The concentration of carbon dioxide (CO2) in the atmosphere has increased due to anthropogenic activities since the last two centuries – after the advent of the industrial revolution. With this, apart from other serious issues affecting our planet, the pH of surface ocean waters has fallen by 0.1 pH units. The pH scale is logarithmic, and even though it might not sound drastic, this change signifies approximately a 30% increase in ocean water acidity.
Our oceans absorb about 30% of CO2 released in the atmosphere. As the levels of atmospheric CO2 increase from human activity, the amount of carbon dioxide taken in by the oceans also increases. A series of chemical reactions occur in the water resulting in the accumulation of hydrogen ions. This process is unfavorable for both – the ocean water and its marine life.
Carbon dioxide, naturally present in our atmosphere, dissolves in oceans. Carbon dioxide and water combine to form carbonic acid (H2CO3). Carbonic acid is a weak acid that dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3–).
Due to the increased levels of CO2 in the atmosphere instigated by man-made factors, there is more carbon dioxide dissolving into the oceans. The average pH of oceans is around 8.1 as of now. This is categorized under alkaline pH, but as seawater continues to absorb more CO2, the pH will eventually decrease – resulting in more acidic oceans.
Ocean acidification negatively affects ocean species, particularly shelled organisms. Oysters, corals, and other shelled marine organisms create hard shells and skeletons by combining calcium and carbonate from seawater. But, as ocean acidification escalates, available carbonate ions (CO32-) bind to excess hydrogen. This leaves fewer carbonate ions that calcifying organisms can procure for building and maintaining their skeletons, shells, and other similar structures. If the pH is further reduced, their shells and skeletons can even begin to dissolve.
Pteropods or sea butterflies form an important part of many food webs. These are consumed by organisms of a variety of sizes – from tiny krill to whales. It was found through experimentation that 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. As per estimates, by 2100, the pH of the surface ocean water could drop to under 7.8 – more than 150% compared to today’s already-corrosive state. Unfortunately, researchers have already discovered severe levels of pteropod shell dissolution in the Southern Ocean, which encircles Antarctica. The effect on other shelled marine species and on ocean water composition could be potentially even more in some specific sensitive parts of our planet, like the Arctic Ocean.
Non-calcifying organisms are affected by changes in ocean chemistry as well. For example, ability of clownfish to detect predators is decreased in more acidic waters. Studies have also shown that larval clownfish are impeded by low pH to locate suitable habitats. When these organisms are at risk, the entire food web is in peril. In addition, ocean acidification may change the way sounds transmit through water, making underwater environs slightly noisier.
As mentioned earlier, estimates of future CO2 levels based on the present emission scenario indicate, by 2100 the surface waters of our oceans could have a pH around 7.8. The last time when the levels were this low was during the middle Miocene – 14 to 17 million years ago. A major extinction event was occurring and the planet was several degrees warmer.
Currently, ocean acidification is affecting every major ocean, including coastal estuaries and waterways. Globally, billions of people rely on food from the ocean as their primary source of protein. Numerous jobs and economies around the world depend on fish and shellfish that live in the ocean. But with just a little fluctuation in pH, all may be lost.
Ocean acidification forms one aspect of global climate change. Anything we do today to mitigate climate change will benefit the tomorrow of our oceans too. Over the past 10 years or so, more focus has been put on the ocean science community, investing time and money to study potential impacts of ocean acidification. However, to research and monitor effects of changing ocean chemistry on economically and ecologically important ecosystems, a strong relationship should exist between scientists, resource executives, policy makers, and the public.
Because sustained efforts to observe ocean acidification on a global level are just beginning, it’s impossible – for now – to predict exactly how it will cascade throughout the food web or affect the overall structure of marine ecosystems. Recognizing this urgency, strengthening marine science to act as a basis for sound decision making and action, policymakers and scientists are and should continue to link arms for the benefit of everyone.
While some species are harmed by ocean acidification, algae and sea-grasses may benefit from higher CO2 levels – just like plants on land, they require carbon dioxide for photosynthesis. There are ongoing studies investigating if growing seaweed can help slow ocean acidification.
While this may serve as a silver lining, the future holds more challenges. By 2050, scientists predict that 86% of the world’s oceans will be warmer and more acidic than anything in modern history. Too much acid that even algae can’t obviate it!
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