- Kelp forests function as major habitat for marine biodiversity, but are in rapid decline worldwide, largely because of climate impacts on the oceans.
- A 2013 marine heat wave known as “the Blob,” combined with the mass die-off of sea stars, caused a 95% loss of Northern California’s kelp forests.
- The loss of sea stars allowed the purple urchins that they thrive on to spread rapidly, converting lush kelp forests into “urchin barrens” in parts of California. There’s been very little recovery since.
- Restoration of kelp forests is extremely difficult and requires far more resources than are currently being committed.
FORT BRAGG, U.S. — In 2024, I was scuba diving in Northern California’s Casper Cove where the Watermen’s Alliance, a group of ex-abalone sports divers, has been culling purple urchins since 2020. It had been six years since abalone season shut down, following the region’s kelp forest collapse.
About 4 meters (13 feet) down, I spotted a few surviving red abalones, their thick-muscled feet showing from under their single shells as they clung to the bottom of boulders. Purple urchins (Strongylocentrotus purpuratus) clustered near them. I also spotted a single white anemone (Diadumene leucolena) and a spiny, bulbous-eyed Cabezon fish (Scorpaenichthys marmoratus) amid scattered stalks of remnant kelp.
A short distance away, it was all bare rock and purple pin-cushion-like urchins. It’s different than watching videos of urchin barrens: Aside from being wet and cold and breathing underwater, I was taken by the scale: it’s so expansive, even with only 2 m (6 ft) of visibility in the murky water.
As far as you go along the northern rim of the cove, there are purple urchins, thousands and tens of thousands, where a kelp forest once thrived. I spread my arms to measure what I guesstimated was a square meter and counted some 120 urchins.
To bring kelp back, that number would have to drop to 2 per square meter. That’s when kelp spores can take hold again.
This requires a lot of urchin-smashing, which these divers were carrying out with rock hammers. To clear one acre (less than half a hectare) on the other side of the cove required three years of diving by dozens of volunteers – and the destruction of more than 150,000 urchins.
With funds from a DDT legal settlement, the nonprofit Bay Foundation has eliminated some 6 million urchins over the past decade, restoring close to 32 hectares (80 acres) of kelp forest in Southern California. It’s the most successful restoration in North America to date.
After 55 minutes in 13° Celsius (55° Fahrenheit) water, we surfaced far off the beach, swam in to where we could stand, made our way through meter-high (3 ft) shore break and across a wide beach.
Sonke Mastrup, the retired head of the California Department of Fish and Game’s invertebrate program, pulled his mask and diving hood over his white hair and bristly white beard. “This is the most kelp I’ve seen here since 2014,” he told me. “Basically, the kelp’s coming back where they culled [cleared out the urchins]. I wasn’t sure that would happen.”
Meanwhile, I’m thinking this is the most depressing dive I’ve ever done in California, reminding me of dives I’ve taken on bleached and dying coral reefs in warmer climes.
“It’s just the massive scale of the problem,” Mastrup explained. “I’m an ecologist by training and this is depressing. You do your best to have an impact and then you see something like this.” He shook his head grimly and walked up the beach to his truck to strip off the rest of his cold-water dive gear.
The collapse
The world’s kelp forests are in serious trouble, although few people realize the massive size or importance of these threatened “rainforests of the sea.” They cover an area larger than the Amazon Rainforest and stretch along close to one-third of the world’s coastlines, providing essential habitat for more than 1,000 species of fish, mammals and invertebrates — including commercially sought-after species such as cod, lobster and salmon.
Kelp forests provide some half a trillion dollars’ worth of goods and services a year, according to a 2023 paper in the journal Nature. Kelp also keeps coastlines healthy, providing powerful protection from storms and coastal erosion, and produces up to 20% of the oxygen we breathe (along with seagrasses and mangroves). And then there’s their inherent value and beauty: Kelp forests are an otherworldly wonder of cathedral light and life for those privileged enough to dive in them.
Unfortunately, warming seas, overfishing and pollution have negatively impacted some 60% of wild kelp beds, from Japan to Norway and Tasmania to Northern California — which is the worst-case scenario to date, with a 95% loss of its bull kelp (Nereocystis luetkeana).
Beginning in 2013, a marine heat wave hit the U.S. West Coast, lasting some three years. Scientists dubbed it “the Blob.” Water temperatures spiked, weakening kelp forests in Oregon and California that normally thrive in cold, nutrient-rich water from ocean upwellings and chilly currents.
Then a disease broke out. Sea star wasting syndrome devastated starfish populations all along the Pacific coast, supercharged by the warmer waters, as confirmed by a 2019 paper in Science Advances.
