The Mechanism of Ocean Acidification and Its Impact on Ecosystems: A Scientific Look at the Coral Reef Crisis

What is ocean acidification: the chemical change CO2 causes in the sea

Ocean acidification is the phenomenon in which the pH of seawater falls as atmospheric carbon dioxide (CO2) dissolves into it. The name "acidification" often causes the misconception that seawater is becoming acidic, but in reality it refers to the process by which seawater remains mildly alkaline while that alkalinity gradually weakens.

The ocean functions as the largest carbon reservoir on Earth and has absorbed about 30% of the carbon dioxide emitted by humanity (IPCC Sixth Assessment Report). This ocean absorption of carbon dioxide plays an important role in mitigating the rise of atmospheric CO2 concentration, but at the same time it is bringing serious impacts to the marine environment.

The discovery of ocean acidification and the history of research

The concept of ocean acidification is relatively new, and serious research only began in the late 1990s. According to long-term observational data from the Japan Meteorological Agency, the pH of surface seawater in the open ocean has been continuously declining at a rate of -0.0010 to -0.0030 per year since the late 1980s (JMA Ocean Acidification Data).

What deserves particular attention is that more than half of the pH change over the past 100 years has been concentrated in the past 40 years. This coincides with the sharp increase in fossil fuel consumption by humanity, showing a strong link between ocean acidification and human activity.

The mechanism of ocean acidification: a detailed look at how pH falls

To understand the mechanism of ocean acidification, we need to know the chemical reaction that occurs when carbon dioxide dissolves into seawater. This chemical reaction proceeds through several stages and ultimately increases the concentration of hydrogen ions in seawater.

The detailed chemical process

This reaction increases the concentration of hydrogen ions (H⁺) in seawater while simultaneously decreasing the concentration of carbonate ions (CO₃²⁻). Because carbonate ions are the raw material for the calcium carbonate (CaCO₃) that coral and other marine organisms use to build their skeletons and shells, this decrease directly affects calcifying organisms.

The equilibrium state of carbonate chemistry

In seawater, these chemical species are constantly seeking equilibrium. According to research by the National Institute for Environmental Studies, the current distribution of carbonate chemical species in seawater is as follows (National Institute for Environmental Studies):

Regional and depth-related differences

The rate at which ocean acidification progresses varies greatly by region and depth. In the Arctic and Antarctic regions, the low temperatures increase the solubility of carbon dioxide, making acidification especially likely to advance. In addition, in the western subarctic North Pacific around Japan, CO₂ released through biological respiration is also accelerating acidification.

In particular, in deep waters, CO₂ absorbed at the surface is carried by physical circulation, and combined with additional oxygen consumption and CO₂ release from biological respiration, acidification is progressing even more rapidly than at the surface in some sea areas.

Serious impact on coral reefs: the reality of impaired calcification

Coral reefs are among the most biodiverse ecosystems on Earth, but their survival is under serious threat from impaired calcification caused by ocean acidification. Calcification is the process by which coral combines calcium ions (Ca²⁺) and carbonate ions (CO₃²⁻) in seawater to form a skeleton of calcium carbonate (CaCO₃).

The scientific mechanism of the calcification process

Coral calcification takes place in a special space called the calcifying fluid, located at the base of each polyp (individual). Groundbreaking research from the University of Tokyo's Atmosphere and Ocean Research Institute revealed that ocean acidification is also lowering the pH of this calcifying fluid (University of Tokyo research announcement).

The calcium carbonate saturation state (Ω: omega) is an index showing how easily calcium carbonate dissolves in seawater. When this value falls below 1.0, calcium carbonate begins to dissolve naturally, making it difficult for coral to form its skeleton. It has been reported that in many tropical sea areas, the Ω value has already fallen from around 3.0 to about 2.5.

Laboratory proof of impaired calcification

The impact of ocean acidification on coral calcification has been clearly demonstrated at the laboratory level. In a long-term experiment observing coral calcification rates for about four years in a tank environment close to acidic conditions, calcification rates were confirmed to drop by as much as 40% compared with a normal alkaline ocean environment.

Differences in impact by coral species

Not all coral is affected by ocean acidification in the same way. Because of differences in crystal structure, calcium carbonate has two main forms: aragonite and calcite. Because many corals build their skeletons from the aragonite form of calcium carbonate, they dissolve more easily and are more strongly affected by ocean acidification.

