications in artificial intelligence’s understanding of cognition in the human brain.

“Recognizing patterns from experiences over time is crucial for the human brain to form memory, predict potential future outcomes and guide behaviors,” said Fried, director of epilepsy surgery at UCLA Health and professor of neurosurgery, psychiatry and biobehavioral sciences at the David Geffen School of Medicine at UCLA. “But how this process is carried out in the brain at the cellular level had remained unknown – until now.”

Previous research, including by Dr. Fried, used brain recordings and neuroimaging to understand how the brain processes spatial navigation, showing in animal and human models that two regions of the brain – the hippocampus and the entorhinal cortex – played key roles. The two brain regions, both important in memory functions, work to interact to create a “cognitive map." The hippocampal neurons act as "place cells" that show when an animal is at a specific location, similar to an 'X' on a map, while the entorhinal neurons act as "grid cells" to provide a metric of spatial distance. These cells found first in rodents were later found in humans by Fried’s group.

Further studies have found similar neural actions work to represent non-spatial experiences such as time, sound frequency and characteristics of objects. A seminal finding by Fried and his colleagues was that of “concept cells” in human hippocampus and entorhinal cortex that responds to particular individuals, places or distinct objects and appear to be fundamental to our ability for memory.  

To examine the brain processing of events in time, the UCLA study recruited 17 participants with intractable epilepsy who had been previously had depth electrodes implanted in their brains for clinical treatment.

Researchers recorded the neural activity of the participants as they underwent a complex procedure that involved behavioral tasks, pattern recognition and image sequencing.

Participants first underwent an initial screening section during which approximately 120 images of people, animals, objects and landmarks were repeatedly shown to them on a computer over about 40 minutes. The participants were instructed to perform various tasks such as determining whether the image showed a person or not. The images, of things like famous actors, musicians and places, were selected partly based on each participant’s preferences.

Following this, the participants underwent a three-phase experiment in which they would perform behavioral tasks in response to images that were arbitrarily displayed on different locations of a pyramid-shaped graph. Six images were selected for each participant.

In the first phase, images were displayed in a pseudo-random order. The next phase had the order of images determined by the location on the pyramid graph. The final phase was identical to the first phase. While watching these images, the participants were asked to perform various behavioral tasks that were unrelated to the positioning of the images on the pyramid graph. These tasks included determining whether the image showed a male or female or whether a given image was mirrored compared to the previous phase.

In their analyses, Fried and his colleagues found the hippocampal-entorhinal neurons gradually began to modify and closely align their activity to the sequencing of images on the pyramid graphs. These patterns were formed naturally and without direct instruction to the participants, according to Fried. Additionally, the neuronal patterns reflected the probability of upcoming stimuli and retained the encoded patterns even after the task was completed.

Lead author of the study was Pawel Tacikowski with co-authors Guldamla Kalendar and Davide Ciliberti.

“This study shows us for the first time how the brain uses analogous mechanisms to represent what are seemingly very different types of information: space and time,” Fried said. “We have demonstrated at the neuronal level how these representations of object trajectories in time are incorporated by the human hippocampal-entorhinal system.”

Article: Human hippocampal and entorhinal neurons encode the temporal structure of experience, Tacikowski et al., Nature, 2024, https://doi.org/10.1038/s41586-024-07973-1

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Journal

Nature

DOI

10.1038/s41586-024-07973-1 

Subject of Research

People

Article Title

Human hippocampal and entorhinal neurons encode the temporal structure of experience

Article Publication Date

25-Sep-2024

 

Brazilian fossils reveal jaw-dropping discovery in mammal evolution

University of Bristol

 

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Riograndia and Brasilodon

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Credit: Jorge Blanco

These fossils, belonging to the mammal-precursor species Brasilodon quadrangularis and Riograndia guaibensis, offer critical insights into the development of the mammalian jaw and middle ear, revealing evolutionary experiments that occurred millions of years earlier than previously thought.

Mammals stand out among vertebrates for their distinct jaw structure and the presence of three middle ear bones. This transition from earlier vertebrates, which had a single middle ear bone, has long fascinated scientists. The new study explores how mammal ancestors, known as cynodonts, evolved these features over time.

Using CT scanning, researchers were able to digitally reconstruct the jaw joint of these cynodonts for the first time. The researchers uncovered a ‘mammalian-style’ contact between the skull and the lower jaw in Riograndia guaibensis, a cynodont species that lived 17 million years before the previously oldest known example of this structure, but did not find one in Brasilodon quadrangularis, a species more closely related to mammals. This indicates that the defining mammalian jaw feature evolved multiple times in different groups of cynodonts, earlier than expected.

