A longer, hotter reality surrounds marine heatwaves, new study finds

First-of-its-kind study shows ecosystems experience far more cumulative heat than standard measurements capture

Virginia Institute of Marine Science

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Periods of sustained warming can last for weeks to months at a time, with a distinct marine heatwave embedded between them and significant implications for estuarine ecosystems and habitats like seagrass beds. Photograph by Jonathan Lefcheck.

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Credit: Jonathan Lefcheck

Anyone who has boiled a pot of water understands that it takes both time and energy to raise the temperature, so it may come as a surprise to learn that scientists have long treated marine heatwaves (MHWs) as isolated events. New research led by William & Mary's VIMS & Batten School is challenging that narrative by providing a framework to characterize an ecosystem’s cumulative heat exposure from MHWs and the extended periods of warm water that precede and follow them.

The study, published in Nature’s Communications Earth & Environment, classifies periods of warming leading up to and following MHWs and provides a framework for assessing cumulative heat exposure. The research aims to provide a more effective measure of ecological impacts by treating MHWs as part of broader warming events, rather than isolated events, based on long-term observations from across 20 U.S. estuaries.

"We started to notice these warm-water anomalies on either side of marine heatwaves and realized that they’re actually embedded within larger periods of warm water," explained lead author Ricardo Utzig Nardi M.S. ‘25, a research specialist working with coauthor Piero Mazzini, assistant professor at VIMS & the Batten School of Coastal & Marine Sciences. “It sounds obvious, but studies usually focus only on the marine heatwave window and that’s not an accurate representation of real-world conditions.”

Nardi found that these periods of sustained warming can last for weeks to months at a time, with a distinct marine heatwave embedded between them. The study revealed that in many cases, these pre- and post-heatwave phases contributed as much or more cumulative heat exposure than the MHW phase itself.

“We can no longer look at marine heatwaves in isolation when assessing the impact of warming events on coastal and oceanic ecosystems,” said Mazzini. “We must acknowledge the larger framework in which they exist and this research provides a way to do just that.”

Decades of estuarine data reveal a pattern of underestimating thermal stress

The study analyzed more than 2,580 MHWs recorded over two decades at 20 estuaries throughout the United States and found that historic assessments underestimate total heat exposure by more than 150% on average, with significant implications for ecosystem health. 

“Consider spending time in the sun,” said Nardi, explaining the importance of cumulative heat exposure. “Your risk of sunburn depends on both sunlight intensity and how long you're exposed to it. A few minutes may cause little harm, but hours of exposure can take a toll. It's similar for marine organisms and warm water. Their biological responses depend not only on how warm the water becomes, but also on how long that warming persists. Prolonged, cumulative heat exposure, especially when combined with other stressors, can create conditions that some species may not be able to survive.”

Results concluded that pre- and post-heatwave warm water anomalies are largely independent of the MHW itself, challenging conventional interpretations of heat stress. The findings establish a new framework for evaluating total heat exposure that can be applied across coastal ecosystems.

The study also identified two distinct categories of MHWs. About two-thirds were “individual” events embedded within roughly 60 days of elevated temperatures beyond the MHW itself. The remaining one-third were "compound" events, producing prolonged periods of warming (approximately 90 days) before and after MHWs and generating more than three times the cumulative heat exposure of the MHW itself.

“As you can see, these are remarkably long periods of thermal exposure,” explained Mazzini. “Many laboratory experiments simulate marine heatwaves by increasing temperatures for a few days or weeks in total. Our findings demonstrate that while these experiments hold value, they often fail to capture the prolonged thermal exposure organisms experience in nature. Our research provides a new framework for designing experiments that reflect natural marine heatwave conditions and quantify cumulative heat exposure, allowing scientists to better evaluate the biological impacts of marine heatwaves across ecosystems.”

A surge of VIMS & Batten research is heating up our understanding of marine heatwaves

This study is the latest publication on MHWs from VIMS & Batten School faculty and researchers. Mazzini and Nathan Shunk, a third-year Ph.D. student, recently published a study that defined “vertical marine heatwaves” and introduced a classification scheme for them in the Bay. Last year, Nardi and Mazzini published another manuscript forecasting an increase in MHWs along the U.S. East Coast and identified relationships between MHWs and large-scale climate patterns, including El Niño and the Pacific Decadal Oscillation.

