Patrick Megonigal, Smithsonian Institution

Firefighters battling the deadly wildfires that raced through the Los Angeles area in January 2025 have been hampered by a limited supply of freshwater. So, when the winds are calm enough, skilled pilots flying planes aptly named Super Scoopers are skimming off 1,500 gallons of seawater at a time and dumping it with high precision on the fires.

Using seawater to fight fires can sound like a simple solution – the Pacific Ocean has a seemingly endless supply of water. In emergencies like Southern California is facing, it’s often the only quick solution, though the operation can be risky amid ocean swells.

But seawater also has downsides.

Saltwater corrodes firefighting equipment and may harm ecosystems, especially those like the chaparral shrublands around Los Angeles that aren’t normally exposed to seawater. Gardeners know that small amounts of salt – added, say, as fertilizer – does not harm plants, but excessive salts can stress and kill plants.

While the consequences of adding seawater to ecosystems are not yet well understood, we can gain insights on what to expect by considering the effects of sea-level rise.

A seawater experiment in a coastal forest

As an ecosystem ecologist at the Smithsonian Environmental Research Center, I lead a novel experiment called TEMPEST that was designed to understand how and why historically salt-free coastal forests react to their first exposures to salty water.

Sea-level rise has increased by an average of about 8 inches globally over the past century, and that water has pushed salty water into U.S. forests, farms and neighborhoods that had previously known only freshwater. As the rate of sea-level rise accelerates, storms push seawater ever farther onto the dry land, eventually killing trees and creating ghost forests, a result of climate change that is widespread in the U.S. and globally.

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In our TEMPEST test plots, we pump salty water from the nearby Chesapeake Bay into tanks, then sprinkle it on the forest soil surface fast enough to saturate the soil for about 10 hours at a time. This simulates a surge of salty water during a big storm.

Our coastal forest showed little effect from the first 10-hour exposure to salty water in June 2022 and grew normally for the rest of the year. We increased the exposure to 20 hours in June 2023, and the forest still appeared mostly unfazed, although the tulip poplar trees were drawing water from the soil more slowly, which may be an early warning signal.

Things changed after a 30-hour exposure in June 2024. The leaves of tulip poplar in the forests started to brown in mid-August, several weeks earlier than normal. By mid-September the forest canopy was bare, as if winter had set in. These changes did not occur in a nearby plot that we treated the same way, but with freshwater rather than seawater.

The initial resilience of our forest can be explained in part by the relatively low amount of salt in the water in this estuary, where water from freshwater rivers and a salty ocean mix. Rain that fell after the experiments in 2022 and 2023 washed salts out of the soil.

But a major drought followed the 2024 experiment, so salts lingered in the soil then. The trees’ longer exposure to salty soils after our 2024 experiment may have exceeded their ability to tolerate these conditions.

Seawater being dumped on the Southern California fires is full-strength, salty ocean water. And conditions there have been very dry, particularly compared with our East Coast forest plot.

Changes evident in the ground

Our research group is still trying to understand all the factors that limit the forest’s tolerance to salty water, and how our results apply to other ecosystems such as those in the Los Angeles area.

Tree leaves turning from green to brown well before fall was a surprise, but there were other surprises hidden in the soil below our feet.

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Rainwater percolating through the soil is normally clear, but about a month after the first and only 10-hour exposure to salty water in 2022, the soil water turned brown and stayed that way for two years. The brown color comes from carbon-based compounds leached from dead plant material. It’s a process similar to making tea.

Our lab experiments suggest that salt was causing clay and other particles to disperse and move about in the soil. Such changes in soil chemistry and structure can persist for many years.

Sea-level rise is increasing coastal exposure

While ocean water can help fight fires, there are reasons fire officials prefer freshwater sources – provided freshwater is available.

U.S. coastlines, meanwhile, are facing more extensive and frequent saltwater exposure as rising global temperatures accelerate sea-level rise that drowns forests, fields and farms, with unknown risks for coastal landscapes.

Patrick Megonigal, Associate Director of Research, Smithsonian Environmental Research Center, Smithsonian Institution

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Virginia Iglesias, University of Colorado Boulder

Investigators are trying to determine what caused several wind-driven wildfires that have destroyed thousands of homes across the Los Angeles area in January 2025. Given the fires’ locations, and lack of lightning at the time, it’s likely that utility infrastructure, other equipment or human activities were involved.