These findings suggest that mammalian ancestors experimented with different jaw functions, leading to the evolution of 'mammalian' traits independently in various lineages. The early evolution of mammals, it turns out, was far more complex and varied than previously understood.

Lead author James Rawson based in Bristol’s School of Earth Sciences explained: "The acquisition of the mammalian jaw contact was a key moment in mammal evolution.

"What these new Brazilian fossils have shown is that different cynodont groups were experimenting with various jaw joint types, and that some features once considered uniquely mammalian evolved numerous times in other lineages as well.”

This discovery has broad implications for the understanding of the early stages of mammal evolution, illustrating that features such as the mammalian jaw joint and middle ear bones evolved in a patchwork, or mosaic, fashion across different cynodont groups.

Dr. Agustín Martinelli, from the Museo Argentino de Ciencias Natural of Buenos Aires, stated: “Over the last years, these tiny fossil species from Brazil have brought marvellous information that enrich our knowledge about the origin and evolution of mammalian features. We are just in the beginning and our multi-national collaborations will bring more news soon.”

The research team is eager to further investigate the South American fossil record, which has proven to be a rich source of new information on mammalian evolution.

Professor Marina Soares of the Museu Nacional, Brazil, stated: “Nowhere else in the world has such a diverse array of cynodont forms, closely related to the earliest mammals.”

By integrating these findings with existing data, the scientists hope to deepen their understanding of how early jaw joints functioned and contributed to the development of the mammalian form.

James added: “The study opens new doors for paleontological research, as these fossils provide invaluable evidence of the complex and varied evolutionary experiments that ultimately gave rise to modern mammals.”

 

Paper:

‘Brazilian fossils reveal homoplasy in the oldest mammalian jaw joint’ by James Rawson et al in Nature.

VIDEP

https://www.eurekalert.org/multimedia/1042939

https://www.eurekalert.org/multimedia/1042940

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Journal

Nature

Method of Research

Computational simulation/modeling

Subject of Research

Animals

Article Title

Brazilian fossils reveal homoplasy in the oldest mammalian jaw joint

Article Publication Date

25-Sep-2024

Disclaimer: AAAS and E

 

World’s oldest cheese reveals origins of kefir

Cell Press

 

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Tarim mummies from the present-day Xinjiang region of Northwestern China

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Credit: Wenying Li

For the first time, scientists successfully extracted and analyzed DNA from ancient cheese samples found alongside the Tarim Basin mummies in China, dating back approximately 3,600 years. The research, published September 25 in the Cell Press journal Cell, suggests a new origin for kefir cheese and sheds light on the evolution of probiotic bacteria.

“This is the oldest known cheese sample ever discovered in the world,” says Qiaomei Fu, the paper’s corresponding author at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences. “Food items like cheese are extremely difficult to preserve over thousands of years, making this a rare and valuable opportunity. Studying the ancient cheese in great detail can help us better understand our ancestors’ diet and culture.”

About two decades ago, a team of archeologists discovered mysterious white substances smeared on the heads and necks of several mummies found in the Xiaohe cemetery in Northwestern China’s Tarim Basin. These mummies dated back to about 3,300 to 3,600 years ago, from the Bronze Age. At the time, scientists thought these substances might be a type of fermented dairy product, but they couldn’t identify exactly what kind.

After more than a decade of advancements in ancient DNA analysis, a team led by Fu has unraveled the mystery.

The researchers successfully extracted mitochondrial DNA from samples found in three different tombs at the cemetery. They identified cow and goat DNA in the cheese samples. Interestingly, the ancient Xiaohe people used different types of animal milk in separated batches, a practice differing from the mixing of milk types common in Middle Eastern and Greek cheesemaking.

Most importantly, Fu and her colleagues managed to recover the DNA of microorganisms from the dairy samples and confirmed that the white substances were in fact kefir cheese. They discovered that the samples contained bacterial and fungal species, including Lactobacillus kefiranofaciens and Pichia kudriavzevii, both commonly found in present-day kefir grains.

Kefir grains are symbiotic cultures containing multiple species of probiotic bacteria and yeast, which ferment milk into kefir cheese, much like a sourdough starter.

Being able to sequence the bacterial genes in the ancient kefir cheese gave the team an opportunity to track how probiotic bacteria evolved over the past 3,600 years. Specifically, they compared the ancient Lactobacillus kefiranofaciens from the ancient kefir cheese with the modern-day species.

