The Biggest Threats to the Ocean

The ocean is the planet's life-support system. It covers roughly 71% of Earth's surface, produces about half the oxygen we breathe, feeds billions of people, and acts as the world's largest climate buffer, absorbing the lion's share of the excess heat trapped by greenhouse gases along with a substantial share of our carbon dioxide emissions. For most of human history it seemed too vast to damage. That assumption no longer holds.

Today the ocean is under pressure from a cluster of human-driven threats that reinforce one another. None acts alone: a reef weakened by warming is more vulnerable to disease and pollution; a coastline stripped of mangroves loses both its fish nursery and its storm defense. Understanding each threat, and how they interlock, is the first step toward a healthier ocean. Here is a detailed look at the eight that matter most.

 

1. Climate Change

 

The single largest driver of ocean change is the warming caused by greenhouse gas emissions. The ocean has absorbed the overwhelming majority of the extra heat humans have added to the climate system, and the consequences are now measurable and accelerating.

2024 was the hottest year on record, and ocean heat content reached its highest level in modern observational records, with the rate of warming over the past two decades running at more than twice the pace of the previous half-century. Warmer water reshapes marine life in several ways at once. It drives species toward cooler waters at the poles or into deeper layers, disrupting fisheries and food webs that took millennia to settle. It intensifies marine heatwaves, prolonged spikes in sea temperature that can kill entire ecosystems in weeks. And it fuels coral bleaching, the process in which heat-stressed corals expel the symbiotic algae that give them color and food.

The most striking recent example is the fourth global coral bleaching event, which ran from early 2023 into 2025 and was confirmed by the U.S. National Oceanic and Atmospheric Administration (NOAA). It was the largest and fastest on record: bleaching-level heat stress hit roughly 84% of the world's reef area across at least 83 countries and territories, spanning the Pacific, Atlantic, and Indian Ocean basins. Reefs can recover from bleaching if temperatures fall back, but repeated, near-annual events leave less and less time to do so, and coral reefs underpin the food and income of close to a billion people.

Warming also melts polar and glacial ice, raising sea levels and threatening coastal communities, and it reduces the amount of oxygen seawater can hold, compounding other stresses described below.

 

2. Plastic Pollution

 

Plastic is the most visible form of ocean pollution and one of the most persistent. Once it enters the water it does not biodegrade. It fragments into ever-smaller pieces, microplastics under five millimeters and then nanoplastics, that spread to every corner of the ocean, from surface gyres to deep-sea sediment and polar ice.

Estimates of how much plastic enters the sea each year vary widely depending on method. International bodies such as the UN Environment Programme and IUCN commonly cite a figure around 11 million tonnes annually, while some older studies put the range as high as 8 to 12.7 million tonnes. More recent high-quality modeling argues the true annual input may be lower, on the order of 1 to 2 million tonnes, with most of it accumulating near shorelines rather than drifting offshore. Whatever the precise number, the stock already in the ocean is enormous: researchers estimate somewhere between 75 and 199 million tonnes of plastic waste are currently in marine environments.

The harm is well documented. Marine animals ingest plastic or become entangled in it, particularly in lost or discarded fishing gear ("ghost gear"), which is among the deadliest forms of debris. Microplastics now turn up throughout marine food chains and, increasingly, in seafood and drinking water consumed by people. Because plastic production is still rising and only a small fraction of plastic worldwide is recycled, the flow into the ocean is unlikely to slow without coordinated changes to how plastic is made, used, and managed.

 

3. Overfishing

 

For decades the ocean was treated as an inexhaustible larder. It is not. Overfishing, catching fish faster than populations can reproduce, has steadily eroded the health of global stocks.

According to the FAO's most comprehensive assessment of marine fishery resources, released in 2025 and covering more than 2,500 stocks, about 35.5% of assessed stocks are now overfished, while 64.5% remain within biologically sustainable limits. That is a sharp decline from 1974, when roughly 90% of stocks were considered sustainably fished, and the share of overfished stocks has crept upward by about one percent per year in recent times. There is a more hopeful number alongside it: because well-managed, high-volume fisheries are mostly healthy, around 77% of the fish actually landed worldwide still comes from sustainable stocks. The lesson is that effective management works: regions with strong, science-based controls (such as the Northeast Pacific) post sustainability rates well above 90%, while poorly managed regions fare far worse.

