This story was originally published by Yale E360 and is reproduced here as part of the Climate Desk collaboration.
In the coastal waters off British Columbia, tribal volunteers from the Haida Nation dive for purple sea urchins amid a dense forest of rippling golden-brown kelp fronds. Sunlight filters through the canopy, creating a mesmerizing dance of light and shadow, as rays and sea lions wend through the kelp maze, sharks glide past, and bright orange garibaldis dash between the swaying fronds.
Kelp forests are biodiversity hot spots teeming with a colorful variety of seaweeds, sponges, crustaceans, and other small ocean animals, many of them found nowhere else. At one time, vast kelp beds grew in nutrient-rich shallow waters along roughly a third of the world’s coastlines, where they helped to reduce the strength of waves, minimized coastal erosion, and provided shelter to fish, invertebrates, and marine mammals.
Today, however, many kelp forests are on life support, victims of water pollution from terrestrial agriculture and coastal development, bottom trawling for fish, and an explosion of kelp-devouring urchins, like those the Haida volunteers are collecting as part of an eradication program. But perhaps the most important driver of kelp decline is the rapid warming of the ocean.
Healthy kelp forests need cool, nutrient-rich seawater to survive. As ocean waters warm, kelp can no longer inhabit parts of their former range. The crisis is escalating quickly. Kelp forests are vanishing twice as fast as coral reefs and four times faster than tropical rainforests. An estimated 40 percent to 60 percent of kelp forests worldwide have been lost or significantly degraded in the last 50 years. These precipitous declines typically received far less scientific scrutiny than higher-profile ecological crises. But kelp has gradually been getting more attention as scientists and the environmental community come to recognize the value of the carbon that coastal ecosystems, including kelp forests, can capture.
A 2023 literature review of more than 180 papers that examined the potential for kelp to store carbon suggested that the climate benefits of these underwater forests may have been “grossly underestimated,” says Albert Pessarrodana, a research fellow at the University of Western Australia and the review’s lead author. “Kelps are one of the fastest growing plants on the planet,” he said in an email interview, “uptaking as much carbon as tropical rainforests per unit of area.”
Much of the carbon that kelp sequesters ends up being released back into the marine environment in the form of leaf litter (kelp, which is a macroalgae, has leaflike structures called blades that are its organs of photosynthesis.) This detritus is typically ingested by fish and other marine organisms and excreted in a matter of days. Still, a small percentage of it ends up in the deep ocean where it remains for centuries, or even millennia. Roughly 62 million tons of carbon is carried into the deep ocean by coastal currents each year, according to two studies published in 2024.
“Unfortunately,” Pessarrodana says, “excessive warming can either kill kelps or severely curtail their growth, reducing their ability to uptake carbon.”
To counter the decline, scientists around the world are breeding new kelp varieties that they transplant as saplings into kelp habitat. The Scripps Institution of Oceanography and the San Diego Zoo have begun using artificial intelligence to digitally replicate kelp ecosystems and assess their vulnerability to climate change. Their findings will help focus resources on areas that are most likely to be saved. The organizations have also established a biobank to preserve kelp varieties for potential use in kelp farming and restoration projects. Researchers at the Woods Hole Oceanographic Institution on Cape Cod are using selective breeding to develop kelp strains with a higher tolerance to warming waters. Similar efforts are underway in China and Australia.
The Nature Conservancy’s Scott Breschkin has been working for the past year to eliminate urchins and replant beds of golden kelp on Australia’s Great Southern Reef, an interconnected system of rocky kelp reefs spanning about 5,000 miles of coastline across southern Australia and the island of Tasmania. Though less famed than the Great Barrier Reef, it is equally biodiverse, boasting thousands of species, some of which are still unknown to science.
As Australia’s coastal waters warm, long-spined urchins are expanding their ranges, leaving virtually lifeless urchin barrens in their wake. “Once the reef transitions to an urchin desert, it is very hard to flip it back to a productive kelp habitat,” Breschkin explained, adding that urchins can persist for decades in a zombie-like state, awakening only occasionally to mow down any kelp sprouts that may appear, which makes it virtually impossible for kelp forests to recover. Eradicating sea urchins, he says, is a critical first step for kelp restoration.
