In once-frigid fjords and along warming coastlines, seaweed forests are rapidly expanding. These underwater ecosystems are now reshaping not only local habitats but the very chemistry of the ocean. From the Arctic to the tropics, seaweed’s quiet rise could become one of the most consequential shifts in marine ecology in decades.

Warming oceans are helping seaweed forests spread into new territories.

A Surge Fueled by Climate Change

Rising ocean temperatures and melting sea ice are creating ideal conditions for seaweed to thrive. In the Arctic, species like brown, red, and green macroalgae are expanding their range into coastal fjords, producing dense biomass that now dominates local sediment samples, according to a study from Kongsfjorden, Svalbard.

But the growth isn’t all good news. As these forests decay, microbial activity can deplete oxygen levels in the surrounding waters, leading to low-oxygen “dead zones.” In warmer, nutrient-rich areas, this decomposition could trigger shifts that make coastal waters more hostile to marine life.

Expanding seaweed growth can lead to oxygen-depleted coastal dead zones.

Nature’s Climate Tech

Despite these risks, seaweed is one of the most promising natural tools in the fight against climate change. As The Conversation reports, seaweed absorbs carbon dioxide through photosynthesis, sequestering carbon in its tissues and helping to offset greenhouse gas emissions. In fact, seaweed and kelp forests may sequester as much carbon as the Amazon rainforest.

This carbon capture power makes seaweed vital for regulating pH in coastal waters. By reducing local ocean acidification, seaweed supports biodiversity and buffers ecosystems that would otherwise suffer from CO₂-induced imbalances, as explained in Free Science.

Kelp forests have declined by up to 95% in some regions.

The Fall and Fight of the Kelp Forests

While some seaweed forests expand, others are disappearing. Kelp forests, once dominant across a quarter of the world’s coastlines, are rapidly vanishing. In North America’s Pacific waters, 95% of kelp forests have died in some regions due to rising temperatures, disease, and the unchecked spread of sea urchins, which devour kelp at alarming rates.

Yet, hope exists in kelp’s resilience. As reported by Scientific American, researchers are now cultivating genetically robust strains of sugar kelp that can withstand rising temperatures, using selective breeding to help repopulate marine habitats and fuel future aquaculture.

Seaweed absorbs CO₂ and helps buffer ocean acidification.

Lessons from Japan and England

Efforts to save seaweed forests are not confined to labs. Along Japan’s Kanagawa coast, where 80% of seaweed beds have vanished, researchers are feeding surplus cabbage to local sea urchins to divert them from ravaging the remaining forests. The project, as Reuters reports, may even turn these pests into a delicacy, increasing incentives for their removal.

In the UK, the Sussex coast saw 96% of its kelp wiped out by decades of trawling and storm surges. But a bylaw banning nearshore trawling has kickstarted a local restoration movement. Divers and community scientists are witnessing the return of mussels, dolphins, and rays—early signs that the kelp forests might one day return, Oceanographic Magazine reports.

Decomposing seaweed contributes to oxygen loss in marine sediments.

The Future of Ocean Forests

What lies ahead is a dual narrative. On one side, seaweed forests expand into warming regions, altering oxygen levels and threatening to destabilize some ecosystems. On the other, these same forests offer a powerful tool to sequester carbon, buffer acidity, restore biodiversity, and protect coastlines.

Innovators like SeaGen, a robotics firm founded to automate seaweed farming and harvesting, are working to scale this natural solution efficiently and safely, The Conversation reports. Meanwhile, communities worldwide—from Tasmania to Argentina—are fighting to bring kelp forests back from the brink.

Whether as climate ally or ecological disruptor, seaweed is no longer a passive background player in ocean ecosystems. Its spread is a story still being written, but one with enormous consequences for life on Earth.