Impact at the ecosystem level

Impaired coral calcification is not merely a problem for individual coral. Coral reefs form the foundation of an ecosystem so diverse that they are called "rainforests of the sea," and about 25% of marine life depends on them for survival. Reduced calcification undermines the structural stability of the reef as a whole, triggering cascading effects on the fish and invertebrates that live there.

What the latest research reveals: measured data from Japan's coral reefs

Groundbreaking research published in 2024 by the University of Tokyo's Atmosphere and Ocean Research Institute was the first in the world to demonstrate that ocean acidification is actually beginning to affect the calcification process in Japan's coral reefs. This research is innovative in that it went a step beyond conventional laboratory-level studies to quantitatively measure long-term change under natural conditions.

An innovative method using boron isotope analysis

The research team analyzed the boron isotope ratio (¹¹B/¹⁰B) contained in the skeletons of massive Porites coral living around Chichijima (Ogasawara Islands) and Kikaijima (Amami Islands). This method is a cutting-edge technique that can accurately reconstruct the seawater pH at the time the coral was calcifying (University of Tokyo research announcement).

The serious changes observed

The analysis revealed that the boron isotope ratio has been declining rapidly in recent years at both sites. This is decisive evidence that the seawater pH decline caused by ocean acidification is also lowering the pH of coral's calcifying fluid, and is beginning to affect the calcification process.

Continuous monitoring by JAMSTEC

The Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has been continuously monitoring ocean acidification in the Tsugaru Strait since 2012. This monitoring revealed that pH is declining near Japan at a rate of -0.0030 to -0.0051 per year, a rate that is high even by global standards (JAMSTEC research report).

Impact on the coral reefs of Okinawa and Amami

In the coral reefs of Okinawa and Amami, bleaching from rising seawater temperature is also progressing alongside ocean acidification, and the compound stress on coral is becoming more serious. According to long-term research by the University of the Ryukyus, coral cover (the proportion of the seafloor covered by coral) around Ishigaki Island has declined from over 50% in the 1980s to below 20% today.

The international significance of these research findings

These research findings from Japan have been highly regarded in the international ocean acidification research community. In particular, by quantitatively demonstrating long-term change under natural conditions—in contrast to previous knowledge, which had been largely based on laboratory-level studies—they are making a major contribution to improving the accuracy of climate change impact assessments and future projections.

The progress of ocean acidification in coral reefs around the world

Ocean acidification is a global phenomenon, and coral reefs around the world are affected to varying degrees. Because the rate of progress and the manner in which impacts appear differ by region, it is important to understand the characteristics of each. About 50% of the world's coral reefs have been lost over the past several decades, and ocean acidification is one of the main causes.

The Great Barrier Reef: a crisis for the world's largest coral reef

Australia's Great Barrier Reef is known as the world's largest coral reef system, but it is a prime example of a reef suffering serious impact from ocean acidification. According to long-term research by the Australian Institute of Marine Science (AIMS), the calcification rate of the Great Barrier Reef has declined by about 14% over the past 30 years (Australian Institute of Marine Science).

Coral reefs in the Caribbean: the front line of rapid degradation

Coral reefs in the Caribbean are among the most rapidly degrading in the world. According to a report by the International Coral Reef Initiative (ICRI), coral cover in the Caribbean has declined from over 50% in the 1970s to below 8% today. In this region, in addition to ocean acidification, increased hurricane frequency and nutrient inflow from land are also compounding factors.

The Indo-Pacific coral reef network

The Indo-Pacific region is where about 75% of the world's coral reefs are concentrated, making it a critically important area. In this region, ocean acidification and high-temperature stress are progressing simultaneously due to the influence of the high-temperature waters known as the Western Pacific Warm Pool (WPWP). In particular, in the sea area known as the "Coral Triangle," encompassing Indonesia, the Philippines, and Papua New Guinea, coral reefs with the world's highest levels of marine biodiversity are under threat.

The special situation in the Arctic and sub-Arctic regions

In the Arctic and sub-Arctic regions, low temperatures increase the solubility of CO₂, causing ocean acidification to progress more rapidly than in other regions. In particular, along the Alaskan coast and in the Bering Sea, sea areas where the aragonite saturation state seasonally falls below 1.0 have already appeared, raising concerns about the impact on cold-water-adapted coral species and other calcifying organisms.

