Climate change has many effects on the ocean. One major effect is the increase in ocean temperatures. This leads to more frequent heatwaves in the ocean. As the ocean warms, sea levels rise because warm water expands and ice on land melts. Other effects include less sea ice, lower pH levels (which means seawater becomes more acidic), and less oxygen in the water. The ocean also becomes more stratified, meaning its layers mix less. These changes can affect ocean currents, such as weakening the Atlantic meridional overturning circulation. The main cause of these changes is the release of greenhouse gases from human activities, such as burning fossil fuels and cutting down forests. Examples of greenhouse gases include carbon dioxide and methane. These gases trap heat, causing the ocean to warm because the ocean absorbs most of the extra heat in the climate system. The ocean also takes in some carbon dioxide from the atmosphere, which lowers the pH of seawater. Scientists estimate that the ocean absorbs about 25% of all human-caused carbon dioxide emissions.

The ocean has layers with different temperatures, with colder water near the bottom. As the ocean surface warms, these temperature differences become more extreme. This reduces mixing between ocean layers, keeping warm water near the surface and slowing the movement of cold, deep water. This makes it harder for the ocean to absorb heat, so more heat stays in the atmosphere and on land. This can increase the energy available for tropical storms and reduce the amount of nutrients in the upper ocean layers, which harms fish. These changes also reduce the ocean’s ability to store carbon. At the same time, differences in salt levels are growing, with salty areas becoming saltier and less salty areas becoming even less salty.

Warmer water holds less oxygen than colder water. This causes oxygen to move from the ocean to the atmosphere. Increased stratification also reduces the flow of oxygen from surface waters to deeper waters, lowering oxygen levels further. The ocean has already lost oxygen throughout its layers, and areas with very low oxygen are growing worldwide.

These changes harm ocean ecosystems, leading to loss of species or changes in where species live. This can affect fishing and tourism. For example, rising ocean temperatures are damaging tropical coral reefs. Coral is sensitive to small temperature changes, causing them to bleach and die. Another example is the loss of sea ice, which harms animals like polar bears that depend on it. These ocean changes add stress to ecosystems already affected by other human activities.

Changes due to rising greenhouse gas levels

In 2020, the amount of carbon dioxide (CO₂) in Earth's atmosphere was over 410 parts per million (ppm). This is about 50% higher than levels before the Industrial Revolution. These high levels and how fast they are rising are the highest seen in the past 55 million years. Scientists have clearly shown that the extra CO₂ comes from human activities, such as burning fossil fuels, industrial processes, and changes in land use. Since the late 1950s, scientists have studied how the ocean absorbs a large part of the CO₂ humans release into the air. Evidence shows that the ocean takes in about a quarter of all human-caused CO₂ emissions.

In 2019, scientists reported new findings about how the ocean and climate are changing. They found that the ocean is warming because of climate change, and this warming is happening faster than before. In 2022, the ocean was the warmest it has ever been in human history. Scientists measure this by looking at how much heat the ocean holds, which was higher in 2022 than in 2021. The warming of the ocean is a direct result of the Earth absorbing more heat than it releases, which is mainly caused by rising greenhouse gas levels. Between the time before the Industrial Revolution and the 2011–2020 decade, the ocean’s surface warmed by 0.68 to 1.01 degrees Celsius.

Most of the heat the ocean gains comes from the Southern Ocean. For example, between the 1950s and 1980s, the Antarctic Southern Ocean warmed by 0.17 degrees Celsius, nearly twice the rate of the global ocean. The speed of warming depends on depth. The upper part of the ocean (above 700 meters) warms the fastest. Between 1981 and 2019, the ocean at 1,000 meters deep warmed by about 0.4 degrees Celsius every 100 years. At 2,000 meters deep, the warming was about 0.1 degrees Celsius every 100 years. In the Antarctic Ocean (at 55°S), the fastest warming was observed at 4,500 meters deep, with a rate of 0.3 degrees Celsius every 100 years.

