A pteropod shell is shown dissolving over time in seawater with a lower pH. When carbon dioxide is absorbed by the ocean from the atmosphere, the chemistry of the seawater is changed. (NOAA)
A pteropod shell is shown dissolving over time in seawater with a lower pH. When carbon dioxide is absorbed by the ocean from the atmosphere, the chemistry of the seawater is changed. (NOAA)
Ocean acidification
National Oceanic and Atmospheric Administration (NAOO)
U.S. Department of Commerce
https://www.noaa.gov/education/resource-collections/ocean-coasts/ocean-acidification
In the 200-plus years since the industrial revolution began, the concentration of carbon dioxide (CO2) in the atmosphere has increased due to human actions. During this time, the pH of surface ocean waters has fallen by 0.1 pH units. This might not sound like much, but the pH scale is logarithmic, so this change represents approximately a 30 percent increase in acidity.
The ocean absorbs about 30% of the carbon dioxide (CO2) that is released in the atmosphere. As levels of atmospheric CO2 increase from human activity such as burning fossil fuels (e.g., car emissions) and changing land use (e.g., deforestation), the amount of carbon dioxide absorbed by the ocean also increases. When CO2 is absorbed by seawater, a series of chemical reactions occur resulting in the increased concentration of hydrogen ions. This process has far reaching implications for the ocean and the creatures that live there.
The pH scale
The pH scale runs from 0 to 14, with 7 being a neutral pH. Anything higher than 7 is basic (or alkaline) and anything lower than 7 is acidic. The pH scale is an inverse of hydrogen ion concentration, so more hydrogen ions translates to higher acidity and a lower pH.
Carbon dioxide and seawater
Carbon dioxide, which is naturally in the atmosphere, dissolves into seawater. Water and carbon dioxide combine to form carbonic acid (H2CO3), a weak acid that breaks (or “dissociates”) into hydrogen ions (H+) and bicarbonate ions (HCO3-).
Because of human-driven increased levels of carbon dioxide in the atmosphere, there is more CO2 dissolving into the ocean. The ocean’s average pH is now around 8.1offsite link, which is basic (or alkaline), but as the ocean continues to absorb more CO2, the pH decreases and the ocean becomes more acidic.
Impacts of ocean acidification on shell builders
Ocean acidification is already impacting many ocean species, especially organisms like oysters and corals that make hard shells and skeletons by combining calcium and carbonate from seawater. However, as ocean acidification increases, available carbonate ions (CO32-) bond with excess hydrogen, resulting in fewer carbonate ions available for calcifying organisms to build and maintain their shells, skeletons, and other calcium carbonate structures. If the pH gets too low, shells and skeletons can even begin to dissolve.
The pteropod, or “sea butterfly,” is a tiny sea snail about the size of a small pea. Pteropods are an important part of many food webs and eaten by organisms ranging in size from tiny krill to whales. When pteropod shells were placed in sea water with pH and carbonate levels projected for the year 2100, the shells slowly dissolved after 45 days. Researchers have already discovered severe levels of pteropod shell dissolutionoffsite link in the Southern Ocean, which encircles Antarctica.
Ocean acidification impacts on fish and seaweeds
Changes in ocean chemistry can affect the behavior of non-calcifying organisms as well. The ability of some fish, like clownfish, to detect predators is decreased in more acidic waters. Studies have shown that decreased pH levels also affect the ability of larval clownfishoffsite link to locate suitable habitat. When these organisms are at risk, the entire food web may also be at risk.
While some species will be harmed by ocean acidification, algae and seagrasses may benefit from higher CO2 conditions in the ocean, as they require CO2 for photosynthesis just like plants on land. There are some ongoing studies examining if growing seaweed can help slow ocean acidification.
Our changing ocean
Estimates of future carbon dioxide levels, based on business-as-usual emission scenarios, indicate that by the end of this century the surface waters of the ocean could have a pH around 7.8. The last time the ocean pH was this low was during the middle Miocene, 14-17 million years ago. The Earth was several degrees warmer and a major extinction event was occurring.