"This study is an example of outstanding science born from a simple question about temperature's relationship to water quality conditions in estuaries," said Mazzini. "It was made possible by NOAA's National Estuarine Research Reserve System and its long-term, high-frequency temperature observations. Comprehensive monitoring programs like these allow us to ask bigger questions and uncover patterns that would otherwise remain hidden."

The new findings also have practical implications for coastal communities and resource managers. Extended periods of warm water can compound the effects of environmental stressors like low-oxygen conditions and harmful algal blooms. This places additional strain on marine ecosystems, from seagrass beds to coral reefs. By incorporating these adjacent warming periods into ecological assessments and laboratory experiments, scientists may develop more realistic predictions of ecosystem vulnerability to better advise conservation and management decisions.

“This research has the potential to shift our understanding of the role warming waters play in ecosystem health by widening our focus beyond the heatwave window, so that we consider the full impact of temperature across time,” explained Nardi. “I’m excited to discover what we may have missed before with this new perspective.”

As MHWs increase in frequency and intensity, understanding their cumulative impact on ecosystems is key to protecting coastal resources and preparing communities for a more resilient future.

 

Overall sea surface temperature (SST) anomaly reflects the pre- and post-event cumulative heat exposure outside of MHW events.

Credit

Ricardo Utzig Nardi

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Journal

Communications Earth & Environment

DOI

10.1038/s43247-026-03739-x 

Method of Research

Computational simulation/modeling

Article Title

Persistent warm water anomalies before and after marine heatwaves amplify heat exposure and associated risks

 

How The Ocean Cleanup is creating the largest-ever database on floating ocean plastic using smart cameras

New study reviews the first five years of operation of the Automated Debris Imaging System (ADIS)

The Ocean Cleanup

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The first five years (2019-2024) of operations of The Ocean Cleanup’s Automated Debris Imaging System (ADIS) has been reviewed and findings published in Environmental Research Communications. In addition to reviewing the performance of the technology, it covers the initial 14,500 km2 of global ocean surface it scanned for plastic debris, and the more than 20,000 large floating objects it has helped identify.
 

Why ADIS matters
ADIS is part of a larger strategy aimed at identifying plastic hotspots and tracking their movements to help The Ocean Cleanup’s efforts to remove plastic from the world’s ocean gyres. Satellite-based methods have proven challenging for the open ocean, where floating marine debris is too dispersed. Specifically, large floating macroplastics (>50 cm) constitute a significant part of the ocean plastic mass, but are poorly quantifiable even by classical methods, such as surface trawls and visual observations, creating a need to develop a method capable of covering large areas of ocean and consistently detecting large plastics and doing so at a low cost.

The first generation of ADIS consisted of a GoPro mounted on the sides of a vessel set to photo-timelapse mode with images backed up on hard drives daily. This data was then brought back to Rotterdam to be processed. Between 2019 and 2024, over 165TB of ocean scans worldwide were collected – 27 million photos in total, gathered from ships crossing the North and South Pacific, North and South Atlantic, and Indian Oceans. These images were fed to an AI model, which recognized and extracted images with objects from the dataset which were then validated manually. From these images, over 20,000 macroplastic items were identified.

The individual plastic detection data from all ADIS cameras is collated into a global dataset and map. This shows how much plastic is found in different areas of the oceans – measured in pieces per square kilometer. An open-access resource, it enables The Ocean Cleanup to better understand the GPGP and also allows other researchers to study the same dataset and map other garbage patches as well.

Testing and Calibrations
In 2022, a ground-truthing expedition to the Great Pacific Garbage Patch (GPGP) was carried out to put ADIS’ detection rate to the test and was compared to UAV aerial imagery and physical trawl samples (manta and mega trawls). During this campaign, ADIS was found to undercount debris 5-6 times less than physical samples, due mostly to color and material bias. However, the consistent nature of the undercount meant it could be used as a correction factor, and data showed that ADIS and physical trawls agreed on where plastic was concentrated, with both methods recording the same peaks and dips in debris density as the research vessel moved through the GPGP. These findings helped guide the development of the next generation of ADIS cameras.

