Blanca Lucio likes to spend her mornings tending to her zucchinis, cucumbers, watermelons and traditional Mexican herbs at a small community garden near downtown Los Angeles. With its cool, damp air, the garden brims with what Lucio calls “magic.”

The only sound comes from green June bugs buzzing by her ears and children playing at the community center across the street.

“Outside of here, you’re exposed to a lot of noise and a lot of pollution,” Lucio said while giving a tour of the garden, a short distance from her home in South-Central L.A. “This space renews me and the other gardeners who grow plants here. I feel more content when I’m here.”

Noise pollution and excessive heat can seem inescapable in L.A. What would the city be without random bursts of fireworks and car sound systems thumping loud enough to shake you from your dreams? And the nearly 365-days-a-year sunshine is practically what defines L.A. sunshine, even though it means commuters often must wait under the blazing sun at bus stops that lack cover.

But just because we’ve grown used to L.A.’s jarring soundscape, shadeless streets and pockets of intense heat, it doesn’t mean they are harmless.

Noise and heat together can pose a special kind of health threat, one that the city’s most vulnerable people are least able to protect against, said Valerie Tornini, a neurobiologist at UCLA.

With climate change ushering in stronger and longer heat waves, a growing body of evidence suggests that excessive heat has become a public health crisis. An estimated 1,300 people die of extreme heat each year, according to the Environmental Protection Agency, and that number will only grow in coming years.

Both heat and noise can harm the nervous system, interfere with metabolism and disrupt sleep patterns. They can also aggravate conditions such as diabetes, hypertension and cardiovascular disease, according to a paper published in Environmental Health Perspectives.

Tornini leads a team of brain researchers trying to figure out how the combination of these two environmental dangers affects brain health and behavior among residents of Central and South L.A.

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Her team is working with the Boston-based nonprofit Prospera Institute and the South L.A. social justice nonprofit Esperanza Community Housing Corp. to collect stories from local Latino and Black Americans, like Lucio, about how they cope.

The collaboration started in 2024 after Tornini, who had been studying the effect of noise and heat on neural development in zebrafish, reached out to Joanne Suarez, who founded Prospera to promote health equity in Black, Latino and Indigenous communities.

Their partnership sprang from a recognition that brain science has lagged behind other disciplines in recognizing the need for community-centered research that treats study participants as equal partners, Tornini said.

The project revolves around two interwoven prompts, she said: “How can it do good and no harm, and how can it serve the cause of justice?”

Joanne Suarez speaks with South L.A. community members about how they’re affected by excessive heat and noise during a focus group at Esperanza Community Housing.

(Carlin Stiehl / Los Angeles Times)

“Sometimes [research] is not aligned with what the community wants and needs,” Tornini said. “I want to listen: What are your concerns? What are your lived experiences? People’s stories and oral histories … can influence the kind of questions that we ask in the lab, and then that data goes back to them.”

That shift in thinking was in evidence on a Saturday morning in July at Mercado La Paloma — a South L.A. food hall that houses the Michelin-starred Mexican seafood restaurant Holbox as well as Esperanza Community Housing’s offices.

A dozen women sat in a circle with Suarez and Tornini for an intimate listening session, held in Spanish, about living with noise and heat.

Suarez invited the women to speak in response to a series of questions printed on a handout. For example: “How do environmental factors like noise and heat impact your health and daily life?” and “Have you noticed changes in your ability to focus, think clearly or even remember things when it’s extremely hot or noisy in your community?”

One woman said it’s hard to mitigate one disturbance without exacerbating the other, such as when she opens the window of her bedroom at night to let in fresh air, only to be kept awake by noise from passing planes and sirens. A mother worried about the effect of sun and heat on her kids during gym class and recess at school. One woman told the group that excessive heat worsens her hypertension headaches, while another said that when it’s hot out, she gets more irritated by noises she can’t control.

Another participant said she fears getting caught in the crossfire of warring gangs in her neighborhood and so won’t sit outside to get fresh air, no matter how hot it gets indoors.

The UCLA initiative is as much an experiment in trust-building as data collection, said Monic Uriarte, a public health advocate and community organizer at Esperanza Community Housing who has lived and worked in L.A.’s urban core for three decades.

Wariness of scientists and healthcare professionals — born of a history of one-sided research that never benefited study volunteers or their communities; nonconsensual lab experiments; and racial discrimination among medical practitioners — is commonplace in some communities of color.