Today, there are two major groups of the Lactobacillus bacteria—one originating Russia and another from Tibet. The Russian type is the most widely used globally, including in the US, Japan, and European countries, for making yogurt and cheese.

The team found that the Lactobacillus kefiranofaciens in the samples was more closely related to the Tibetan group, challenging a long-held belief that kefir originated solely in the North Caucasus mountain region of modern-day Russia.

“Our observation suggests kefir culture has been maintained in Northwestern China’s Xinjiang region since the Bronze Age,” Fu says.

The study also revealed how Lactobacillus kefiranofaciens exchanged genetic material with related strains, improving its genetic stability and milk fermentation capabilities over time. Compared with ancient Lactobacillus, modern-day bacteria are less likely to trigger an immune response in the human intestine. This suggests that the genetic exchanges also helped Lactobacillus become more adapted to human hosts over thousands of years of interaction.

“This is an unprecedented study, allowing us to observe how a bacterium evolved over the past 3,000 years. Moreover, by examining dairy products, we’ve gained a clearer picture of ancient human life and their interactions with the world,” says Fu. “This is just the beginning, and with this technology, we hope to explore other previously unknown artifacts.”

###

This work was supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences (CAS), the Feng Foundation of Biomedical Research, and the Fundamental Research Funds for the Central Universities.

Cell, Liu et al., “Bronze Age cheese reveals human-Lactobacillus interactions over evolutionary history” https://cell.com/cell/fulltext/S0092-8674(24)00899-7

Cell (@CellCellPress), the flagship journal of Cell Press, is a bimonthly journal that publishes findings of unusual significance in any area of experimental biology, including but not limited to cell biology, molecular biology, neuroscience, immunology, virology and microbiology, cancer, human genetics, systems biology, signaling, and disease mechanisms and therapeutics. Visit http://www.cell.com/cell. To receive Cell Press media alerts, contact press@cell.com.

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Journal

Cell

DOI

10.1016/j.cell.2024.08.008 

Method of Research

Experimental study

Subject of Research

Not applicable

Article Title

Bronze Age cheese reveals human-Lactobacillus interactions over evolutionary history

Article Publication Date

25-Sep-2024

Bronze age lactobacillus genomes clarify kefir history

Chinese Academy of Sciences Headquarters

 

image: 

A mummy from the Xiaohe cemetery, and dairy remains are scattered around the neck of the mummy

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Credit: Copyright: LI Wenying, Xinjiang Cultural Relics and Archaeology Institute

Food fermentation is the oldest production practice using microorganisms in human history. Milk fermentation, for example, can be traced back to 6000–4000 BC in India, and Mediterranean populations produced and consumed cheese as early as 7000 before present (BP).

Despite the long history of human consumption of fermented products, though, little has been known about the history of the use of fermentative microorganisms and the history of related cultural transmission. In particular, the evolutionary trajectories, especially functional adaptation, of these fermented microorganisms through long-term interactions with humans has been unclear.

Recently, a group of Chinese scientists has added to our knowledge of the dispersion of early dairy fermentation practices, however, by completing the world’s first metagenomic study of Bronze Age kefir cheese unearthed from Xiaohe cemetery—the most ancient dairy remains known to date. Molecular evidence obtained from ancient fermented dairy residues serves as a powerful tool for understanding past human-fermentative microbial interactions.

In this study, the scientists extracted high-quality genomes of Lactobacillus kefiranofaciens (L. kefiranofaciens) from three ancient cheese samples unearthed in Xiaohe cemetery in Xinjiang, dating back to about 3,500 years ago. The group’s findings were published online in Cell on Sept. 25, in an article entitled “Bronze Age cheese reveals human-Lactobacillus interactions over evolutionary history.” It was featured as a Cell Highlighted Paper.

“We  has been working on this project for over 11 years,” said FU Qiaomei, the last corresponding author and director of the Molecular Paleontology Laboratory at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences. The key is she  designed probes to enrich Lactobacillus DNA, which increases the target DNA from <1% to 64-80% and enables the reconstruction of the whole genome of Lactobacillus.

“This is an unprecedented study, allowing us to observe how a bacterium evolved over the past 3,000 years. Moreover, by examining dairy products, we’ve gained a clearer picture of ancient human life and their interactions with the world,” says FU. “This is just the beginning, and with this technology, we hope to explore other previously unknown artifacts.”

History of the use and spread of kefir yogurt among ancient Xinjiang populations

The cheese sample excavated from Xiaohe Cemetery in Xinjiang is the earliest cheese product ever found. It was previously identified through paleoproteomics as kefir cheese—a fermented milk product made using kefir grains, which contain fermentative microbes.