ADIS2.0
While the current paper publication deals with all data collected by the first generation of ADIS, the second generation of ADIS has been collecting data worldwide since 2024. ADIS2.0 autonomously recognizes floating debris and takes detailed pictures of the debris along with GPS coordinates. It automates the processing and offloading of data and is suitable for long-term deployments without relying on experts to maintain it.

As a result, ADIS2.0 has quietly been collecting even more data than ADIS1 – which can be visualized on an online data collection map. If we keep running and expanding ADIS2.0 long enough, it will allow us to create not only a static but a periodic map of plastic around the world, making it possible to compare between years and investigate trends over time. In the past two years we’ve already mapped an extra 39,431 km2, bringing it up to almost 54,000km2 overall.

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Journal

Environmental Research Communications

DOI

10.1088/2515-7620/ae8152 

Method of Research

Case study

Subject of Research

Not applicable

Article Title

Automated debris imaging system: calibrated global monitoring of floating marine litter using ship-based cameras and deep learning

Article Publication Date

3-Jul-2026

COI Statement

The authors are or were employed by The Ocean Cleanup, a non-profit organization aimed at advancing scientific understanding and developing solutions to rid the oceans of plastic, headquartered in the Netherlands.

 

The Great Barrier Reef has a microbiome too

We can now measure changes in the thousands of different and invisible microbes that contribute to healthy reef communities.

Australian Institute of Marine Science

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Seawater sampling on the Great Barrier Reef

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Credit: © AIMS | Neal Cantin

In 2012, Nature revealed that humans provide a home to thousands of different species of microbes, in a series of papers that opened a path to understanding the impact of the human microbiome on our health.

Today, a paper in Nature documents over 800,000 microbial genomes, identifying over 500 new bacterial species and over 300,000 distinct viruses in the waters of the Great Barrier Reef.

This invisible community was documented using DNA found in samples of seawater from 48 reefs, by a research team led by The University of Queensland and the Australian Institute of Marine Science (AIMS).

The authors believe this is the first time that the microbiome of the open waters of reef systems on the Great Barrier Reef has been comprehensively surveyed. They have created the Great Barrier Reef Microbial Genomes Database which is available to all reef researchers.

“Now, we can start to explore what makes a healthy reef microbiome, and how these invisible communities respond to changes on the Great Barrier Reef such as bleaching, storms, sediment, fishing and other stresses,” Professor Philip Hugenholtz, a microbiologist at the University of Queensland and a senior author on the paper said.

Researchers from James Cook University, University of Melbourne and University of Tasmania also contributed to the study.

________________________

 

Twenty years ago, it was almost impossible to identify and document the members of the vast but invisible communities of microbes that live in association with humans, plants and animals and are essential to life on Earth.

Dr Yun Kit Yeoh is a senior author on the Nature paper and a senior research scientist at AIMS. His career studying microbes has taken him from plants to humans and now to the microbes of the Reef.

“Advances in DNA sequencing have allowed us to document and start to modify the communities of microbes contributing, for example, to plant health and human health. The latest advances have now enabled us to study microbiomes across a massive ecosystem, the Great Barrier Reef,” he said.

“These microbes are important. Microalgae produce most of the oxygen we breathe and underpin food chains in the open oceans,” Yun Kit says. “They are eaten by krill and other zooplankton which are then consumed by other animals from the smallest coral polyps to the largest whales. But, until now, we couldn’t see these communities,” Dr Yeoh said.

Dr Steven Robbins, team lead at the University of Queensland and first author on the paper, said collecting and sequencing microbial DNA in the oceans was “challenging”.

“The ocean likes to mix everything up,” he said. “A single drop of water can contain thousands of different, but often very closely related microbes—they’re really complex communities. And many ocean microbes are adapted to low-nutrient conditions typical of ocean ecosystems, which leads to them having low levels of G and C, two of the four bases that form the double helix of DNA. It’s the combination of that low-GC and the complexity of the communities that has hindered this kind of research.

