A few miles west of San Diego is a stretch of ocean that’s rather unremarkable from the surface. The water is cold and blue. There’s some green seaweed peeking out.

Sink below the waves, however, and a whole other realm appears. Under the sea here, near Point Loma, is a forest as beautiful as any other. It’s made not of trees but of strands of giant kelp, a species of algae that can grow taller than a 10-story building.

Tethered to the seafloor and buoyed by air-filled chambers, the kelp strands undulate with the current, moving in slow motion. Schools of fish, seals, and other aquatic critters weave through the stalks like birds through a forest canopy.

“Diving into a forest is like descending into a cathedral,” Jarrett Byrnes, a marine ecologist at the University of Massachusetts Boston, said of kelp forests. Light from the surface filters through blades of kelp as if they’re stained glass, said Byrnes, who’s been diving kelp forests for more than 20 years. “It’s just amazing.”

A forest of bull kelp near Carmel-by-the-Sea in California in the fall of 2022.

Our planet has a number of great forests — the Amazon, for example, or the boreal forest of Canada and Russia. These iconic ecosystems not only support an incredible diversity of wildlife but store vast amounts of carbon that might otherwise heat up the planet. It’s not hyperbole to say that our existence depends on them.

But equally important are the forests of the sea. Found in cold waters across roughly a quarter of the world’s coasts, kelp forests are the foundation of many marine ecosystems. They underpin coastal fisheries, helping sustain the seafood industry. They also absorb enormous amounts of pollution and help sequester planet-warming gases. A recent study valued these benefits at roughly $500 billion a year, worldwide.

Yet for all they are worth, scientists know surprisingly little about kelp forests. Globally, data on how they’re responding to climate change and other threats, such as the spread of non-native species, is incomplete. Conservation efforts — which have been ramping up in recent years, especially on land — have largely overlooked these marine environments.

What biologists do know suggests that many of these forests are in trouble. And a lengthy new review published this week by the United Nations indicates that kelp forests have declined globally. “Kelp have suffered widespread losses across much of their range,” the report states — and climate change stands to make things worse.

The full story, however, is much more complicated.

A spectacular undersea world that we all rely on

Kelp forests have a lot in common with their land-based counterparts. They form three-dimensional structures that provide homes to animals. They often have a canopy. And kelp stalks themselves look a bit like trees: They have root-like anchors, a central structure similar to a trunk, and leaf-like blades.

Yet there are a few key differences. For example, kelp is not a plant but a kind of algae, a group of aquatic organisms in an entirely different kingdom of life (home to things like pond scum and Florida’s red tide). They also grow far faster than trees — as fast as two feet per day, depending on the species.

Young bull kelp growing near Carmel-by-the-Sea in April.

A curious harbor seal checks out photographer Jennifer Adler during a dive near Monastery Beach in Carmel.

Some kelp species, like the giant kelp common off the coast of San Diego, reach the surface to form a canopy. Others top off many feet down, creating an understory. And these forests are quite widespread, covering an area of ocean up to five times greater than that of all coral reefs, according to the new report by the United Nations Environment Programme (UNEP).

These forests support a stunning diversity of life. Kelp blades and anchors (known as holdfasts, the closest thing they have to roots) provide shelter for young fish, a place for adult fish to spawn, and food for invertebrates like urchins and other creatures. One study found that a single stalk of kelp in Norway supported roughly 80,000 organisms across 70 distinct species. Over 1,000 species of plants and animals are found in some kelp forests in California.

The distribution of common varieties of kelp (in green) around the world.United Nations Environment Programme

This is especially relevant for people who eat seafood. Research has demonstrated that many popular commercial species including pollack, lobster, and abalone spend at least part of their lives in kelp forests and depend on their existence. A new study in the journal Nature illustrates just how valuable these environments are to the seafood industry: A single hectare of forest contributes an average of roughly $30,000 a year to fisheries, the authors found.

