New study unveils 16,000 years of climate history in the tropical Andes

A new study that explores ancient temperatures and rainfall patterns in the tropical Andes of South America has revealed how 16,000 years of climate history in this part of the world was driven by carbon dioxide levels and ocean currents from global climate events.

Led by Brown University researchers, the study marks the first high-resolution temperature record covering the past 16,000 years in the tropical Andes and could help scientists predict and mitigate future climate impacts in tropical regions of the planet. The work is described in the Proceedings of the National Academy of Science.

"Usually when we study climate change in the past, we emphasize the Northern Hemisphere or Southern Hemisphere because of the outsized role they play in affecting climates all over the globe," said Boyang Zhao, a scientist in Brown's Department of Earth, Environmental and Planetary Sciences and the study's first author. "One of the biggest questions we are getting at is what are the driving factors behind temperature history in this part of the tropics, so that we can begin to potentially apply that data and knowledge to other tropical regions."

Along with future implications, the new study provides a unique look at the way distinct and distant parts of the world influence temperature and weather elsewhere, emphasizing how regional climates are connected to global climate changes.

"Our evidence here suggests that temperatures in this region of the world are more influenced by the Southern Hemisphere -- so places like Antarctica," Zhao said.

The study focused on an analysis of sediment samples from Laguna Llaviucu, a lake located in Ecuador's Cajas National Park. The measurements from the sediment samples showed that temperature variations in the tropical Andes closely aligned with climate events that saw the planet's temperatures rise and fall during the past 16,000 years.

Overall, the evidence showed that the main driver in these temperature fluctuations was the concentration of CO2. Researchers saw evidence that the tropical Andean temperatures track with Antarctic temperatures, which are mainly controlled by carbon dioxide concentrations. The findings showed that about 17,000 to 14,000 years ago, tropical Andean temperatures rose when carbon dioxide increased and that the temperatures remained relatively stable about 12,000 years ago when carbon dioxide levels remained relatively stable.

Zhao said that the findings support previous research identifying carbon dioxide as a key driver of global temperature changes since the last Ice Age.

The study also highlighted the role of ocean currents that move warm water from the tropics to the North Atlantic. During a cooling period known as the Antarctic Cold Reversal 14,500 years ago, the northward current was strengthened, causing it to transport more heat northward and cool sea surface temperatures in the south. The researchers found that this cooling effect extended to the tropical Andes.

To reconstruct past climate conditions, the researchers analyzed lipid biomarkers and hydrogen isotopes collected in 2009 from Laguna Llaviucu by scientist Mark Bush from the Florida Institute of Technology. Lipid biomarkers are chemical compounds that provide clues about past temperatures and rainfall patterns. The high-resolution data, combined with accurate dating techniques, allowed the team to create a detailed timeline of climate changes over the past 16,000 years.

According to the researchers, this is the first time organic biomarkers have been used to put together a quantitative climate history for tropical South America. It is traditionally difficult to reconstruct temperatures from the tropics.

The study also points out a number of regional differences in temperature patterns, like how the tropical Andes and Southeast Asia cooled during certain historical periods while other regions like Africa did not.The research also shows how local factors may help to counteract the global effects of rising CO2 levels by looking at differences between past temperature models and what the sediment data show.

The researchers from Brown plan to continue to explore past temperature patterns in this part of the world that historically lacks complete climate records.

"Mountain environments are some of the most sensitive regions on Earth to climate change," said James Russell, a professor of Earth, environmental, and planetary sciences at Brown who oversaw the new research. "In fact, in the near future, high tropical mountains, such as the Andes, are predicted to experience a rate of warming second only to the Arctic, which is the fastest warming part of the planet."

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Retreat of tropical glaciers foreshadows changing climate's effect on the global ice

As they are in many places around the globe, glaciers perched high in the Andes Mountains are shrinking. Now, researchers at the University of Wisconsin-Madison and their collaborators have uncovered evidence that the high-altitude tropical ice fields are likely smaller than they've been at any time since the last ice age ended 11,700 years ago.

That would make the tropical Andes the first region in the world known to pass that threshold as a result of the steadily warming global climate. It also makes them possible harbingers of what's to come for glaciers globally.

"We think these are the canary in the coal mine. The tropics would probably be the first place you'd expect ice to disappear, and that's what we're seeing," says Shaun Marcott, a professor of geoscience at UW-Madison. Marcott guided the research with colleagues at Boston College and Tulane University. Andrew Gorin, a former Boston College graduate student who is now at University of California, Berkeley, led the study, which appears in the Aug. 2, 2024, issue of the journal Science.

Glaciers grow slowly over time in regions where summer weather isn't warm enough to melt all of the previous winter's snowfall. Over time, unmelted snow collects and gets compacted and begins to move under its own weight, resulting in the year-round ice that defines a glacier.

Satellite imagery and on-the-ground observations have provided conclusive evidence for decades that high-altitude glaciers in the Andes are steadily shrinking as warmer temperatures cause them to melt more quickly than falling snow can replenish them.

What has remained unclear, though, is whether the glaciers' dwindling footprints are anomalously small compared to the rest of the period that began at the end of the last ice age, known as the Holocene. Meanwhile, glaciers in other parts of the world were smaller at some points in the early Holocene, when the global climate was warmer and drier than recent millennia.

"We knew that glaciers ebbed and flowed in the past, so we wanted to learn how the behavior of glaciers today -- melting due to human-caused climate change -- stacks up against their long-term fluctuations," says Andy Jones, a UW-Madison doctoral student and study co-author.

To answer this question, the team of scientists analyzed the geochemistry of bedrock from areas near the edges of four glaciers in the high tropical Andes, choosing sites that satellite imagery showed were exposed by melting ice in only the last two or three decades.

The team specifically looked for evidence of two unique isotopes -- basically chemical flavors -- of a pair of elements with the bedrock's quartz crystals: beryllium-10 and carbon-14. These isotopes are only present in rock that has spent time at or near the Earth's surface as they result from interactions between the rock and cosmic rays, which are high-energy particles that constantly rain down on the planet from outer space.

Bedrock accumulates beryllium-10 and carbon-14 once it's exposed to the surface, so measuring the isotopes' concentrations in rock crystals near glaciers can be useful for understanding the previous extent of ice coverage. The team found "remarkably low" concentrations of both isotopes in nearly all samples, suggesting that melting ice has exposed bedrock near the glaciers for the first time only recently in most of the sampled locations.

Additional analyses -- and the fact that the extremely low concentrations were consistent across sample sites -- made the researchers confident that melting ice, rather than erosion, exposed the bedrock.

"It's highly unlikely this is from erosion," says Marcott. "Because the multiple locations we went to all show the same thing."

This consistency points to a single likely conclusion, according to Marcott: The world's tropical glaciers, more than 99% of which are located in the Andes, are the first to shrink beyond what's been seen in the recent geologic past.

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Ice age climate analysis reduces worst-case warming expected from rising CO2

As carbon dioxide accumulates in the atmosphere, the Earth will get hotter. But exactly how much warming will result from a certain increase in CO2 is under study. The relationship between CO2 and warming, known as climate sensitivity, determines what future we should expect as CO2 levels continue to climb.

New research led by the University of Washington analyzes the most recent ice age, when a large swath of North America was covered in ice, to better understand the relationship between CO2 and global temperature. It finds that while most future warming estimates remain unchanged, the absolute worst-case scenario is unlikely.

The open-access study was published April 17 in Science Advances.

"The main contribution from our study is narrowing the estimate of climate sensitivity, improving our ability to make future warming projections," said lead author Vince Cooper, a UW doctoral student in atmospheric sciences. "By looking at how much colder Earth was in the ancient past with lower levels of greenhouse gases, we can estimate how much warmer the current climate will get with higher levels of greenhouse gases."

The new paper doesn't change the best-case warming scenario from doubling CO2 -- about 2 degrees Celsius average temperature increase worldwide -- or the most likely estimate, which is about 3 degrees Celsius. But it reduces the worst-case scenario for doubling of CO2 by a full degree, from 5 degrees Celsius to 4 degrees Celsius. (For reference, CO2 is currently at 425 ppm, or about 1.5 times preindustrial levels, and unless emissions drop is headed toward double preindustrial levels before the end of this century.)

As our planet heads toward a doubling of CO2, the authors caution that the recent decades are not a good predictor of the future under global warming. Shorter-term climate cycles and atmospheric pollution's effects are just some reasons that recent trends can't reliably predict the rest of this century.

"The spatial pattern of global warming in the most recent 40 years doesn't look like the long-term pattern we expect in the future -- the recent past is a bad analog for future global warming," said senior author Kyle Armour, a UW associate professor of atmospheric sciences and of oceanography.

Instead, the new study focused on a period 21,000 years ago, known as the Last Glacial Maximum, when Earth was on average 6 degrees Celsius cooler than today. Ice core records show that atmospheric CO2 then was less than half of today's levels, at about 190 parts per million.