California’s wildfires have become increasingly destructive in recent years. Research my colleagues and I have conducted shows U.S. wildfires are up to four times larger and three times more frequent than they were in the 1980s and ’90s. Fast-moving fires have been particularly destructive, accounting for 78% of structures destroyed and 61% of suppression costs between 2001 and 2020.

Lightning strikes are a common cause of U.S. wildfires, but the majority of wildfires that threaten communities are started by human activities.

A broken power line started the deadly 2023 Maui fire that destroyed the town of Lahaina, Hawaii. Metal from cars or mowers dragging on the ground can spark fires. California’s largest fire in 2024 started when a man pushed a burning car into a ravine near Chico. The fire destroyed more than 700 homes and buildings.

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What makes these wildfires so destructive and difficult to contain?

The answer lies in a mix of wind speed, changing climate, the legacy of past land-management practices, and current human activities that are reshaping fire behavior and increasing the risk they pose.

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Fire’s perfect storm

Wildfires rely on three key elements to spread: conducive weather, dry fuel and an ignition source. Each of these factors has undergone pronounced changes in recent decades. While climate change sets the stage for larger and more intense fires, humans are actively fanning the flames.

Climate and weather

Extreme temperatures play a dangerous role in wildfires. Heat dries out vegetation, making it more flammable. Under these conditions, wildfires ignite more easily, spread faster and burn with greater intensity. In the western U.S., aridity attributed to climate change has doubled the amount of forestland that has burned since 1984.

Compounding the problem is the rapid rise in nighttime temperatures, now increasing faster than daytime temperatures. Nights, which used to offer a reprieve with cooler conditions and higher humidity, do so less often, allowing fires to continue raging without pause.

Finally, winds contribute to the rapid expansion, increased intensity and erratic behavior of wildfires. Wind gusts push heat and embers ahead of the fire front and can cause it to rapidly expand. They can also create spot fires in new locations. Additionally, winds enhance combustion by supplying more oxygen, which can make the fire more unpredictable and challenging to control. Usually driven by high winds, fast-moving fires have become more frequent in recent decades.

Fuel

Fire is a natural process that has shaped ecosystems for over 420 million years. Indigenous people historically used controlled burns to manage landscapes and reduce fuel buildup. However, a century of fire suppression has allowed vast areas to accumulate dense fuels, priming them for larger and more intense wildfires.

Invasive species, such as certain grasses, have exacerbated the issue by creating continuous fuel beds that accelerate fire spread, often doubling or tripling fire activity.

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Additionally, human development in fire-prone regions, especially in the wildland-urban interface, where neighborhoods intermingle with forest and grassland vegetation, has introduced new, highly flammable fuels. Buildings, vehicles and infrastructure often ignite easily and burn hotter and faster than natural vegetation. These changes have significantly altered fuel patterns, creating conditions conducive to more severe and harder-to-control wildfires.

Ignition

Lightning can ignite wildfires, but humans are responsible for an increasing share. From unattended campfires to arson or sparks from power lines, over 84% of the wildfires affecting communities are human-ignited.

Human activities have not only tripled the length of the fire season, but they also have resulted in fires that pose a higher risk to people.

Lightning-started fires often coincide with storms that carry rain or higher humidity, which slows fires’ spread. Human-started fires, however, typically ignite under more extreme conditions – hotter temperatures, lower humidity and stronger winds. This leads to greater flame heights, faster spread in the critical early days before crews can respond, and more severe ecosystem effects, such as killing more trees and degrading the soil.

Human-ignited fires often occur in or near populated areas, where flammable structures and vegetation create even more hazardous conditions. Homes and the materials around them, such as wooden fences and porches, can burn quickly and send burning embers airborne, further spreading the flames.

As urban development expands into wildlands, the probability of human-started fires and the property potentially exposed to fire increase, creating a feedback loop of escalating wildfire risk.

Whiplash weather

A phenomenon known as whiplash weather, marked by unusually wet winters and springs followed by extreme summer heat, was especially pronounced in Southern California in recent years.

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A wet spring in 2024 fostered vegetation growth, which then dried out under scorching summer temperatures, turning into highly combustible fuel. This cycle fueled some of the biggest fires of the 2024 season, several of which were started by humans.