Ocean acidification is currently affecting the entire ocean, including coastal estuaries and waterways. Billions of people worldwide rely on food from the ocean as their primary source of protein. Many jobs and economies in the U.S. and around the world depend on the fish and shellfish that live in the ocean.
Current research
Ocean acidification is one aspect of global climate change. Anything we do to mitigate climate change today will benefit the future of the ocean as well. Over the last decade, there has been much focus in the ocean science community on studying the potential impacts of ocean acidification. NOAA’s Ocean Acidification Program serves to build relationships between scientists, resource managers, policy makers, and the public in order to research and monitor the effects of changing ocean chemistry on economically and ecologically important ecosystems such as fisheries and coral reefs.
Because sustained efforts to monitor ocean acidification worldwide are only beginning, it is currently impossible to predict exactly how ocean acidification impacts will cascade throughout the marine food web and affect the overall structure of marine ecosystems. With the pace of ocean acidification accelerating, scientists, resource managers, and policymakers recognize the urgent need to strengthen the science as a basis for sound decision making and action.
海洋酸性化
ウィキペディア
https://ja.wikipedia.org/wiki/海洋酸性化
海洋酸性化とは、主に大気中において以前よりも濃度が上昇した二酸化炭素が、より多く海洋へと溶け込んだことによって引き起こされる、海水のpH低下のことである。
機序
産業革命以降200年以上にわたって、化石燃料の燃焼により大気中の二酸化炭素濃度は増加しつづけている。産業革命以前は約280 ppmで安定していた二酸化炭素濃度は、2011年には390 ppmを超えた。さらに2016年には400 ppm、つまり、0.04 %を観測史上初めて超えた。つまり、たったの5年で0.001 %も大気中に二酸化炭素が増えるなど、この増加には歯止めがかかっていない。
ところで、海水中へと溶け込んだ二酸化炭素(CO2(aq))は、下記の平衡状態となる。
この平衡が成り立っている状態において、大気中の二酸化炭素が増えたことによって海水へとより多くの二酸化炭素が溶け込むと、溶存態の二酸化炭素(CO2(aq))が増える。あとは上記の平衡状態がルシャトリエの法則に従った移動を起こすため、海水中の水素イオンが増加し、水素イオン指数が低下する。事実、1751年から2004年までの間に、海洋表面の海水のpHは、約8.25だったものが、約8.14にまで低下した。
考えられる影響
上記の平衡の移動に伴い重炭酸イオンと炭酸イオン濃度はそれぞれ低下する。炭酸イオンは貝殻やサンゴの骨格などの構成要素であり、炭酸イオンの減少によってサンゴ・貝類・ウニ・円石藻など、炭酸カルシウムである方解石やアラレ石の構造を作る生物が影響を受ける。従って海洋の酸性化が進むと、海洋の生物多様性が低下することが懸念されている。
持続可能な開発目標(SDGs)の達成項目14.3
国連が2030年までに達成すべきとして採択したSDGsの17の目標のうち目標14において、達成目標の「14.3」としてあらゆるレベルでの科学的協力の促進などを通じて、海洋酸性化の影響に対処し最小限化するとして、海洋酸性化の進行を食い留めることがうたわれている。
人類の安全な活動領域を定めるプラネタリー・バウンダリーによれば、アラレ石（アラゴナイト）が海洋酸性化の指標として使われている。アラレ石の水準が産業革命以前の80%を下回ると危険とされ、サンゴ礁の絶滅の危機や、それによる海洋の生物多様性の喪失につながる。プラネタリー・バウンダリーは持続可能な開発目標（SDGs）に採用されている。
日本での海洋酸性化
日本沿岸部でも海洋酸性化が進んでいることが、海洋研究開発機構などの調査により報告されている。