"The paleoclimate record includes long periods that were on average much warmer or colder than the current climate, and we know that there were big climate forcings from ice sheets and greenhouse gases during those periods," Cooper said. "If we know roughly what the past temperature changes were and what caused them, then we know what to expect in the future."

Researchers including co-author Gregory Hakim, a UW professor of atmospheric sciences, have created new statistical modeling techniques that allow paleoclimate records to be assimilated into computer models of Earth's climate, similar to today's weather forecasting models. The result is more realistic temperature maps from previous millennia.

For the new study the authors combined prehistoric climate records -- including ocean sediments, ice cores, and preserved pollen -- with computer models of Earth's climate to simulate the weather of the Last Glacial Maximum. When much of North America was covered with ice, the ice sheet didn't just cool the planet by reflecting summer sunlight off the continents, as previous studies had considered.

By altering wind patterns and ocean currents, the ice sheet also caused the northern Pacific and Atlantic oceans to become especially cold and cloudy. Analysis in the new study shows that these cloud changes over the oceans compounded the glacier's global cooling effects by reflecting even more sunlight.

In short, the study shows that CO2 played a smaller role in setting ice age temperatures than previously estimated. The flipside is that the most dire predictions for warming from rising CO2 are less likely over coming decades.

"This paper allows us to produce more confident predictions because it really brings down the upper end of future warming, and says that the most extreme scenario is less likely," Armour said. "It doesn't really change the lower end, or the average estimate, which remain consistent with all the other lines of evidence."

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Ice cores provide first documentation of rapid Antarctic ice loss in the past

Researchers from the University of Cambridge and the British Antarctic Survey have uncovered the first direct evidence that the West Antarctic Ice Sheet shrunk suddenly and dramatically at the end of the Last Ice Age, around eight thousand years ago.

The evidence, contained within an ice core, shows that in one location the ice sheet thinned by 450 metres -- that's more than the height of the Empire State Building -- in just under 200 years.

This is the first evidence anywhere in Antarctica for such a fast loss of ice. Scientists are worried that today's rising temperatures might destabilize parts of the West Antarctic Ice Sheet in the future, potentially passing a tipping point and inducing a runaway collapse. The new study, published in Nature Geoscience, sheds light on how quickly Antarctic ice could melt if temperatures continue to soar.

"We now have direct evidence that this ice sheet suffered rapid ice loss in the past," said Professor Eric Wolff, senior author of the new study from Cambridge's Department of Earth Sciences. "This scenario isn't something that exists only in our model predictions and it could happen again if parts of this ice sheet becomes unstable."

The Antarctic ice sheets, from west to east, contain enough freshwater to raise global sea levels by around 57 metres. The West Antarctic Ice Sheet is considered particularly vulnerable because much of it sits on bedrock that lies below sea level.

Model predictions suggest that a large part of the West Antarctic Ice Sheet could disappear in the next few centuries, causing sea levels to rise. Exactly when and how quickly the ice could be lost is, however, uncertain.

One way to train ice sheet models to make better predictions is to feed them with data on ice loss from periods of warming in Earth's history. At the peak of Last Ice Age 20,000 years ago, Antarctic ice covered a larger area than today. As our planet thawed and temperatures slowly climbed, the West Antarctic Ice Sheet contracted to more or less its current extent.

"We wanted to know what happened to the West Antarctic Ice Sheet at the end of the Last Ice Age, when temperatures on Earth were rising, albeit at a slower rate than current anthropogenic warming," said Dr Isobel Rowell, study co-author from the British Antarctic Survey. "Using ice cores we can go back to that time and estimate the ice sheet's thickness and extent."

Ice cores are made up of layers of ice that formed as snow fell and was then buried and compacted into ice crystals over thousands of years. Trapped within each ice layer are bubbles of ancient air and contaminants that mixed with each year's snowfall -- providing clues as to the changing climate and ice extent.

The researchers drilled a 651-metre-long ice core from Skytrain Ice Rise in 2019. This mound of ice sits at the edge of the ice sheet, near the point where grounded ice flows into the floating Ronne Ice Shelf.

After transporting the ice cores back to Cambridge at -20oC, the researchers analysed them to reconstruct the ice thickness. First, they measured stable water isotopes, which indicate the temperature at the time the snow fell. Temperature decreases at higher altitudes (think of cold mountain air), so they were able to equate warmer temperatures with lower-lying, thinner ice.

They also measured the pressure of air bubbles trapped in the ice. Like temperature, air pressure also varies systematically with elevation. Lower-lying, thinner ice contains higher pressure air bubbles.

These measurements told them that ice thinned rapidly 8,000 years ago. "Once the ice thinned, it shrunk really fast," said Wolff, "this was clearly a tipping point -- a runaway process."

They think this thinning was probably triggered by warm water getting underneath the edge of the West Antarctic Ice Sheet, which normally sits on bedrock. This likely untethered a section of the ice from bedrock, allowing it to float suddenly and forming what is now the Ronne Ice Shelf. This then allowed neighbouring Skytrain Ice Rise, no longer restrained by grounded ice, to thin rapidly.

The researchers also found that the sodium content of the ice (originating from salt in sea spray) increased about 300 years after the ice thinned. This told them that, after the ice thinned, the ice shelf shrunk back so that the sea was hundreds of kilometres nearer to their site.

"We already knew from models that the ice thinned at around this time, but the date of this was uncertain," said Rowell. Ice sheet models placed the retreat anywhere between 12,000 and 5,000 years ago and couldn't say how quickly it happened. "We now have a very precisely dated observation of that retreat which can be built into improved models," said Rowell.

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What turned Earth into a giant snowball 700m years ago? Scientists now have an answer

Australian geologists have used plate tectonic modelling to determine what most likely caused an extreme ice-age climate in Earth's history, more than 700 million years ago.

The study, published in Geology, helps our understanding of the functioning of the Earth's built-in thermostat that prevents the Earth from getting stuck in overheating mode.

It also shows how sensitive global climate is to atmospheric carbon concentration.

"Imagine the Earth almost completely frozen over," said the study's lead author, ARC Future Fellow Dr Adriana Dutkiewicz.

"We now think we have cracked the mystery: historically low volcanic carbon dioxide emissions, aided by weathering of a large pile of volcanic rocks in what is now Canada; a process that absorbs atmospheric carbon dioxide."

The project was inspired by the glacial debris left by the ancient glaciation from this period that can be spectacularly observed in the Flinders Ranges in South Australia.

A recent geological field trip to the Ranges, led by co-author Professor Alan Collins from the University of Adelaide, prompted the team to use the University of Sydney EarthByte computer models to investigate the cause and the exceptionally long duration of this ice age.

The extended ice age, also called the Sturtian glaciation after the 19th century European colonial explorer of central Australia, Charles Sturt, stretched from 717 to 660 million years ago, a period well before the dinosaurs and complex plant life on land existed.

Dr Dutkiewicz said: "Various causes have been proposed for the trigger and the end of this extreme ice age, but the most mysterious aspect is why it lasted for 57 million years -- a time span hard for us humans to imagine."

The team went back to a plate tectonic model that shows the evolution of continents and ocean basins at a time after the breakup of the ancient supercontinent Rodina.

They connected it to a computer model that calculates CO2 degassing of underwater volcanoes along mid-ocean ridges -- the sites where plates diverge and new ocean crust is born.

They soon realised that the start of the Sturtian ice age precisely correlates with an all-time low in volcanic CO2 emissions.

In addition, the CO2 outflux remained relatively low for the entire duration of the ice age.

Dr Dutkiewicz said: "At this time, there were no multicellular animals or land plants on Earth. The greenhouse gas concentration of the atmosphere was almost entirely dictated by CO2 outgassing from volcanoes and by silicate rock weathering processes, which consume CO2."

Co-author Professor Dietmar Müller from the University of Sydney said: "Geology ruled climate at this time. We think the Sturtian ice age kicked in due to a double whammy: a plate tectonic reorganisation brought volcanic degassing to a minimum, while simultaneously a continental volcanic province in Canada started eroding away, consuming atmospheric CO2.

"The result was that atmospheric CO2 fell to a level where glaciation kicks in -- which we estimate to be below 200 parts per million, less than half today's level."

The team's work raises intriguing questions about Earth's long-term future.

A recent theory proposed that over the next 250 million years, Earth would evolve towards Pangea Ultima, a supercontinent so hot that mammals might become extinct.

However, the Earth is also currently on a trajectory of lower volcanic CO2 emissions, as continental collisions increase and the plates slow down.

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Ice age could help predict oceans' response to global warming

A team of scientists led by a Tulane University oceanographer has found that deposits deep under the ocean floor reveal a way to measure the ocean oxygen level and its connections with carbon dioxide in the Earth's atmosphere during the last ice age, which ended more than 11,000 years ago.

The findings, published in Science Advances, help explain the role oceans played in past glacial melting cycles and could improve predictions of how ocean carbon cycles will respond to global warming.