“I love higher education, but we are tired of being guinea pigs for different studies,” Uriarte said. “We need this kind of collaboration — a space for our community to share, in our own words, the experience of living in South Los Angeles.”

She’s excited about the prospect of volunteers being able to cite whatever findings result from the research when asking city officials for noise mitigation for their homes, tree plantings or more open spaces.

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Living and commuting in L.A. means navigating an environment that can make you want to cover your ears and run for the shadows.

The relentless flow of vehicles and Metro light-rail trains drowned out Blanca Lucio’s voice as she gave a tour of South-Central L.A., walking past auto-body shops and restaurants at the intersection of San Pedro Street and Washington Boulevard.

Not far away in the downtown jewelry district, sidewalk vendors selling wares as varied as avocados, roasted corn, cellphone cases and brass lanterns shielded themselves from the intense midday sun with beach umbrellas, or by clustering in the shadows of high-rises.

During L.A.’s recent heat wave, when temperatures regularly surpassed 90 degrees, a woman selling rose bouquets out of buckets at Pershing Square looked beleaguered while standing in the paltry shade of a tree. A man pushing a cooler full of 50-cent bottled waters wiped sweat from his forehead and tried to cool down with a Spanish fan.

A woman sleeps on a bench in Los Angeles’ Pershing Square in June 2024.

(Brian van der Brug / Los Angeles Times)

There was no escape from the onslaught of car horns, rumbling motors and pedestrians blasting music from speakers stuffed in backpacks.

About 10 miles south is the Harbor Freeway transit terminal, an important hub for commuters who need to catch a bus or train in South L.A.

The terminal is located on a raised platform in the middle of a concrete tangle of ramps and the elevated lanes of the 105 Freeway. The commotion and noise are unnerving; cars speed by so close you can feel whooshes as they pass.

But even if you don’t have to wait daily for transport while being inundated with the sounds of a Los Angeles freeway, you may be forced to endure some noise pollution seemingly designed to disturb the peace. On any given evening in the city, drivers and bikers amp up the soundscape by revving their engines while waiting at traffic stops, then slam on the gas when their light turns green, screeching down the street.

Nighttime also brings the piercing sound of street takeovers. Drivers draw crowds of spectators as they perform stunts such as “doughnuts” — spinning their cars in circles until their tires burn rubber marks on the pavement. The phenomenon has become such a problem countywide — with shootings and cars set on fire at some of them — that officials have vowed to crack down on the illegal gatherings.

L.A. is notoriously noisy and hot, but experiences like these are widespread across the U.S.

About 95 million Americans, nearly one-third of the U.S. population, are subjected to transportation-related noise pollution, with Latino, Black and Asian communities disproportionately exposed to it, according to data compiled by researchers at the University of Washington.

Noise is measured in decibels, with a middle range of 50-60 considered a normal level of ambient sound that doesn’t pose a risk to health. Most people experience noise at this level while doing routine things such as working at an office or walking down a street with little to no traffic. Emergency sirens, lawn mowers and music in a nightclub, by contrast, can exceed 90 decibels.

While grating noises and intolerable heat may be experienced in pockets across the city, making it hard to draw direct comparisons, some whole sections of L.A. feel conspicuously beset by these environmental disturbances. Other neighborhoods feel more insulated.

A pedestrian crosses a median as traffic passes along San Vicente Boulevard in Brentwood.

(Genaro Molina / Los Angeles Times)

The urban core and South L.A. — where the median household income ranges from $48,000 to $62,000 a year and Latino and Black people make up the majority of the population, according to the U.S. census — is a wall of sound and a bubble of heat. But farther west in predominantly white Brentwood, where the median annual household income is more than $160,000, walls of semi-tropical foliage insulate many private homes from intrusive noises and overhanging trees form of canopies of coolness over gently curving streets.

Take a sunset walk along the gently sloped, flower-scented streets above busy Sunset Boulevard in Brentwood — you will be immersed in a stillness broken only by birds chirping in the treetops. To the south, along the historic canals of Venice, ocean breezes cool the air and the prevailing sound is of fountains trickling in homeowners’ yards.