A study from 2025 predicted that rising ocean temperatures and other climate changes will more than double the harm to marine ecosystems by mid-century. This will affect areas like the Arctic, Antarctic, tropical regions, and coasts, where many species and people depend on the ocean. Marine heatwaves also harm ocean life. For example, after a marine heatwave in the Pacific Northwest from 2019 to 2021, Bering Sea snow crab numbers dropped by 84% between 2018 and 2022, a loss of 9.8 billion crabs.

Scientists say marine heatwaves will become more common, last longer, cover more areas, and be more intense. This is because ocean surface temperatures will keep rising as the planet warms. The IPCC Sixth Assessment Report (2022) said that marine heatwaves have become more frequent, more intense, and longer since the 1980s. It also said these changes are very likely caused by human activities. A 2019 IPCC report confirmed that marine heatwaves have doubled in frequency and are now longer, more intense, and cover more areas. The 2022 report predicted that by 2081–2100, marine heatwaves will be four times more frequent than in 1995–2014 under a lower emissions scenario, or eight times more frequent under a higher emissions scenario.

Ocean temperatures vary by location. They are warmer near the equator and cooler near the poles. Changes in how much heat the ocean holds best show how the ocean is warming. Compared to 1969–1993, the ocean has absorbed more heat between 1993 and 2017.

Ocean heat content (OHC) or ocean heat uptake (OHU) measures the energy the ocean absorbs and stores. It is a key sign of global warming. Scientists calculate OHC by measuring ocean temperatures at many places and depths and combining these measurements.

Between 1971 and 2018, the ocean absorbed over 90% of the extra heat from global warming. Scientists estimate that from 1961 to 2022, the Earth’s warming trend was 0.43 ± 0.08 W/m², increasing by about 0.15 ± 0.04 W/m² every decade. By 2020, about one-third of the extra heat had reached depths below 700 meters. The five highest heat measurements to 2,000 meters deep all happened between 2020 and 2024. The main cause of this increase is human-caused greenhouse gas emissions.

Ocean acidification is the gradual decrease in the ocean’s pH level. From 1950 to 2020, the average pH of the ocean’s surface dropped from about 8.15 to 8.05. Human CO₂ emissions are the main cause of this change. As CO₂ from the air dissolves in the ocean, it forms carbonic acid, which breaks into bicarbonate ions and hydrogen ions. The hydrogen ions lower the ocean’s pH, making it more acidic. However, the ocean is still alkaline, with a pH higher than 8. Organisms that build shells or skeletons, like mollusks and corals, are especially affected because they rely on calcium carbonate for their structures.

A change in pH by 0.1 means a 26% increase in hydrogen ions in the ocean. The pH and carbonate levels vary with depth and location. Colder and higher latitude waters absorb more CO₂, which can increase acidity and lower pH and carbonate levels in these areas. Other factors that influence CO₂ exchange between the atmosphere and ocean include ocean currents, upwelling zones, proximity to rivers, sea ice, and emissions from fossil fuels and agriculture.

Lower ocean pH can harm marine life. Scientists have seen reduced shell-building, weaker immune systems, and less energy for reproduction in some species. Ocean acidification can affect ecosystems that support food and livelihoods for many people. About one billion people rely on services like fishing, tourism, and coastal management provided by coral reefs. Continued acidification may threaten ocean-based food chains.

Some parts of the climate system respond slowly to warming. For example, acidification in the deep ocean will last for thousands of years, and the ocean will continue to hold heat for centuries even if greenhouse gas emissions stop. Similarly, sea level rise will continue for centuries or longer because ice sheets melt slowly and the ocean

Effects on the physical environment

Many coastal cities will face coastal flooding in the coming decades and beyond. Local ground sinking, which can happen naturally or be made worse by human actions, can make flooding worse. By 2050, coastal flooding could threaten hundreds of millions of people, especially in Southeast Asia.

Sea levels have been rising since the end of the Last Glacial Maximum, which was about 20,000 years ago. From 1901 to 2018, the average sea level rose by 15 to 25 centimeters (6 to 10 inches), with an increase of 2.3 millimeters (0.091 inches) each year since the 1970s. This rise was faster than it had been in at least the past 3,000 years. The rate increased to 4.62 millimeters (0.182 inches) per year between 2013 and 2022. Human-caused climate change is the main reason for this faster rise. Between 1993 and 2018, melting ice sheets and glaciers contributed 44% of the sea level rise, and 42% came from water expanding as it warmed.