Beyond depleting target species, destructive fishing methods cause collateral damage. Bottom trawling drags heavy gear across the seabed, flattening habitats that can take centuries to form. Bycatch, the unintended capture of turtles, seabirds, sharks, and marine mammals, kills vast numbers of non-target animals. And the removal of key species can unravel entire food webs from the top down.

 

4. Ocean Acidification

 

Often called climate change's "evil twin," ocean acidification is a direct chemical consequence of carbon dioxide emissions. The ocean absorbs roughly 30% of the CO₂ humans release into the atmosphere. When that CO₂ dissolves in seawater it forms carbonic acid, releasing hydrogen ions that lower the water's pH.

Since the start of the industrial era, average surface-ocean pH has fallen by about 0.1 units. That sounds tiny, but the pH scale is logarithmic, so the change represents roughly a 30% increase in acidity, a faster shift in ocean chemistry than anything seen in tens of millions of years. If emissions continue unchecked, the IPCC projects pH could fall enough to make the ocean around 150% more acidic by 2100.

The biological stakes are high for calcifiers such as corals, oysters, clams, sea snails, and many plankton, which build shells and skeletons from calcium carbonate. More acidic water makes the carbonate building blocks harder to obtain and can even dissolve existing shells. Because some of these organisms sit at the base of the food web, the effects ripple upward to the fish and fisheries that depend on them. Acidification is also a key part of a dangerous triple squeeze on marine life, alongside warming and oxygen loss, a combination associated with mass extinctions in Earth's deep past.

 

5. Habitat Destruction

 

Some of the ocean's most valuable ecosystems sit right at the edge where land meets sea, and that is exactly where human development concentrates. Coral reefs, mangrove forests, seagrass meadows, and salt marshes are being degraded by coastal construction, dredging, land reclamation, destructive fishing, and pollution.

The losses are substantial. Global mangrove cover has declined by roughly 35 to 40% over recent decades, largely cleared for aquaculture ponds, farmland, and shoreline development, though the annual rate of loss has slowed in recent years thanks to growing protection efforts. Seagrass meadows have fared similarly: studies estimate that close to 30% of mapped seagrass area has disappeared since records began, with the rate of decline accelerating to around 7% per year since 1990. Salt marshes have seen even steeper historic losses in some regions.

These are not just scenic habitats. Mangroves and seagrasses are "blue carbon" powerhouses, storing carbon far more efficiently per unit area than terrestrial forests; when they are destroyed, that stored carbon is released. They also serve as nurseries for commercially important fish, filter pollutants, and shield coastlines from storm surge and erosion. Losing them weakens the ocean's biodiversity, its climate buffering, and the natural defenses that protect coastal communities.

 

6. Agricultural Runoff

 

When fertilizers and animal waste wash off farmland and flow down rivers to the sea, they carry a heavy load of nitrogen and phosphorus. These nutrients trigger explosive blooms of algae. When the algae die and decompose, bacteria consume the surrounding oxygen, leaving dead zones, areas of water so depleted of oxygen (a condition called hypoxia) that fish, crabs, and other mobile animals must flee or suffocate.

The number of documented dead zones has climbed roughly tenfold over the past half-century, with scientists identifying well over 400 worldwide and some estimates running considerably higher once undocumented zones are included. The Gulf of Mexico hosts one of the largest recurring examples, fed by nutrients draining from the vast Mississippi River basin; in 2023 it spanned about 8,185 square miles, roughly the size of New Jersey. The Baltic Sea, Arabian Sea, and Chesapeake Bay are other notorious cases.

Climate change is making the problem worse on two fronts: warmer water naturally holds less oxygen, and heavier rainfall flushes more nutrients off the land in concentrated pulses. The encouraging news is that dead zones respond to action: coordinated reductions in agricultural and sewage runoff have shrunk hypoxic zones in places such as the North Sea and parts of the United States.