That dryness continued in Southern California through the fall and into early winter, with very little rainfall. Soil moisture in the Los Angeles region was about 2% of historical levels for that time of year when the fires began on Jan. 7, 2025.

As the factors that can drive wildfires converge, the potential for increasingly severe wildfires looms ever larger. Severe fires also release large amounts of carbon from trees, vegetation and soils into the atmosphere, increasing greenhouse gas emissions and exacerbating climate change, contributing to more extreme fire seasons.

This is an update to an article originally published Oct. 8, 2024.

Virginia Iglesias, Interim Earth Lab Director, University of Colorado Boulder

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Luke Montrose, Colorado State University

Millions of people across the Los Angeles area are being exposed to wildfire smoke as fires burn through homes and vehicles. The fires in January 2025 have burned thousands of structures, along with the building materials, furniture, paints, plastics and electronics inside them.

When materials like these burn, they can release toxic chemicals with the potential to harm people breathing the air downwind.

A 2023 study of smoke from fires in the wildland-urban interface – areas where urban neighborhoods bleed into the wildlands – found it contained a vast array of chemicals harmful to humans, including hydrogen chloride, polycyclic aromatic hydrocarbons, dioxins and a range of toxic organic compounds, including known carcinogens such as benzene, as well as toluene, xylenes, styrene and formaldehyde. The researchers also found metals in the smoke, including lead, chromium, cadmium and arsenic, which are known to affect several body systems, such as the brain, liver, kidney, skin and lungs.

The short-term effects of exposure to smoke like this can trigger asthma attacks and cause lung and cardiac problems.

But smoke can also have long-term effects, and those are less well understood. As an environmental toxicologist who focuses on wildfire smoke health effects, I, along with many of my colleagues, am increasingly concerned about the impact of long-term and repeated exposures to wildfire smoke that more people are now facing.

Long-term smoke exposure is increasing

Nationwide, the acreage burned in wildfires in the U.S. has nearly doubled each decade since 1990. That is changing how people are exposed to wildfire smoke.

Communities have found themselves blanketed in smoke for days and weeks at a time increasingly often. In 2023, massive wildfires in Canada repeatedly spread thick smoke into many U.S. communities. Controlled burns, which firefighters set to clear away flammable brush and reduce the severity of future wildfires, also add smoke to the air.

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Wildfire smoke is now the leading source of PM2.5 – microscopic particulate matter than can penetrate into the lungs – in the western U.S.

This growing exposure increases the need to understand the long-term consequences of living and working in wildfire-risk areas.

Dose, duration and frequency matter

When scientists study the health risks of wildfire smoke, they tend to use analysis methods that were developed to assess health effects caused by low-level, chronic, urban air pollution exposures – picture car exhaust or smokestack emissions. However, these approaches fail to capture the dynamic and intense nature of wildfire smoke.

Researchers suspect there are differing consequences for people exposed to smoke at varying intensities and durations. Repeated exposure to wildfire smoke may also have compounding health effects over time.

To study the long-term impact of wildfire smoke, scientists need to know how much smoke people were exposed to, for how long and how often. That’s not an experiment anyone can conduct on humans in a lab, but the data can be gathered from communities being affected by wildfires.

Right now, however, this kind of data collection is rare.

Most studies that have explored long-term exposure, such as its impact on dementia or pregnancy, have used an average exposure over years rather than detailed data on exposures.

A few have focused on specific events. For example, a study of residents who had been exposed to six weeks of smoke during the 2017 Rice Ridge Fire near Seeley Lake, Montana, found their lung function was significantly reduced for at least two years after the fire. That was a forest fire, and while burning vegetation is bad, it’s generally thought to be less toxic than burning buildings.

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Thinking differently about smoke exposure

Improving understanding of the long-term effects of wildfire smoke will require thinking differently about smoke.

If epidemiologists can begin clearly defining the negative health effects from wildfire smoke exposure in terms of dose, duration and frequency in their studies, taking into account the dynamic and episodic nature, then toxicologists can model these human experiences in animal experiments.

These experiments would have the potential to improve the understanding of the long-term health risks and then help scientists develop effective guidelines and strategies to mitigate harmful exposures.

Luke Montrose, Assistant Professor of Environmental and Radiological Health Sciences, Colorado State University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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