By reconstructing the fermentation microbial community, the study confirms that the cheese was made using lactic acid bacteria and yeasts. The researchers also discovered that the milk used to make the cheese came from a clade of goats widely distributed in Eurasia during the post-Neolithic period, distinct from contemporary domesticated goats from inland East Asia.

This finding suggests that ancient peoples from the Tarim Basim probably learned kefir production techniques from Eurasian steppe populations.

The spread of dairy fermentation technology largely accompanied human migration and interaction, a process that drove the evolution of lactic acid bacteria. However, this study also reveals a new route of spread of L. kefiranofaciens by analyzing the phylogenetic relationships of ancient L. kefiranofaciens in Xinjiang.

The researchers found that the L. kefiranofaciens used in fermentation comprised two clades. The first mainly consisted of strains from Europe (e.g., the area occupied by modern Germany) and coastal areas and islands in Asia (e.g., the areas occupied by modern Guangdong, Taiwan Island, Japan, and Singapore). This distribution conformed to the dispersal route from the Caucasus to Europe on the one hand and to the coastal areas of Asia and Southeast Asia, on the other hand. The other clade mainly consisted of strains distributed in inland East Asia (including Tibet).

The reconstructed ancient strains are located at the base of the clade, suggesting an additional route of diffusion of kefir production technology from Xinjiang to inland East Asia through techno-cultural exchange.

The divergence of the two clades of L.  kefiranofaciens is likely the result of the spread of their common ancestor initially domesticated in different populations, representing the migration and interactions that occurred during the use and domestication of fermentative microorganisms by different ancient populations.

“It is exciting to see how much information can be retrieved from these cheeses,” said YANG Yimin, a professor at the University of Chinese Academy of Sciences. “Organic residues open a window into past human behaviors and culture that were lost in history and records.”

Domestication and evolution of Lactobacillus by the Xiaohe population

This study also found that utilization and domestication by humans played an important role in the evolution of L. kefiranofaciens. Comparative analysis of the genomes of L. kefiranofaciens from the Bronze Age and modern times allows us to characterize the adaptive evolution of Lactobacillus kefiranofaciens over the past few thousands of years.

Extensive horizontal gene transfer is one of the main mechanisms of adaptive evolution in lactic acid bacteria. By comparing the functional profiles of ancient and modern L. kefiranofaciens, the study found three main trajectories of L. kefiranofaciens evolution: 1) adaptation to environmental stress, with the emergence of drug-resistance-related genes in modern strains; 2) enhancement of bacterial genome defense mechanisms, with modern L. kefiranofaciens showing gene clusters such as the R-M system and the associated toxin-antitoxin system, which can counteract the introduction of exogenous DNA and thus reduce related fitness costs; and 3) adaptations related to the human intestinal environment; for example, modern strains gained gene clusters that potentially interact with the host gut, likely due to their long-term interactions with humans.

This study also found that long-term human use and domestication of L. kefiranofaciens likely also influenced its evolution. Compared to Bronze Age L. kefiranofaciens, modern strains from Tibet show two horizontally transferred gene clusters associated with alleviation of the intestinal inflammatory response. This not only contributes to the survival of L. kefiranofaciens in the human gut but also promotes intestinal function.

Considering ancient populations could easily spread kefir production technology through kefir grains, the presence of these gene clusters is likely related to the preference among populations at the time for kefir grains carrying different clades of L.kefiranofaciens.

“Human-microbial interaction is always fascinating,” said LIU Yichen, one of the first authors and an associate professor at the IVPP Molecular Paleontology Laboratory. “Fermentative microbes played such an important role in the daily life of these ancient humans, and they propagated these microbes for thousands of years without knowing the existence of them for most of the time.”

By using ancient DNA information about fermentative microorganisms closely related to past human activities and culture, this study offers a novel framework for using ancient DNA to examine the cultural interactions of past humans.

The research was supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences, the Peak Biomedical Fund, and the Fundamental Research Operating Expense Fund of the Central Universities.

The M25 dairy sample 

The Bronze Age dairy remains (aged about 3500 BP) from tombs M25 of the Xiaohe cemetery

The Bronze Age dairy remains (aged about 3500 BP) from tombs M29 of the Xiaohe cemeter

Credit

Copyright: YANG Yimin, University of the Chinese Academy of Sciences, Beijing

Usage Restrictions

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Journal

Cell

DOI

10.1016/j.cell.2024.08.008 

Article Title

Bronze Age cheese reveals human-Lactobacillus interactions over evolutionary history

Article Publication Date

25-Sep-2024

 CLIMATE CRISIS

One in two El Niño events could be extreme by mid-century

University of Colorado at Boulder

With the climate pattern known as El Niño in full force from mid-2023 to mid-2024, global temperatures broke records for 12 months in a row. As one of the strongest El Niño events on record, it was likely the main culprit of unprecedented heat, floods and droughts worldwide.

