Climate & Environment

Katrina 20 years later: What we've learned

Lisa Marshall — Wed, 20 Aug 2025 17:35:21 +0000

Katrina 20 years later: What we've learned Lisa Marshall Wed, 08/20/2025 - 11:35

Lisa Marshall

Driving into New Orleans two weeks after Hurricane Katrina made landfall, Lori Peek was overcome with sorrow.

Lori Peek stands near the Lower Ninth Ward in New Orleans prior to the 10th anniversary of Hurricane Katrina. Credit: Charles Varley

Trees were flattened for miles. Water marks up to 20 feet high blackened apartment buildings. Holes in rooftops revealed where desperate families had used axes to try to escape the rising floods.

“You looked up at these houses, and you just had to pause and be silent,” recalled Peek, who just months before Katrina’s landfall had earned her doctorate in sociology from �山��.

Weeks after the storm, she traveled to Louisiana to begin a collaborative study of Katrina’s impacts on children. “Not knowing if these people had survived, or what had happened to the children and their parents...these are images I will never forget.”

Twenty years later, Peek has co-authored three books and nearly 20 research papers about the historic storm, which killed more than 1,800 people, displaced an estimated 1.2 million residents from the Gulf Coast, and separated 5,000 children from their families.

Now, as director of CU’s Natural Hazards Center and professor of sociology, she views Katrina as a “pivotal turning point” for the country. It not only revealed what’s possible—meteorologically—amid a warming climate, she said, but it also pulled back the curtain on social inequities that make some populations more vulnerable to natural hazards.

Nearly 20 years after Katrina, �山�� Today caught up with Peek to discuss what the disaster taught us—and what we have yet to learn.

What was unique about Katrina?

A New Orleans resident seeks shelter on a rooftop during Hurricane Katrina. Credit: Charles Varley

The storm itself was a monster. Even though it was only a Category 3 when it made its second landfall in Louisiana on Aug. 29, 2005, it was at one point a Category 5 and, at that time, one of the most intense Atlantic hurricanes ever recorded.

We’re still talking about Katrina 20 years later, though, not because of its meteorological properties, but due to the social impacts. This was the first time in New Orleans history that the city was placed under a mandatory evacuation order. It was the first major natural disaster that was captured in a 24-hour news cycle, so people's immense suffering was on full display. It starkly revealed inequalities along race, class, gender and age lines.

How so?

People were told they were supposed to leave New Orleans, but at least 100,000 did not, and research has revealed that in most cases, they didn’t evacuate because they didn't have the resources.

It was the end of the month, and many people reported they didn't have money for gas or they didn't have a car. Others were caring for elders or children. New Orleans also had one of the highest disability rates in the country, and some couldn’t get access to transportation or couldn’t leave without their caregiver.

At the end of the day, the images that people saw on TV were overwhelmingly of Black Americans, low-income Americans, children, people with disabilities and elders suffering. People were stuck on rooftops in sweltering heat or wading through toxic floodwaters. Thousands ended up in so-called “refuges of last resort”—like the Superdome or Convention Center, which were not meant to house that many people for a week.

Were certain groups more likely to perish?

Yes. Even though people over age 65 only made up about 12% of the population of New Orleans, they made up 67% of those who died.

What happened in the months and years after the storm?

At one point, data indicated that Katrina survivors landed in every single state in the nation, and even in every county in Colorado. But when you look at who was displaced the farthest and who was or was not able to return, it, again, revealed how pre-existing inequalities shape people’s recovery.

Even 10 years after Katrina, data showed that low-income people, Black residents, single mothers and people with disabilities were least likely to be able to return home. It’s important to say that these survivors were not passive. Much research documented the incredible ingenuity and strength of families that worked tirelessly to rebuild their lives after the catastrophe.

What about the children?

Of the 5,000 children separated from their caregivers during the chaos of Katrina, the last child was not reunited with her family until April of 2006—more than six months after the storm. Even two years after, one study showed that some 160,000 children were still displaced from their home school district and not back into stable education and housing.

The boys and girls who we followed for nearly a decade for our book, "Children of Katrina," experienced loss of community and separation of family, but they also were generous and creative as they worked to contribute to the recovery.

New Orleans residents line up outside the Louisiana Superdome, seeking shelter, after Hurrican Katrina left much of the city underwater.

What changes resulted from Katrina?

It was a reckoning for the emergency management community.