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By contrast, noises associated with law enforcement are such familiar nuisances on the relatively bare streets of South L.A. that they are treated as if they are part of the natural environment. The late artist 2Pac rapped about the menacing presence of “ghetto bird” police helicopters in 1996‘s “To Live and Die in L.A.,” and Compton-born rapper Kendrick Lamar referenced ghetto birds and samples the piercing wail of police sirens on “XXX,” released in 2017.

“Basically, the Blacker the neighborhood, the more flight hours; the more Latinx the neighborhood, the more flight hours … and the Blacker the neighborhood, the lower the helicopters are flying,” said Nick Shapiro, a multidisciplinary environmental researcher at UCLA.

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Shapiro has spent years using L.A. Police Department flight data to map helicopter trajectories across the city in studies of “sonic inequality” that his team conducted jointly with residents of South L.A.

Helicopter noise is an issue citywide — even in typically serene, higher-income neighborhoods. The noise is a problem for outdoor TV and film productions too, Shapiro said.

Still, Shapiro said, “there’s pretty extreme inequality between Malibu and Watts.”

Meanwhile, it’s even worse for those in South L.A. who live in the L.A. International Airport flight path and have to contend with both helicopters and the earsplitting sonic reality of jets landing and taking off.

West Century Boulevard runs along the airport’s flight path, meaning that every couple of minutes, a low-flying jet cuts a trail of the high-frequency whines and low-frequency roars on its approach to the airport, sending decibel levels into the 90s. Because of all the broad, shadeless streets that define many of South L.A.’s neighborhoods, the hot summer sun seems to bear down more intensely on these communities of color too.

Plane spotters get a close-up view of planes on their final approach to Los Angeles International Airport.

(Gina Ferazzi / Los Angeles Times)

One sunny afternoon in August, Charles Lewis, a retired store clerk, sat in a folding chair under a solitary shade tree and watched a steady stream of cars and trucks rush past him on Century. As one plane after another shrieked across the cloudless sky, jet-shaped shadows raced across the pavement, alongside cars.

Lewis lives close by but lamented that sidewalks along residential streets closer to his home are too exposed to the sun. He’s witnessed shade gradually disappear in the 40 years he has lived in the neighborhood and believes law enforcement agencies are partly to blame.

Los Angeles Police Department Deputy Chief Donald Graham acknowledged that his agency has asked city crews to trim publicly maintained trees to improve street lighting and deter illegal activity in specific trouble spots.

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“We’re always trying to balance the beautification of the city and the need to have a tree canopy with public safety,” he said.

The cacophony of the boulevard offers little in the way of tranquility, but Lewis said the noise from jets is so bad at home that he has to turn up the volume on his TV and wait for aircraft to pass to have a conversation without yelling.

At least his perch on Century provides a refuge from the excessive heat.

“This is the only shade I have,” Lewis said.

Nearby, the late-day sun felt oppressive along a busy, tree-less stretch of Slauson Avenue near the 110 Freeway. Two women at a food stand squinted in the sunlight as they cooked whole chickens on a hot grill to serve with freshly made tortillas and beans and rice.

A metro train traveling on the K Line passes a mural of the late rapper Nipsey Hussle that is located on Crenshaw Boulevard at Slauson Avenue in Los Angeles.

(Mel Melcon / Los Angeles Times)

Too busy filling orders to talk, the women laughed and said they’ve given up trying to stay cool while working on days like this.

Meanwhile, six miles north, things weren’t much better. At the junction of Olympic Boulevard and Western Avenue in Koreatown, a search for both shade and quiet was an exercise in futility. The sparse landscaping on the thoroughfares left sidewalks exposed to the bright sun, and the constant rumble of trucks and buses assaulted the eardrums.

A mile away, in the flats of Hollywood near Paramount Studios, the block letters of the district’s famous hilltop sign appeared like a vision through the smoggy air above a bustling intersection at Melrose Avenue and Vine Street — though on a recent August day, the 85-degree temperatures, blazing sunlight and din of speeding vehicles made it that much more difficult to savor the view.

Traditional lab-based brain research has too often discounted the health challenges that come with navigating an ecosystem as complex and inequitable as L.A.’s, said Helena Hansen, professor and interim chair of psychiatry and behavioral sciences at UCLA’s Geffen School of Medicine.

The noise and heat study, along with the analysis of helicopter noise, are part of a broader effort to incorporate information about social and physical conditions into research design, she said.

“We’re really trying to rethink the way science is done,” she said.

