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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Ocean deoxygenation | 2/4 | https://en.wikipedia.org/wiki/Ocean_deoxygenation | reference | science, encyclopedia | 2026-05-05T07:35:51.047511+00:00 | kb-cron |
While oxygen minimum zones (OMZs) occur naturally, they can be exacerbated by human impacts like climate change and land-based pollution from agriculture and sewage. The prediction of current climate models and climate change scenarios is that substantial warming and loss of oxygen throughout the majority of the upper ocean will occur. Global warming increases ocean temperatures, especially in shallow coastal areas. As water temperature increases, its ability to hold oxygen decreases, resulting in lower oxygen concentrations. This compounds the effects of eutrophication in coastal zones described above. Open ocean areas with no oxygen have grown more than 1.7 million square miles in the last 50 years, and coastal waters have seen a tenfold increase in low-oxygen areas in the same time. Measurements of dissolved oxygen in coastal and open-ocean waters over the past 50 years have revealed a marked decline in oxygen content. This decline is associated with expanding spatial extent, expanding vertical extent, and prolonged duration of oxygen-poor conditions in all regions of the global oceans. Examinations of the spatial extent of OMZs in the past through paleoceanographical methods clearly show that the spatial extent of OMZs has expanded through time, and this expansion is coupled to ocean warming and reduced ventilation of thermocline waters. Research has sought to model potential changes to OMZs driven by rising global temperatures and human impacts. This is challenging due to the many factors that could contribute to changes in OMZs. The factors used for modeling change in OMZs are numerous, and in some cases hard to measure or quantify. Some of the processes being studied are changes in oxygen gas solubility as a result of rising ocean temperatures, as well as changes in the amount of respiration and photosynthesis occurring around OMZs. Many studies have concluded that OMZs are expanding in multiple locations, but fluctuations of modern OMZs are still not fully understood. Existing Earth system models project considerable reductions in oxygen and other physical-chemical variables in the ocean due to climate change, with potential ramifications for ecosystems and humans. The global decrease in oceanic oxygen content is statistically significant and emerging beyond the envelope of natural fluctuations. This trend of oxygen loss is accelerating, with widespread and obvious losses occurring after the 1980s. The rate and total content of oxygen loss varies by region, with the North Pacific emerging as a particular hotspot of deoxygenation due to the increased amount of time since its deep waters were last ventilated (see thermohaline circulation) and related high apparent oxygen utilization (AOU). Estimates of total oxygen loss in the global ocean range from 119 to 680 T mol decade−1 since the 1950s. These estimates represent 2% of the global ocean oxygen inventory. Melting of gas hydrates in the bottom layers of water may result in the release of more methane from sediments and subsequent consumption of oxygen by aerobic respiration of methane to carbon dioxide. Another effect of climate change on oceans that causes ocean deoxygenation is circulation changes. As the ocean warms from the surface, stratification is expected to increase, slowing ocean circulation and further deoxygenating the ocean.
=== Estimates for the future === The results from mathematical models show that global ocean oxygen loss rates will continue to accelerate up to 125 T mol year−1 by 2100 due to persistent warming, a reduction in ventilation of deeper waters, increased biological oxygen demand, and the associated expansion of OMZs into shallower areas.
== Variations ==
=== Expanding oxygen minimum zones (OMZ) === Several areas of the open ocean have naturally low oxygen concentration due to biological oxygen consumption that cannot be supported by the rate of oxygen input to the area from physical transport, air-sea mixing, or photosynthesis. These areas are called oxygen minimum zones (OMZs), and there is a wide variety of open ocean systems that experience these naturally low oxygen conditions, such as upwelling zones, deep basins of enclosed seas, and the cores of some mode-water eddies. Ocean deoxygenation has led to suboxic, hypoxic, and anoxic conditions in both coastal waters and the open ocean. Since 1950, more than 500 sites in coastal waters have reported oxygen concentrations below 2 mg liter−1, which is generally accepted as the threshold of hypoxic conditions. The extent of OMZs has expanded in tropical oceans during the past half century. Oxygen-poor waters of coastal and open ocean systems have largely been studied in isolation of each other, with researchers focusing on eutrophication-induced hypoxia in coastal waters and naturally occurring (without apparent direct input of anthropogenic nutrients) open ocean OMZs. However, coastal and open ocean oxygen-poor waters are highly interconnected and therefore both have seen an increase in the intensity, spatial extent, and temporal extent of deoxygenated conditions.
The spatial extent of deoxygenated conditions can vary widely. In coastal waters, regions with deoxygenated conditions can extend from less than one to many thousands of square kilometers. Open ocean OMZs exist in all ocean basins and have similar variation in spatial extent; an estimated 8% of global ocean volume is within OMZs. The largest OMZ is in the eastern tropical north Pacific and comprises 41% of this global volume, and the smallest OMZ is found in the eastern tropical North Atlantic and makes up only 5% of the global OMZ volume.