Ocean Acidification

This section looks at an ocean acidification definition, causes of ocean acidification, ocean acidification effectssolutions to ocean acidification and ocean acidification facts

Ocean acidification definition: the ongoing decrease in the pH (and therefore, increase in acidity) of the Earth's oceans, caused by the uptake and storage of carbon dioxide (CO2) from the atmosphere.

Causes of ocean acidification

To address a basic misconception: pH is a measure of acidity or alkalinity of water soluble substances (pH stands for 'potential of hydrogen'). A pH value is a number from 0 to 14, with 7 as the middle (neutral) point. Values below 7 indicate acidity which increases as the number decreases, 1 being the most acidic. The pH scale is logarithmic. This means a change factor of 10-fold is behind each whole number jump.

 

Oceanic pH has moved in the last two hundred and fifty years from a value of around 8.25 to a current value of around 8.15. It is therefore true to say that ocean acidification could also be equally expressed as a move towards pH neutrality. That of course does not garner headlines. The disturbing fact is that whatever the appropriate label is, marine ecosystems are showing significant signs of stress as a result of increasing concentrations of carbonic acid caused by carbon dioxide storage.

Ocean acidification: the carbon cycle

The carbon cycle involves biological, geological and chemical processes - it is an amalgamation of systems through which carbon is exchanged among the biosphere (Earth's surface), pedosphere (soil strata), geosphere  (atmosphere) and hydrosphere (water systems).

 

The global carbon cycle is usually divided into the following major reservoirs of carbon interconnected by pathways of exchange:

 

The atmosphere.


The terrestrial biosphere.


The oceans, including dissolved inorganic carbon and living and non-living marine biota.


The sediments, including fossil fuels, fresh water systems and non-living organic material.


The Earth's interior, carbon from the Earth's mantle and crust. These carbon stores interact with the other components through geological processes.

 

The carbon exchanges between reservoirs occur as the result of various chemical, physical, geological, and biological processes. The ocean contains the largest active pool of carbon near the surface of the Earth.

 

The natural flows of carbon between the atmosphere, ocean, terrestrial ecosystems, and sediments is fairly balanced, so that carbon levels would be roughly stable without human influence.

 

 

Ocean acidification history

While ongoing ocean acidification is man-made in origin, it has occurred previously in Earth's history.

 

The most notable example is the Paleocene-Eocene Thermal Maximum (PETM), which occurred approximately 56 million years ago.

 

Much of what’s known about corals in an acidifying ocean was discovered in the Arizona desert in the 1990s, in a sealed environment designed to mimic conditions on Earth, called Biosphere 2.

 

Over time, carbon dioxide levels soared and the pH of the “ocean,” simulated inside a stainless-steel tank, dropped.

 

A then-Columbia University scientist, Chris Langdon, tried to correct the ocean’s pH by adding baking soda and baking powder. Boosting the alkalinity not only restored pH but also restored coral growth.

 

To test his hypothesis, that coral reef-building depended on the saturation state of water, he spent three years measuring coral growth in varied states of saturation. His paper, published in 2000, generated such a stir that he spent another two years replicating the results. 

 

A German marine biologist, Ulf Riebesell, documented a similar effect on coccolithophores, a species of phytoplankton covered in plate-like armor made of calcite (Kolbert, 2006).

 

The urgency of the problem became apparent. In 2004, scientists from around the world gathered for the first-ever symposium on ocean acidification to compare notes and discuss research priorities for the future.

 

The journal Nature called it, “a turning point in expanding awareness among scientists about acidification.”

Ocean acidification chemistry

When carbon dioxide (CO2) dissolves, it reacts with water to form a chemical cocktail comprised of:

 

Dissolved free carbon dioxide (CO2(aq)).

 

Carbonic acid (H2CO3).

 

Bicarbonate (HCO−3).

 

Carbonate (CO3−2).

 

The ratio of these elements depends on factors such as seawater temperature and alkalinity.