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At the listening session in July, the idea of breaking down the barrier between laboratory science and real life was on full display. Nearly all the women nodded in agreement when one brought up her struggle to focus on tasks or relax because of heat and noise. It was clear that for these Angelenos, stress is the norm — peace the exception.

Lucio was among those who attended. She is participating in the UCLA study not just to help the researchers, she said, but also to make living in L.A. more comfortable and healthier for herself and her neighbors.

The surrounding neighborhood, just across a busy freeway from the University of Southern California’s campus, is one of several in Central L.A. that the budding citizen scientist has surveyed as part of her own study of the area’s spotty tree canopy.

“We need more trees,” Lucio said. “I’ve noticed people walking around searching for shade and clustering in the few spots where they can find it…. I’ve even seen dogs searching for shade in this neighborhood.”

Trees provide a canopy for travelers along Grayburn Avenue in Los Angeles’ Leimert Park neighborhood.

(Brian van der Brug / Los Angeles Times)

It’s little slices of life and firsthand observations such as these that the UCLA scientists and Prospera facilitator want to heed as they pursue their research. The group just secured additional funding for further study and possibly to record accounts of lived experiences on video, Suarez said. For now, Tornini, the brain scientist, just wants to keep the line of communication open with participants.

“The goal is for this to be a living relationship that is shaped mutually,” Tornini said. “What the community does with this information is within their own power. And if they ask — how can we help?”

Working from a dock on St. Helena Island, S.C., on a sweltering day this summer, Ed Atkins pulled in a five-foot cast net from the water and dumped out a few glossy white shrimp from the salt marsh.

Mr. Atkins, a Gullah Geechee fisherman, sells live bait to anglers in a shop his parents opened in 1957. “When they passed, they made sure I tapped into it and keep it going,” he said. “I’ve been doing it myself now for 40 years.”

These marshes, which underpin Mr. Atkins’s way of life, are where the line between land and sea blurs. They provide a crucial nursery habitat for many marine species, including commercial and recreational fisheries.

But these vast, seemingly timeless seascapes have become some of the world’s most vulnerable marine habitats, according to a new study published on Thursday in the journal Science that adds up and maps the ways human activity is profoundly reshaping oceans and coastlines around the world.

Soon, many of Earth’s marine ecosystems could be fundamentally and forever altered if pressures like climate change, overfishing, ocean acidification and coastal development continue unabated, according to the authors.

It’s “death by a thousand cuts,” said Ben Halpern, a marine biologist and ecologist at the University of California, Santa Barbara, and one of the authors of the new study. “It’s going to be a less rich community of species. And it may not be something we recognize.”

Among the other ecosystems at high risk are sea grass meadows, rocky intertidal zones and mangrove forests. These parts of the ocean, near shore, are the ones people most depend on. They provide natural defenses against storm damage. And the vast majority of commercial and recreational fishing, which together support more than two million jobs in the United States alone, takes place in shallower coastal waters.

There’s also an intangible cultural richness at stake. The culture of Gullah Geechee people like Mr. Atkins, a community descended from enslaved West Africans forced to work the rice and cotton plantations of the Southeastern coast, for example, is inextricably linked to fishing and the seashore.

“We have our own language, we have our own food ways, we have our own ecological system here,” said Marquetta Goodwine, the elected head of the Gullah Geechee people and a leader in efforts to protect and restore the coastline. That distinctive culture, she said, depends on things like the oyster beds, the native grasses and the maritime forests that characterize the seashore and the scores of tidal and barrier islands here, collectively known as the Sea Islands.

“You don’t have that, you don’t have a Sea Island,” said Ms. Goodwine, who also goes by Queen Quet. “You don’t have a Sea Island, you don’t have Gullah Geechee culture.”

A Poorer Ocean

The new study tries to measure just how much various human-caused pressures are squeezing, shifting and transforming coastal and marine habitats.

The research began in the early 2000s, when widespread coral bleaching was raising alarm among marine scientists. In response, Dr. Halpern and his colleagues set out to map the parts of the ocean that were healthiest and least affected by humans and, conversely, which parts were the most affected.

The inherent challenge was comparing marine habitats, from coral reefs to the deep ocean floor, and their responses to different human activities and pressures, like fishing and rising temperatures, all on a common scale. They came up with what researchers call an impact score that’s based on a formula incorporating the location of each habitat, the intensities of the various pressures on that habitat, and the vulnerabilities of each habitat to each form of pressure.