Ocean currents are caused by differences in temperature, salt, and wind. Warm air rises near the equator and moves toward the poles, where it cools and sinks. Cool air near the poles then moves back toward the equator, creating large wind patterns called Hadley cells. These wind patterns drive surface currents that push water toward colder areas. When the water cools, it becomes denser and sinks, forming deep water in the North Atlantic and Antarctic regions.

These currents help move water around the ocean. However, global warming changes this process. As glaciers and ice caps melt, more fresh water enters high-latitude areas where deep water forms. This lowers the density of surface water, slowing how quickly it sinks.

The Atlantic Meridional Overturning Circulation (AMOC) may have weakened since the preindustrial era, but scientists are not certain. Climate change projections suggest the AMOC is likely to weaken further during the 21st century. This weakening could greatly affect global climate, especially in the North Atlantic.

Changes in ocean currents impact the ocean’s ability to absorb carbon dioxide and support marine life. Currents move nutrients that help ocean plants grow. Because the AMOC moves water very slowly (taking hundreds to thousands of years to circulate the entire ocean), it responds slowly to climate change.

Ocean stratification, or the layering of water based on density, is important for ocean health. Warmer surface water is less dense than colder deep water, creating layers that limit mixing between the ocean’s surface and deeper areas. Since 1970, ocean stratification has increased due to global warming and changes in salt levels. In tropical areas, evaporation increases salt levels, while melting ice in polar regions decreases salt levels.

Water density depends on temperature, salt, and pressure. Warmer surface water is less dense than deep water, causing stratification. This affects the AMOC and the movement of nutrients from deep water to the surface, which supports ocean productivity and helps oxygen from the atmosphere reach the deep sea.

Climate change affects oxygen levels in both coastal and open ocean areas. Some parts of the open ocean naturally have low oxygen, called oxygen minimum zones. These zones are isolated from atmospheric oxygen and lose oxygen as organic matter breaks down. Warming oceans reduce water circulation and oxygen solubility, causing these zones to expand.

Overall, ocean oxygen levels have decreased by 2% over 50 years since the 1960s. These low-oxygen areas are more common in the Pacific Ocean. Low oxygen harms marine life, and these zones are expected to grow larger in the future.

Coastal areas also face oxygen problems. Increased nutrients from rivers can lead to excessive organic matter sinking, causing extreme oxygen loss, or "dead zones." These zones are growing due to more nutrients and increased ocean stratification from climate change.

Satellite images show that oceans are becoming greener as climate change continues. This may be due to changes in plankton populations. These changes could also make oceans darker, as water properties change and less light reaches deeper layers.

Climate change is causing the ocean to warm, which affects Earth’s climate and weather. Warmer oceans can make tropical storms and monsoons stronger, increase weather extremes, and change rainfall patterns. Some areas may get wetter, while others become drier. Wind changes may also increase wave heights in some regions.

Human-caused climate change continues to warm the ocean, which stores past warming effects. This increases ocean heat and surface temperatures, making tropical storms more intense, larger, longer-lasting, and causing heavier rainfall. For example, Hurricane Harvey in 2017 was more severe because of warmer ocean temperatures.

Climate change affects tropical storms in several ways: stronger rainfall and winds, more frequent intense storms, and storms reaching maximum intensity farther from the equator. Tropical storms rely on warm, moist air for energy. As ocean temperatures rise, more of this energy becomes available, making storms more powerful.

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Impacts on biological processes

Photosynthesis in the surface ocean releases oxygen and uses carbon dioxide. This process is mainly done by phytoplankton, which are tiny, floating algae. After plants grow, bacteria break down the organic matter made by photosynthesis, using oxygen and releasing carbon dioxide. When some organic matter sinks to deep ocean water, where it does not touch the atmosphere, oxygen levels decrease, and carbon dioxide, carbonate, and bicarbonate increase. This movement of carbon dioxide in the ocean is an important part of the global carbon cycle.