 

These different forms of dissolved inorganic carbon (inorganic carbons are derived from ores and minerals rather than from living matter) are transferred from an ocean's surface to its interior.

 

Dissolving carbon dioxide in seawater increases the hydrogen ion (H+
) concentration in the ocean, and thus decreases ocean pH, as follows:

 

CO2 (aq) + H2O <-> H2CO3 <-> HCO3− + H+ <-> CO32− + 2 H+.

 

Or:

 

Dissolved free carbon dioxide AND seawater CREATING carbonic acid CREATING bicarbonate AND hyrogen ion CREATING carbonate AND 2 hydrogen ions. 

 

Since the industrial revolution began, it is estimated that surface ocean pH has dropped by slightly more than 0.1 units on the logarithmic scale of pH, representing about a 30% increase in H+.

 

It is expected to drop by a further 0.3 to 0.5 pH units (an additional doubling to tripling of today's post-industrial acid concentrations) by 2100 as the oceans absorb more man-made carbon dioxide, the impacts being most severe for coral reefs and the Southern Ocean. 

Ocean acidification effects

If we continue emitting CO2 at the same rate, by 2100 ocean acidity will increase by about 150 percent, a rate that has not been experienced for at least 400,000 years. ” — UK Ocean Acidification Research Programme, 2015.

 

“Nearly everything with a calcium carbonate shell or skeleton disappeared.......coral reefs weren’t seen again for two million years. You have to go back to events like this, many tens of millions of years ago, to find anything comparable to what we are doing to ocean chemistry today with our carbon dioxide emissions.”  — Ken Caldeira, the person who first coined the term 'ocean acidification'.

 

Full credit in this section attributable to: International Geosphere-Biosphere Programme (IGBP). Information sourced from the IGBP report "Ocean Acidification Summary for Policymakers 2013". IGBP was launched in 1987 to coordinate international research on global-scale and regional-scale interactions between Earth's biological, chemical and physical processes and their interactions with human systems. 

 

Ocean acidification impact on calcifying organisms

Calcification
Changes in ocean chemistry can have extensive direct and indirect effects on organisms and their habitats. One of the most important repercussions of increasing ocean acidity relates to the production of shells and plates out of calcium carbonate. This process is called calcification and is important to the biology and survival of a wide range of marine organisms. 

 

These calcifying organisms span the food chain from autotrophs to heterotrophs and include organisms such as coccolithophores (a form of phytoplankton), corals, foraminifera (single-celled protists with shells), echinoderms (including star fish and sea urchins), crustaceans and molluscs.

 

As ocean pH falls, the concentration of carbonate ions required for calcification to occur increases, and when carbonate becomes undersaturated, structures made of calcium carbonate are vulnerable to dissolution.  Saturation measures the thermodynamic potential for a mineral to form or to dissolve. The shells and skeletons of many marine organisms are made from either calcite or aragonite; both are forms of calcium carbonate. Scientists are particularly interested in aragonite, which is produced by many corals and some molluscs, because it is more soluble than calcite. Organisms grow shells and skeletons more easily when carbonate ions in water are abundant – “supersaturated”. Unprotected shells and skeletons dissolve when carbonate ions in water are scarce – “undersaturated”. 

 

When exposed in experiments to pH reduced by 0.2 to 0.4, larvae of a temperate brittlestar, a relative of the common sea star, fewer than 0.1 percent survived more than eight days. The devastating impact of rising acidity on entire species and entire populations is well made in this example.

Ocean acidification impact on the ecosystem

Species-specific impacts of ocean acidification have been seen in laboratory and field studies on organisms from the poles to the tropics. Many organisms show adverse effects, such as reduced ability to form and maintain shells and skeletons, as well as reduced survival, growth, abundance and larval development. Conversely, evidence indicates that some organisms tolerate ocean acidification and that others, such as some seagrasses, may even thrive.

 

Within decades, large parts of the polar oceans will become corrosive to the unprotected shells of calcareous marine organisms.

 

Changes in carbonate chemistry of the tropical ocean may hamper or prevent coral reef growth within decades.

