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.
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 (NADW) in the north and Antarctic Bottom Water (AABW) 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 (AMOC) may have weakened since the preindustrial era, according to modern observations and paleoclimate reconstructions (the AMOC is part of a global thermohaline circulation), 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 (which is affected by water temperature) as well as ocean productivity because the currents transport nutrients (see Impacts on phytoplankton and net primary production). Because the AMOC deep ocean circulation is slow (it takes hundreds to thousands of years to circulate the entire ocean), it is slow to respond to climate change.
Changes in ocean stratification are significant because they ca…

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The process of photosynthesis in the surface ocean releases oxygen and consumes carbon dioxide. This photosynthesis in the ocean is dominated by phytoplankton – microscopic free-floating algae. After the plants grow, bacterial decomposition of the organic matter formed by photosynthesis in the ocean consumes oxygen and releases carbon dioxide. The sinking and bacterial decomposition of some organic matter in deep ocean water, at depths where the waters are out of contact with the atmosphere, leads to a reduction in oxygen concentrations and increase in carbon dioxide, carbonate and bicarbonate. This cycling of carbon dioxide in oceans is an important part of the global carbon cycle.

The photosynthesis in surface waters consumes nutrients (e.g. nitrogen and phosphorus) and transfers these nutrients to deep water as the organic matter produced by photosynthesis sinks upon the death of the organisms. Productivity in surface waters therefore depends in part on the transfer of nutrients from deep water back to the surface by ocean mixing and currents. The increasing stratification of the oceans due to climate change therefore acts generally to reduce ocean productivity. However, in some areas, such as previously ice covered regions, productivity may increase. This trend is already observable and is projected to continue under current projected climate change. In the Indian Ocean for example, productivity is estimated to have declined over the past sixty years due to climate warming and is projected to continue.

Ocean productivity under a very high emission scenario (RCP8.5) is very likely to drop by 4-11% by 2100. The decline will show regional variations. For example, the tropical ocean NPP will decline more: by 7–16% for the same emissions scenario. Less organic matter will likely sink from the upper oceans into deeper ocean layers due to increased ocean stratification and a reduction in nutrient supply. The reduction in ocean productivity is due to the "combined effects of warming, stratification, light, nutrients and predation".
Although the drivers of harmful algal blooms (HABs) are poorly understood, they appear to have increased in range and frequency in coastal areas since the 1980s. This is the result of human induced factors such as increased nutrient inputs (nutrient pollution) and climate change (in particular the warming of water temperatures). The parameters that affect the formation of HABs are ocean warming, marine heatwaves, oxygen loss, eutrophication and water pollution. These increases in HABs are of concern because of the impact of their occurrence on local food security, tourism and the economy.

It is however also possible that the perceived increase in HABs globally is simply due to more severe bloom impacts and bett…

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While some mobile marine species can migrate in response to climate change, others such as corals find this much more difficult. A coral reef is an underwater ecosystem characterised by reef-building corals. Reefs are formed by colonies of coral polyps held together by calcium carbonate. Coral reefs are important centres of biodiversity and vital to millions of people who rely on them for coastal protection, food and for sustaining tourism in many regions.

Warm water corals are clearly in decline, with losses of 50% over the last 30–50 years due to multiple threats from ocean warming, ocean acidification, pollution and physical damage from activities such as fishing. These pressures are expected to intensify.

The warming ocean surface waters can lead to bleaching of the corals which can cause serious damage and/or coral death. The IPCC Sixth Assessment Report in 2022 found that: "Since the early 1980s, the frequency and severity of mass coral bleaching events have increased sharply worldwide". Marine heatwaves have caused coral reef mass mortality. It is expected that many coral reefs will suffer irreversible changes and loss due to marine heatwaves with global temperatures increasing by more than 1.5 °C.

Coral bleaching occurs when thermal stress from a warming ocean results in the expulsion of the symbiotic algae that resides within coral tissues. These symbiotic algae are the reason for the bright, vibrant colors of coral reefs. A 1-2°C sustained increase in seawater temperatures is sufficient for bleaching to occur, which turns corals white. If a coral is bleached for a prolonged period of time, death may result. In the Great Barrier Reef, before 1998 there were no such events. The first event happened in 1998 and after that, they began to occur more frequently. Between 2016 and 2020 there were three of them.

Apart from coral bleaching, the reducing pH value in oceans is also a problem for coral reefs because ocean acidification reduces coralline algal biodiversity. The physiology of coralline algal calcification determines how the algae will respond to ocean acidification.