The former head of the Federal Emergency Management Agency was fired in the midst of the disaster, and, later, the government accepted responsibility for the failures that occurred at multiple levels.

In response, the emergency management community, nationwide, began to update their planning guidance to take much more careful account of vulnerable populations and better plan for the whole community.

This was a wake-up call for personnel in other countries, too. They were surprised that something like this could happen in the United States.

Learn more

Learn more

The Natural Hazards Center will host a webinar, Hurricane Katrina at 20: Looking Back and Moving Forward, at 11 a.m. Friday, Aug. 29.

Could it happen again today?

Absolutely. Meteorologists have obviously recorded even bigger and more rapidly intensifying storms since. More people live in coastal areas now. And because we're also seeing rising economic inequality, we have more people without the means to take recommended preparedness actions.

We’ve also had a rollback of hazard mitigation and preparedness planning grants and initiatives recently and a loss of many of our federal officials who have been most responsible for leading mitigation and preparedness. This leaves us more exposed and vulnerable at a time when the risk of severe storms is rising.

What advice do you have, as we reflect on this somber anniversary?

Go back and look at the images from Katrina. Look at the city of New Orleans, a vital city, underwater, but then home in and look at those people's faces. Know that there were actions that could have been taken in advance to lessen the suffering.

There are all kinds of lessons to be learned, but we're running out of time. The next disaster is right around the corner.

�山�� Today regularly publishes Q&As with our faculty members weighing in on news topics through the lens of their scholarly expertise and research/creative work. The responses here reflect the knowledge and interpretations of the expert and should not be considered the university position on the issue. All publication content is subject to edits for clarity, brevity and university style guidelines.

On Aug. 29, 2005, Hurricane Katrina tore through New Orleans, killing more than 1,800 people and displacing 1.2 million. Natural Hazards Center Director Lori Peek reflects on what we learned—or should have.

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New research paves way for greener construction practices

Megan Maneval — Tue, 19 Aug 2025 19:46:00 +0000

New research paves way for greener construction practices Megan Maneval Tue, 08/19/2025 - 13:46

College of Engineering and Applied Science

A study led by doctoral student Daniel Donado-Quintero shows that setting carbon benchmarks can encourage asphalt producers to lower emissions and optimize production processes—supporting Colorado’s Buy Clean Act and other efforts.

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Beyond Arrakis: Dune researchers confront real-life perils of shifting sand formations

Daniel William Strain — Mon, 18 Aug 2025 18:50:15 +0000

Beyond Arrakis: Dune researchers confront real-life perils of shifting sand formations Daniel William… Mon, 08/18/2025 - 12:50

Daniel Strain

Elk graze in Great Sand Dunes National Park and Preserve. (Credit: National Park Service)

Last summer, Stephanie McNamara got her first glimpse of Great Sand Dunes National Park and Preserve in southern Colorado. The park is a monument to sand, where dunes stretch across 30 square miles and tower nearly 750 feet high, making them the tallest such formations in North America.

McNamara, a graduate student in geophysics at �山��, had recently joined a project studying features like these. But it was one thing to read about dunes and another to experience them in person.

“We have what we call saltating grains, which are small sand grains that skip across the surface of the dune. You could feel those on your legs, then, all of a sudden, they’re in your mouth,” McNamara said. “Unless you’re in a dune field, it’s hard to think about everything that’s blowing around.”

The graduate student is part of a research effort led by Nathalie Vriend, associate professor at the Paul M. Rady Department of Mechanical Engineering at �山��. Among other questions, the lab explores how dunes evolve over time—shifting and surging across the landscape through processes like those that were scraping McNamara’s legs.

The sci-fi classic book Dune, and its recent film adaptations, takes place mostly on the fictional world of Arrakis—a desert planet where sand stretches endlessly in every direction. But dunes abound even on a water world like Earth. Sand dunes occupy an estimated 5% of our planet’s land area. They sit on six continents and come in all shapes and sizes. The most common dunes, barchans, are shaped like crescent moons, while others look like stars or castle walls. Some dunes even sing.

Just like the dunes in the national park, these formations are relentlessly, sometimes stubbornly, alive.

Understanding exactly how dunes move has become an urgent pursuit, Vriend said. The world is growing hotter and dryer, and deserts across the globe are spreading, swallowing homes, farmland and even entire villages.

