Ocean Health: Acidification, Plastic, and Oil Spills

Ocean health refers to the condition of marine ecosystems and their ability to provide benefits to people, and it is threatened by acidification (a fall in the pH of seawater as it absorbs carbon dioxide), plastic pollution and oil spills, in addition to warming, overfishing and nutrient pollution. In brief: the oceans have absorbed about a quarter to a third of human carbon dioxide emissions, lowering the average surface pH from about 8.2 to about 8.1, an increase in acidity of around 26–30%. Millions of tonnes of plastic enter the sea every year, and oil spills such as Deepwater Horizon (2010) cause severe local damage. Each pressure is made worse by the others.

The ocean regulates climate, produces oxygen and feeds billions of people, so its health is vital. This guide explains the causes and effects of acidification, plastic and oil pollution, and other threats, the responses, compares the problems in a table, works through calculations, outlines the history and corrects common mistakes.

What is ocean acidification?

When carbon dioxide dissolves in seawater, it reacts with water to form carbonic acid, which releases hydrogen ions and reduces the concentration of carbonate ions, which are the building blocks for the shells and skeletons of many marine organisms. The average pH of surface water has fallen from about 8.2 in the pre-industrial period to about 8.1 today. Because the pH scale is logarithmic, a fall of 0.1 means about a 26% increase in hydrogen ion concentration (10^0.1 = 1.26), often rounded to about 30%. The ocean is still slightly alkaline (above 7), so “acidification” refers to the move toward lower pH. Under high-emission scenarios, the surface pH could fall by a further 0.3 or so by 2100.

Effects: organisms that build calcium carbonate shells and skeletons, such as corals, oysters, mussels, sea urchins, some plankton (pteropods and coccolithophores), find it harder to build and maintain them, and in some places shells dissolve. Fish behavior and physiology can be affected, and the food web may change. Cold waters, such as the Arctic and Southern Ocean, take up more CO₂ and are affected earlier. Shellfish hatcheries on the west coast of the United States have seen larval die-offs linked to acidified upwelled water. The only way to stop acidification is to cut CO₂ emissions.

How does plastic pollution affect the ocean?

Plastic production has grown from about 2 million tonnes in 1950 to over 400 million tonnes a year, and a study in 2015 (Jambeck and colleagues) estimated that around 4.8 to 12.7 million tonnes of plastic waste entered the oceans in 2010, with a mid-estimate of about 8 million tonnes a year. Most of it comes from land, from poorly managed waste, particularly in rapidly urbanizing countries with limited waste collection, and rivers carry a large share. Some also comes from fishing gear and ships. Plastic drifts with the currents and accumulates in the centers of the subtropical gyres, and the best-known “garbage patch” in the North Pacific covers an area estimated at 1.6 million km², though it is not a solid island of waste but a diffuse area with a higher concentration of floating debris, much of it abandoned fishing gear.

Effects: animals such as turtles, seabirds, seals and whales are entangled in nets and lines or die after swallowing plastic, which fills their stomachs. Plastic breaks into microplastics (smaller than 5 mm) and nanoplastics that are eaten by plankton and shellfish and enter the food web, and they have been found in the deepest trenches and in Arctic ice. Plastics also carry chemicals and can transport invasive species. The long-term effects on ecosystems and human health are still being studied. Solutions focus on reducing the production and use of single-use plastics, improving waste collection and recycling, producer responsibility, cleaning up rivers before the plastic reaches the sea, and international agreements. A UN treaty on plastic pollution has been under negotiation since 2022.

What are the effects of oil spills?

Oil enters the ocean from tanker and pipeline accidents, offshore drilling, routine shipping discharge, and natural seeps. The best-known disasters include the Torrey Canyon (1967) off the coast of Britain, the Exxon Valdez (1989) in Alaska, which released about 37,000 tonnes (about 11 million US gallons) of crude oil and killed hundreds of thousands of seabirds, and the Deepwater Horizon blowout in the Gulf of Mexico (2010), the largest accidental marine oil spill in history, which released about 4.9 million barrels, roughly 780 million liters, over 87 days, and killed 11 workers. Oil coats birds and mammals, destroying insulation, poisons fish and invertebrates, smothers shorelines and wetlands, and can damage ecosystems for years or decades, as at Prince William Sound. Fishing and tourism are disrupted.