 

The far-reaching effects of ocean acidification are predicted to impact food webs, biodiversity, aquaculture and hence man.

 

Species differ in their potential to adapt to new environments. Ocean chemistry may be changing too rapidly for many species or populations to adapt through evolution.

 

Multiple stressors – ocean acidification, warming, decreases in oceanic oxygen concentrations (deoxygenation), increasing UV-B irradiance due to stratospheric ozone depletion, overfishing, pollution and eutrophication – and their interactions are creating significant challenges for ocean ecosystems.

 

We do not fully understand the biogeochemical feedbacks to the climate system that may arise from ocean acidification.

 

Predicting how whole ecosystems will change in response to rising CO2 levels remains challenging. While we know enough to expect changes in marine ecosystems and biodiversity within our lifetimes, we are unable to make reliable, quantitative predictions of socio-economic impacts.

Ocean acidification impact on humans

Societies depend on the ocean for various ecosystem services: provisioning services, such as food; regulating services, such as carbon absorption from the atmosphere; cultural services, such as recreation; and supporting services, such as nutrient cycling.

 

While much is known about the effects of ocean acidification on individual organisms, the potential responses of whole ecosystems are largely unknown. Thus, although deleterious consequences are expected for shellfish and warm water corals (high confidence) and fisheries (low confidence), it is difficult to quantify how the ecosystems and fisheries will change and how societies will adapt to and manage the changes. 

 

The capacity of the ocean to act as a carbon sink decreases as it acidifies. The capacity of the ocean to absorb CO2 decreases as ocean pH decreases; that is, the buffering capacity of seawater decreases. This reduced capacity is a concern for stabilising CO2 emissions and implies that larger emissions cuts will be needed to meet targets to mitigate climate change

 

Declines in shellfisheries will lead to significant economic losses. By 2100, estimated global annual economic losses due to declines in mollusc production from ocean acidification could be more than $130 billion (US dollars, at 2010 price levels) for a business-as-usual CO2 emissions trend, according to one estimate. Molluscs appear to be one of the most sensitive groups of organisms studied under ocean acidification regimes. 

 

Negative socio-economic impacts of coral reef degradation are expected, but the size of the costs is uncertain Substantial economic losses are likely to occur due to the loss of tropical coral reef extent from ocean acidification (by 2100, the scarcity of corals will push the value of their losses over $1 trillion per year, in US dollars at 2010 price levels, according to one estimate). A large proportion of these losses will occur in vulnerable societies and small island states that economically rely on coral reefs. Coral reef losses will negatively affect tourism, food security, shoreline protection and biodiversity. But ocean acidification is not the only stressor. Reefs are already under pressure from warmer temperatures (which cause coral bleaching), habitat destruction, overfishing, sedimentation and pollution.

 

Impacts of ocean acidification on ecosystems may affect top predators and fisheries. It is uncertain how changes in phytoplankton and zooplankton abundance and distribution will propagate through marine ecosystems to affect fish and fisheries, on which many societies depend. Also, very little is known about the direct effects of ocean acidification on fish that are the target of commercial and subsistence fishing, which results in high uncertainties in predicting changes in fisheries in the future. However, this area is key for research, as fisheries support the livelihoods of about 540 million people, or 8% of the world’s population.

 

A final sobering thought

The legacy of historical fossil fuel emissions on ocean acidification will be felt for centuries. The increase in atmospheric CO2 is occurring too quickly to be stabilised by natural feedbacks such as the dissolution of deep-sea carbonates, which acts on time-scales of thousands of years, or the weathering of terrestrial carbonate and silicate rocks, which operates on time-scales of tens to hundreds of thousands of years. Global-scale projections of the changing chemistry of seawater can be made with high accuracy from scenarios of atmospheric CO2 levels. Even if man-made CO2 emissions stopped today, the ocean’s pH would not recover to its preindustrial level for centuries.

Demystifying ocean acidification and biodiversity impacts: Why are the oceans becoming more acidic and how does that threaten biodiversity?