“In this study, we used new “long-read” sequencing technologies that bypass these issues and make it much easier to assemble the jigsaw puzzle of each microbial genome. What was a puzzle with tens of thousands of pieces becomes a simple puzzle, with far fewer pieces for the average microbial genome.

Highlights of the team’s discoveries include:

• Identifying 5,283 different bacterial and archaeal genomes, representing 876 distinct species of which two-thirds are new to public databases and available to study for the first time.
• Identifying 362,802 different kinds of viruses including Crassvirales, a virus that infects bacteria and was first found in human guts in 2014. It was originally thought to be a potential indicator of the presence of human or animal waste, but as this study shows, it is also found in the open ocean waters of the Reef.
• Recovery of complete chromosomes from Bathycoccus and Ostreococcus directly from seawater, two of the most abundant microalgae on the Reef.

“This study will complement long-term monitoring of the Great Barrier Reef, which has been conducted by AIMS for over forty years,” says Yun Kit.

The Great Barrier Reef Microbial Genomics Database was developed with the support of the Integrated Marine Observing System (IMOS) with funding from the Queensland Government. It is hosted by the IMOS Australian Ocean Data Network.

 

Caption

Additional images available at https://aimsimages.fotoware.com.au/fotoweb/albums/al2Wx7fBQ30KRhsY/

Credit

@AIMS as noted

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Journal

Nature

DOI

10.1038/s41586-026-10778-z 

Method of Research

Observational study

Subject of Research

Not applicable

Article Title

The planktonic microbiome of the Great Barrier Reef

Article Publication Date

22-Jul-2026

 

Gut microbiomes follow distinct trajectories during urbanization across South Asia

An international team of researchers studied communities across India and Sri Lanka to understand the impact of urbanization, local diets, and culture on the gut microbiome.

University of Chicago

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Urbanization is one of the most consequential demographic shifts of the 21st century. As communities move from traditional or rural to urbanized ways of life, changes in diet, lifestyle, and environmental exposures can influence many aspects of human biology and health. Research has shown that urbanization can alter the gut microbiome, the community of microorganisms inhabiting the digestive tract, but whether this transition follows a common pattern across different global populations has remained unclear.

South Asia is uniquely suited to address this question because it combines extraordinary cultural, dietary, and ecological diversity with some of the world's fastest rates of urbanization. As lifestyles change, the region is also experiencing a rapid rise in cardiometabolic disorders, such as type 2 diabetes, heart disease, and obesity, making it increasingly important to understand how urbanization shapes the gut microbiome. Despite this, South Asian populations remain underrepresented in global microbiome research.

A new study published in Gut Microbes addresses this gap through one of the most geographically and culturally diverse surveys of the South Asian gut microbiome to date. An international team of researchers from India, the United States, and Sri Lanka reports findings from the South Asian MicroBiome ARray (SAMBAR), a population-scale cohort of 575 adults from ten communities across India and Sri Lanka: Spitians from the Indian Himalayas; Gond, Kolam, and Koya from Central India; Kani from South India; Mizo and Pochury Naga from Northeast India; and Adivasi alongside urban Sinhalese and urban Sri Lankan Tamil communities from Sri Lanka.

By recruiting rural and urban participants across these communities, the researchers were able to investigate whether gut microbiome changes accompanying urbanization follow similar or distinct trajectories across South Asia and how these trajectories, in turn, compare with urbanization-related gut microbiome shifts reported elsewhere in the world.

The study adopted a community-engaged approach throughout the research process. “Establishing SAMBAR required extensive fieldwork across India and Sri Lanka, bringing together researchers, local collaborators, and participating communities spanning ten geographically and culturally distinct populations,” said Nagarjuna Pasupuleti, PhD, one of the first authors of the study who led field operations and data generation. “These collaborations were fundamental not only to making this study possible, but also to ensuring that study findings were returned to participating communities through accessible presentations and discussions.”

