Ocean Breathing

INTRODUCTION

The ocean is changing not only in temperature, but also in chemistry.

As atmospheric carbon dioxide increases, the ocean absorbs part of that CO₂. This has helped slow the accumulation of carbon dioxide in the atmosphere, but it comes with a consequence: when CO₂ dissolves in seawater, it changes the ocean’s carbonate chemistry and lowers pH. This process is known as ocean acidification.

At the same time, oxygen concentrations are declining in many parts of the ocean. Warmer water holds less dissolved oxygen, while increased stratification can reduce the mixing that transports oxygen from the surface into deeper waters.

In coastal environments, excess nutrients from agriculture, wastewater and other sources can stimulate large algal blooms. When this organic material dies and decomposes, microorganisms consume oxygen, sometimes creating severely oxygen-depleted waters known as hypoxic zones.

Acidification and oxygen loss can also occur together. Both can place marine organisms under physiological stress and reduce the amount of suitable habitat available to them.

These processes are largely invisible to us from above the surface, but they are changing some of the fundamental chemical conditions under which marine life evolved.

NOAA and IPCC identify atmospheric CO₂ uptake as the primary global driver of acidification and warming, stratification and nutrient pollution as important drivers of deoxygenation and coastal hypoxia.

WHY IT MATTERS

Marine organisms evolved within particular chemical conditions. Changes in pH and oxygen can affect growth, metabolism, reproduction and survival.

Shell-building organisms can be especially vulnerable to acidification. Oysters, clams, pteropods, corals and other organisms use calcium carbonate minerals to construct shells or skeletons. Acidification reduces the availability of carbonate ions needed for this process.

All aerobic marine animals require oxygen. When oxygen concentrations fall sufficiently, animals may experience physiological stress, leave affected areas or die if they cannot escape.

Low oxygen can reduce available habitat. Fish and other mobile organisms may become compressed into smaller areas where oxygen conditions remain suitable.

Food webs can be affected. Changes to plankton, shellfish, corals and other organisms can influence predators and fisheries that depend on them.

Coastal economies can be affected. Fisheries and shellfish aquaculture depend on environmental conditions that allow commercially important species to grow and reproduce.

Acidification and deoxygenation therefore represent more than chemical measurements. They can influence biodiversity, fisheries, food security and coastal livelihoods.

NOAA documents reduced carbonate availability under acidification, while IPCC assesses impacts of acidification and deoxygenation across marine organisms and fisheries. NOAA

KEY FACTS

The ocean is naturally alkaline, not acidic. Its average surface pH is around 8.1. “Ocean acidification” means that pH is decreasing and seawater is becoming more acidic relative to its previous state.

Human-produced atmospheric CO₂ is the primary global driver of ocean acidification. When the ocean absorbs additional CO₂, seawater chemistry changes.

The ocean has absorbed roughly one-third of human-related CO₂ emissions since the 1700s.

Ocean acidification reduces carbonate-ion availability. This can make shell and skeleton formation more difficult for some calcifying organisms.

The ocean is losing oxygen. IPCC assessments conclude that deoxygenation has occurred across most regions of the open ocean since the mid-20th century.

The upper 1,000 metres experienced an estimated global mean oxygen decline of approximately 0.5–3.3% between 1970 and 2010.

Hypoxia means abnormally low dissolved oxygen. Severe low-oxygen conditions can create areas commonly called “dead zones,” where many organisms cannot survive.

Acidification, warming and deoxygenation can interact. Marine organisms increasingly experience several environmental stressors simultaneously rather than one in isolation.

The pH and CO₂ figures are reported by NOAA; the historical oxygen-loss range comes from the IPCC assessment.

Main Threats

Rising atmospheric carbon dioxide. Continued CO₂ emissions drive long-term ocean acidification as more carbon dioxide enters seawater.

Ocean warming. Warmer water generally contains less dissolved oxygen and increases metabolic oxygen demand for many organisms.

Increasing stratification. Stronger separation between surface and deeper waters can reduce vertical mixing and limit the replenishment of oxygen at depth.

Nutrient pollution. Fertilizers, sewage and other nutrient sources can stimulate excessive algal growth in coastal waters.

Decomposition of organic matter. When large quantities of algae and other organic material die, microbial decomposition consumes dissolved oxygen and releases CO₂.

Coastal acidification. Coastal chemistry can be affected by atmospheric CO₂ alongside nutrient inputs, freshwater, upwelling and local biological processes.