The losses included the sunflower sea stars (Pycnopodia helianthoides) that can grow to the size of a serving platter, weigh up to 5.9 kilograms (13 pounds) and sprout up to 24 arms. This voracious starfish is the main urchin predator in northern California and southern Oregon — where another urchin predator, the sea otter (Enhydra lutris), was extirpated by 18th– and 19th-century fur traders.
The absence of some 99% of sunflower sea stars, which literally melted away within days to weeks of contracting the disease, allowed small purple urchins to proliferate. The direct cause of the wasting disease remained a mystery for more than a decade until this August, when scientists identified the bacteriumVibrio pectenicida as the cause, which they reported in Nature, Ecology & Evolution. By then, many of California’s giant kelp (Macrocystis pyrifera) and bull kelp forests, which can grow more than a foot a day, had been decimated.
An urchin call for action
With the last of their predators gone, purple urchins emerged from their crevices in the ocean’s rocky substrate where they normally hid, living on kelp fragments that drifted past. Their numbers quickly multiplied, growing by an estimated 10,000%.
These voracious kelp eaters took out almost all of Northern California’s heat-weakened bull kelp along roughly 563 kilometers (350 miles) of coastline, while also impacting the giant kelp forests of Central and Southern California. The spiky creatures decimated the complex, nearshore marine kelp forests, replacing them with urchin barrens, leaving the seabed carpeted with small, pastel-colored urchins.
While there has been some natural recovery in Central and Southern California — thanks in part to the presence of some 3,000 sea otters and other predators including large gray-and-red sheephead fish (Bodianus pulcher) and lobsters — Northern California’s bull kelp remains largely a memory more than a decade after the Blob first appeared. The summer of 2025 saw some limited recovery in a few coves where restoration efforts are ongoing, but nothing at significant scale.
It’s incredibly difficult to get rid of purple urchins. After they consume all available kelp and seaweed, they can go into a years-long dormant state of starvation that includes a lower metabolic rate, shrunken gonads and reproductive shutdown. So, these urchin barrens are literally made up of barren urchins that can survive an additional 20 to 40 years or more, reawakening to feed on any new or reintroduced kelp.
That’s why some people call them “zombie urchins.” Although, in truth, the problem lies not with these native globular animals — a species of Echinoids that have been around some 450 million years — but with the disruption caused by fossil-fuel-fired climate change.
Other factors have contributed to kelp’s decline, including overharvesting and runoff pollution that obscures water clarity and blocks sunlight, interfering with photosynthesis. But marine heat waves are responsible for most of today’s kelp loss.
Too hot to handle
Meanwhile, the frequency and intensity of these heat waves is increasing as the ocean continues to warm: 90% of human-generated atmospheric heat is being absorbed by the ocean. That’s why 2023 saw an unprecedented increase in global marine heat waves that lasted four times longer than average, according to a recent study published in the journal Science.
Given that climate change produces winners and losers (though far more of the latter), South Africa and Norway have been winners. Both countries have seen a bit of climate-linked kelp recovery. In South Africa, changing winds have brought increased cold water upwellings, and in Norway’s case, it’s expanded habitat for urchin-eating crabs.
The decline of wild kelp has also sparked unprecedented collaboration between normally fractious groups. In the working waterfront town of Fort Bragg, California, for instance, I spoke with commercial fishermen, divers and scientists, Indigenous leaders, environmental activists and members of the Coast Guard and NOAA who have all begun to work together to restore their lost kelp ecosystem and the cultures and livelihoods it supported.
These collaborative efforts have included culling urchins and selecting larger purple urchins for rearing as seafood, in places including a converted shipping container on the Fort Bragg waterfront.
Marine labs in Bodega Bay, California, and elsewhere are growing kelp and out-planting spores and sprouts in the ocean on ropes or on gravel. Other labs are rearing sunflower sea stars for reintroduction to the wild. An initial release took place in Washington’s San Juan Islands in 2024. The most controversial of these efforts, however, is the proposed reintroduction of sea otters to their historic range, despite objections from fishermen.
These approaches have shown some promising results, but not at the scale needed. They have proved more fruitful where major financial resources have been committed to the effort. Southern California’s 32-hectare restoration has cost more than $4 million to date.
South Korea’s fisheries agency invests $29 million a year to ensure marine health and food security. It has restored 20,000 hectares (about 50,000 acres) since 2009, with a goal of 50,000 hectares (125,000 acres) by 2030.
The Australian-based Kelp Forest Alliance wants to see 1 million hectares (2.5 million acres) restored and 4 million hectares (10 million acres) protected by 2040.