Influence of local diets, environments, and culture

The researchers found that geography and community affiliation explained more of the variation in gut microbiome composition than rural or urban lifestyle alone. Although urbanization consistently influenced the gut microbiome, the magnitude and nature of these changes differed among communities, reflecting differences in local diets, environments, and cultural practices.

“By studying rural and urban participants from multiple communities, we could ask not only whether urbanization changes the gut microbiome, but whether that transition unfolds similarly across populations,” said Shreya Ramachandran, PhD, postdoctoral researcher at the University of Chicago and one of the first authors of the study who led the data analyses. “We learned that South Asian communities are heterogeneous, and different communities have unique microbial signatures that reflect their cultures and diets and respond to urbanization in different ways.”

The study identified associations between gut bacteria and dietary patterns. For instance, bacteria such as Bifidobacterium were associated with wheat and yogurt consumption. In some communities, this genus was abundant in both rural and urban cohorts, reflecting a tradition of pastoralism and dairy consumption; in some, it was significantly more present in the urban cohorts, reflecting the increase in consumption in cities.

The diverse study population also allowed researchers to identify culturally specific links between diet and bacteria, including a significantly higher abundance of the genus Bacillus among Pochury participants, who regularly consume a Bacillus-heavy fermented soybean condiment called axone.

After accounting for the cultural heterogeneity, the study design allowed the researchers to identify broad patterns of microbial shifts associated with urbanization. For instance, multiple communities showed decreased diversity and an increase in Megamonas in urban cohorts – a genus associated with fat metabolism and with metabolic outcomes including obesity.

Expanding microbiome research globally

The researchers also compared the SAMBAR data with previously published microbiome datasets from other world regions. South Asian communities occupied a distinct position within global gut microbiome diversity, reinforcing the importance of expanding microbiome research beyond populations that have historically dominated biomedical studies.

“South Asia is undergoing rapid and heterogeneous urbanization alongside a growing burden of cardiometabolic disease. Understanding how the gut microbiome changes during these transitions will be essential for interpreting future links between lifestyle, metabolism, and health across the region,” said Niraj Rai, PhD, group leader at the Birbal Sahni Institute of Palaeosciences and one of the senior authors.

“As microbiome-based diagnostics and therapies continue to develop, they must be informed by data that reflect the diversity of human populations and the different pathways through which lifestyles are changing,” said Maanasa Raghavan, PhD, Assistant Professor of Human Genetics at the University of Chicago and one of the senior authors. “Resources like SAMBAR help broaden that perspective by bringing historically underrepresented populations into global microbiome research.”

The authors hope that SAMBAR will serve as a foundational resource for future studies investigating how changing diets, environments, and lifestyles influence the gut microbiome and health across South Asia, while providing a framework for microbiome studies in other underrepresented populations.

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Journal

Gut Microbes

DOI

10.1080/19490976.2026.2700042 

Method of Research

Data/statistical analysis

Subject of Research

Cells

Article Title

Distinct trajectories of urbanization shape the human gut microbiome across South Asia

Article Publication Date

14-Jul-2026

 

Arctic ozone soars in 2024 due to series of extraordinary atmospheric events

Ocean-Land-Atmosphere Research (OLAR)

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image: 

The graph depicts the total column ozone (TCO), or the total amount of ozone in a vertical column extending from the Earth's surface to space, over time. The annual (blue) and March month (red) TCO measurements are illustrated in the graph by satellite (top panel), reanalysis (second from the top), and ground-based (Lerwick, Oslo, Sodankylä, and Scoresbysund) observations in the Arctic between 1978–2024. The pink lines in the background show the observed high TCO years. 

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Credit: Authors and OLAR

Scientists discover the atmospheric processes behind the highest Arctic stratospheric ozone levels observed in more than 45 years, providing new insight into how large-scale climate variability shapes the ozone layer.

The Arctic ozone layer is a protective layer of gas that sits in the stratosphere above the North Pole, filtering ultraviolet radiation from the sun. Recently, scientists discovered that the Arctic ozone layer recorded its highest ozone levels in March 2024 since satellite observations began in 1979. What puzzled researchers was why.