Serious impact on small island nations

In small island nations of the Pacific and Indian Oceans, coral reefs function as natural breakwaters, playing an important role in protecting islands from coastal erosion. If ocean acidification impairs coral reef growth, combined with sea level rise it threatens the very survival of the islands themselves. In countries such as the Maldives, Tuvalu, and Kiribati, this issue is recognized as a crisis at the level of national survival.

Impact on calcifying organisms overall: threats to marine life beyond coral

The impact of ocean acidification is not limited to coral reefs. The ocean is home to a wide variety of organisms with shells or skeletons made of calcium carbonate, each affected by ocean acidification in its own way. These organisms are important components of the marine food chain, and the impact on them ripples through the entire marine ecosystem.

Pteropods: the tiny sea snails supporting the marine ecosystem

Pteropods are marine sea snails that serve as an important food source for many marine animals. Pteropods living in the Arctic Ocean and Southern Ocean in particular have already been observed with dissolving shells, making them one of the organism groups most directly affected by ocean acidification (Nature journal research report).

Coccolithophores: the complex impact on marine primary producers

Coccolithophores are a type of marine phytoplankton covered in tiny plates of calcium carbonate called coccoliths. While these organisms are important contributors to marine primary production, their calcification also has the effect of lowering the alkalinity of seawater. Research by the National Institute for Environmental Studies has shown that ocean acidification has complex effects on both the calcification and photosynthesis of coccolithophores (National Institute for Environmental Studies).

Foraminifera: indicator organisms essential to paleoclimate research

Foraminifera are single-celled organisms that float in the ocean and possess shells made of calcium carbonate. These organisms have been used as important indicators in paleoclimate research, but ocean acidification may impair their shell formation. In particular, benthic foraminifera living in the deep sea are expected to be greatly affected by the acidification of deep waters.

Shellfish and crustaceans: direct impact on fisheries

Bivalves such as oysters, mussels, and scallops, as well as crustaceans such as crabs and shrimp, also have shells or exoskeletons made of calcium carbonate and are therefore affected by ocean acidification. In particular, impaired shell formation during the larval stage can seriously affect population maintenance. On the U.S. West Coast, mass die-offs of larvae have been reported in oyster farming, with links to ocean acidification pointed out.

Amplification of impact through the food chain

Calcifying organisms play an important role in supporting the foundation of the marine food chain. For example, pteropods are a major food source for many fish, seabirds, and marine mammals, and their decline affects predators higher up the food chain as well. Pink salmon, a species of Alaskan salmon that feeds mainly on pteropods, is a notable example of how ocean acidification can indirectly affect fishery resources.

The special situation in the deep sea

In the deep sea, a pattern of ocean acidification progression different from that at the surface has been observed. In deep waters, acidification progresses as CO₂ absorbed at the surface is carried by physical circulation, but because CO₂ released through biological respiration is also added, change is occurring even more rapidly than at the surface in some locations. The impact on deep-sea cold-water coral and other deep-sea calcifying organisms remains an important research question that has not yet been fully clarified.

Future projections from the IPCC Sixth Assessment Report

The IPCC Sixth Assessment Report (AR6), published between 2021 and 2023, is the most comprehensive report summarizing the latest scientific findings on ocean acidification. The report presents projections of ocean acidification progress through the end of this century based on various emissions scenarios, and the results issue a serious warning about the future of marine ecosystems.

Future projections by emissions scenario

IPCC AR6 provides future projections under five main scenarios based on Shared Socioeconomic Pathways (SSPs). Even under the most optimistic SSP1-1.9 scenario (achieving the 1.5°C target), ocean acidification continues, while under the most pessimistic SSP5-8.5 scenario, catastrophic changes are projected (IPCC AR6 Working Group I Report).

Detailed regional projections

IPCC AR6 also provides detailed projections at the regional level. In particular, in tropical and subtropical coral reef areas, the sea areas where the aragonite saturation state falls below the 3.0 threshold needed for calcification are projected to expand significantly by the 2050s. In the Arctic Ocean, sea areas where the aragonite saturation state already seasonally falls below 1.0 exist, and further expansion is considered certain.