Under the world’s current trajectory, the study found, by the middle of the century about 3 percent of the total global ocean is at risk of changing beyond recognition. In the nearshore ocean, which most people are more familiar with, the number rises to more than 12 percent.

That future will look different in different regions. Tropical and polar seas are expected to face more pronounced effects than temperate, mid-latitude ones. Human pressures are expected to increase faster in offshore zones, but coastal waters will continue to experience the most serious effects, the researchers forecast.

There are also countries that are considered more vulnerable because they depend more heavily on resources from the ocean: Togo, Ghana and Sri Lanka top the list in the study.

Across the whole ocean, scientists generally agree that many places will look ecologically poorer, with less biodiversity, Dr. Halpern said. That’s mainly because the number of species that are resilient against climate change and other human pressures is simply far fewer than the number of more vulnerable species.

The study found that the biggest pressures, both now and in the future, are ocean warming and overfishing. But the researchers most likely underestimated the effects of fishing, they wrote, because their model assumes that fishing activity will hold steady rather than increase. They also focused only on the species actually targeted by fishing fleets and did not include by-catch, the unwanted species swept up in gear like gill nets and discarded, or habitat destruction from bottom trawling.

The effects of some other human activities aren’t well represented either, including seabed drilling and mining, which are expanding quickly offshore.

Another limitation of the Science study is the fact that the researchers simply added together the pressures from human activities in a linear way to arrive at their estimate of cumulative effects. In reality, those effects might add up to more than the sum of their parts.

“Some of these activities, they might be synergistic, they might be doubling,” said Mike Elliott, a marine biologist and emeritus professor at the University of Hull in England who was not involved in the study. “And some might be antagonistic, might be canceling.”

Even so, Dr. Elliott said he agreed with the broad conclusions of the new study. Scientists could argue about whether the cumulative effects of human activities will double or triple, he said, “but it will be more, because we’re doing more in the sea.”

“If we wait until we’ve got perfect data,” he added, “we’ll never do anything.”

‘Time to Scale It Up’

One of the benefits of such studies is that they can help inform better ocean planning and management, including initiatives like 30×30, the global effort to place 30 percent of the world’s land and seas under protection by 2030.

In South Carolina, one place that has already been set aside is the ACE Basin, a largely undeveloped 350,000-acre wetland on the state’s southern coast that is named for the Ashepoo, Combahee and Edisto rivers, which thread through it.

Riding a boat across the enormous basin can be disorienting. The world flattens as the sun beats down and salt marsh stretches in every direction. Almost everything is a vivid blue or green, like an abstract painting or a map come to life.

White wading birds dot the green marsh grasses, and occasional groups of gray bottlenose dolphins break the blue surface of the water.

Sometimes the dolphins corral their fish prey onto the mud and temporarily beach themselves for a meal, using the salt marsh islands like giant dinner plates. This behavior, called strand feeding, is rarely seen outside the Southeast.

On a recent visit, in one tucked-away corner of the marsh, something emerged from the mud at low tide: a wall, built with concrete blocks now nearly hidden by thousands of shells. They’re called oyster castles, and they look like something out of a storybook about mermaids.

The blocks were placed by volunteers from the Boeing assembly plant in nearby North Charleston. The effort was organized by the Nature Conservancy and the South Carolina Department of Natural Resources as part of a growing string of living shorelines projects, which aim to stabilize the coast using natural materials like shellfish and native vegetation, in South Carolina and beyond.

The oyster castles are meant to protect the landscapes behind them from erosion, sea level rise and storm surges. Scientists from the Nature Conservancy have been experimenting with a variety of methods for years, and are beginning to see results. Behind the oyster castles, which allow water to pass through and deposit sediment, mud had piled up significantly higher than elsewhere. And in the mud, marsh grass has taken root and grown tall.

“We’ve been testing and piloting things for so long, and now is the time to scale it up,” said Elizabeth Fly, director of resilience and ocean conservation at the Nature Conservancy’s South Carolina chapter.

In fact, the state’s oyster shell recycling program has now built small living shorelines at more than 200 sites, all with the help of volunteers, and often working with other groups, like the Gullah Geechee Nation. There’s a living shoreline taking shape at the Charleston wastewater treatment plant. Another at the entrance to the exclusive Kiawah Island Golf Resort. They’re at Marine Corps bases, at boat launches and at docks.