Oceans adjust atmospheric CO2 as ice ages transition to warmer climates by releasing the greenhouse gas from carbon stored within the deep ocean.

The research demonstrates a striking correlation between global ocean oxygen contents and atmospheric CO2 from the last ice age to today -- and how carbon release from the deep sea may rise as the climate warms.

"The research reveals the important role of the Southern Ocean in controlling the global ocean oxygen reservoir and carbon storage," said Yi Wang, lead researcher and an assistant professor of Earth and Environmental Sciences at Tulane University School of Science and Engineering.

Wang specializes in marine biogeochemistry and paleoceanography.

"This will have implications for understanding how the ocean, especially the Southern Ocean, will dynamically affect the atmospheric CO2 in the future," she said.

Wang conducted the study with colleagues from the Woods Hole Oceanographic Institution, the world's leading independent nonprofit organization dedicated to ocean research, exploration and education.

She worked for the institute before joining Tulane in 2023.

The team analyzed seafloor sediments collected from the Arabian Sea to reconstruct average global ocean oxygen levels thousands of years ago.

They precisely measured isotopes of the metal thallium trapped in the sediments, which indicate how much oxygen was dissolved in the global ocean at the time the sediments formed.

"Study of these metal isotopes on glacial-interglacial transitions has never been looked at before, and these measurements allowed us to essentially recreate the past," Wang said.

The thallium isotope ratios showed the global ocean lost oxygen overall during the last ice age compared to the current warmer interglacial period.

Their study revealed thousand-year global ocean deoxygenation during abrupt warming in the Northern Hemisphere, whereas the ocean gained more oxygen when abrupt cooling occurred during the transition from the last ice age to today.

The researchers attributed the observed ocean oxygen changes to Southern Ocean processes.

"This study is the first to present an average picture of how the oxygen content of the global oceans evolved as Earth transitioned from the last glacial period into the warmer climate of the last 10,000 years," said Sune Nielsen, associate scientist at WHOI and co-author of the research.

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Using evidence from last Ice Age, scientists predict effects of rising seas on coastal habitats

The rapid sea level rise and resulting retreat of coastal habitat seen at the end of the last Ice Age could repeat itself if global average temperatures rise beyond certain levels, according to an analysis by an international team of scientists from more than a dozen institutions, including Rutgers.

In a study published in Nature, scientists reported how ancient coastal habitats adapted as the last glacial period ended more than 10,000 years ago and projected how they are likely to change with this century's predicted sea level rise. They conducted their analysis by examining the ocean sediments of ancient shorelines from a time when oceans rose rapidly, mainly because of melting ice sheets in the Northern Hemisphere. This examination allowed them to infer how ancient coastal habitats changed and formed the basis of improved predictions about the present.

"Every ton of carbon dioxide humankind emits turns up the global thermostat, which in turn increases the pace of global sea level rise," said Robert Kopp, a Distinguished Professor in the Department of Earth and Planetary Sciences in the Rutgers School of Arts and Sciences and an author of the study. "The faster the oceans rise, the greater the threat to tidal marshes, mangroves and coral reefs around the world. For example, in our analysis, most tidal marshes are likely to be able to keep up with sea level rise under 1.5 degrees Celsius [2.7 degrees Fahrenheit] of warming, but two-thirds are unlikely to be able to keep up with 2 degrees Celsius [3.6 degrees Fahrenheit] of warming."

The temperature ranges mentioned in the study are significant because they relate directly to the Paris Agreement, an international treaty on climate change adopted in 2015, said Kopp, who is also the director of the Megalopolitan Coastal Transformation Hub and co-director of the University Office of Climate Action. The goal of the Paris treaty is to substantially reduce carbon emissions worldwide to limit the global temperature increase in this century to 2 degrees Celsius above preindustrial levels while pursuing efforts to limit the increase even further to 1.5 degrees Celsius.

The study predicted higher global temperatures will provoke sea level rises that will lead to instability and profound changes to coastal ecosystems, including tidal marshes, mangrove forests, coral reefs and coral islands.

Tidal marshes -- low-lying areas flooded and drained by tidal salt water -- protect many of the world's coastlines. They sequester pollutants, absorb carbon dioxide and protect nearby communities from storm surge and flooding. They are common along the Atlantic shores of North America. Large expanses of tidal marshes line New Jersey's coast.

"This new paper provides evidence from geological history that, without mitigation and under current projections, tidal marshes will not have the capacity to adjust," said Judith Weis, a Professor Emerita of Biological Sciences at Rutgers-Newark who isn't an author of the study but is an expert on tidal marshes. "For many tidal marshes in New Jersey, this is not a prediction but a description of the present situation, in which sea level is rising faster than the marshes can increase their elevation. This makes it even more vital to reduce climate change as rapidly as possible."

Tidal marshes and mangrove forests adapt to rising seas by accumulating sediment and moving slowly inland.

"Mangroves and tidal marshes act as a buffer between the ocean and the land -- they absorb the impact of wave action, prevent erosion and are crucial for biodiversity of fisheries and coastal plants," said Neil Saintilan, the paper's lead author and a professor at Macquarie University in Sydney, Australia. "When the plants become water-logged due to higher sea levels, they start to flounder."

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Scientists zero in on timing, causes of ice age mammal extinctions in southern California

The end of the last Ice Age also marked the end for more than three dozen genera of large mammals in North America, from mammoths and mastodons to bison and saber-toothed cats. Details concerning the precise timing and circumstances, however, have remained murky ever since.

A team of scientists that included Texas A&M University archaeologist Dr. Michael Waters recently focused on the well-known Rancho La Brea Tar Pits in southern California in their quest to provide answers to these questions, resulting in the most exact and detailed timeline for the extinctions that happened during the latter part of the Pleistocene period in North America, along with some foreboding insight into the area's present and future. Their work is featured on the cover of the current issue of Science.

Waters, a distinguished professor in the Department of Anthropology and director of the Center for the Study of the First Americans (CSFA), along with roughly a dozen fellow researchers, examined the timing and cause of the extinction of a variety of large mammals, known as megafauna, that got stuck in tar at Rancho La Brea, ensuring the preservation of their bones. The team used the radiocarbon dating method to date 169 bones from seven different animals -- bison, horse, camel and ground sloths as well as the carnivores that ate them, including the saber-toothed cat, dire wolf and American lion. They also compared those findings to regional pollen and charcoal records along with continent-wide data on human and large mammal populations.

Armed with their new data, the researchers subsequently used time-series modeling to produce the most detailed chronobiology to date, showing the relationships between climate and vegetation change, fire activity, human demographics and megafauna extinctions -- groundbreaking results they report in the Aug. 18 edition of the world-leading academic journal.

Waters says the team's findings reveal that Ice Age mammal populations in southern California were steady from 15,000 to around 13,250 years ago. Afterward, there was a sharp decline in the population of the seven animals studied, and they all became extinct between 13,070 to 12,900 years ago.

In an interesting modern-day parallel, this extinction event corresponds with a change in the environment from 13,300 to 12,900 years ago marked by warming and drying that made the land more vulnerable to fires in southern California. Charcoal records show that fires increased around 13,500 years ago and peaked between 13,200 and 12,900 years ago. Studies show that humans arrived in North America's Pacific coast 16,000 to 15,000 years ago and lived alongside the megafauna for 2,000 to 3,000 years before their extinction.

While humans hunted animals during this period, Waters says the impact of hunting on the demise of the megafauna likely was minor because of the low population of humans on the landscape. However, the fires would have been devastating, resulting in the loss of habitat causing the rapid decline and extinction of the megafauna in southern California. The study suggests these fires were ignited by humans, which had increased in number by that time.

"Fire is a way that small numbers of humans can have a large impact over a broad area," said Waters, who also cautions that climate changes observed in present-day California are similar to those of the late Pleistocene.

"This study has implications for the changes we see in southern California today," Waters added. "The temperatures are rising, and the area is drying. We also see a dramatic increase in fires. It appears that history may be repeating itself."

While Waters acknowledges that this is the story of extinction at Rancho La Brea, he says it has the potential to offer insights into when extinctions happened across all of North America.

"Mammoths and mastodons survived in many parts of North America until around 12,700 years ago," he added. "These animals were hunted by the Clovis people between about 13,000 and 12,700 years ago. We are now dating megafauna remains from other locations to give a broader understanding of the Rancho La Brea research in the context of North America."

The museum at La Brea Tar Pits holds the world's largest collection of fossils from the Ice Age and has been central to the study of animal and plant life at the end of the Pleistocene epoch for more than a century. Its naturally occurring asphalt pools entrapped and preserved the bones of thousands of individual animals representing dozens of megafaunal species during the last 60,000 years, enabling scientists to determine when different species disappeared from the ecosystem and why.

