Ocean Acidification
Safe Operating Space
Ocean acidification is the phenomenon of increasing acidity (decreasing pH) in ocean water due to the absorption of atmospheric CO2. This process harms calcifying organisms, impacting marine ecosystems, and reduces the ocean's efficiency in acting as a carbon sink. The indicator for Ocean Acidification, the current aragonite saturation state, is within the Safe Operating Space but is close to crossing the safe boundary.
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Importance
Our oceans absorb CO2 from the atmosphere, which results in a chemical change in sea water. The rapid increase in CO2 levels since industrialization has led to our oceans becoming more acidic, causing biodiversity loss and the degradation of delicate ecosystems and coral reefs. According to the current definition of the PB, ocean acidification is just within the Safe Operating Space (2.80; lower values mean higher acidification), but it is close to crossing the safe boundary. Several new studies suggest that even these current conditions may be problematic for multiple marine organisms, suggesting a need for re-evaluating the safe boundary.
Impacts
Crossing the safe boundary for Ocean Acidification has multiple impacts:
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Corals struggle to build their skeletons, weakening reef structures. Mollusks and other shellfish have difficulty forming shells impacting their survival and growth. Some organisms (pteropods) at high latitudes already have damaged shells.
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The availability of calcifying organisms changes, disrupting marine food webs and affecting species that rely on them for food. Coral reefs, which are biodiversity hotspots, suffer, leading to the loss of habitat for many marine species. Changes in carbonate chemistry reduce the ocean's capacity to sequester carbon, weakening its ability to mitigate global warming.
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At low latitudes, where the aragonite saturation state is still relatively high, the absolute rate of reduction is highest. This can pose a risk as tropical corals become stressed when aragonite saturation falls below 3, especially in combination with other stressors, such as marine heat waves. At high latitudes, the aragonite saturation state is naturally lower and acidification drives some areas to become undersaturated with respect to aragonite, creating corrosive conditions for aragonite shells.
Control Variables
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Aragonite saturation state (Ω)
Aragonite is a form of calcium carbonate used by many calcifying organisms (e.g., corals and shellfish) to construct their shells or skeletons. The aragonite saturation state measures the current carbonate ion concentration against the concentration needed to form stable aragonite. The aragonite saturation state is sensitive to changes in CO2 concentration because the uptake of anthropogenic CO2 by the ocean leads to the formation of carbonic acid. This acid dissociates, producing hydrogen ions that convert carbonate ions into bicarbonate ions, thereby reducing the carbonate ion concentration. Consequently, this process lowers the aragonite saturation state, making it a reliable indicator of the impact of increased CO2 on ocean chemistry and marine ecosystems. Ocean acidification is approaching its PB, with the surface aragonite saturation state declining significantly towards the PB, posing a growing threat to marine ecosystems.
Global Map of Ocean Acidification Indicator Aragonite Saturation State Change
Ocean acidification is affecting oceans worldwide, with the effects being most pronounced in the Southern Ocean and the Arctic Ocean. Some areas have already become undersaturated with respect to aragonite, posing a risk to vulnerable calcifying organisms that play an important role in the food web.
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Key Drivers
Human-caused CO2 emissions are the primary driver of Ocean Acidification. There are strong regional and yearly variations due to phenomena like El Niño. Overall, Ocean Acidification exemplifies that one process (increased CO2 concentration) can affect more than one Planetary Boundary (Ocean Acidification and Climate Change).
Connected Tipping Points
If the status of this Planetary Boundary continues to deteriorate, it will push many tipping elements toward tipping, including:
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The death of warm-water coral reefs
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Deoxygenation of marine environments
Read more about other Boundaries
Planetary Boundary process inside Earth's safe operating space
Planetary Boundary process outside Earth's safe operating space
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