Ocean acidification solutions

Government actions to reduce ocean acidification: what is needed?

Ocean acidification is not explicitly governed by international treaties. United Nations (UN) processes and international and regional conventions are beginning to note ocean acidification (London Convention/Protocol, UN Convention on the Law of the Sea, Convention on Biological Diversity and others).

 

Negotiators to the UN Framework Convention on Climate Change (UNFCCC) have begun to receive regular reports from the scientific community on ocean acidification, and the issue is now covered in the assessment reports of the Intergovernmental Panel on Climate Change (IPCC).

 

In June 2012, the UN Conference on Sustainable Development (Rio+20) recognised ocean acidification as a threat to economically and ecologically important ecosystems and human wellbeing.

 

However, there are still no international mechanisms or adequate funding to deal specifically with mitigation or adaptation to ocean acidification.

Personal actions to reduce ocean acidification

Refer to the the global warming, aerosol and air pollution 'personal actions' sections.

 

In short, cutting your carbon footprint will help. Top ten tips:

 

Smart use of domestic gas.

 

Smart use of domestic electricty.

 

Smart use of personal vehicle use.

 

Comprehensive home insulation.

 

Smart use of domestic appliances.

 

Eco-savvy food choices: local produce, less beef, avoid processed food and try and minimse food wastage.

 

Clothing: fabric choices - natural fabrics have a much higher carbon footprint than man-made alternatives.

 

Paper and packaging: reduce, re-use, recycle.

 

Air travel - less is more environmentally speaking.

 

Public transport - more is more.

What should inform future policy development?

The primary cause of ocean acidification is the release of atmospheric CO2 from human activities. The only known realistic mitigation option on a global scale is to limit future atmospheric CO2 levels.

 

Appropriate management of land use and land-use change can enhance uptake of atmospheric CO2 by vegetation and soils through activities such as restoration of wetlands, planting new forests and reforestation.

 

The impacts of other stressors on ocean ecosystems such as higher temperatures and deoxygenation – also associated with increasing CO2 – will be reduced by limiting increases in CO2 levels.

 

The shellfish aquaculture industry faces significant threats and may benefit from a risk assessment and analysis of mitigation and adaptation strategies. For example, seawater monitoring around shellfish hatcheries can identify when to limit the intake of seawater with a lower pH, hatcheries can be relocated, or managers can select larval stages or strains that are more resilient to ocean acidification for breeding.

 

At local levels, the effects of ocean acidification on ecosystem resilience may be constrained by minimising other local stressors through the following:

 

• Developing sustainable fisheries management practices such as regulating catches to reduce overfishing and creating long-term bycatch reduction plans. If implemented and enforced, this type of management has been shown to sustain ecosystem resilience.

 

• Adopting sustainable management of habitats, increased coastal protection, reduced sediment loading and application of marine spatial planning.

 

• Establishing and maintaining Marine Protected Areas (MPAs) that help manage endangered and highly vulnerable ecosystems to enhance their resilience against multiple environmental stressors.

 

• Monitoring and regulating localised sources of acidification from runoff and pollutants such as fertilisers.

 

• Reducing sulphur dioxide and nitrous oxide emissions from coal-fired power plants and ship exhausts that have significant acidifying effects locally.

Ocean acidification facts

Oceans absorb between 25-50% of the carbon dioxide that is relased into the atmosphere through human activity

That equates to roughly 20-25 million tons of carbon dioxide daily. The primary driver of this accelerated carbon dioxide absorption process is man's insatiable desire to burn fossil fuels.

Carbon dioxide dissolves in seawater, carbonic acid is formed. Oceans are acidifying at their fastest rate for 300 million years

Over the last 300 million years, the average pH value of the world's oceans have produced stable averages or around 8.25 units. In 1996, oceanic pH was estimated at 8.14 units. 

Oceanic pH values have decreased by around 30% since the start of the industrial revolution

The relatively small-sounding drop in pH values masks the fact that acidity-causing H+ ion concentrations have risen between 25 to 35% since 1751 to achieve this.

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