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Insolation affected ice age climate dynamics

In past ice ages, the intensity of summer insolation affected the emergence of warm and cold periods and played an important role in triggering abrupt climate changes, a study by climate researchers, geoscientists, and environmental physicists suggests. Using stalagmites in the European Alps, they were able to demonstrate that warm phases appeared primarily when the summer insolation reached maxima in the Northern Hemisphere. Study participants included scientists from Germany, Austria, and Switzerland led by researchers from Heidelberg University and the GFZ German Research Centre for Geosciences Potsdam.

Past ice ages in the Northern Hemisphere were marked by sudden transitions between cold and warm phases, each lasting several thousand years. The reason for these fluctuations has yet to be resolved, but research does point to effects relating to the size of the continental ice sheets. Greenland ice records 25 such warm-cold cycles between 115,400 and 14,700 years ago. Investigating stalagmites in the Melchsee-Frutt cave system in the Swiss Alps, the researchers were able to investigate for the first time and with high precision 16 such fluctuations in the penultimate glacial period 185,000 to 130,000 years ago.

Stalagmites in caves are crucial archives in climate research and offer clues as to changes in temperature, precipitation, and vegetation cover. "We are able to precisely determine their age and hence analyse the chronological sequence of abrupt ice age climate fluctuations, which we identify using oxygen isotope values," explains Prof. Dr Norbert Frank of the Institute of Environmental Physics at Heidelberg University. "Our investigations targeted whether, in addition to ice volumes in the Northern Hemisphere, orbitally driven changes in the global distribution of insolation could have influenced the abrupt changes in climate," states study head Dr Jens Fohlmeister, who earned his doctorate in environmental physics at Heidelberg University and worked at the GFZ German Research Centre for Geosciences Potsdam and the Potsdam Institute for Climate Impact Research during the investigations.

The researchers studied the transitions of warm-cold cycles in the penultimate ice age by analysing the age and oxygen isotope composition of stalagmites from the Melchsee-Frutt cave system. "Based on the newly acquired data, we were able to show that warm phases occurred mainly during the peak phase of summer insolation in the Northern Hemisphere, even when the sea level, which is dependent on the volume of the continental ice sheets, remained close to its minimum during peak glacial periods," explains Dr Fohlmeister. Model simulations confirmed these findings. In accordance with the research data from the cave system, the simulations predict the frequency as well as the duration of warm phases at the corresponding sea level and existing insolation.

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'Warm Ice Age' changed climate cycles

Approximately 700,000 years ago, a "warm ice age" permanently changed the climate cycles on Earth. Contemporaneous with this exceptionally warm and moist period, the polar glaciers greatly expanded. A European research team including Earth scientists from Heidelberg University used recently acquired geological data in combination with computer simulations to identify this seemingly paradoxical connection. According to the researchers, this profound change in the Earth's climate was responsible for the change in the climate cycles, thus representing a critical step in the later climate evolution of our planet.

Geological ice ages -- called glacial periods -- are characterised by the development of large ice sheets in the Northern Hemisphere. In the past 700,000 years, phases shifted between distinct glacial and warm periods about every 100,000 years. Before then, however, the Earth's climate was governed by 40,000-year cycles with shorter and weaker glacial periods. The change in the climate cycles occurred in the Middle Pleistocene Transition period, which began approximately 1.2 million years ago and ended about 670,000 years ago. "The mechanisms responsible for this critical change in the global climate rhythm remain largely unknown. They cannot be attributed to variations in the orbital parameters governing the Earth's climate," explains Associate Professor Dr André Bahr of the Institute of Earth Sciences at Heidelberg University. "But the recently identified 'warm ice age', which caused the accumulation of excess continental ice, did play a critical role."

For their investigations, the researchers used new climate records from a drill core off Portugal and loess records from the Chinese Plateau. The data was then fed into computer simulations. The models show a long-term warming and wetting trend in both subtropical regions for the past 800,000 to 670,000 years. Contemporaneous with this last ice age in the Middle Pleistocene Transition period, the sea surface temperatures in the North Atlantic and tropical North Pacific were warmer than in the preceding interglacial, the phase between the two ice ages. This led to higher moisture production and rainfall in Southwest Europe, the expansion of Mediterranean forests, and an enhanced summer monsoon in East Asia. The moisture also reached the polar regions where it contributed to the expansion of the Northern Eurasian ice sheets. "They persisted for some time and heralded in the phase of sustained and far-reaching ice-age glaciation that lasted until the late Pleistocene. Such expansion of the continental glaciers was necessary to trigger the shift from the 40,000-year cycles to the 100,000-year cycles we experience today, which was critical for the Earth's later climate evolution," states André Bahr.

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Evidence of Ice Age human migrations from China to the Americas and Japan

Scientists have used mitochondrial DNA to trace a female lineage from northern coastal China to the Americas. By integrating contemporary and ancient mitochondrial DNA, the team found evidence of at least two migrations: one during the last ice age, and one during the subsequent melting period. Around the same time as the second migration, another branch of the same lineage migrated to Japan, which could explain Paleolithic archeological similarities between the Americas, China, and Japan. The study appears May 9 in the journal Cell Reports.

"The Asian ancestry of Native Americans is more complicated than previously indicated," says first author Yu-Chun Li, a molecular anthropologist at the Chinese Academy of Sciences. "In addition to previously described ancestral sources in Siberia, Australo-Melanesia, and Southeast Asia, we show that northern coastal China also contributed to the gene pool of Native Americans."

Though it was long assumed that Native Americans descended from Siberians who crossed over the Bering Strait's ephemeral land bridge, more recent genetic, geological, and archeological evidence suggests that multiple waves of humans journeyed to the Americas from various parts of Eurasia.

To shed light on the history of Native Americans in Asia, a team of researchers from the Chinese Academy of Sciences followed the trail of an ancestral lineage that might link East Asian Paleolithic-age populations to founding populations in Chile, Peru, Bolivia, Brazil, Ecuador, Mexico, and California. The lineage in question is present in mitochondrial DNA, which can be used to trace kinship through the female line.

The researchers scoured over 100,000 contemporary and 15,000 ancient DNA samples from across Eurasia to eventually identify 216 contemporary and 39 ancient individuals belonging to the rare lineage. By comparing the accumulated mutations, geographic locations, and carbon-dated age of each of these individuals, the researchers were able to trace the lineage's branching path. They identified two migration events from northern coastal China to the Americas, and in both cases, they think that the travelers probably set dock in America via the Pacific coast rather than by crossing the inland ice-free corridor (which would not have opened at the time).

The first radiation event occurred between 19,500 and 26,000 years ago during the Last Glacial Maximum, when ice sheet coverage was at its greatest and conditions in northern China were likely inhospitable for humans. The second radiation occurred during the subsequent deglaciation or melting period, between 19,000 and 11,500 years ago. There was a rapid increase in human populations at this time, probably due to the improved climate, which may have fueled expansion into other geographical regions.

The researchers also uncovered an unexpected genetic link between Native Americans and Japanese people. During the deglaciation period, another group branched out from northern coastal China and traveled to Japan. "We were surprised to find that this ancestral source also contributed to the Japanese gene pool, especially the indigenous Ainus," says Li.

This discovery helps to explain archeological similarities between the Paleolithic peoples of China, Japan, and the Americas. Specifically, the three regions share similarities in how they crafted stemmed projectile points for arrowheads and spears. "This suggests that the Pleistocene connection among the Americas, China, and Japan was not confined to culture but also to genetics," says senior author Qing-Peng Kong, an evolutionary geneticist at the Chinese Academy of Sciences.

Though the study focused on mitochondrial DNA, complementary evidence from Y chromosomal DNA suggests that male ancestors of Native Americans also lived in northern China at around the same time as these female ancestors.

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Massive iceberg discharges during the last ice age had no impact on nearby Greenland, raising new questions about climate dynamics

During the last ice age, massive icebergs periodically broke off from an ice sheet covering a large swath of North America and discharged rapidly melting ice into the North Atlantic Ocean around Greenland, triggering abrupt climate change impacts across the globe.

These sudden episodes, called Heinrich Events, occurred between 16,000 and 60,000 years ago. They altered the circulation of the world's oceans, spurring cooling in the North Atlantic and impacting monsoon rainfall around the world.

But little was known about the events' effect on nearby Greenland, which is thought to be very sensitive to events in the North Atlantic. A new study from Oregon State University researchers, just published in the journal Nature, provides a definitive answer.

"It turns out, nothing happened in Greenland. The temperature just stayed the same," said the study's lead author, Kaden Martin, a fourth-year doctoral candidate in OSU's College of Earth, Ocean, and Atmospheric Sciences. "They had front-row seats to this action but didn't see the show."

Instead, the researchers found that these Heinrich events caused rapid warming in Antarctica, at the other end of the globe.

The researchers anticipated Greenland, in close proximity to the ice sheet, would have experienced some kind of cooling. To find that these Heinrich Events had no discernible impact on temperatures in Greenland is surprising and could have repercussions for scientists' understanding of past climate dynamics, said study co-author Christo Buizert, an assistant professor in the College of Earth, Ocean, and Atmospheric Sciences.