Vriend’s research group uses a variety of tools, including ground-penetrating radar and computer simulations, to reveal the physics and mathematics that govern dunes. The team ultimately wants to answer a pressing question: Can humans efficiently shift or even halt the flow of the planet’s largest dunes?

“Here in the U.S., we encountered this problem 100 years ago when we had the Dust Bowl, when dust and sand was ravaging through farms and completely destroying harvests,” Vriend said. “We’re still facing this issue today, and, as a result, we need to understand how sediment moves, how it transports.”

Stephanie McNamara at Great Sand Dunes National Park and Preserve. (Credit: Stephanie McNamara)

Desert songs

Vriend had her own moment of discovery early during her time as a doctoral student at the California Institute of Technology in Pasadena. As part of a research trip, the mechanical engineer hiked to the top of one of the Mojave Desert’s famous singing dunes. These dunes create an eerie humming noise that can echo for miles in every direction.

“I remember going down this dune, and this sound started to boom out on the desert floor, and my whole body started to vibrate,” said Vriend, who came to �山�� in 2022. “Being a tiny part in a big, big landscape like a dune landscape is very humbling.”

Years later, Vriend and her advisor at Caltech, Melany Hunt, helped uncover what makes singing dunes sing—it has to do with how small avalanches of sand vibrate as they slide down the face of a dune.

Vriend and her former graduate student, Karol Bacik, summarized the state of science on the dynamics of dunes in a paper published online ahead of print in the Annual Review of Fluid Mechanics.

She noted that the physics that underlie large dunes is surprisingly complex.

“If [sand] rests on your hand, it forms a little heap, and it’s solid. Then you pour it out, and it acts like a fluid and flows,” Vriend said. “If you throw it up in the air, and the wind is blowing, it flies away like a gas.”

Which all makes tracking the flow of dunes a tricky undertaking. Depending on their size and the prevailing winds, dunes can move anywhere from a few feet to dozens of feet every year.

In a previous experiment, Vriend and her colleagues set out to discover what might happen when the winds blowing on a dune suddenly reverse direction. She led the experiment as a Royal Society University Research Fellow at the University of Cambridge in the United Kingdom.

To simulate the complicated dynamics of a dune field, the team turned to a massive apparatus called a circular flume. The machine is, essentially, a ring of water that measures more than 6 feet across and spins like a turntable. The spinning flume creates underwater currents that wash over piles of sand, not unlike winds blowing in the desert.

The researchers discovered that nearly two-thirds of the dune never moves at all during such a reversal. Instead, only the top layer of sand seems to shift, flowing from one direction to the other—a bit like a baker icing, then re-icing a cake.

Stephanie McNamara works on a dune in Chile where researchers have planted different kinds of obstacles. (Credit: Stephanie McNamara)

Researchers in Chile will explore whether obstacles can shift the path of dunes. (Credit: Stephanie McNamara)

Nathalie Vriend taking samples from a dune. (Credit: Nathalie Vriend)

Grains of sand

Winds blowing over a dune can move grains of sand in three main ways. What path these grains take depends on their size, although the exact size in each category can change from site to site, said Stephanie McNamara, graduate student in geophysics at �山��.

Credit: Nathalie Vriend

Creep
The largest grains roll and bounce over the face of a dune—a process known as creep.

Saltation
Medium-sized grains undergo a phenomenon called saltation. They jump into the air then land back down, knocking into more grains in the process, somewhat like a game of billiards.

Suspension
Then there are the smallest grains. Winds suspend these particles high in the air like a gas. “They can even leave the dune field,” McNamara said.

Spreading sand

For millions of people around the world, these dynamics aren’t just the stuff of scientific papers or sci-fi blockbusters. Rather, they can become a matter of survival, McNamara said.

In the United States, for example, sand dunes around Lake Michigan engulf houses, roads and other infrastructure. In Mauritania, sand from the Sahara Desert has consumed entire communities and is threatening the nation’s capital of Nouakchott, home to roughly 1.5 million people.

Those trends will likely only get worse in the decades to come, the researchers added.

That’s because, in many regions of the world, deserts are growing and spreading at an alarming rate—the result of a confluence of factors, from warming temperatures due to climate change to human land-use practices like deforestation. According to one study, this rapid “desertification” may have already affected the lives of more than 200 million people.

But can humans efficiently alter the flow of dunes, a process that has continued unabated for countless eons?

