Effects of climate change on oceans
There are many effects of climate change on oceans. One of the most important is an increase in ocean temperatures. More frequent marine heatwaves are linked to this. The rising temperature contributes to a rise in sea levels due to the expansion of water as it warms and the melting of ice sheets on land. Other effects on oceans include sea ice decline, reducing pH values and oxygen levels, as well as increased ocean stratification. All this can lead to changes of ocean currents, for example a weakening of the Atlantic meridional overturning circulation. The main cause of these changes are the emissions of greenhouse gases from human activities, mainly burning of fossil fuels and deforestation. Carbon dioxide and methane are examples of greenhouse gases. The additional greenhouse effect leads to ocean warming because the ocean takes up most of the additional heat in the climate system. The ocean also absorbs some of the extra carbon dioxide that is in the atmosphere. This causes the pH value of the seawater to drop. Scientists estimate that the ocean absorbs about 25% of all human-caused emissions.
The various layers of the oceans have different temperatures. For example, the water is colder towards the bottom of the ocean. This temperature stratification will increase as the ocean surface warms due to rising air temperatures. Connected to this is a decline in mixing of the ocean layers, so that warm water stabilises near the surface. A reduction of cold, deep water circulation follows. The reduced vertical mixing makes it harder for the ocean to absorb heat. So a larger share of future warming goes into the atmosphere and land. One result is an increase in the amount of energy available for tropical cyclones and other storms. Another result is a decrease in nutrients for fish in the upper ocean layers. These changes also reduce the ocean's capacity to store carbon. At the same time, contrasts in salinity are increasing. Salty areas are becoming saltier and fresher areas less salty.
Warmer water cannot contain the same amount of oxygen as cold water. As a result, oxygen from the oceans moves to the atmosphere. Increased thermal stratification may reduce the supply of oxygen from surface waters to deeper waters. This lowers the water's oxygen content even more. The ocean has already lost oxygen throughout its water column. Oxygen minimum zones are increasing in size worldwide.
These changes harm marine ecosystems, and this can lead to biodiversity loss or changes in species distribution. This in turn can affect fishing and coastal tourism. For example, rising water temperatures are harming tropical coral reefs. The direct effect is coral bleaching on these reefs, because they are sensitive to even minor temperature changes. So a small increase in water temperature could have a significant impact in these environments. Another example is loss of sea ice habitats due to warming. This will have severe impacts on polar bears and other animals that rely on it. The effects of climate change on oceans put additional pressures on ocean ecosystems which are already under pressure by other impacts from human activities.
Changes due to rising greenhouse gas levels
Presently, atmospheric carbon dioxide levels of more than 410 parts per million are nearly 50% higher than preindustrial levels. These elevated levels and rapid growth rates are unprecedented in the geological record's 55 million years. The source for this excess is clearly established as human-driven, reflecting a mix of fossil fuel burning, industrial, and land-use/land-change emissions. The idea that the ocean serves as a major sink for anthropogenic has been discussed in scientific literature since at least the late 1950s. Several pieces of evidence point to the ocean absorbing roughly a quarter of total anthropogenic emissions.The latest key findings about the observed changes and impacts from 2019 include:
Rising ocean temperature
It is clear that the ocean is warming as a result of climate change, and this rate of warming is increasing. The global ocean was the warmest it had ever been recorded by humans in 2022. This is determined by the ocean heat content, which exceeded the previous 2021 maximum in 2022. The steady rise in ocean temperatures is an unavoidable result of the Earth's energy imbalance, which is primarily caused by rising levels of greenhouse gases. Between pre-industrial times and the 2011–2020 decade, the ocean's surface has heated between 0.68 and 1.01 °C.The majority of ocean heat gain occurs in the Southern Ocean. For example, between the 1950s and the 1980s, the temperature of the Antarctic Southern Ocean rose by 0.17 °C, nearly twice the rate of the global ocean.
The warming rate varies with depth. The upper ocean is warming the fastest. At an ocean depth of a thousand metres the warming occurs at a rate of nearly 0.4 °C per century. In deeper zones of the ocean, at 2000 metres depth, the warming has been around 0.1 °C per century. The warming pattern is different for the Antarctic Ocean, where the highest warming has been observed at a depth of 4500 m.