"If anything, our findings raise more questions than answers," said Buizert, a climate change specialist who uses ice cores from Greenland and Antarctica to reconstruct and understand the Earth's climate history. "This really changes how we look at these massive events in the North Atlantic. It's puzzling that far-flung Antarctica responds more strongly than nearby Greenland."

Scientists drill and preserve ice cores to study past climate history through analysis of the dust and tiny air bubbles that have been trapped in the ice over time. Ice cores from Greenland and Antarctica provide important records of Earth's atmospheric changes over hundreds of thousands of years.

Records from ice cores from those regions have served as pillars for scientists' understanding of past climate events, with ice collected from both locations often telling similar stories, Martin said.

The impact of Heinrich Events on Greenland and Antarctica was not well understood, spurring Martin and Buizert to try to find out more about what was happening in those parts of the world.

The core used for the latest study was collected in 1992 from the highest point of Greenland, where the ice sheet is around 2 miles thick. Since then, the core has been in storage in the National Science Foundation Ice Core Facility in Denver.

Advancement in scientific tools and measurements over the last few decades gave Martin, Buizert and their colleagues the opportunity to re-examine the core using new methods.

The analysis shows that no changes in temperatures occurred in Greenland during Heinrich Events. But it also provides a very clear connection between Heinrich Events and the Antarctic response.

"When these big iceberg discharges happen in the Arctic, we now know that Antarctica responds right away," Buizert said. "What happens in one part of the world has an effect on the rest of the world. This inter-hemispheric connection is likely caused by change in global wind patterns."

The finding challenges the current understanding of global climate dynamics during these massive events and raises new questions for researchers, Buizert said. The researchers' next step is to take the new information and run it through climate models to see if the models can replicate what occurred.

"There has to be a story that fits all of the evidence, something that connects all the dots," he said. "Our discovery adds two new dots; it's not the full story, and it may not be the main story. It is possible that the Pacific Ocean plays an important role that we haven't figured out yet."

The ultimate goal is to better understand how the climate system is connected and how the components all interact, the researchers said.

"While Heinrich Events are not going to happen in the future, abrupt changes in the globally interconnected climate system will happen again," Martin said. "Understanding the global dynamics of the climate system can help us better project future impacts and inform how we respond and adapt."

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Wisconsin cave holds tantalizing clues to ancient climate changes, future shifts

Even in their dark isolation from the atmosphere above, caves can hold a rich archive of local climate conditions and how they've shifted over the eons. Formed over tens of thousands of years, speleothems -- rock formations unique to caves better known as stalagmites and stalactites -- hold secrets to the ancient environments from which they formed.

A newly published study of a stalagmite found in a cave in southern Wisconsin reveals previously undetected history of the local climate going back thousands of years. The new findings provide strong evidence that a series of massive and abrupt warming events that punctuated the most recent ice age likely enveloped vast swaths of the Northern Hemisphere.

The research, conducted by a team of scientists at the University of Wisconsin-Madison, appears March 2 in the journal Nature Geoscience. It's the first study to identify a possible link between ice age warm-ups recorded in the Greenland ice sheet -- known as Dansgaard-Oeschger events -- and climate records from deep within the interior of central North America.

"This is the only study in this area of the world that is recording these abrupt climate events during the last glacial period," says Cameron Batchelor, who led the analysis while completing her PhD at UW-Madison. Batchelor is now a postdoctoral fellow with the National Science Foundation working at the Massachusetts Institute of Technology.

The study is based on an exceptionally detailed chemical and physical analysis of a stalagmite that formed in the Cave of the Mounds, a tourist attraction and educational destination.

"At Cave of the Mounds our mission is to interpret this geologic wonder for our many annual visitors," says Joe Klimczak, general manager of the cave, which is a designated national natural landmark. "We are thrilled to deepen our understanding of the cave thanks to this world-class research and very exciting results."

The stalagmite Batchelor and her team analyzed grew extremely slowly -- taking roughly 20,000 years to reach the length of a human pinky finger.

The finger-length subterranean rock formed from a complex process that began in the sky. Water that originally fell as precipitation from the atmosphere soaked into the ground and percolated through soil and cracks in bedrock, dissolving tiny bits of limestone along the way. Some of that dissolved limestone was then left behind as countless drips of water fell from the ceiling of Cave of the Mounds, gradually accumulating into thousands of exceedingly thin layers of a mineral called calcite.

"And because those calcite layers are formed from that original precipitation, they're locking in the oxygen in the H2O originating from that precipitation," says Batchelor.

Therein lies the key to reconstructing an ancient climate record from a small, otherwise unremarkable rock. The oxygen trapped in the calcite exists in a couple varieties -- known as isotopes -- that scientists can use to glean information about the environmental conditions present during the precipitation events that formed it. That includes the temperature and possible sources of rain and snow that fell atop the Cave of the Mounds over thousands of years.

Batchelor's team used a specialized imaging technique that allowed them to identify layers within the stalagmite representing annual growth bands -- much like how tree rings record a season's worth of growth. Using another technique, they identified the isotopes in the tiny layers, revealing that present-day southern Wisconsin experienced a number of very large average temperature swings of up to 10 C (or about 18 F) between 48,000 and 68,000 years ago. Several of the temperature swings occurred over the course of around a decade.

While the dating information is not precise enough to definitively tie the temperature swings to the Dansgaard-Oeschger events recorded in Greenland ice cores, the researchers can say with confidence they occurred within similar timeframes. The team also performed climate simulations that bolstered the hypothesis that warming events occurred tens of thousands of years ago in the region of North America that includes present-day Wisconsin, and that the climate records from Cave of the Mounds and the Greenland ice sheet are indeed linked.

This potential link is exciting for Batchelor because it offers a climate story about central North America that has so far gone untold. Previous research from the mid-continent has not resolved signals of these large temperature swings, also called excursions.

"One theory was that the mid-continent is relatively immune to abrupt climate changes, and that maybe that's because it's surrounded by landmass, and there's some type of buffering happening," says Batchelor. "However, when we went and measured, we saw these really large excursions, and we were like, 'Oh, no, something is definitely happening.'"

That something -- a rapidly changing climate -- is unfolding yet again today, thanks to humans and our use of fossil fuels. Batchelor says she hopes her work in Wisconsin, and now a cave in the Canadian subarctic that she is studying for her postdoc, helps fill a big data gap about the history and potential future of abrupt climate changes in the mid-continent of North America.

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Oldest human genome from southern Spain

A new study reports on genomic data from a 23,000-year-old individual who lived in what was probably the warmest place of Europe at the peak of the last Ice Age. The oldest human genome recovered from the southern tip of Spain adds an important piece of the puzzle to the genetic history of Europe.

An international team of researchers has analysed ancient human DNA from several archaeological sites in Andalucía in southern Spain. Thestudy reports on the oldest genome to date from Cueva del Malalmuerzo in southern Spain, as well as the 7,000 to 5,000-year-old genomes of early farmers from other well-known sites, such as Cueva de Ardales.

The Iberian Peninsula plays an important role in the reconstruction of human population history. As a geographic cul-de-sac in the southwest of Europe, it is on one hand considered a refuge during the last Ice Age with its drastic temperature fluctuations. On the other hand, it may have been one of the starting points for the recolonisation of Europe after the glacial maximum. Indeed, previous studies had reported on the genomic profiles of 13,000 to 8,000-year-old hunter-gatherers from the Iberian Peninsula and provided evidence for the survival and continuation of a much older Palaeolithic lineage that has been replaced in other parts of Europe and is no longer detectable.

After an organism's death, its DNA is only preserved for a certain period of time and under favourable climatic conditions. Extracting DNA from ancient remains from hot and dry climates is a huge challenge for researchers. In Andalucía, in the south of present-day Spain, climatic conditions are similar to those in North Africa -- however, DNA has successfully been recovered of 14,000-year-old human individuals from a cave site in Morocco. The new study fills crucial temporal and spatial gaps. Researchers can now directly investigate the role of the southern Iberian Peninsula as a refuge for Ice Age populations and potential population contacts across the Strait of Gibraltar during the last Ice Age, when sea-levels were much lower than today.

In the right place at the right time

The genetic ancestry of individuals from central and southern Europe who lived before the Last Glacial Maximum (24,000 to 18,000 years before today) differs from the ones who recolonised Europe afterwards. However, the situation in western Europe has not been clear until now due to a lack of genomic data from critical time periods. The 23,000-year-old individual from Cueva del Malalmuerzo near Granada finally adds data from the time when large parts of Europe were covered by massive ice sheets. The study describes a direct genetic link between a 35,000-year-old individual from Belgium and the new genome from Malalmuerzo. "Thanks to the high quality of our data we were able to detect traces of one of the first genetic lineages that settled Eurasia 45,000 years ago. Importantly, we found similarities with a 35,000-year-old individual from Belgium whose ancestry we can now trace further to the 23,000-year-old individual from southern Iberia," explains first author Vanessa Villalba-Mouco of the Max Planck Institute for Evolutionary Anthropology.