Photosynthesis in surface waters uses nutrients like nitrogen and phosphorus. These nutrients are sent to deep water when organic matter from photosynthesis sinks after the organisms die. Surface water productivity depends partly on nutrients returning to the surface through ocean mixing and currents. Climate change is increasing ocean stratification, which generally reduces ocean productivity. However, in some areas, such as regions that were once covered by ice, productivity may increase. This trend is already happening and is expected to continue. For example, productivity in the Indian Ocean has decreased over the past sixty years due to warming and is projected to keep declining.

Under a very high emission scenario (RCP8.5), ocean productivity is likely to drop by 4-11% by 2100. This decline will vary by region. For example, tropical ocean productivity may fall by 7–16% under the same scenario. Less organic matter will sink to deeper layers because of increased ocean stratification and fewer nutrients. The drop in productivity is due to the combined effects of warming, stratification, light, nutrients, and predation.

The full effects of changes in calcification caused by ocean acidification are complex. However, many calcifying species may be harmed by acidification. Acidification makes it harder for organisms that build shells to access carbonate ions, which are needed for their hard shells. These organisms range from autotrophs to heterotrophs and include coccolithophores, corals, foraminifera, echinoderms, crustaceans, and molluscs.

All marine ecosystems will face changes in acidification and other ocean biogeochemical changes. Acidification may force some organisms to use resources for calcification instead of other functions. For example, the oyster Magallana gigas experiences metabolic changes and altered calcification rates due to energy trade-offs caused by pH imbalances.

Although the causes of harmful algal blooms (HABs) are not fully understood, they have become more common and widespread in coastal areas since the 1980s. This is linked to human activities, such as nutrient pollution and climate change, especially warmer water temperatures. Factors affecting HABs include ocean warming, marine heatwaves, oxygen loss, eutrophication, and water pollution. These increases are concerning because HABs impact food security, tourism, and the economy.

It is also possible that the observed rise in HABs is due to more severe effects and better monitoring, not necessarily climate change.

Impacts on coral reefs and fisheries

Coral reefs are underwater ecosystems formed by colonies of reef-building corals. These corals are held together by calcium carbonate. Coral reefs support a wide variety of marine life and are essential for millions of people who depend on them for coastal protection, food, and tourism.

Warm water corals are declining rapidly. Over the past 30–50 years, about half of these corals have been lost due to rising ocean temperatures, ocean acidification, pollution, and physical damage from human activities like fishing. These threats are expected to become even worse in the future.

When ocean temperatures rise, corals can bleach. Bleaching happens when corals expel the algae that live inside their tissues and give corals their bright colors. A temperature increase of 1–2°C can cause bleaching, which turns corals white. If bleaching lasts too long, corals may die. According to the IPCC Sixth Assessment Report from 2022, mass coral bleaching events have become more frequent and severe since the 1980s. Marine heatwaves have caused large-scale coral deaths. If global temperatures rise more than 1.5°C, many coral reefs may experience permanent damage or loss.

Ocean acidification, which lowers the pH of seawater, also harms coral reefs. It reduces the diversity of coralline algae, which are important for reef structure. The ability of these algae to form hard structures depends on how they respond to changes in ocean chemistry.

Coral growth depends on the balance of minerals in the water. Corals build their skeletons using aragonite, a type of calcium carbonate. If the surrounding water has lower levels of aragonite, corals must work harder to maintain the right balance inside their bodies. This slows their growth. If aragonite levels are too low, corals may stop growing entirely. By 2050–60, about 70% of North Atlantic cold-water corals may live in water that is corrosive to their skeletons due to rising carbon emissions.

Climate change affects fisheries in many ways. Rising ocean temperatures, acidification, and lower oxygen levels harm marine ecosystems. Freshwater ecosystems are also affected by changes in water temperature, flow, and fish habitats. These effects vary by region. Climate change is changing where fish live and how productive they are. It is expected to reduce the availability and trade of fish products, with the greatest impacts in tropical regions, especially in the South Pacific.

Climate change affects the sustainability of fisheries and aquaculture, the livelihoods of people who rely on fishing, and the ocean’s ability to capture and store carbon. Rising sea levels harm coastal fishing communities, while changes in rainfall and water use affect inland freshwater fisheries. Climate change also increases risks of floods, diseases, parasites, and harmful algal blooms, which can damage aquaculture production and infrastructure.