Many of these efforts are part of a sprawling network called the South Atlantic Salt Marsh Initiative, which includes the Pew Charitable Trusts, the Department of Defense, other federal agencies and state governments. The network spans one million acres of salt marsh across four Southeastern states.

Amid those efforts to reinforce and protect marine ecosystems, and as scientists work to better understand the pressures that are altering the oceans, people in coastal communities everywhere are already living changes large and small.

The day after Mr. Atkins demonstrated his fishing methods, the town of Mount Pleasant, S.C., 80 miles up the coast, held its annual Sweetgrass Festival to celebrate the region’s traditional Gullah Geechee baskets. Dozens of artists braved the heat in booths at a waterfront park, showing off and selling baskets woven from sweetgrass, bulrush, palmetto leaves and pine needles.

One artist and teacher, Henrietta Snype, displayed baskets made by five generations of her family, from her grandmother down to her own grandchildren.

Ms. Snype started making baskets at age 7. Now, at 73, she takes pride in upholding the tradition and teaching others the craft and its history. But she feels the world around her changing.

She said she had noticed the climate shifting for many years now. Big hurricanes seem to have become more frequent and seem to do more damage. And making baskets is harder, too.

Traditionally, the men in basket-making families went out into the dunes, marshes and woods to gather the materials they needed. But lately, Ms. Snype said, the plants have been harder to find. Sweetgrass is diminishing, and harvesters have trouble getting access to built-up and privately owned parts of the coastline.

“The times bring on a lot of change,” she said.

In 2003, the Nobel Prize winner David Baltimore, then president of Caltech, paused to reflect on his role as one of the world’s most decorated scientists.

“People keep e-mailing me to ask, ‘What is the meaning of life?’” Baltimore told an interviewer, with amusement. “And they want me to e-mail them back quickly with an answer!”

Baltimore was then 65, an age when many people are retired from public life, yet he was still actively leading one of the world’s top research universities. Others, he said, found their meaning “in friends, in dogs, in religion, in the self-reflectiveness of writing, etc. But Caltech people largely find it in the continual contest with nature.”

It was a contest that Baltimore waged right to the end of his life as a scientist, businessman and internationally respected conscience of the new world of biological engineering. He died Saturday at his home in Woods Hole, Mass., according to his wife, as reported by the New York Times. Baltimore was 87.

His death concludes one of the most illustrious careers in 20th century science. The bearded scientist with the penetrating blue eyes played a role, usually a leading one, in virtually every important national debate over the use and potential misuse of the science of genetic engineering, whether it was gene-splicing and the search for an AIDS vaccine, or the dangers of tinkering with the human genome.

But it was as a working scientist that he made his most enduring contributions, the role he was most proud of.

“When you are a scientist, and you are trying to prove or disprove a notion, you work at the bench doing the dullest, most routine things over and over and over again,” Baltimore once explained.

“I can’t tell you how many ways things go wrong. All the time you are doing this because there is an idea behind it.”

In a statement, Caltech President Thomas Rosenbaum said Baltimore’s “contributions as a virologist, discerning fundamental mechanisms and applying those insights to immunology, to cancer, to AIDS, have transformed biology and medicine.”

“David’s profound influence as a mentor to generations of students and postdocs, his generosity as a colleague, his leadership of great scientific institutions, and his deep involvement in international efforts to define ethical boundaries for biological advances, fill out an extraordinary intellectual life,” he added.

David Baltimore was born March 7, 1938, in New York City, the son of a garment industry merchant, Richard Baltimore, and Gertrude Lipschitz-Baltimore.

Richard’s family was Orthodox Jewish, from Lithuania, and though the Baltimores in America were not overtly religious, the family communicated a moral code that influenced their son’s concern for the underprivileged.

This led him to take public stands on social issues, such as the AIDS epidemic and nuclear proliferation, that other scientists shunned. In 1970, while performing experiments that would win him the Nobel Prize, he shut down his lab for a week and joined demonstrators in Boston against the Vietnam War-era invasion of Cambodia.

In high school, Baltimore enrolled in a summer program at the prestigious Jackson Laboratory at Bar Harbor, Maine, where he made a discovery that altered his life and set him on the path to science.

“It was the process of research. I discovered that I could investigate the unknown as a high school student, that the frontier of knowledge was actually very close and very accessible,” he said, many years later.