The individual from Cueva del Malalmuerzo not only links to earlier periods of settlement but also to the hunter-gatherers of southern and western Europe who lived long after the last Ice Age. It also confirms the important role of the Iberian Peninsula as a refuge for human populations during the last Ice Age. From there, humans migrated northwards and eastwards once the ice sheets had retreated. "With Malalmuerzo, we managed to find the right place and the right time period to trace a Palaeolithic human group back to one of the proposed Ice Age refugia. It is remarkable to find such a long-lasting genetic legacy on the Iberian Peninsula, especially since this pre-Ice Age ancestry had long since disappeared in other parts of Europe," adds senior author Wolfgang Haak of the Max Planck Institute for Evolutionary Anthropology.

More puzzle pieces of human history

Interestingly, the authors did not find any genetic link between the southern Iberian Peninsula and North Africa -- despite a distance of only 13 kilometres across the Mediterranean Sea, and parallels in the archaeological record. "In Malalmuerzo, we found no evidence of a genetic contribution from North African lineages, and conversely, there is no evidence of a genetic contribution from southern Spain in the genomes of the 14,000-year-old individuals from the Taforalt cave in Morocco," adds Gerd-Christian Weniger from the University of Cologne. "Why the Strait of Gibraltar was a barrier at the end of the last Ice Age is still one of the unresolved questions of archaeological research in the western Mediterranean region."

The study also includes a number of younger individuals from the Neolithic, a time period when the first farmers arrived in Europe from the Near East. The characteristic genetic ancestry of Anatolian Neolithic groups is indeed detectable in the individuals from Andalucía, suggesting that these early farmers spread over large geographic distances. "Neolithic people from southern Iberia, however, show a higher proportion of hunter-gatherer lineages. Hence, interaction between the last hunters and the first farmers may have been much closer than in other regions," says co-author Jose Ramos-Muñoz from Universidad de Cádiz.

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Ice Age survivors

The team analysed the genomes of 356 prehistoric hunter gatherers from different archaeological cultures -- including new data sets of 116 individuals from 14 different European and Central Asian countries. Modern humans began to spread across Eurasia around 45,000 years ago but previous research showed that the first modern humans that arrived in Europe did not contribute to later populations. This study focuses on the people who lived between 35,000 and 5,000 years ago and that are, at least partially, the ancestors of the present-day population of Western Eurasia, including -- for the first time -- the genomes of people who lived during the Last Glacial Maximum (LGM), the coldest phase of the last Ice Age, around 25,000 years ago.

Climatic refugium or dead end?

Surprisingly, the research team found that populations from different regions associated with the Gravettian culture, which was widespread across the European continent between 32,000 and 24,000 years ago, were not closely related to each other. They were linked by a common archaeological culture: they used similar weapons and produced similar portable art. Genetically, however, the populations from western and southwestern Europe (today's France and Iberia) differed from contemporaneous populations from central and southern Europe (today's Czech Republic and Italy).

Furthermore, the gene pool of the western Gravettian populations is found continuously for at least 20,000 years: their descendants who are associated with the Solutrean and Magdalenian cultures stayed in southwestern Europe during the coldest period of the last Ice Age (between 25,000 and 19,000 years ago) and later spread north-eastward to the rest of Europe. "With these findings, we can for the first time directly support the hypothesis that during the Last Glacial Maximum people found refuge in the climatically more favourable region of southwestern Europe" says first author Cosimo Posth.

The Italian peninsula was previously considered to be another climatic refugium for humans during the LGM. However, the research team found no evidence for this, on the contrary: hunter-gatherer populations associated with the Gravettian culture and living in central and southern Europe are no longer genetically detectable after the LGM. People with a new gene pool settled in these areas, instead. "We find that individuals associated with a later culture, the Epigravettian, are genetically distinct from the area's previous inhabitants," says co-author He Yu. "Presumably, these people came from the Balkans, arrived first in northern Italy around the time of the glacial maximum and spread all the way south to Sicily."

Large-scale genetic replacement

The analysed genomes also show that the descendants of these Epigravettian inhabitants of the Italian peninsula spread across the rest of Europe about 14,000 years ago, replacing populations associated with the Magdalenian culture. The research team describes a large-scale genetic replacement that may have been caused, in part, by climatic changes that forced people to migrate: "At that time, the climate warmed up quickly and considerably and forests spread across the European continent. This may have prompted people from the south to expand their habitat. The previous inhabitants may have migrated to the north as their habitat, the 'mammoth' steppe, dwindled," says Johannes Krause, the study's senior author.

Furthermore, the findings show that there had been no genetic exchange between contemporaneous hunter-gatherer populations in western and eastern Europe for more than 6,000 years. Interactions between people from central and eastern Europe can only be detected again from 8,000 years ago. "At that time, hunter-gatherers with distinct ancestries and appearances started to mix with each other. They were different in many aspects, including their skin and eye colour," says He Yu.

During this time agriculture and a sedentary lifestyle spread from Anatolia to Europe. "It is possible that the migration of early farmers into Europe triggered the retreat of hunter-gatherer populations to the northern edge of Europe. At the same time, these two groups started mixing with each other, and continued to do so for around 3,000 years," Krause says.

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What crocodile DNA reveals about the Ice Age

What drives crocodile evolution? Is climate a major factor or changes in sea levels? Determined to find answers to these questions, researchers from McGill University discovered that while changing temperatures and rainfall had little impact on the crocodiles' gene flow over the past three million years, changes to sea levels during the Ice Age had a different effect.

"The American crocodile tolerates huge variations in temperature and rainfall. But about 20,000 years ago - when much of the world's water was frozen, forming the vast ice sheets of the last glacial maximum - sea levels dropped by more than 100 metres. This created a geographical barrier that separated the gene flow of crocodiles in Panama," says postdoctoral fellow José Avila-Cervantes, working under the supervision of McGill professor Hans Larsson.

The researchers point out that the crocodiles are good swimmers, but they can't travel long distances on land. As a result, the Caribbean and Pacific crocodile populations were isolated from each other, and consequently have undergone different genetic mutations.

The team compared the climate tolerance of living populations of American crocodiles (Crocodylus acutus) to the paleoclimate estimates for the region over the past 3 million years - the time span of extreme climate variation during the Ice Age.

"This is one of the first times Ice Age effects have been found in a tropical species. It's exciting to discover effects of the last Ice Age glaciation still resonate in the genomes of Pacific and Caribbean American crocodiles today," says Larsson, Professor of Biology at the Redpath Museum of McGill University.

"Discovering that these animals would have easily tolerated the climate swings of the Ice Age speaks to their resilience over geological time. Only humans in recent decades of hunting and land development seem to really affect crocodiles," he says. The findings offer new insight into how environmental drivers affect genetic evolution and where conservation efforts of particular crocodile populations in Panama should be focused.

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Bering Land Bridge formed surprisingly late during last ice age

A new study that reconstructs the history of sea level at the Bering Strait shows that the Bering Land Bridge connecting Asia to North America did not emerge until around 35,700 years ago, less than 10,000 years before the height of the last ice age (known as the Last Glacial Maximum).

The new findings, published the week of December 26 in Proceedings of the National Academy of Sciences, indicate that the growth of the ice sheets -- and the resulting drop in sea level -- occurred surprisingly quickly and much later in the glacial cycle than previous studies had suggested.

"It means that more than 50 percent of the global ice volume at the Last Glacial Maximum grew after 46,000 years ago," said Tamara Pico, assistant professor of Earth and planetary sciences at UC Santa Cruz and a corresponding author of the paper. "This is important for understanding the feedbacks between climate and ice sheets, because it implies that there was a substantial delay in the development of ice sheets after global temperatures dropped."

Global sea levels drop during ice ages as more and more of Earth's water gets locked up in massive ice sheets, but the timing of these processes has been hard to pin down. During the Last Glacial Maximum, which lasted from about 26,500 to 19,000 years ago, ice sheets covered large areas of North America. Dramatically lower sea levels uncovered a vast land area known as Beringia that extended from Siberia to Alaska and supported herds of horses, mammoths, and other Pleistocene fauna. As the ice sheets melted, the Bering Strait became flooded again around 13,000 to 11,000 years ago.

The new findings are interesting in relation to human migration because they shorten the time between the opening of the land bridge and the arrival of humans in the Americas. The timing of human migration into North America remains unresolved, but some studies suggest people may have lived in Beringia throughout the height of the ice age.

"People may have started going across as soon as the land bridge formed," Pico said.

The new study used an analysis of nitrogen isotopes in seafloor sediments to determine when the Bering Strait was flooded during the past 46,000 years, allowing Pacific Ocean water to flow into the Arctic Ocean. First author Jesse Farmer at Princeton University led the isotope analysis, measuring nitrogen isotope ratios in the remains of marine plankton preserved in sediment cores collected from the seafloor at three locations in the western Arctic Ocean. Because of differences in the nitrogen composition of Pacific and Arctic waters, Farmer was able to identify a nitrogen isotope signature indicating when Pacific water flowed into the Arctic.