In a new study published Sep. 25 in the journal Nature, a University of Colorado Boulder climate scientist and collaborators reveal that the planet could see more frequent extreme El Niño events by 2050 if greenhouse gas emissions continue to increase. 

“It’s pretty scary that 2050 is not very far away,” said Pedro DiNezio, the paper’s co-lead author and associate professor in the Department of Atmospheric and Oceanic Sciences. “If these extreme events become more frequent, society may not have enough time to recover, rebuild and adapt before the next El Niño strikes. The consequences would be devastating.” 

Shifting wind and soaring temperatures

El Niño occurs when water temperatures along the equator in the Pacific Ocean rise by at least 0.9 °F above average for an extended period. 

The seemingly marginal temperature change can shift wind patterns and ocean currents, triggering unusual weather worldwide, including heat waves, floods and droughts. 

When the area warms by 3.6°F above average, scientists classify the El Niño event as extreme. Since the U.S. National Oceanic and Atmospheric Administration began collecting data in the 1950s, the agency has recorded up to four extreme El Niño events. 

During an extreme El Niño, the impacts on global weather tend to be more severe. For example, during the winter of 1997-98,  El Niño brought record rainfall to California, causing devastating landslides that killed more than a dozen people. Over the same period, the planet lost about 15% of its coral reefs due to prolonged warming.   

Last winter El Niño almost reached extreme magnitude, DiNezio said. 

“El Niño events are difficult to simulate and predict because there are many mechanisms driving them. This has hindered our ability to produce accurate predictions and help society prepare and reduce the potential damage,” they said.

Prior research suggests that climate change is intensifying and increasing the frequency of extreme weather events, possibly linked to changes in El Niño patterns. However, due to limited data, scientists have yet to confirm whether El Niño will strengthen with warming.

DiNezio and their team set out to simulate El Niño events in the past 21,000 years—since the peak of Earth’s last Ice Age—using a computer model.

The model shows that during the Ice Age, when Earth’s climate was colder, extreme El Niño events were very rare. As the planet warmed since the end of the Ice Age, the frequency and intensity of El Niño have been increasing. 

The team validated the model by comparing the simulated data with past ocean temperature data retrieved from fossilized shells of foraminifera, a group of single-celled organisms ubiquitous in the oceans long before human existence. By analyzing the type of oxygen compounds preserved in these fossilized shells, the team reconstructed how El Niño drove ocean temperature fluctuations across the Pacific Ocean for the past 21,000 years. The ancient record aligned with the model’s simulations. 

“We are the first to show a model that can realistically simulate past El Niño events, enhancing our confidence in its future predictions.  We are also proud of the robust technique we developed to evaluate our model, but unfortunately, it brought us no good news,” DiNezio added. 

The model predicts that if society continues to pump greenhouse gases into the atmosphere at the current rate, one in two El Niño events could be extreme by 2050. 

The control knob

Despite El Niño’s complexity, the model reveals that a single mechanism has controlled the frequency and intensity of all El Niño events as the planet has warmed since the last Ice Age.

When the eastern Pacific Ocean water warms from natural fluctuation, the winds that always blow east to west over the equatorial Pacific weaken due to changes in air pressure above the ocean. But during an El Niño, weakened winds allow warm water to flow east, and the warmer water weakens the winds even more, creating a feedback loop known as the Bjerknes feedback.

DiNezio’s research suggests that as the atmosphere warms rapidly from greenhouse gas emissions, the planet experiences a stronger Bjerknes feedback, leading to more frequent extreme El Niño events.

With the most recent El Niño now in the past, DiNezio emphasized that society needs to focus on taking measures to reduce the impact from future extreme El Niño events, including cutting emissions and helping communities, particularly those in the developing countries, become resilient to extreme weather. 

“We now understand how these extreme events happen, and we just need the will to reduce our reliance on fossil fuels,” they said. “Our findings emphasize the urgent need to limit warming to 1.5 °C to avoid catastrophic climate impacts.” 
 

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Journal

Nature

DOI

10.1038/s41586-024-07984-y 

Article Title

Future increase in extreme El Niño supported by past glacial changes

Article Publication Date

25-Sep-2024