Jono Wilson, the director of ocean science for The Nature Conservancy’s California chapter, works with KelpWatch.org, a partnership of academic institutions and government agencies that is using satellite imagery and drones to monitor the distribution of canopy-forming kelps along California’s Pacific coast and assess where kelp restoration efforts have been successful. These undersea forests are often characterized by boom-and-bust cycles, flourishing and retreating as ecological conditions change, Wilson says. But recent climate-driven losses have been unprecedented. A 2013 to 2015 ocean warming event known as “the Blob” reduced kelp populations in Northern California by 95 percent.
Since 2015, water temperatures in California’s kelp forests have not dropped below 57 degrees F, a rough threshold beyond which kelp cannot thrive. Higher temperatures disrupt the kelp reproductive cycle, affecting their ability to produce viable offspring. Like corals, kelp bleaches when stressed, losing the chlorophyll that allows it to photosynthesize.
Kelp diebacks transform their ecosystems. “Kelp provide habitat and food for thousands of species,” says Wilson. “They are nurseries for abalone and economically important fish species like cod and rockfish. They are meccas for kayakers and recreational scuba divers.” The Nature Conservancy estimates that kelp forests contribute $250 million in economic value to California annually.
While kelp forests in Southern California are faring reasonably well — the dominant species, called giant kelp, grows quickly and can reach up to 200 feet tall — purple sea urchins are wiping out kelp beds dominated by bull kelp along the coasts of Central and Northern California. Wilson and his team are exploring ways to control those urchins. They are developing new kinds of more efficient urchin traps — round mesh devices baited with fish — and working with fertilizer companies to create a viable market for urchin shells, which contain calcium, nitrogen, and other plant nutrients. The Norwegian seafood firm Ava Ocean is currently using crushed urchin shells to produce a mineral-rich alternative to traditional bone-meal fertilizers.
There have also been attempts to support the native sunflower sea star, a voracious consumer of sea urchins. Populations of this predatory starfish, which can grow as large as a car tire, have been reduced by 90 percent since the outbreak, in 2013, of sea star wasting disease.
Scientists recently isolated the bacterium responsible for the epidemic, giving them a better shot at helping sea stars recover. Aquariums in California and Oregon have successfully treated their affected sea stars with antibiotics, and scientists are hoping to learn how to breed disease-resistant sunflower starfish that can be released into the wild.
There have also been efforts since the late 1980s to breed and reintroduce sea otters, another urchin predator, to the coastal waters where they once flourished. Driven to the edge of extinction by the fur trade in the 19th and early 20th centuries, otters have now substantially recovered in parts of their former range. They now number well over 3,000 individuals in Northern California. Otters are also staging a comeback along the coast of Washington and British Columbia. Kelp forests where the otters have been released are faring noticeably better than kelp forests without otters.
So far, kelp restoration projects in the U.S. have been small scale, with most covering less than a hundred acres. Such projects are “very expensive and subject to zoning laws that make [them] hard to initiate,” says Kyle Cavanaugh, a coastal geographer at the UCLA Institute of the Environment and Sustainability. “These localized projects need to be scaled up if we hope to turn the tide on the loss of kelp habitat.”
Kelp restoration is happening on a far larger scale in East Asia. More than half of South Korea’s kelp forests have been lost or badly degraded over the past century. But thanks to the world’s largest kelp restoration projects, the nation now boasts 71,660 acres of kelp forest. Its goal is to eventually re-green 75 percent of its coastline. Farmers in South Korea harvest nearly a billion dollars worth of seaweed each year, cutting only the upper fronds and blades of the macroalgae. There are also more than 700 restoration projects in Japan where kelp, known as kombu, is a staple in Japanese cuisine, most commonly used in soups.
Kelp’s value as a food source, and to a lesser extent as an ingredient in cosmetics, skincare products, and biodegradable packaging, may be one key to its survival. But if we don’t find a way to slash emissions of greenhouse gases, experts say, the long-term prospects of kelp and other key ocean ecosystems may be bleak. “It’s likely that we’ll see more destructive marine heat waves and warmer waters overall,” says Cavanaugh. “Beds of far less-productive turf algae will replace giant kelp and prevent it from becoming reestablished.”
In Maine and in other coastal regions around the world, carpet-like turf algae is already replacing kelp, according to a paper published last month by the Bigelow Laboratory for Ocean Sciences in Maine. This is “a radical ecological transformation,” says the University of Maine’s Shane Farrell, the study’s lead author. “The good news is we now understand what’s driving this shift, and this will help us predict when and where it will happen next and create different conservation strategies to combat it.”