 

Ozone is a molecule of three highly reactive oxygen atoms (O3). In the atmosphere, it serves a protective function, but on the ground, ozone is a harmful gas that forms when pollutants from tailpipes, power plants and factories react with sunlight. Inhaling ozone can cause severe respiratory issues.

 

The researchers found record-high total column ozone of 477 Dobson Units (DU), a standard measure of the total amount of ozone, from merged satellite observations, supported by exceptionally high measurements from monitoring stations across the Arctic, including Lerwick, Scotland; Oslo, Norway; Sodankylä, Finland; and Scoresbysund, a deep inlet in the Greenland Sea. Balloon observations and satellite measurements also confirmed unusually high ozone concentrations throughout the lower and middle stratosphere.

 

Rather than simply reporting these observations, a team of scientists from CORAL at the Indian Institute of Technology Kharagpur in Kharagpur, India, investigated what caused the record-setting levels of ozone above the Arctic.

 

The team published their paper, entitled “Intense Wave Activity and Climate Oscillations Drive Record-High Arctic Ozone in March 2024,”on July 21st  in Ocean-Land-Atmosphere Research.

 

“Although the record ozone values were evident in satellite and ground-based observations, the atmospheric processes responsible for this extraordinary event were not well understood. Our goal was to identify the mechanisms that led to this unprecedented increase and determine how large-scale climate variability influenced the Arctic ozone layer,” said Jayanarayanan Kuttippurath, associate professor at CORAL, IIT Khargapur and lead scientist of the research study.

 

The team performed a comprehensive dynamical analysis, which uses mathematical, analytical, and numerical methods to model how a system evolves over time, that showed exceptionally strong planetary waves, or large, meandering waves that form naturally in rotating fluids like the Earth's atmosphere and oceans, propagating from the lower atmosphere into the stratosphere, triggering three warming events during the winter of 2023–2024. These warming events weakened and disturbed the Arctic polar vortex, allowing ozone-rich air to accumulate over the polar region.

 

The study further demonstrated that this sequence of events was amplified by the combined influence of a strong El Niño, or warmer than average water across the equatorial Pacific; the Madden–Julian Oscillation, a massive, eastward-moving atmospheric disturbance that travels along the equator, circling the entire globe every 30 to 60 days; and the Quasi-Biennial Oscillation, a regular variation of stratospheric winds over the equator that alternate between easterlies and westerlies.

 

The team’s integration of advanced wave diagnostics with observations from multiple independent datasets has provided the most robust explanation of the atmospheric mechanisms responsible for the record-breaking Arctic ozone levels to date.

 

"This was not simply an unusual ozone year," said Kuttippurath. "It was the result of an exceptionally rare alignment of atmospheric processes operating from the Earth's surface to the upper atmosphere."

 

The findings highlight the growing importance of atmospheric dynamics in controlling Arctic ozone variability. “As the recovery of the ozone layer continues under the Montreal Protocol [a 1987 global treaty designed to protect Earth's ozone layer], year-to-year ozone levels will increasingly depend on natural climate variability and large-scale atmospheric circulation rather than changes in ozone-depleting substances alone,” said Anjali Sathyanath, research scholar at CORAL, IIT Kharagpur and lead author of the study.

 

The research also has broader implications for seasonal climate prediction. Improving our understanding of how planetary waves, tropical climate modes that determine the recurring shifts in temperature, atmospheric pressure, and rainfall patterns between the Tropic of Cancer and the Tropic of Capricorn, and the polar vortex interact could improve forecasts of Arctic stratospheric conditions and their influence on global weather patterns.

 

“In the longer term, we hope this research will contribute to a new generation of Earth System Models capable of providing more reliable predictions of atmospheric extremes in a warming climate,” said Kuttippurath.

 

This study did not receive any specific funding.

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Journal

Ocean-Land-Atmosphere Research

DOI

10.34133/olar.0164 

Method of Research

Observational study

Article Title

Intense Wave Activity and Climate Oscillations Drive Record-High Arctic Ozone in March 2024

Article Publication Date

21-Jul-2026