Humans have tried various approaches over the years to doing just that, from putting up fences to planting vegetation. But those strategies have limitations, and some can even be counterproductive. McNamara, Vriend and their colleagues want to use their knowledge of dune dynamics to improve the process.

Earlier this year, McNamara traveled to Chile where she joined researchers led by Tomás Trewhela at the Universidad Adolfo Ibañez. They worked on sand dunes around Viña del Mar, a community on the Pacific Ocean. The team planted several large obstacles, including poles with arms made from pool noodles, into the dune. The group is now monitoring the experiment to investigate how various obstacles may aggravate or stall sand transport.

Vriend is currently reinstalling the circular flume in her lab on the �山�� campus—an infrastructural challenge since the machine weighs 400 pounds.

“Can we actually affect where a dune is going to migrate?” McNamara said. “What happens if we have a dune, and we put rocks in front of it, for example?”

The graduate student saw first-hand why questions like these are so important during a research trip in June to the Indonesian island of Java. There, she met people whose lives had been overturned by geological disasters, including residents who had lost their homes to repeated flooding.

“The human impact is what drives my research,” McNamara said. “It’s really going to stick with me—talking to these families and seeing where they’re living, hearing about the impossible decisions they have to make.”

Across the globe, deserts are spreading, engulfing homes, roads and even entire villages in sand. Engineers at �山�� are exploring how humans can shift the paths of these towering formations.

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Which tree species are best at cooling down the city?

Megan Maneval — Thu, 14 Aug 2025 20:04:12 +0000

Which tree species are best at cooling down the city? Megan Maneval Thu, 08/14/2025 - 14:04

INSTAAR

INSTAAR doctoral student Advyth Ramachandran is presenting preliminary findings at a conference in Baltimore. His work seeks to understand the cooling effects of various urban tree species in �山��.

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‘Cyborg jellyfish’ could aid in deep-sea research, inspire next-gen underwater vehicles

Amber Elise Carlson — Thu, 14 Aug 2025 18:09:03 +0000

‘Cyborg jellyfish’ could aid in deep-sea research, inspire next-gen underwater vehicles Amber Elise Carlson Thu, 08/14/2025 - 12:09

Amber Carlson

Nicole Xu stands behind the main jellyfish tank in her lab. (Credit: Glenn Asakawa)

In a towering aquarium in a darkened laboratory, moon jellyfish (Aurelia aurita) hover as if floating in space.

The glow of neon lights illuminates their translucent, bell-shaped bodies as they expand and contract rhythmically, their graceful tentacles flowing in wavelike patterns.

�山�� engineer Nicole Xu watches them with fondness. Xu, an assistant professor in the Paul M. Rady Department of Mechanical Engineering, first became fascinated with moon jellies more than a decade ago because of their extraordinary swimming abilities. Today, Xu has developed a way to harness their efficiency and ease at moving through the water in ways that could make some types of aquatic research much easier.

She fits the jellies with microelectronic devices that activate key swimming muscles, enabling researchers to steer them toward remote ocean areas that are hard to access in any other way. Eventually, she plans to add sensors to the devices that can gather critical data on temperature, pH and other environmental characteristics.

“Think of our device like a pacemaker on the heart,” Xu said. “We're stimulating the swim muscle by causing contractions and turning the animals toward a certain direction.”

Going where humans can’t go

Nicole Xu reaches her hand into the tank and touches one of the moon jellyfish. (Credit: Glenn Asakawa)

As climate change accelerates, ocean waters are becoming less hospitable for a variety of marine life. The ocean is getting warmer and more acidic as it absorbs growing amounts of atmospheric carbon dioxide.

Measuring changes in the ocean is essential to understanding how human activities are impacting all life on Earth. But because the ocean is so vast and deep, some parts are hard to study without prohibitively expensive equipment. The cyborg jellies could offer a way for humans to wade into these relatively uncharted waters.

Moon jellyfish are the most energy-efficient animals on the planet. They’re prehistoric, with a simple body structure that has stayed the same for more than 500 million years. As invertebrates, they also lack a brain or spinal cord, though they do have basic organs and a pair of overlapping nerve nets. Importantly, the jellies do not have nociceptors, or sensory receptors that can detect potentially harmful stimuli.

Moon jellies can range from a centimeter to more than a foot in diameter. Their short, fine tentacles help them sting and catch prey like zooplankton, crustacean larvae and small fish. But thankfully for Xu, their sting cells can’t penetrate human skin.