 

7. Chemical and Industrial Pollution

 

Beyond plastic and nutrients, the ocean receives a steady stream of toxic substances: heavy metals like mercury and lead, persistent organic pollutants from industry and pesticides, untreated sewage, pharmaceutical residues, and oil from spills and routine discharges.

Many of these contaminants are dangerous precisely because they persist and concentrate. Mercury, for instance, is converted by marine bacteria into methylmercury, which accumulates in tissue and biomagnifies up the food chain, meaning long-lived top predators such as tuna, swordfish, and sharks carry the highest concentrations, which is why health advisories caution against eating too much of them. Persistent organic pollutants behave similarly, lodging in the fatty tissue of marine mammals and seabirds. Oil spills cause acute, dramatic damage to coastlines and wildlife, but chronic, low-level inputs from runoff and shipping add up to far more oil entering the sea over time. Together these pollutants impair reproduction, weaken immune systems, and ultimately reach humans through the seafood we eat.

 

8. Deep-Sea Mining

 

The newest threat on this list is also the least understood. The deep seabed holds vast deposits of metals such as nickel, cobalt, copper, and manganese, bound up in potato-sized polymetallic nodules and in crusts and vents. The Clarion-Clipperton Zone in the Pacific alone is estimated to contain on the order of 21 billion tonnes of nodules. As demand for battery and electronics metals grows, companies are eager to extract them.

The problem is that the deep ocean is one of the least-explored environments on Earth, home to slow-growing, fragile ecosystems that may take centuries or longer to recover from disturbance, if they recover at all. Mining would stir up vast sediment plumes, destroy seafloor habitat, and introduce noise and light into a realm adapted to permanent darkness, with consequences scientists cannot yet fully predict.

Governance remains unsettled. The International Seabed Authority (ISA), which oversees mining in international waters under the Law of the Sea, has spent more than a decade negotiating a "mining code" but has not adopted final commercial regulations; its July 2025 talks ended without consensus, with negotiations set to resume in 2026. Meanwhile some actors are moving to press ahead outside that framework. A prominent company has sought permits through the United States rather than the ISA, and in early 2026 U.S. authorities finalized revisions to their own domestic seabed-mining rules. At the same time, a growing coalition of states (several dozen at last count) has called for a moratorium or precautionary pause until the environmental risks are far better understood. How this tension resolves will shape whether one of the planet's last wildernesses is opened to industry.

 

How the Threats Compound One Another

 

What makes these eight threats so dangerous is that they rarely act in isolation. Warming, acidification, and oxygen loss form a "triple squeeze" that stresses marine life on multiple fronts at once. A coral reef bleached by heat is more easily smothered by sediment from coastal dredging or weakened by nutrient pollution. Overfished ecosystems lose resilience, making them slower to bounce back from a heatwave or a pollution event. Mangrove loss removes a defense that would otherwise filter runoff and buffer the very storms that climate change is intensifying. The cumulative pressure is greater than the sum of its parts.

 

Reasons for Hope

 

The picture is serious but not hopeless, and many of these threats respond well to action. Well-managed fisheries demonstrate that depleted stocks can recover. Targeted cuts to agricultural and sewage runoff have shrunk dead zones. Marine protected areas, mangrove and seagrass restoration, and international agreements, from a global treaty on plastics to negotiations over the high seas and the deep seabed, offer levers that work when they are properly enforced. Above all, because so much of the damage traces back to greenhouse gas emissions, cutting carbon is the most powerful single step for the ocean: it slows warming, eases acidification, and reduces oxygen loss simultaneously.

The ocean has sustained life on this planet for billions of years and has shown a remarkable capacity to recover when pressure is relieved. The choices made over the next decade, about emissions, fishing, pollution, and the deep sea, will largely determine which version of the ocean future generations inherit.

Note: Figures in this article reflect the most current authoritative assessments available as of mid-2026, drawing on sources including NOAA, the UN FAO, the IPCC, UNEP, and the International Seabed Authority. Estimates for some metrics, particularly annual plastic inputs, vary between studies, and ranges are noted where relevant.