Unlike coral restoration, where scientists and diver technicians are thrilled to see a reintroduced branching coral grow 30 centimeters (12 inches) in a year, it’s not uncommon for a restored giant kelp stipe (stem) to grow from 5 up to 30 m (15-100 ft) in a single year.
Jared Huffman is a former environmental attorney and member of Congress representing California’s 2nd congressional district, which encompasses all the coastal counties north of San Francisco. He’s also the top Democrat on the House Natural Resources Committee. “I think I have the most important kelp district in America,” he says. “I’ve seen when you have a healthy kelp forest what it looks like. Towns like Fort Bragg — when there’s abalone diving underway, when the kelp forest is healthy and everything is thriving — it’s a different place than it is during the last few years when all the kelp is gone.”
He’s introduced a “Help Our Kelp” bill that would mandate $5 million a year for kelp forest research and restoration. This would be about one-sixth of what South Korea is investing but is still significant. Almost no one else on Capitol Hill is presently raising the issue of the U.S. or global kelp crisis or calling for legislative action.
I ask if he thinks his bill is going to go anywhere during this second Trump term. “[Republicans] have talked about giving it a hearing and putting it on the ‘markup’ agenda for committee members to work on before sending it to the full House for a vote. This is not a radical issue to the House Republicans, at least so far … It’s not one of these bills that I’d say we’re just doing as a ‘messaging bill’ that we [Democrats] have no chance of passage. I think we’ve got a real chance with this one.”
Sheila Semans, executive director of the Noyo Center for Marine Science in Fort Bragg, is working on kelp restoration and building a more sustainable blue economy for her city of 7,000. After generations of logging and fishing, the city saw its timber mill close in 2002 and much of its fishing fleet disappear.
“The kelp crisis will be a focus of attention for years to come,” she told me at their field station in Noyo Harbor, a converted café with a “Kelp Wanted” sign in the window. “We’re not bringing back the kelp like we used to have it. I used to get nervous going abalone diving because the kelp was so thick. But there is nothing so beautiful as diving in a kelp forest, and I worry I may not get to do that again in my lifetime.”
“Fort Bragg was damaged and has not been healed,” adds Javier Silva, a member of the Sherwood Valley band of Pomos (a California Native American tribe) who have lived in the region since time immemorial. “A lot of the people who made the money aren’t here anymore. They left, and left nothing. After the redwoods and abalone and salmon and crabs and kelp were taken, all they left behind was the garbage.
“In the current climate change reality, we all, everyone, has to look at changes that can and will happen in their lives,” he continued, as we walked along Noyo Harbor Beach. “You can be on the ocean, but you can’t believe that means you own it.”
Silva argued that rather than believing we own the ocean, we need to understand how much we owe the ocean. The fight to restore California’s — and the world’s — depleted kelp forests may be one of the multigenerational efforts needed to begin to pay back that debt, and perhaps resurrect this coast’s marine ghost forests.
Banner image: Kelp forests are one of the world’s most important ecosystems, providing homes and nurseries for innumerable species. Image courtesy of Monterey Bay Aquarium.
David Helvarg is an author and founder of Blue Frontier. He’s executive producer of the documentary ‘Sequoias of the Sea.’ His next book, ‘Forest of the Sea: The Remarkable Life and Imperiled Future of Kelp’ will published by Island Press in spring 2026.
More to explore:
Eger, A. M., Marzinelli, E. M., Beas-Luna, R., Blain, C. O., Blamey, L. K., Byrnes, J. E. K., … Vergés, A. (2023). The value of ecosystem services in global marine kelp forests. Nature Communications, 14(1894). doi:10.1038/s41467-023-37385-0
Harvell, C. D., Montecino-Latorre, D., Caldwell, J. M., Burt, J. M., Bosley, K., Keller, A., … Gaydos, J. K. (2019). Disease epidemic and a marine heat wave are associated with the continental-scale collapse of a pivotal predator (Pycnopodia helianthoides). Science Advances, 5(1). doi:10.1126/sciadv.aau7042
Prentice, M. B., Crandall, G. A., Chan, A. M., Davis, K. M., Hershberger, P. K., Finke, J. F., … Gehman, A. M. (2025). Vibrio pectenicida strain FHCF-3 is a causative agent of sea star wasting disease. Nature, Ecology & Evolution. doi:10.1038/s41559-025-02797-2
Dong, T., Zeng, Z., Pan, M., Wang, D., Chen, Y., Liang, L., … Zhang, D. (2025). Record-breaking 2023 marine heatwaves, Science, 389(6758). doi:10.1126/science.adr0910
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