Multiple environmental stressors. Warming, acidification and oxygen loss can occur simultaneously, while pollution, habitat destruction and overfishing place additional pressure on ecosystems.

These processes can reinforce one another, making some coastal and marine environments particularly vulnerable.

NOAA specifically notes that nutrient-driven blooms can both consume oxygen and release CO₂ during decomposition, linking coastal hypoxia and acidification. Ocean Acidification

SOLUTIONS

Reduce carbon dioxide emissions. Rapid reductions in greenhouse-gas emissions are the most important long-term solution for limiting further ocean acidification, warming and climate-driven deoxygenation.

Reduce nutrient pollution. Better fertilizer management, wastewater treatment and stormwater control can reduce the nutrient loads that contribute to coastal algal blooms and hypoxia.

Protect and restore coastal ecosystems. Wetlands, seagrass meadows, mangroves and other coastal habitats can improve water quality and support healthier marine ecosystems.

Improve wastewater infrastructure. Reducing untreated sewage and nutrient discharge can directly improve conditions in vulnerable coastal waters.

Expand ocean monitoring. Sensors, autonomous floats, research vessels and long-term observing stations can measure pH, oxygen, temperature, carbon chemistry and biological responses.

Develop adaptation strategies for fisheries and aquaculture. Monitoring and early-warning systems can help shellfish hatcheries and other marine industries respond to periods of unfavorable water chemistry.

Protect ecosystem resilience. Reducing overfishing, pollution and habitat destruction gives marine ecosystems a better chance of coping with climate-driven chemical changes.

Coordinate action from watersheds to the ocean. Coastal hypoxia demonstrates that activities far inland can ultimately influence marine environments through rivers and runoff.

IPCC reports that mitigation limits future exposure to warming, acidification and deoxygenation, while NOAA highlights monitoring and science-based coastal management as important adaptation tools. IPCC

WHAT YOU CAN DO

Support climate action. Reducing CO₂ emissions addresses the primary global cause of ocean acidification and helps limit warming-driven oxygen loss.

Reduce unnecessary fertilizer use. Excess nutrients can travel through streams and rivers before reaching coastal waters and contributing to algal blooms and hypoxia.

Support effective wastewater treatment. Modern wastewater and stormwater systems can reduce nutrient and organic pollution entering aquatic ecosystems.

Protect coastal habitats. Wetlands, seagrasses and other coastal ecosystems support water quality, biodiversity and ecosystem resilience.

Choose seafood responsibly. Sustainable fisheries management can reduce additional pressure on ecosystems already affected by climate and chemical changes.

Support ocean monitoring and science. Long-term measurements are essential because changes in pH and oxygen cannot usually be seen from the surface.

Learn the difference between acidification and an acidic ocean. The ocean remains alkaline overall. The problem is the rapid shift toward lower pH and the resulting changes in carbonate chemistry.

Connect land and ocean action. Decisions about energy, agriculture, wastewater and pollution on land can ultimately affect conditions beneath the ocean surface.

Individual action is useful, but the largest changes require coordinated climate policy, pollution control, scientific monitoring and effective marine management.

SOURCES

NOAA — What Is Ocean Acidification?
Explains CO₂ absorption, changes in seawater chemistry, coastal acidification and biological impacts. Ocean Acidification

NOAA — What Is Ocean Acidification?

NOAA — Ocean Acidification
Overview of changing ocean chemistry and reduced availability of calcium-carbonate minerals used by many marine organisms. NOAA

NOAA — Ocean Acidification

NOAA Ocean Service — Hypoxia
Explanation of low-oxygen waters, dead zones, nutrient pollution and the biological processes that consume dissolved oxygen. National Ocean Service

NOAA — Hypoxia

NOAA NCCOS — Coastal Ocean Acidification
Research on interactions among acidification, hypoxia, harmful algal blooms, nutrients and climate change in coastal environments. NCCOS

NOAA — Coastal Ocean Acidification

IPCC — Oceans and Coastal Ecosystems and Their Services
Comprehensive scientific assessment of ocean warming, acidification, deoxygenation and their effects on marine ecosystems. IPCC

IPCC — Oceans and Coastal Ecosystems

UN Sustainable Development Goal 14.3 — Ocean Acidification
International framework for monitoring average marine acidity and progress toward minimizing the impacts of ocean acidification. Sustainable Development Goals

UNEP — SDG 14.3 Ocean Acidification

Ocean Acidification & Hypoxia