Pico, whose expertise is in sea level modeling, then compared Farmer's results with sea level models based on different scenarios for the growth of the ice sheets.

"The exciting thing to me is that this provides a completely independent constraint on global sea level during this time period," Pico said. "Some of the ice sheet histories that have been proposed differ by quite a lot, and we were able to look at what the predicted sea level would be at the Bering Strait and see which ones are consistent with the nitrogen data."

The results support recent studies indicating that global sea levels were much higher prior to the Last Glacial Maximum than previous estimates had suggested, she said. Average global sea level during the Last Glacial Maximum was about 130 meters (425 feet) lower than today. The actual sea level at a particular site such as the Bering Strait, however, depends on factors such as the deformation of the Earth's crust by the weight of the ice sheets.

"It's like punching down on bread dough -- the crust sinks under the ice and rises up around the edges," Pico said. "Also, the ice sheets are so massive they have gravitational effects on the water. I model those processes to see how sea level would vary around the world and, in this case, to look at the Bering Strait."

The findings imply a complicated relationship between climate and global ice volume and suggest new avenues for investigating the mechanisms underlying glacial cycles.

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Florida mints radiated as peninsula sank and resurfaced during ice ages

During the ice ages of the Pleistocene, the Florida peninsula regularly grew to twice its current size as glaciers expanded near the planet's poles, only to be reduced to a series of islands as melting ice returned to the sea during warm periods. All told, glaciers advanced and retreated 17 times, and according to a new study, the resulting environmental instability may have contributed to the incredible plant diversity found in Florida today.

Researchers from the Florida Museum of Natural History came to this conclusion while studying scrub mints, a unique group of plants endemic to the southeastern U.S. that radiated during the ice ages. Cyclically marooned on isolated islands as seas rose and fell, mint populations became genetically isolated and diverged over time, generating multiple new species.

Despite their long winning streak, scrub mints have recently been hit head on with the threat of extinction due to human-mediated habitat destruction and impending climate change. Of the 13 species endemic to Florida, eight are listed as either state or federally endangered.

"The most remarkable thing about this group is how rare they are," said lead author Andre Naranjo, who completed the study while working as a doctoral student in the University of Florida's department of biology. "One species, Conradina etonia, only grows within a 30-square mile area, and if you were to pave over that, that'd be it. The species would be gone."

Other scrub mints share a similar pattern. Lakela's mint (Dicerandra immaculata) has been reduced to just a single population, most of which is located on privately owned lands. Scrub balm (Dicerandra frutescens) is restricted to Highlands County, FL where it grows along an elevated ridge increasingly checkered by development. Further west, the Apalachicola rosemary (Conradina glabra) has been reduced to just ten known locations in a single region of the Florida panhandle.

The current plight of scrub mints and other groups like them offers only a partial explanation of why they have been pushed so far to the sidelines. To get the full picture, says Naranjo, you have to take a much longer view of how species have fared over time, one that covers several million years of their natural history.

Naranjo wanted to know where scrub mints came from, when they originated and how they ended up with their current distributions. Building on his previous work, Naranjo used a new method for reconstructing historical environments developed by co-author Ryan Folk, a former postdoctoral associate at the Florida Museum of Natural History who joined the faculty at Mississippi State University in 2019.

By inputting information about the plants' current habitats, such as temperature, precipitation and soil type, Naranjo could then trace their geographic history. The result was a detailed map that pinpointed the most suitable environments for each of the 22 species, half of which are endemic to Florida.

Nearly four million years ago, a scrub mint species growing in the Apalachicola River Basin of Florida shed a fine layer of seeds on the sandy soil below. Each no larger than a coarse grain of sand, the seeds don't often travel far, which researchers suspect is a major cause of their rarity. But they're also equipped with an opportunistic deployment mechanism that occasionally enables long-distance transport.

Scrub mint seed coats are perforated with glands that exude small amounts of viscous oil, Naranjo said. "When it rains really hard, the water forms little streams that drain the sand away from the scrub habitats. If the seeds land in these streams, their mucilaginous coating reduces friction, which helps carry them a few meters away from the parent plant."

Whether all at once or in stages, a seed or seeds from the original population in Apalachicola somehow traveled potentially hundreds of miles east, ultimately leading to the establishment of mints in the Altamaha region of Georgia. Seeds from these newly established populations may have floated down rivers and streams into peninsular Florida, where they washed ashore on the ancient Lake Wales and Atlantic Coastal Ridges.

Throughout the ice ages, the population that remained in the west radiated into the false rosemaries (genus Conradina), while those in the east gave rise to the genus Dicerandra. The groups occasionally crisscrossed in a complex migration pattern that resulted in distant relatives sharing the same environment, a family reunion on a millennial timescale.

Scrub mints are merely one example of unique Florida plants that originated in the peninsula that are now imperiled due to habitat destruction, fire suppression and competition with invasive species. The Lake Wales Ridge, where many scrub mints evolved and which functioned as an ark for plants and animals retreating from rising seas, has lost more than 85% of its natural habitat to urbanization and agriculture.

Florida is also part of the North American Coastal Plain, which was listed in 2015 as one of Earth's 36 biodiversity hotspots, defined as a region harboring at least 1,500 endemic species and which has lost 70% or more of its original vegetation.

"We need to start thinking about conservation in a broader context than just individual species," Naranjo said, emphasizing the focus instead should be shifted toward preserving entire regions and environments. "Our hope is that this research can be used as a rubric to study other endemic plants and further refine a comprehensive conservation approach for those areas most at risk of being developed."

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Footprints claimed as evidence of ice age humans in North America need better dating, new research shows

The wide expanse of an ancient lakebed in New Mexico holds the preserved footprints of life that roamed millennia ago. Giant sloths and mammoths left their mark, and alongside them, signs of our human ancestors. Research published in September 2021 claimed that these footprints are "definitive evidence of human occupation of North America" during the last ice age, dating back to between 23 and 21 thousand years ago. Now, a new study disputes the evidence of such an early age.

Scientists from DRI, Kansas State University, the University of Nevada, Reno, and Oregon State University caution in Quaternary Research that the dating evidence is insufficient for claims that would so radically alter our understanding of when, and how, humans first arrived in North America. Using the same dating method and materials, the new study shows that the footprints could have been left thousands of years later than originally claimed.

"I read the original Science article on the human footprints at White Sands and was initially struck not only by how tremendous the footprints were on their own, but how important accurate dating would be," says Charles Oviatt, emeritus professor of geology at Kansas State University and one of the new study's authors. "I saw potential problems with the scientific tests of the dates reported in the Science paper."

"It really does throw a lot of what we think we know into question," says David Rhode, Ph.D., a paleoecologist at DRI and co-author of the new study. "That's why it's important to really nail down this age, and why we're suggesting that we need better evidence."

Archaeologists and historians use a number of methods to determine the timing of historic events. Based on these methods, scientists tend to agree that the earliest known dates of humanity's colonization of North America lie between 14 and 16 thousand years ago, after the last ice age. If the original claims are correct, current chronological models in fields as varied as paleogenetics and regional geochronology would need to be reevaluated.

"23 to 21 thousand years ago is in a timeframe where you need to really pay attention to how people got into North America," says Rhode. "At that time, there was a huge, mile-high mountain range of ice covering Canada to the north, and the pathway down the Pacific Coast wasn't very accommodating either -- so it may have been that people had to come here much earlier than that."

By studying ancient DNA from human fossils and using rates of genetic change (a sort of molecular clock using DNA), paleogeneticists surmise that the American Southwest was first occupied no earlier than 20 thousand years ago. If the footprints are older, it throws into question the use and integrity of these genetic models. It's possible that the ages from one study at a single site in a New Mexico lake basin are valid, and that age estimates from a variety of other fields are invalid, the authors write, but more robust evidence is needed to confirm the claims.

At the center of the debate are the tiny seeds of an aquatic plant used to age the footprints. The timeframe for the seeds was identified using radiocarbon dating methods, in which researchers examine a type of carbon known as Carbon-14. Carbon-14 originates in the atmosphere and is absorbed by plants through photosynthesis. These carbon isotopes decay at a constant rate over time, and comparing the amount of Carbon-14 in the atmosphere to the amount present in fossilized plant material allows scientists to determine their approximate age. But the plant species used, Ruppia cirrhosa, grows underwater and therefore obtains much of its carbon for photosynthesis not directly from the atmosphere as terrestrial plants do, but from dissolved carbon atoms in the water.

"While the researchers recognize the problem, they underestimate the basic biology of the plant," says Rhode. "For the most part, it's using the carbon it finds in the lake waters. And in most cases, that means it's taking in carbon from sources other than the contemporary atmosphere -- sources which are usually pretty old."

This method is likely to give radiocarbon-based age estimates of the plant that are much older than the plants themselves. Ancient carbon enters the groundwater of the Lake Otero basin from eroded bedrock of the Tularosa Valley and the surrounding mountains, and occurs in extensive calcium carbonate deposits throughout the basin.

