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Oceans

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Ocean acidification

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Estimated annual mean sea surface anthropogenic dissolved inorganic carbon concentration for the present day (normalised to year 2002) from the Global Ocean Data Analysis Project v2 (GLODAPv2) climatology.
Annual mean sea surface dissolved oxygen from the World Ocean Atlas 2009. Dissolved oxygen here is in mol O2m−3.

Ocean acidification poses a severe threat to the earth's natural process of regulating atmospheric CO2 levels.[1] Atmospheric CO2 emissions have increased by almost 50% from preindustrial levels of 280 ppm (part per million) to nearly 420 ppm today. Due to high proportion of Earth that oceans represent and the buffering capacity of seawater for CO2, ocean absorbs up to 25% of atmospheric carbon dioxide, lessening the effects of climate change.[1] Oceanic uptake of CO2 decreases with increasing atmospheric CO2 concentrations as the buffering capacity becomes reduced.[2][3] As atmospheric CO2 is mixed in with seawater it forms carbonic acid, which then dissociates into free hydrogen ions (H+), bicarbonate (HCO3-), and carbonate ions (CO32-). As H+ ions increase, so does the ocean's acidity, decreasing its ph by up to 0.1 per 100 ppm of atmospheric CO2.[1] Following gas exchange with the atmosphere, CO2 becomes aqueous and mixes in with the surface layers of the ocean as dissolve inorganic carbon (DIC) before being transported by ocean currents to deeper waters. Ocean pH has already decreased from 8.2 to 8.1 since preindustrial levels and is expected to continue decreasing with time.[4] The increase of ocean acidity also decelerates the rate of calcification in salt water, leading to slower growing reefs which supports approximately 25% of marine life.[5][1] As seen with the great barrier reef, the increase in ocean acidity in not only killing the coral, but also the wildly diverse population of marine inhabitants which coral reefs support.[6]

Dissolved oxygen

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Another issue faced by increasing global temperatures is the decrease of the ocean's ability to dissolve oxygen, one with potentially more severe consequences than other repercussions of global warming.[7] Ocean depths between 100 meters and 1,000 meters are known as "oceanic mid zones" and host a plethora of biologically diverse species, one of which being zooplankton.[8] Zooplankton feed on smaller organisms such as phytoplankton, which are an integral part of the marine food web.[9] Phytoplankton perform photosynthesis, receiving energy from light, and provide sustenance and energy for the larger zooplankton, which provide sustenance and energy for the even larger fish, and so on up the food chain.[9] The increase in oceanic temperatures lowers the ocean's ability to retain oxygen generated from phytoplankton, and therefore reduces the amount of bioavailable oxygen that fish and other various marine wildlife rely on for their survival.[8] This creates marine dead zones, and the phenomenon has already generated multiple marine dead zones around the world, as marine currents effectively "trap" the deoxygenated water.

Algal blooms

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Climate change and a warming ocean can increase the frequency and the magnitude of algal bloom. There is evidence that harmful algal blooms have increased in recent decades, resulting in impacts ranging from public health, tourism, aquaculture, fisheries, to ecosystem Such events may result in changes in temperature, stratification, light, ocean acidification, increased nutrients, and grazing.[10] As climate change continues, harmful algal blooms will likely exhibit spatial and temporal shifts under future conditions.[10] Spatially, species may experience range expansion, contraction, or latitudinal shifts, while temporally, the seasonal windows of growth may expand or shorten.[10] In 2019, the biggest Sargassum bloom ever seen created a crisis in the Tourism industry in North America. The event was probably caused by Climate Change and Fertilizers. Several Caribbean countries, even considered declaring a state of emergency due to the impact on tourism. The bloom can benefit the marine life, but, can also block the sunlight necessary for it.[11]

Impact on calcifying organisms

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Marine calcifying organisms use CO32- ions to form their shells and reefs. As ocean acidification continues, calcium carbonate (CaCO3) saturation states, a measure of CO32- in seawater are lowered, inhibiting calcifying organisms from building their shells and structures.[12] Increased anthropogenic CO2 invasion into the ocean results in fewer carbonate ions for shell and reef-forming organisms due to an increase in H+ ions, resulting in fewer and smaller calcifying organisms.[4][13]

Impact on phytoplankton

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Phytoplankton are vital to Earth systems forming the base of nearly every marine food web, as well as producing nearly 50% of oxygen in the atmosphere.[14] Critical for global ecosystem functioning and services, phytoplankton are vary with environmental parameters such as, temperature, water column mixing, nutrients, sunlight, and consumption by grazers.[15] Climate change results in fluctuations and modification of these parameters, which in turn may impact phytoplankton community composition, structure, and annual and seasonal dynamics.[15] These impacts then affect the entire marine system. Satellite measurement and chlorophyll observations show decline in the number of phytoplankton, microorganisms that produce half of the earth's oxygen, absorb half of the world carbon dioxide and serve foundation of the entire marine food chain. The decline is probably linked to climate change.[16][17][18] However, there are some measurements that show increases in the number of phytoplankton.[19]

Coral bleaching

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The warming ocean surface waters can lead to bleaching of the corals which can cause serious damage and/or coral death. Coral bleaching occurs when . In the Great Barrier Reef, before 1998 there were no such events. The first event happened in 1998 and after it they begun to occur more and more frequently so in the years 2016 - 2020 there were 3 of them.[20]

Combined impact

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Eventually the planet could warm to such a degree that the ocean's ability to dissolve oxygen would no longer exist, resulting in a worldwide dead zone.[8] Dead zones, in combination with ocean acidification, may usher in an era where marine life in most forms would cease to exist, resulting in a sharp decline in the amount of oxygen generated through photosynthesis in surface waters.[8] This disruption to the food chain will cascade upward, thinning out populations of primary consumers, secondary consumers, tertiary consumers, etc., as primary consumers being the initial victims of these phenomenon.

Marine wildlife

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This section is an excerpt from Effects of climate change on oceans.[edit]
Overview of climatic changes and their effects on the ocean. Regional effects are displayed in italics.[21]

There are many effects of climate change on oceans. One of the main ones 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 melting ice sheets. 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 (AMOC).[22] The main root cause of these changes are the emissions of greenhouse gases from human activities, mainly burning of fossil fuels. 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.[23] 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.[24] Scientists estimate that the ocean absorbs about 25% of all human-caused CO2 emissions.[24]

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.[25]: 471  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.[26] At the same time, contrasts in salinity are increasing. Salty areas are becoming saltier and fresher areas less salty.[27]

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.[28] The ocean has already lost oxygen throughout its water column. Oxygen minimum zones are increasing in size worldwide.[25]: 471 

These changes harm marine ecosystems, and this can lead to biodiversity loss or changes in species distribution.[22] 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.[22]

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