Though they’re often found near coastlines, close to their favorite food sources, moon jellies live in diverse habitats worldwide and can swim to incredible depths: They’ve been found in some of the lowest places on Earth, including the Mariana Trench, which sits roughly 36,000 feet beneath the western Pacific Ocean’s surface at its deepest point.

Xu co-created the biohybrid robotic jellyfish concept with her former academic advisor about five years ago, and she first tested them in the field in 2020, steering them around shallow ocean waters off the coast of Woods Hole, Mass.

On top of the implications for ocean and climate research, Xu believes we can draw inspiration from the jellyfish.

“There’s really something special about the way moon jellies swim. We want to unlock that to create more energy-efficient, next-generation underwater vehicles,” she said.

Striving for ethical research

From left: Nicole Xu and graduate students Marshall Graybill and Charlie Fraga stand next to the main jellyfish tank in Xu's lab. (Credit: Glenn Asakawa)

Today, Xu spends much of her time studying precisely how moon jellies move through the water with such ease.

Xu, research associate Yunxing Su and graduate student Mija Jovchevska published a new study late last month that involved adding biodegradable particles to a jellyfish tank. The researchers then shone a laser through the tank to illuminate the suspended particles in the water and visualize how water flows when jellies swim.

In the past, researchers have used synthetic tracers such as silver-coated glass beads to look at underwater flow patterns, but the new study suggests biodegradable particles, such as corn starch, could be more sustainable, more affordable and less toxic alternatives.

She and graduate student Charlie Fraga are also working on making the jellyfish easier to steer in the wild. Going forward, Xu hopes to design other nature-inspired tools for studying the ocean.

There’s more to learn about the ethics of studying invertebrates. In a paper published earlier this year, Xu and others pointed out the need for more investigation of how research affects invertebrates. It was once widely believed that invertebrates couldn’t feel pain, but there is growing evidence that some do react to aversive stimuli.

Through all of her research, Xu says she strives to minimize harm to the animals she works with. When they’re stressed, moon jellies may secrete extra mucus, and they often stop reproducing. But Xu’s jellies have not shown increased mucus production, and small polyps—baby jellyfish the size of a pinhead whose tentacles are just beginning to form—line the inside of Xu’s jellyfish tanks.

“It's our responsibility as researchers to think about these ethical considerations up front,” Xu said. “But as far as we can tell, the jellyfish are doing well. They're thriving.”

�山�� engineer Nicole Xu fits moon jellyfish with microelectronic devices that enhance their natural swimming ability and will one day be able to gather data.

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Human emissions drove the megadrought in the western US

Yvaine Ye — Thu, 14 Aug 2025 15:16:51 +0000

Human emissions drove the megadrought in the western US Yvaine Ye Thu, 08/14/2025 - 09:16

Yvaine Ye

Greenhouse gas and aerosol emissions from human activity have been driving the prolonged drought in the western United States through a complicated connection with the Pacific Ocean, according to a new �山��-led study.

For more than two decades, an extreme dry spell has drained the Colorado River, devastated local farms, and intensified wildfires across the American Southwest. The new prediction, published Aug. 13 in Nature, could help water managers region develop better water use plans or invest in infrastructure accordingly, with relief potentially still decades away.

“Our results show that the drought and ocean patterns we’re seeing today are not just natural fluctuations—they’re largely driven by human activity,” said Jeremy Klavans, postdoctoral researcher in �山��’s Department of Atmospheric and Oceanic Sciences and lead author of the study.

Worst drought in 12 centuries

The water level of Lake Mead on the Colorado River dropped to about a third of its total capacity in 2021 and reached the lowest point in the lake’s history in 2023. (Credit: European Union, Copernicus Sentinel-2 Imagery)

The drought hitting the Colorado River Basin states and California is directly linked to a climate pattern of the north Pacific Ocean, known as the Pacific decadal oscillation (PDO).

The PDO is a natural fluctuation of the Pacific that waxes and wanes every two decades or so. In its positive phase, waters in the eastern Pacific Ocean along the U.S. West Coast tend to be warmer, whereas waters near Japan are colder. In its negative phase, the pattern flips, bringing cold water to the eastern Pacific.

Since the 1990s, the PDO has been stuck in a negative phase, an unusually long stretch for a typical cycle, Klavans said.