Irreversibility and tipping points

One particularly important point made in IPCC AR6 is the irreversibility of ocean acidification. Even if atmospheric CO₂ emissions were to stop completely, it would take a timescale of thousands to tens of thousands of years for the CO₂ accumulated in the ocean to return to the atmosphere. It is also emphasized that coral reef ecosystems have tipping points, and that once they collapse, returning to their original state becomes extremely difficult.

Assessment of compound impacts

AR6 provides detailed assessment not only of the impact of ocean acidification alone, but also of its compound effects with rising water temperature and ocean deoxygenation. It shows that these factors interact with one another and can produce far more serious consequences than any single factor acting alone. In coral reefs in particular, the simultaneous progression of acidification and bleaching from high water temperature is projected to significantly reduce resilience.

Projected socioeconomic impact

The IPCC AR6 Working Group II Report also assesses the impact of ocean acidification on human society. Global fishery production may decline by up to 25% by 2050, and the impact on shellfish aquaculture and small-scale fisheries in particular is projected to become severe. It is also pointed out that the decline in coastal protection function, combined with sea level rise, will significantly increase coastal disaster risk.

The effectiveness and limits of mitigation measures

IPCC AR6 also assesses the effectiveness of various mitigation measures. Even under the most ambitious emissions reduction scenario (SSP1-1.9), it is unavoidable that ocean acidification will continue to progress until around the middle of this century. However, in the long term, the effects of emissions reductions clearly emerge, and it is shown that the degree of ocean acidification by 2100 can be significantly reduced.

Measures to prevent ocean acidification and what we can do

Ocean acidification is a global problem, but solving it requires measures at every scale, from international efforts to individual actions. The fundamental solution is to reduce atmospheric CO₂ concentrations, but adaptation measures to increase the resilience of marine ecosystems are also necessary at the same time. Here we introduce concrete measures grounded in scientific evidence.

International efforts and policy frameworks

Measures against ocean acidification are being advanced within the framework of the United Nations Framework Convention on Climate Change (UNFCCC) and the Paris Agreement. The 2015 Paris Agreement agreed to hold the increase in global average temperature to well below 2°C above pre-industrial levels and to pursue efforts to limit it to 1.5°C. Achieving this goal would have the effect of significantly mitigating the progress of ocean acidification (United Nations Framework Convention on Climate Change).

Technological solutions: the state and challenges of carbon removal technology

Research is progressing on technological solutions such as Direct Air Capture (DAC), which directly removes CO₂ from the atmosphere, and ocean alkalinization. However, these technologies are still at the experimental stage, and many challenges remain for large-scale deployment. According to analysis by the International Energy Agency (IEA), these technologies are not expected to be put into practical use at a meaningful scale until at least the 2030s (International Energy Agency).

Establishing and strengthening the management of marine protected areas

Establishing and effectively managing Marine Protected Areas (MPAs) is an important adaptation measure to increase the resilience of coral reefs and calcifying organisms. Within protected areas, restrictions on fishing and the inflow of pollutants can reduce stress factors other than ocean acidification and improve the ecosystem's natural resilience. According to the International Union for Conservation of Nature (IUCN), effectively managed marine protected areas have been reported to have coral reef recovery rates 2-3 times higher than areas outside protection.

Concrete actions individuals can take

Because the fundamental cause of ocean acidification lies in rising atmospheric CO₂ concentration, individual carbon reduction actions have a direct effect. According to a survey by Japan's Ministry of the Environment, the annual CO₂ emissions per person in Japan are about 10 tons, roughly twice the global average. Through changes in individual behavior, it is possible to significantly reduce these emissions.

Choosing sustainable seafood

To protect fishery resources affected by ocean acidification, it is important to choose seafood caught through sustainable fishing practices. Choosing products certified by the Marine Stewardship Council (MSC) or the Aquaculture Stewardship Council (ASC) can reduce the burden on marine ecosystems. Choosing local products can also reduce CO₂ emissions from transportation.

Supporting scientific research and participating in citizen science

Supporting ocean acidification research and participating in citizen science projects are also important contributions. For example, the "Floating Forests" project engages citizen scientists in monitoring ocean change using satellite imagery. Participating in local beach cleanup activities can also contribute to reducing marine plastic pollution.