It is predicted that climate change could reduce global fish community biomass by up to 30% by 2100.

Impacts on marine mammals

Some effects on marine mammals, especially those in the Arctic, are very direct, such as loss of habitat, temperature stress, and exposure to severe weather. Other effects are more indirect, such as changes in how diseases spread, changes in body condition due to predator-prey relationships, changes in exposure to toxins and CO₂ emissions, and increased human interactions. Even though ocean warming can greatly affect marine mammals, scientists still do not fully understand how vulnerable these animals are to global warming.

Marine mammals have evolved to live in the ocean, but climate change is changing their natural habitat. Some species may not adapt quickly enough, which could lead to their extinction.

It has been generally believed that Arctic marine mammals are the most vulnerable to climate change because of the large loss of sea ice in the Arctic. However, research has shown that species in the North Pacific Ocean, the Greenland Sea, and the Barents Sea are actually the most vulnerable to global warming. The North Pacific is already a place where human activities, such as shipping, pollution, and oil and gas development, harm marine mammals. Now, it is also a place where marine mammals face added risks from global warming. These combined threats could cause lasting harm to the ecosystems in this region.

Marine animals usually live in waters with more stable temperatures than land animals, so they may be more sensitive to changes in ocean temperature. As the ocean warms, some species may move to cooler areas to find suitable habitats. If ocean temperatures keep rising, some species may leave certain areas or shrink their global range. Changes in the number of some species can affect the food available to marine mammals, causing them to move to new areas. If a species cannot move to a cooler area or adapt to warmer water, it may face extinction.

The loss of Arctic sea ice reduces the habitat available for marine mammals, raises water and air temperatures, and increases the frequency of severe weather. Losing sea ice habitat can reduce the number of seals, which are a key food source for marine mammals like polar bears. Changes in sea ice can also affect animal health by altering how diseases spread, changing the balance of prey and predators, and increasing exposure to harmful substances due to more human activity in the Arctic.

Sea level rise is also important when considering how global warming affects marine mammals because it changes coastal areas that many species depend on.

Seals are another type of marine mammal that are affected by climate change. Like polar bears, some seal species rely on sea ice for breeding and raising their young. In 2010 and 2011, sea ice in the Northwest Atlantic reached record lows, and harp seals and ringed seals that bred on thin ice had higher death rates. In South Georgia, Antarctic fur seals experienced extreme population declines over 20 years, during which scientists recorded higher-than-normal sea surface temperatures.

Climate change has had a major impact on dolphins. For example, in the Mediterranean Sea, rising ocean temperatures, salt levels, and changes in ocean currents have reduced food supplies, leading to a sharp drop in the population of short-beaked common dolphins, which are now classified as endangered. In Shark Bay, Western Australia, a marine heatwave in 2011 caused a significant decline in the local population of Indo-Pacific bottlenose dolphins. River dolphins are also greatly affected by climate change because of higher water temperatures, drier conditions, and more acidic water.

Dolphins live in many parts of the world, making them vulnerable to climate change in different ways. One major effect is the rise in ocean temperatures, which has caused many dolphin species to move to cooler waters. Another effect is the increase in harmful algae blooms, which has led to the deaths of many bottlenose dolphins.

Human-caused climate change is a growing threat to right whales. Scientific studies show that climate change affects their reproduction, movement patterns, access to food, interactions with humans, and overall health.

Changes in ocean currents and water temperatures caused by climate change have altered how right whales find food and use their habitats. Warmer waters have reduced the number of an important food source, the zooplankton Calanus finmarchicus. This decline in food affects the health of right whales in several ways. The most direct effects are on the survival and ability to reproduce of individual whales. Lower numbers of C. finmarchicus have been linked to poor health, malnutrition, and difficulty giving birth and raising calves.

Potential feedback effects

Rising ocean temperatures can affect methane clathrate reservoirs found in ocean floor sediments. These reservoirs store large amounts of the greenhouse gas methane, which could be released if ocean temperatures increase. However, it is currently considered unlikely that methane in underwater clathrates will cause a noticeable change in emissions during this century.

In 2004, the worldwide estimate of ocean methane clathrates was between one and five million cubic kilometers.