That has had profound impacts on the United States. The cold air and water along the U.S. West Coast hold less moisture than warm air, causing a reduction in precipitation. This extended cool phase also pushed storms that would have brought water to the region farther north.

As a result, scientists estimated that about 93% of the western United States is experiencing drought, with 70% facing severe dry conditions. Prior studies have shown that the past two decades have been the driest in the American Southwest in at least 1,200 years.

Scientists had long thought that the PDO was entirely determined by natural forces, such as the heat exchanges between the ocean and the air. Even the latest report from the Intergovernmental Panel on Climate Change (IPCC), a body of experts convened by the United Nations, said the PDO is controlled by natural forces with high confidence.

If that theory was correct, the PDO should have flipped from negative to positive in 2015 after a strong El Niño event warmed the Pacific.

Instead, the PDO shifted positive for a short time following the El Niño before reverting to the negative phase again.

New reality

To understand why the PDO has been stuck, Klavans and his team used a large collection of climate simulation programs to predict what would happen in the future.

Using a new suite of over 570 simulations, the team found that between 1870 and 1950, changes in the PDO were almost entirely driven by internal forces. But since the mid-20th century, greenhouse gas and aerosol emissions have accounted for more than half of the variations in the PDO.

The team discovered that existing climate models tend to overestimate the role of internal factors on the PDO while underestimating the influence of external factors, such as emissions. After correcting the imbalance, the team found that emissions, and their impacts on the PDO, have been responsible for nearly all of the precipitation decline in the western United States over the past three decades.

“People have been trying for a long time to find out why this part of the country is so dry, and we have an answer for that finally,” Klavans said.

Because the same imbalance has been shown in other regions, Klavans said the study’s implications could go far beyond the Pacific. For example, the North Atlantic Oscillation, a similar fluctuation over the Atlantic Ocean, is driving drought in places like Spain. He added that improving climate models to capture the role of external forces could help scientists predict future changes in precipitation across the globe.

As for the American Southwest, the outlook is grim. If greenhouse gas emissions continue to rise, the PDO will likely remain in its negative phase, and the drought will persist for at least the next three decades, Klavans said.

“With this information, water planners could set new expectations and make proper investments in water infrastructure now, knowing this drought is here to stay,” Klavans said.

For example, some Californian cities are already building desalination plants to turn seawater into drinking water.

“This study can allow us to better quantify the costs of continued greenhouse gas emissions for Americans,” Klavans said. “That can only help our region plan for a better future.”

New research revealed that changes in the Pacific Ocean are driving the unrelenting dry spell in the American Southwest, and it might not let up for the next three decades.

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The water level of the Reflection Canyon, a section of the Colorado River, was extremely low in 2021. (Credit: Jay Huang/Flickr)

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The water level of the Reflection Canyon, a section of the Colorado River, was extremely low in 2021. (Credit: Jay Huang/Flickr)

New report evaluates how we can use energy better in buildings at a community scale

Megan Maneval — Mon, 11 Aug 2025 14:43:17 +0000

New report evaluates how we can use energy better in buildings at a community scale Megan Maneval Mon, 08/11/2025 - 08:43

RASEI Fellow Gregor Henze is a co-author and co-editor on a new report from the International Energy Agency evaluating approaches aiming to use energy more efficiently in buildings and districts.

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Air pollution and warming are changing Colorado's remote alpine lakes

Megan Maneval — Thu, 07 Aug 2025 15:01:21 +0000

Air pollution and warming are changing Colorado's remote alpine lakes Megan Maneval Thu, 08/07/2025 - 09:01

INSTAAR

The Oleksy lab has taken over a 42-year-old monitoring project in Rocky Mountain National Park. The lab's investigations reveal how remote alpine watersheds are changing in the Anthropocene.

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Historical practices led to pollution disparities in Denver

Megan Maneval — Wed, 30 Jul 2025 19:23:53 +0000

Historical practices led to pollution disparities in Denver Megan Maneval Wed, 07/30/2025 - 13:23

New work, led by former CIRES and �山�� doctoral student Alex Bradley, shows that modern pollution patterns and the burdens they place on communities in Denver depend heavily on historical changes, including city planning, industry and discriminatory redlining practices.

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Rainy tropics could face unprecedented droughts as an Atlantic current slows

Yvaine Ye — Wed, 30 Jul 2025 15:00:00 +0000

Rainy tropics could face unprecedented droughts as an Atlantic current slows Yvaine Ye Wed, 07/30/2025 - 09:00

Yvaine Ye

Some of the rainiest places on Earth could see their annual precipitation nearly halved if climate change continues to alter the way ocean water moves around the globe.