After graduating from Swarthmore College, Baltimore earned his doctorate from the Rockefeller Institute (now University), before doing three years of research at the Salk Institute in La Jolla, where he met his future wife, Alice Shih Huang. His postdoctoral student, Huang collaborated in his research on animal viruses, later becoming a full professor at Harvard Medical School. At this time, Baltimore was particularly interested in the poliovirus, which attacks the RNA (ribonucleic acid) in cells.

“He was on the cutting edge of molecular biology,” said science historian Daniel Kevles, his friend and colleague. “There was no molecular biology to speak of and very little virology. … It was a brave field of work.”

At the time, it was an ironclad rule in molecular biology that genetic information was a one-way street, flowing from the double-helix structure of DNA to the single-stranded RNA, which the cell’s machinery uses to make proteins. But some biologists were beginning to question that assumption, and Baltimore joined the hunt for evidence that genetic information might flow in both directions, which, if true, held enormous potential for understanding the spread of viruses.

After leaving the Salk, Baltimore returned to Boston and became an associate professor of microbiology at MIT. As it became apparent that not all viruses behaved alike, Baltimore launched a new classification system, one that is still in use, grouping them by families according to their genomes and replication systems.

It was during this work that he discovered an enzyme that enabled a virus made of RNA to be copied into DNA, a process known as reverse transcription. The discovery of reverse transcriptase was greeted with overheated predictions that science had at last found a cure for cancer. The thinking went, if one could use RNA to code DNA, scientists could seize control of the body’s defenses.

Baltimore knew his work did not augur a cure for cancer, but the discovery of reverse transcriptase was nonetheless important because it led to an understanding of how genes can modify cells, turning normal cells into cancer cells. Reverse transcriptase is also used by a unique family of viruses, known as retroviruses, to replicate themselves. This finding would be critical to understanding the AIDS virus, HIV, which is a retrovirus, and devising anti-HIV treatments.

Baltimore’s discovery was attended by great fanfare and led to his promotion to full professor at MIT. In 1973, he was awarded a lifetime research professorship by the American Cancer Society, and a year later was elected to the National Academy of Sciences and the American Academy of Arts and Sciences. Finally, in 1975, with Howard Temin, a friend and colleague who had discovered reverse transcriptase around the same time, Baltimore was awarded the Nobel Prize for physiology or medicine.

With the prize came fame; people began referring to Baltimore as the most influential biologist of his generation. To the general public, who did not necessarily understand what he had done, only that it was important, he became, at the age of 37, a full-fledged savant.

The award had a profound effect on colleagues.

“I don’t see it as a burden, but you can’t get away from it,” Baltimore said. “I know that when I talk to young scientists, they are looking at me and saying, ‘God, I am talking to a Nobel Prize winner.’ I try to break that down. It gets harder every year.”

His new celebrity status gave him a platform to address issues of broad cultural and scientific importance, a role Baltimore embraced.

In the 1970s, when people became concerned that gene-splicing techniques could lead to the production of super viruses, Baltimore organized a conference at Asilomar near Monterey to design a self-regulating system to monitor those experiments. In the early 1980s, he led the fight against a crash program to map all human genes, fearing, once again, unknown consequences. In each case, when it was shown the dangers had been overestimated, he then led the effort to relax federal restrictions. He became an early champion of federal AIDS research and chaired a national commission that concluded the federal government’s response to the epidemic was dangerously inadequate.

As his reputation grew, he took leadership roles on political issues. When Pope John Paul II wanted to warn President Reagan of the danger of nuclear weapons, Baltimore was one of four scientists the pontiff appointed to carry his message.

In 1984, Baltimore was chosen founding director of the new Whitehead Institute for Biomedical Research, molding it into one of the world’s leading institutions of its kind. Following that success, he was appointed president of the Rockefeller University.

Along the way, he became not only a respected link between the government and scientists but also a key player in the burgeoning biotechnology industry. His early involvement in the industry made him a “relatively wealthy man,” according to a 1997 Times magazine profile.

The profile described a man in the fullness of middle age, harvesting the benefits he had earned, drinking the best wines and single-malt scotch, driving appropriately luxurious but not ostentatious vehicles. “With his wife, Dr. Alice Huang, he shares a luxury duplex condominium on Union Wharf, which has a commanding view of Boston Harbor,” it said.