A study published in 2025 projected that rising ocean temperatures, together with other climate-driven stressors, will more than double cumulative impacts on marine ecosystems by mid-century. It particularly affects in the Arctic, Antarctic, tropical regions, and coastal areas where biodiversity and human reliance are highest.
Marine heatwaves
also take their toll on marine life: For example, due to fall-out from the 2019-2021 Pacific Northwest marine heatwave, Bering Sea snow crab populations declined 84% between 2018 and 2022, a loss of 9.8 billion crabs.Ocean heat content
The ocean temperature varies from place to place. Temperatures are higher near the equator and lower at the poles. As a result, changes in total ocean heat content best illustrate ocean warming. When compared to 1969–1993, heat uptake has increased between 1993 and 2017.Ocean acidification
Time scales
Many ocean-related elements of the climate system respond slowly to warming. For instance, acidification of the deep ocean will continue for millennia, and the same is true for the increase in ocean heat content. Similarly, sea level rise will continue for centuries or even millennia even if greenhouse gas emissions are brought to zero, due to the slow response of ice sheets to warming and the continued uptake of heat by the oceans, which expand when warmed.Effects on the physical environment
Sea level rise
Many coastal cities will experience coastal flooding in the coming decades and beyond. Local subsidence, which may be natural but can be increased by human activity, can exacerbate coastal flooding. Coastal flooding will threaten hundreds of millions of people by 2050, particularly in Southeast Asia.Changing ocean currents
are caused by temperature variations caused by sunlight and air temperatures at various latitudes, as well as prevailing winds and the different densities of salt and fresh water. Warm air rises near the equator. Later, as it moves toward the poles, it cools again. Cool air sinks near the poles, but warms and rises again as it moves toward the equator. This produces Hadley cells, which are large-scale wind patterns, with similar effects driving a mid-latitude cell in each hemisphere. Wind patterns associated with these circulation cells drive surface currents which push the surface water to higher latitudes where the air is colder. This cools the water, causing it to become very dense in comparison to lower latitude waters, causing it to sink to the ocean floor, forming North Atlantic Deep Water in the north and Antarctic Bottom Water in the south.Driven by this sinking and the upwelling that occurs in lower latitudes, as well as the driving force of the winds on surface water, the ocean currents act to circulate water throughout the sea. When global warming is factored in, changes occur, particularly in areas where deep water is formed. As the oceans warm and glaciers and polar ice caps melt, more and more fresh water is released into the high latitude regions where deep water forms, lowering the density of the surface water. As a result, the water sinks more slowly than it would normally.
The Atlantic Meridional Overturning Circulation may have weakened since the preindustrial era, according to modern observations and paleoclimate reconstructions, but there is too much uncertainty in the data to know for certain. Climate change projections assessed in 2021 indicate that the AMOC is very likely to weaken over the course of the 21st century. A weakening of this magnitude could have a significant impact on global climate, with the North Atlantic being particularly vulnerable.
Any changes in ocean currents affect the ocean's ability to absorb carbon dioxide as well as ocean productivity because the currents transport nutrients. Because the AMOC deep ocean circulation is slow, it is slow to respond to climate change.
Increasing stratification
Changes in ocean stratification are significant because they can influence productivity and oxygen levels. The separation of water into layers based on density is known as stratification. Stratification by layers occurs in all ocean basins. The stratified layers limit how much vertical water mixing takes place, reducing the exchange of heat, carbon, oxygen and particles between the upper ocean and the interior. Since 1970, there has been an increase in stratification in the upper ocean due to global warming and, in some areas, salinity changes. The salinity changes are caused by evaporation in tropical waters, which results in higher salinity and density levels. Meanwhile, melting ice can cause a decrease in salinity at higher latitudes.Temperature, salinity and pressure all influence water density. As surface waters are often warmer than deep waters, they are less dense, resulting in stratification. This stratification is crucial not just in the production of the Atlantic Meridional Overturning Circulation, which has worldwide weather and climate ramifications, but it is also significant because stratification controls the movement of nutrients from deep water to the surface. This increases ocean productivity and is associated with the compensatory downward flow of water that carries oxygen from the atmosphere and surface waters into the deep sea.