The authors demonstrated this effect by examining Ruppia plant material with a known age from the same region. Botanists collected living Ruppia plants from a nearby spring-fed pond in 1947 and archived them at the University of New Mexico herbarium. Using the same radiocarbon dating method, the plants that were alive in 1947 returned a radiocarbon date suggesting they were about 7400 years old, an offset resulting from the use of ancient groundwater by the plant. The authors note that if the ages of the Ruppia seeds dated from the human footprints were also offset by roughly 7400 years, their real age would be between 15 and 13 thousand years old -- a date which aligns with ages of several other known early North American archaeological sites.

The dating of the footprints can be resolved through other methods, including radiocarbon dating of terrestrial plants (which use atmospheric carbon and not carbon from groundwater) and optically stimulated luminescence dating of quartz found in the sediment, the authors write.

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Earth-sun distance dramatically alters seasons in the equatorial Pacific in a 22,000-year cycle

Weather and climate modelers understand pretty well how seasonal winds and ocean currents affect El Niño patterns in the eastern equatorial Pacific Ocean, impacting weather across the United States and sometimes worldwide.

But new computer simulations show that one driver of annual weather cycles in that region -- in particular, a cold tongue of surface waters stretching westward along the equator from the coast of South America -- has gone unrecognized: the changing distance between Earth and the sun.

The cold tongue, in turn, influences the El Niño-Southern Oscillation (ENSO), which impacts weather in California, much of North America, and often globally.

The Earth-sun distance slowly varies over the course of the year because Earth's orbit is slightly elliptical. Currently, at its closest approach -- perihelion -- Earth is about 3 million miles closer to the sun than at its farthest point, or aphelion. As a result, sunlight is about 7% more intense at perihelion than at aphelion.

Research led by the University of California, Berkeley, demonstrates that the slight yearly change in our distance from the sun can have a large effect on the annual cycle of the cold tongue. This is distinct from the effect of Earth's axial tilt on the seasons, which is currently understood to cause the annual cycle of the cold tongue.

Because the period of the annual cycle arising from the tilt and distance effects are slightly different, their combined effects vary over time, said lead researcher John Chiang, UC Berkeley professor of geography.

"The curious thing is that the annual cycle from the distance effect is slightly longer than that for tilt -- around 25 minutes, currently -- so over a span of about 11,000 years, the two annual cycles go from being in phase to out of phase, and the net seasonality undergoes a remarkable change, as a result," Chiang said.

Chiang noted that the distance effect is already incorporated into climate models -- though its effect on the equatorial Pacific was not recognized until now -- and his findings will not alter weather predictions or climate projections. But the 22,000-year phase cycle may have had long-term, historical effects. Earth's orbital precession is known to have affected the timing of the ice ages, for example.

The distance effect -- and its 22,000-year variation -- also may affect other weather systems on Earth. The ENSO, which also originates in the equatorial Pacific, is likely affected because its workings are closely tied to the seasonal cycle of the cold tongue.

"Theory tells us that the seasonal cycle of the cold tongue plays a key role in the development and termination of ENSO events," said Alyssa Atwood, a former UC Berkeley postdoctoral fellow who is now an assistant professor at Florida State University in Tallahassee. "Because of this, many of ENSO's key characteristics are synced to the seasonal cycle."

For example, ENSO events tend to peak during Northern Hemisphere winters, she said, and they don't typically persist beyond northern or boreal spring months, which scientists refer to as the "spring predictability barrier." Because of these linkages, it is reasonable to expect that the distance effect could also have a major impact on ENSO -- something that should be examined in future studies.

"Very little attention has been paid to the cold tongue seasonal cycle because most people think it's solved. There's nothing interesting there," Chiang said. "What this research shows is that it's not solved. There's still a mystery there. Our result also begs the question whether other regions on Earth may also have a significant distance effect contribution to their seasonal cycle."

"We learn in science classes as early as grade school that the seasons are caused by the tilt of Earth's axis," added co-author Anthony Broccoli of Rutgers University. "This is certainly true and has been well understood for centuries. Although the effect of the Earth-sun distance has also been recognized, our study indicates that this 'distance effect' may be a more important effect on climate than had been recognized previously."

Chiang, Atwood and Broccoli and their colleagues reported their findings today in the journal Nature.

Two distinct yearly cycles affect Pacific cold tongue

The main driver of global weather changes is seasonal change. Earth's equator is tilted relative to its orbit around the sun, so the Northern and Southern hemispheres are illuminated differently. When the sun shines directly overhead in the north, it's warmer in the north and colder in the south, and vice versa.

These yearly changes have major effects on the Pacific equatorial trade winds, which blow from southeast to northwest across the south and equatorial Pacific and push surface waters westward, causing upwelling of cold water along the equator that creates a tongue of cold surface water that stretches from Ecuador across the Pacific -- almost one-quarter the circumference of the planet.

The yearly hemispheric changes in seasonal temperature alters the strength of the trades, and thus cause a yearly cycle in the temperature of the cold tongue. This, in turn, has a major influence on ENSO, which typically peaks during Northern Hemisphere winter.

The occurrence of El Niño -- or its opposite, La Niña -- helps determines whether California and the West Coast will have a wet or dry winter, but also whether the Midwest and parts of Asia will have rain or drought.

"In studying past climates, much effort has been dedicated to trying to understand if variability in the tropical Pacific Ocean -- that is, the El Niño/La Niña cycle -- has changed in the past," Broccoli said. "We chose to focus instead on the yearly cycle of ocean temperatures in the eastern Pacific cold tongue. Our study found that the timing of perihelion -- that is, the point at which the earth is closest to the sun -- has an important influence on climate in the tropical Pacific."

In 2015, Broccoli, co-director of the Rutgers Climate Institute, along with his then-graduate student Michael Erb, employed a computer climate model to show that the distance changes caused by Earth's elliptical orbit dramatically altered the cold tongue yearly cycle. But climate modelers mostly ignored the result, Chiang said.

"Our field is focused on El Niño, and we thought that the seasonal cycle was solved. But then we realized that the result by Erb and Broccoli challenged this assumption," he said.

Chiang and his colleagues, including Broccoli and Atwood, examined similar simulations using four different climate models and confirmed the result. But the team went further to show how the distance effect works.

Earth's 'marine' and 'continental' hemispheres

The key distinction is that changes in the sun's distance from Earth don't affect the Northern and Southern hemispheres differently, which is what gives rise to the seasonal effect due to Earth's axial tilt. Instead, they warm the eastern "continental hemisphere" dominated by the North and South American and African and Eurasian landmasses, more than it warms the Western Hemisphere -- what he calls the marine hemisphere, because it is dominated by the Pacific Ocean.

"The traditional way of thinking about monsoons is that the Northern Hemisphere warms up relative to the Southern Hemisphere, generating winds onto land that bring monsoon rains," Chiang said. "But here, we're actually talking about east-west, not north-south, temperature differences that cause the winds. The distance effect is operating through the same mechanism as the seasonal monsoon rains, but the wind changes are coming from this east-west monsoon."

The winds generated by this differential heating of the marine and continental hemispheres alter the yearly variation of the easterly trades in the western equatorial Pacific, and thereby the cold tongue.

"When Earth is closest to the sun, these winds are strong. In the offseason, when the sun is at its furthest, these winds become weak," Chiang said. "Those wind changes are then propagated to the Eastern Pacific through the thermocline, and basically it drives an annual cycle of the cold tongue, as a result."

Today, Chiang said, the distance effect on the cold tongue is about one-third the strength of the tilt effect, and they enhance one another, leading to a strong annual cycle of the cold tongue. About 6,000 years ago, they canceled one another, yielding a muted annual cycle of the cold tongue. In the past, when Earth's orbit was more elliptical, the distance effect on the cold tongue would have been larger and could have led to a more complete cancellation when out of phase.

Though Chiang and his colleagues did not examine the effect of such a cancellation, this would potentially have had a worldwide effect on weather patterns.

Chiang emphasized that the distance effect on climate, while clear in climate model simulations, would not be evident from observations because it cannot be readily distinguished from the tilt effect.

"This study is purely model based. So, it is a prediction," he said. "But this behavior is reproduced by a number of different models, at least four. And what we did in this study is to explain why this happens. And in the process, we've discovered another annual cycle of the cold tongue that's driven by Earth's eccentricity."

Atwood noted that, unlike the robust changes to the cold tongue seasonal cycle, changes to ENSO tend to be model-dependent.

"While ENSO remains a challenge for climate models, we can look beyond climate model simulations to the paleoclimate record to investigate the connection between changes in the annual cycle of the cold tongue and ENSO in the past," she said. "To date, paleoclimate records from the tropical Pacific have largely been interpreted in terms of past changes in ENSO, but our study underscores the need to separate changes in the cold tongue annual cycle from changes in ENSO."

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