The benefits don’t stop there! As they grow, kelp forests, like those on land, absorb a lot of pollution including fertilizer runoff from farmland and compounds of carbon (much of which enters the ocean from the atmosphere). They’re helping offset much of our planet-warming emissions, for free. The Nature study, which examined a handful of services that kelp provide, conservatively estimates that these habitats sequester at least 4.9 megatons of carbon from the atmosphere each year. “Pound for pound,” kelp remove as much as (or more) carbon than other ecosystems, such as terrestrial forests or mangroves, the study’s lead author, marine scientist Aaron Eger, told Vox.

Combined, the services provided by kelp forests globally — supporting fisheries, cleaning up pollution, and sequestering carbon — are worth half a trillion dollars a year, the study found. And that doesn’t take into account other potential benefits, such as coastal protection (kelp forests may tamp down waves, helping limit the impact of storms, the UNEP report found).

In short: Kelp help.

A forest of bull kelp near Carmel-by-the-Sea in the fall of 2022.Jennifer Adler

Are kelp forests at risk?

The simple answer is yes.

The most recent global analysis, based on data through 2012, found that global kelp forests are declining on average at a rate of about 1.8 percent per year. A more recent review that only considers long-term data (which is more reliable; kelp forests can vary a lot from year to year) points to a more troubling trend. It finds that more than 60 percent of the kelp forests scientists have studied over a period of 20 years or more have declined.

“On the whole, when we look at kelp forests over a long time period and in temperate latitudes, we’re seeing strong declines,” said Kira Krumhansl, a marine ecologist who led the global analysis and was a co-author of the more recent review. Those declines are most severe in regions closer to the equator where the water is warmer, such as Baja California, Western Australia, and southern New England, said Krumhansl, who works at Fisheries and Oceans Canada.

This pattern points to a major force behind shrinking kelp forests: climate change.

Giant kelp can grow up to two feet per day.

Kelp like to grow in cold, nutrient-rich water, yet rising global temperatures — fueled by power plants, gas-powered cars, and so on — are making the oceans warmer and fueling epic marine heat waves. That has pushed some kelp forests past their thermal limits, especially if they’re already in the warmer reaches of their range.

An added twist is that warm waters tend to hold fewer nutrients, which makes it harder for forests to grow, according to Byrnes, the biologist at UMass Boston.

Another reason why kelp forests have declined is overfishing and the loss of marine predators. Cod, lobster, and sea otters, among other animals, prey on sea urchins. Urchins, in turn, graze on kelp. When fishing nets capture urchin predators, urchins proliferate and mow down kelp forests.

Purple sea urchins devour stalks of kelp near Portuguese Beach in Mendocino, California.

As urchins mow down forests of kelp, they create “urchin barrens.”

This process can create what are called urchin barrens, eerie stretches of sea floor covered in little more than prickly orbs. You can find these barrens all over the world, from California to Tasmania to Japan. (Degraded kelp forests are also increasingly being replaced by mats of algae that form a turf on the sea floor and prevent the kelp from recovering.)

Two things that make the story of kelp more complicated

Although research shows that, on average, kelp forests are declining worldwide, some marine biologists are hesitant to make sweeping conclusions about the global trend.

One reason why is that kelp forests vary dramatically from place to place. Many forests have eroded or vanished entirely, though some seem to be fine or are even expanding. “Every spot on Earth has a different story,” said Tom Bell, a marine scientist at the Woods Hole Oceanographic Institution.

Stalks of bull kelp grow near Carmel-by-the-Sea, California, in fall of 2022.

A recent study co-authored by Bell documented steep declines in kelp forests along the West Coast, following a spate of intense marine heat waves between 2014 and 2016. While forests in parts of Northern and Central California have yet to recover these losses, some of those off the coast of Oregon have grown substantially.

“In some locations, we found jaw-dropping recovery in canopy-forming kelps,” said Vienna Saccomanno, a co-author of the recent study and ocean scientist at The Nature Conservancy, an environmental group. “These places are important glimmers of hope.”

