Plastic pollution in the ocean contributes to climate change through several interconnected pathways: it directly releases greenhouse gases as it breaks down, it disrupts the ability of marine life to absorb and store carbon, and it creates new microbial habitats that alter the ocean’s chemical cycles. The scale of the problem is significant. The ocean absorbs roughly 2.6 billion metric tons of carbon dioxide from the atmosphere each year, and plastic pollution is increasingly interfering with the biological processes that make that possible.
Plastic Releases Greenhouse Gases as It Degrades
When plastic floats in the ocean and is exposed to sunlight, it doesn’t just break into smaller pieces. It also releases methane and ethylene, both potent greenhouse gases. A 2018 study published in PLOS One tested every common type of plastic and found that all of them produced measurable amounts of methane and ethylene under solar radiation in seawater.
Low-density polyethylene, one of the most common plastics found in ocean debris (used in grocery bags, cling wrap, and squeeze bottles), was by far the worst offender. It emitted roughly 45 times more methane and 27 times more ethylene than high-density polyethylene when exposed to sunlight in water. Over a 212-day experiment, emission rates from low-density polyethylene actually increased over time rather than tapering off, suggesting that as plastic weathers and cracks, it exposes more surface area and releases gases at accelerating rates.
These emissions from individual pieces of plastic are tiny. But with an estimated 150 million metric tons of plastic already in the ocean and millions more entering each year, the cumulative effect adds up. And because plastic persists for centuries, this is a source of greenhouse gas emissions that compounds over time.
Microplastics Weaken the Ocean’s Carbon Absorption
Phytoplankton, the microscopic algae that float near the ocean surface, are responsible for roughly half of all photosynthesis on Earth. They pull enormous quantities of CO2 out of the atmosphere, functioning as a planetary carbon sink. Microplastics and nanoplastics interfere with this process in multiple ways.
Tiny plastic particles inhibit the growth of key algae species. Lab studies on marine diatoms and green algae have shown that polystyrene nanoplastics suppress their ability to grow and fix carbon. The mechanism is partly indirect: nanoplastics interfere with how algae absorb essential trace metals like copper, which are critical for photosynthesis. As nanoplastic concentrations increase, the algae’s capacity to sequester CO2 measurably drops.
There’s also a physical shading effect. Plastic accumulating near the ocean surface, along with the organic films that form around it, reduces how much sunlight penetrates the water. Phytoplankton need that light for photosynthesis. Less light means less CO2 absorption. At the same time, floating plastic creates a thicker barrier at the air-sea interface, physically impeding the exchange of gases between the atmosphere and the ocean.
Disrupted Biological Pump Keeps Carbon Near the Surface
The ocean buries carbon in its depths through a process scientists call the biological pump. Here’s how it normally works: phytoplankton absorb CO2 near the surface, zooplankton eat the phytoplankton, and the dense fecal pellets those tiny animals produce sink rapidly to the deep ocean, carrying carbon with them. This vertical transport locks carbon away from the atmosphere for centuries or longer.
Microplastics disrupt this process at a critical step. When zooplankton like copepods eat microplastics alongside their normal food, their fecal pellets come out significantly less dense. Research on the copepod species Calanus helgolandicus found that after ingesting polystyrene microplastics, their fecal pellets sank 2.25 times more slowly and were far more prone to breaking apart. Slower sinking means the pellets spend more time in shallow water, where other organisms eat them and bacteria decompose them, releasing the carbon back into the water column instead of burying it in the deep ocean.
This is a subtle but important feedback loop. The carbon that would have been locked away in deep sediments instead gets recycled near the surface, where it can eventually return to the atmosphere as CO2.
The Plastisphere Creates New Greenhouse Gas Sources
Every piece of plastic in the ocean quickly becomes colonized by microorganisms, forming what researchers call the “plastisphere,” a distinct microbial ecosystem that doesn’t exist on natural surfaces in the same way. These communities don’t just passively ride along on plastic debris. They actively alter ocean chemistry in ways that can amplify greenhouse gas production.
Bacteria in the plastisphere break down organic carbon that leaches from the plastic itself and from dissolved organic matter in surrounding seawater. This decomposition releases CO2 and methane as end products. The process also consumes oxygen, contributing to localized oxygen depletion.
Perhaps more concerning, plastisphere communities in freshwater environments (which eventually flow into the ocean) show elevated populations of denitrifying bacteria. These microbes convert nitrogen compounds into nitrous oxide, a greenhouse gas with 298 times the warming potential of CO2 over a 100-year period. The plastisphere essentially provides a floating habitat that concentrates the types of microbial activity most likely to produce potent greenhouse gases.
How These Effects Compound With Ocean Warming
Plastic pollution doesn’t act in isolation. It interacts with the other stresses already weakening the ocean’s ability to absorb carbon. Warmer water holds less dissolved CO2. Ocean acidification stresses the same phytoplankton that microplastics are already inhibiting. These pressures are additive, and in some cases they may amplify each other.
The numbers underscore why this matters. Between 2009 and 2018, the ocean absorbed about 2.6 billion metric tons of carbon per year. Meanwhile, greenhouse gas emissions from the entire plastic lifecycle (production through disposal) are projected to reach the equivalent of 1.34 billion metric tons of carbon per year by 2030, and 2.8 billion by 2050 under current trends. If ocean plastic simultaneously reduces the ocean’s ability to absorb carbon while generating its own emissions, the gap between what we emit and what the planet can absorb grows from both directions.
The full climate impact of ocean plastic remains difficult to quantify precisely because the mechanisms span biology, chemistry, and physics across the entire water column. What is clear is that plastic pollution is not just a wildlife or pollution issue. It is actively undermining one of Earth’s most important climate regulation systems.