In a new �山��-led study published July 30 in Nature, scientists revealed that even a modest slowdown of a major Atlantic Ocean current could dry out rainforests, threaten vulnerable ecosystems and upend livelihoods across the tropics.

“That’s a stunning risk we now understand much better,” said lead author Pedro DiNezio, associate professor in �山��’s Department of Atmospheric and Oceanic Sciences, adding that parts of the Amazon rainforest could see up to a 40% reduction in annual precipitation.

The AMOC makes up half of the global thermohaline circulation, a large, conveyor belt–like ocean current system driven by temperature and salinity. (Credit: Avsa/Wikimedia)

The ocean conveyor belt

The Atlantic Meridional Overturning Circulation (AMOC) is a massive system of ocean currents that moves water through the Atlantic Ocean, transporting warm, salty water from the tropics to the North Atlantic. The AMOC plays an important role in regulating the climate by redistributing heat from the southern to the northern hemisphere. It also makes sure the tropical rain belt, a narrow band of heavy precipitation near the equator, stays north of it.

As the climate warms, melting polar ice and increasing rainfall will dilute the ocean’s surface waters, making them less dense and potentially slowing down the circulation. The impact of a weakened AMOC on the tropics remains uncertain, because scientists have only been monitoring the system directly for two decades.

As a technician at a National Oceanic and Atmospheric Administration (NOAA) lab in Miami in 2005, DiNezio helped calibrate some of the earliest measurements of AMOC. At the time, he had no idea that he’d be studying that very same system two decades later.

“A few years ago, this monitoring system recorded signs of a decline in the AMOC, but it later rebounded. So we weren’t sure if it was just a fluke. The problem is, we haven’t been measuring the ocean long enough to detect meaningful long-term change,” DiNezio said.

While scientists are uncertain whether the AMOC has already begun to decline, climate models predict the system will eventually weaken because of climate change.

Predicting the future

DiNezio and his team set out to explore how a future slowing of these critical ocean currents could impact global precipitation patterns.

“Changes in rainfall are very difficult to predict, because so many factors are involved in making rain, like moisture, temperature, wind and clouds. Many models struggle to predict how the pattern will change in a warming world,” DiNezio said.

Pedro DiNezio

The team turned to climate records from about 17,000 years ago, when the AMOC last slowed down significantly due to natural causes. Evidence of precipitation preserved in cave formations, as well as lake and ocean sediments revealed how rainfall patterns responded to the slowdown during that period.

Drawing on that data, DiNezio’s team identified the computer models that best captured those ancient rainfall shifts and used them to predict how the patterns could change in the future.

Their best models predict that as the AMOC weakens and cools the northern Atlantic, this temperature drop would spread toward the tropical Atlantic and into the Caribbean. This change, on top of rising global temperatures, will lead to significant reductions in precipitation over Central America, the Amazon, and West Africa.

“This is bad news, because we have these very important ecosystems in the Amazon,” said DiNezio. The Amazon rainforest contains almost two years of global carbon emissions, making it a major carbon sink on Earth. “Drought in this region could release vast amounts of carbon back into the atmosphere, forming a vicious loop that could make climate change worse.”

While DiNezio said the AMOC is unlikely to stop completely, even a small reduction in its strength could lead to changes across the entire tropical region, increasing the risk of reaching a tipping point. But how fast and how much it slows depends on the degree of future climate change.

“We still have time, but we need to rapidly decarbonize the economy and make green technologies widely available to everyone in the world. The best way to get out of a hole is to stop digging,” DiNezio said.

Beyond the story

Our sustainability impact by the numbers:

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New research warns that global rainfall patterns could shift dramatically as a result of climate change.

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The Amazon rainforest along the Urubu River in Amazonas State, Brazil. (Credit: Andre Deak/Flickr)

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The Amazon rainforest along the Urubu River in Amazonas State, Brazil. (Credit: Andre Deak/Flickr)

Climate & Environment

Katrina 20 years later: What we've learned

What was unique about Katrina?

How so?

Were certain groups more likely to perish?

What happened in the months and years after the storm?

What about the children?

What changes resulted from Katrina?

Could it happen again today?

What advice do you have, as we reflect on this somber anniversary?

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