In person, “Baltimore’s practiced elegance frames a fierce pride and a sometimes brutal intellect, softened only by his insistence that professional criticism be leavened by personal respect.”

And then, the entire edifice crumbled as Baltimore became the focus and fall guy for one of the more infamous investigations of scientific misconduct in the last half of the 20th century. A colleague wrote a paper claiming sensational results. When others could not reproduce those results, allegations of fraud were aired, causing Congress to get involved. With the decline of the space program, biology had emerged as the preeminent science, and Congress was becoming skeptical about how millions of dollars in federal research grants were being spent.

The whiff of scandal was attached to Baltimore himself, even though his work was never questioned. Still, his refusal to admit error, or to abandon his problematic colleague, came to symbolize for many the arrogance of the new mandarins of the biological sciences.

“The Baltimore case is reminiscent of the Watergate scandal,” the New York Times wrote.

Four federal investigations and a grand jury probe later, Baltimore’s colleague, and Baltimore himself, were exonerated. The ordeal had consumed a decade of his life. Then, within months, everything changed. He was chosen to coordinate the federal effort to develop an AIDS vaccine and then appointed president of Caltech. It was a breathtaking reversal of fortune.

“It is even more breathtaking,” Baltimore said in 1997, shortly after taking the Caltech job, “to live through it.”

Kevles, a professor at Caltech at the time, recalled that when Baltimore’s name was announced to the assembled faculty, “the room erupted in cheers. I had never seen the biologists look so ecstatic. It legitimized their field.”

In his eight years as president, Baltimore raised the university’s profile, both as a place where cutting-edge biology is done and as a respected voice on pressing national scientific debates. Under his leadership, Caltech raised more than $1.1 billion. He cited the gift of $600 million to the school by Intel Corp. co-founder Gordon Moore and his wife, Betty, as the “decisive moment” of his presidency.

“Caltech is a wonderful place, the best place to do science I have ever seen,” Baltimore said in 2005, when he announced his resignation. “I will have done what I can do [as president], and it is time for somebody else to be thinking about it.”

As for what would come next, Baltimore said, “I have a fairly extensive life in science and in business that I will pursue.”

If he thought his return to the laboratory would be a placid coda to his career, he was soon proved wrong, by yet another advance in genetic engineering, this one called CRISPR. “I’ve seen revolution after revolution in biology,” Baltimore said in 2016. “This one is a big deal.”

As one writer noted, if the gene-splicing technology of the 1970s spurred images of laboratory-hatched plagues from the “Andromeda Strain” novel and movie, CRISPR inspired comparisons to “Brave New World.” MIT’s Technology Review wrote of labs in which “man rebuilds creation to suit himself” and warned of “a path toward a dystopia of superpeople.”

Just as he did decades earlier, Baltimore took a leadership role in starting a public discussion about how to manage the powerful new tool. “At Asilomar, we had identified the genetic modification of humans as the biggest coming issue,” Baltimore said. “We just didn’t know when it would come.”

A statement drafted by participants at a meeting in Napa in early 2015 spoke of the promise of “curing genetic disease” but also warned of “unknown risks to human health and well-being.”

The statement listed 18 authors, with Baltimore at the top. Though he wrote an op-ed for the Wall Street Journal entitled, “Let’s Hit ‘Pause’ Before Altering Humankind,” Baltimore admitted later that genome-editing would in all probability take place sooner rather than later.

After retiring as president of Caltech, he remained on staff in an emeritus capacity, and was appointed the Robert Andrews Millikan professor of biology. He finally shuttered his lab in 2019 but remained active in business. He helped found a number of companies, including Calimmune and Immune Design, which carried on the work he began in immunology and virology. Though he was most visible for his public advocacy of cancer and AIDS research, it was his work as a “lab-based, working biologist” that gave him the most pleasure, and for which he hoped to be remembered.

Besides the Nobel Prize, he received the National Medal of Science in 1999, and the Warren Alpert Foundation Prize in 2000. He was the 1999 recipient of the National Medal of Science and published more than 700 peer-reviewed articles.

He was also a member of numerous scientific advisory boards, including Amgen, the Broad Institute, Ragon Institute, and Regulus. Baltimore was past-president and chair of the American Assn. of the Advancement of Science.

He is survived by his wife, Alice, and daughter, T.K. Baltimore.

Johnson is a former Times staff writer. City News Service contributed to this report.