Kelp forests also appear to be growing in parts of the Arctic. The ocean there is warming, making the water more tolerable for kelp (yet still cold enough for these algae to survive). Melting ice, meanwhile, frees up space for forests to take root. But this trend is not universal or well understood. Melting ice can also make the water cloudier, potentially limiting the growth of kelp forests.

Some parts of the ocean, including the coast of South Africa, are also anomalously cooling, causing kelp forests to increase, Byrnes said.

The other barrier to describing clear trends in kelp forests is a lack of data. Much of the planet’s kelp forests have yet to be mapped, and they’re rarely monitored, according to UNEP. The 2016 global analysis — which remains the most comprehensive assessment to date — only analyzed data for about a third of the regions home to kelp forests. Information is especially limited in places like the tip of South America and in parts of Africa, Byrnes said.

“The lack of information about certain parts of the world really worries me,” Byrnes said. “We don’t know what’s happening. And sometimes it can be alarming.”

Photographer Effie Benjamin freedives in a bull kelp forest near Carmel Point, California, in April 2023.

Kelp forests are harder to monitor than most other ecosystems. Often, marine biologists get in the water and count kelp stands by hand, which is expensive, labor intensive, and requires some special skills. Kelp forests can extend for miles.

“It is a challenge to monitor [kelp forests], and that’s partly why they haven’t been as much of a focus for conservation or engagement,” Krumhansl said. “Nobody actually sees them. They’re not like a forest on land that you can walk through and see the changes happening for yourself.”

To an extent, tech is helping fix this problem. Instead of diving into a forest, scientists can now analyze images of the ocean taken by satellites for subtle changes in color that correspond to kelp forest canopies.

Bell’s recent analysis was based entirely on this approach. He used satellite-based data from Kelpwatch, a website he and other scientists designed to make this kind of data freely accessible. (You can do a similar analysis yourself on the website, though for now there’s only data for the west coast of North America.)

There’s one big caveat to this new era of kelp forest monitoring: Satellites, at least for now, can only detect canopy-forming kelp. And just a portion of kelp species form canopies, Byrnes said. That means we may not have a clear picture of these habitats for decades.

The future of kelp forests

In the years to come, kelp forests may still face a raft of problems including overfishing and the spread of invasive species. But none are likely to be more threatening to their long-term existence than climate change.

The oceans are warming, and marine heat waves — extended periods of abnormally hot temperatures — are almost certainly becoming more common. Since the 1980s, the frequency of marine heat waves has doubled, according to a 2021 report by the Intergovernmental Panel on Climate Change, a UN group that studies warming.

That’s a problem for kelp, Krumhansl said. “They are cold-adapted species,” she said. “So the future doesn’t look great.”

Yet there are things that countries and environmental advocates can do to lessen the damage and give kelp forests a chance at survival.

One approach is to protect kelp forests with marine parks. Right now, these ecosystems are underrepresented in the world’s network of protected areas, according to UNEP, yet they’ve been shown to help kelp forests recover. By safeguarding marine predators, such as lobsters and sea otters, parks can keep urchin populations under control.

Marine ecologist Rachael Karm collects bits of bull kelp for a research project at Russian Gulch State Beach in Jenner, California.

Iris Flores, a doctoral researcher at the University of California Santa Cruz, collects purple sea urchins at Stillwater Cove off Pebble Beach, California, to study the impacts of coastal upwelling on the grazing behavior of sea urchins.

Another approach is to manually remove urchins from a reef — an activity that is oddly satisfying to watch — or kill them en masse with poison, which can be highly effective in restoring kelp forests, according to a recent review. (There are plenty of other approaches to kelp forest restoration and an entire organization working toward that cause.)

The good news is that when you take away some of these threats, kelp forests can quickly bounce back, Byrnes said. Kelp is resilient. And again, it grows incredibly quickly. “Kelp is just phenomenal,” Byrnes said. “As long as the conditions are right, it will grow and it will thrive.”

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Benji Jones