Response methods include booms (floating barriers) to contain the oil, skimmers to remove it, dispersants that break it into droplets (whose use is controversial because of toxicity), controlled burning, shoreline cleanup and bioremediation, which speeds up the natural breakdown by bacteria. Prevention is more effective: double-hulled tankers (required after the Exxon Valdez accident under the US Oil Pollution Act of 1990), better navigation and rules on drilling safety. The international MARPOL convention (1973/78) regulates pollution from ships.

What other threats exist?

• Ocean warming and marine heat waves: the ocean has absorbed over 90% of the extra heat trapped by greenhouse gases, causing coral bleaching, shifts in species and sea-level rise.
• Deoxygenation and dead zones: nutrients (nitrogen and phosphorus) from fertilizers and sewage cause algal blooms, which use up oxygen when they decay. The Gulf of Mexico dead zone, fed by the Mississippi River, averages about 15,000 km².
• Overfishing and habitat destruction (discussed in the article on marine resources).
• Chemical pollution: heavy metals, persistent organic pollutants and noise from ships and seismic surveys.
• Invasive species carried in ships’ ballast water, and noise that disturbs marine mammals.

What can be done?

Responses combine local, national and global action: cutting greenhouse gas emissions (the only way to stop acidification and warming), reducing land-based pollution through better wastewater treatment, farming practices and waste management, a global plastics treaty, stricter shipping and drilling rules, marine protected areas and sustainable fisheries to make ecosystems more resilient, restoration of habitats such as mangroves, seagrass and oyster reefs, and monitoring with satellites and buoys. The UN Decade of Ocean Science for Sustainable Development (2021–2030) and SDG 14 (Life Below Water) provide frameworks.

The problems compared

Worked example: pH, plastic and spill size

The values are approximate or hypothetical and illustrate the calculations.

1. pH and acidity: pH = −log₁₀[H⁺]. A change from pH 8.2 to 8.1 multiplies the hydrogen ion concentration by 10^{(8.2 − 8.1)} = 10^0.1 = 1.259, an increase of about 26%. A fall to pH 7.8 would be a rise of 10^0.4 = 2.51 times, an increase of 151%.
2. Plastic input: if 8 million tonnes enter the ocean each year, the amount over 10 years is 80 million tonnes. Per day, it is 8,000,000 ÷ 365 = about 21,900 tonnes.
3. Plastic on a coast: a beach of 500 m has 15 kg of plastic litter per 100 m. The total is 5 × 15 = 75 kg. A cleanup that removes 60 kg removes 60 ÷ 75 = 80%.
4. Oil volume: one barrel is 159 liters. For 4.9 million barrels, the volume is 4,900,000 × 159 = 779 million liters, or 779,000 m³.
5. Slick area: an oil spill of 40,000 tonnes with a density of 0.9 t/m³ has a volume of 40,000 ÷ 0.9 = 44,400 m³. If it spreads in a layer 0.1 mm (0.0001 m) thick, the area is 44,400 ÷ 0.0001 = 444,000,000 m², which is 444 km².
6. Dead zone area: a dead zone of 15,000 km² is equal to about 15,000 ÷ 1,200 = 12.5 times the area of a city of 1,200 km².

How have these issues developed?

Public awareness of ocean pollution grew after the Torrey Canyon spill in 1967 and the 1969 Santa Barbara blowout, which helped to launch the modern environmental movement. The MARPOL convention was adopted in 1973 and amended in 1978. The first reports of plastic in the ocean appeared in the 1970s, and the “garbage patch” was described in the late 1990s by Charles Moore. The concept of ocean acidification was introduced in 2003 by Ken Caldeira and Michael Wickett, and was documented widely after 2005. The Deepwater Horizon disaster in 2010 led to new rules on drilling safety. In the 2020s, the UN has led a negotiation of a binding treaty on plastic pollution, and the 2023 High Seas Treaty and the 2022 global biodiversity framework are part of wider efforts to protect marine life.

Which mistakes are common?

Mistake 1: Thinking that the ocean is becoming acidic

It remains slightly alkaline, with a pH of about 8.1. It is becoming less alkaline, and the change is large enough to harm many organisms.

Mistake 2: Picturing the garbage patch as a floating island of plastic

It is a large area with a higher concentration of mostly small, scattered pieces, which makes it hard to clean up.

Mistake 3: Assuming that large oil spills are the main source of ocean oil pollution

Major accidents are dramatic, but chronic sources, such as runoff, shipping discharges and natural seeps, contribute most of the oil by volume over time.

Mistake 4: Seeing each threat in isolation

Warming, acidification, pollution and overfishing interact, and a stressed ecosystem is less able to cope with any additional pressure.