CO2 and Ocean Acidification: Causes, Impacts, Solutions

Published Jan 30, 2019 Updated Feb 6, 2019

Translucent pterapod shells.
NOAA Environmental Visualization Laboratory
Table of Contents

Carbon pollution is changing the ocean’s chemistry, slowing its ability to uptake CO2, making it more acidic, and harming shellfish and other marine life we depend on.

The ocean has absorbed about 29 percent of global CO2 emissions since the end of the preindustrial era. In the last decade (from 2008-2017), we’ve dumped into the atmosphere about 40 gigatons of emissions of heat-trapping gases each year from the burning of fossil fuels and land-use change—or the equivalent to 252 million blue whales.

What is ocean acidification?

When carbon dioxide enters the ocean, it dissolves in saltwater. First, it forms carbonic acid. Then, this carbonic acid breaks apart – or “dissociates” – producing bicarbonate ions and hydrogen ions. Ocean acidification results from an increased concentration of hydrogen ions and a reduction in carbonate ions due to the absorption of increased amounts of CO2. Clams, mussels, crabs, corals, and other sea life rely on carbonate ions to grow their shells and thrive.


Acidity is a measure (in units of pH) of the concentration of hydrogen ions in a solution, in this case, ocean water.

For millions of years, the exchange of CO2 between the surface of the ocean and the atmosphere remained constant. In the past 150 years, humans have greatly increased the amount of CO2 in the atmosphere by burning fossil fuels and changing land-use practices. As a result, the ocean has absorbed about 29 percent of this additional carbon.

This added CO2 has had significant effects on the ocean. Surface waters are now 30 percent more acidic than they were at the start of the industrial era. Ocean acidification is now happening at a faster rate than at any point in the last 66 million years, and possibly in the last 300 million years. And projections show that by the end of this century, ocean surface waters could be more than twice as acidic as they were at the end of last century if we do not reduce our carbon emissions.

CO2 concentrations drive rising temperatures and acidification

The rising concentration of carbon dioxide in the atmosphere is driving up ocean surface temperatures and causing ocean acidification. Although warming and acidification are different phenomena, they interact to the detriment of marine ecosystems. These changes to the ocean aren’t occurring at the same rates everywhere: there are significant differences across gradients of temperature, latitude, and depth.

The rate at which water absorbs CO2 decreases as water temperature increases. This means that polar regions like Alaska, where ocean water is relatively cold, can take up more CO2 than the warmer tropics. As a result, polar surface waters are generally acidifying faster than those in other latitudes, and on average, warmer regions of the ocean are releasing CO2 into the atmosphere instead of absorbing it.

The regional differences in ocean acidification can also be partially explained by the effects of ocean circulation patterns. Due to prevailing wind patterns and other natural phenomena, the ocean upwells nutrient-rich and more acidic or corrosive deep waters. Under natural conditions, an infusion of nutrient-rich, cool, and corrosive deep waters into the upper layers is beneficial to coastal ecosystems. But in regions with acidifying waters, an infusion of cooler deep waters (which also tend to be more acidic) amplifies the effects of existing acidification.

In other regions, usually the tropics, rising temperatures in surface waters is slowing down the exchange of carbon between deep waters and surface waters. Here wind plays a key role: it mixes upper and deeper waters and carries the CO2-saturated waters to deeper areas of the ocean. With rising surface temperatures, it’s harder for winds to mix these layers, which become increasingly stratified, meaning that they sit on top of one another. Consequently, in locations with warmer waters, upper layers are becoming more saturated with CO2 and unable to absorb more, and lower layers have less oxygen (known as deoxygenation).

Ocean acidification affects marine life

Coastal and marine ecosystems are under tremendous stress from climate change. Ocean acidification, paired up with other climate impacts like warming waters, deoxygenation, melting ice, and coastal erosion, pose real threats to the survival of many marine species.

Ocean acidification is particularly detrimental to species that build their skeletons and shells from calcium carbonate (like clams, mussels, crabs, phytoplankton, and corals), and that constitute the bottom of the food chain. Acidification reduces the availability of carbonate ions in ocean water, which provide the building blocks these organisms need to make their shells and skeletons, significantly reducing the chances for their offspring to survive.

In the presence of other climate stressors, ocean acidification makes it harder for species to bounce back. Take the problem of coral bleaching, for example. Corals maintain a mutualistic relationship with photosynthetic algae living in their tissue: corals provide shelter for the algae and each provide the other with nutrients necessary for their survival. But when water temperatures get too high, corals expel these algae, leaving them more vulnerable to disease and less able to maintain and build their skeletal structure.

Ocean acidification hinders the ability of corals to recover from these bleaching events because it reduces the amount of calcium carbonate available that corals need to grow back to health. A report by the Intergovernmental Panel on Climate Change finds that 99 percent of the world’s warm-water coral reefs could disappear if global average temperatures rise 2°C or more above pre-industrial levels.

Ocean acidification affects the economy, people’s livelihoods, and communities

Shellfish fisheries are also feeling the impacts of carbon pollution. In places like the Pacific Coast of the United States, warming waters and ocean acidification are expected to reduce the Dungeness crab populations, the highest- revenue fishery in Oregon and Washington.

Fisheries in the Northwest are already feeling the impacts of warming waters, which are wreaking havoc in the region and causing multimillion-dollar losses to local economies. Warming ocean temperatures have caused a rapid increase of toxic algal blooms. Toxic algae produce domoic acid, a dangerous neurotoxin, that builds up in the bodies of shellfish, posing a risk to human health. As a result, many West Coast fisheries have been forced to shut down. With increasingly acidic waters, and the subsequent reduction of the minerals that shellfish need to grow, these fisheries face serious challenges into the future.

Regarding warming, the Pacific Coast Federation of Fishermen's Associations (PCFFA) filed a lawsuit against 30 fossil fuel companies in 2018. PCFFA alleges that these companies promoted and profited from increased oil, coal, and natural gas consumption despite being aware of the link between increased heat-trapping emissions and climate change. It is the first case brought by an industry association against fossil fuel companies.

Why should we all care, and what can you do?

If you live in Kansas or Oklahoma, you may think that ocean acidification doesn’t affect you. But it does. Ocean acidification impacts important sectors of the US economy, like fisheries and tourism, it affects food supply, and makes global warming worse by hindering the oceans’ ability to absorb CO2. For communities that depend on coastal resources, their way of life and cultural identity are on the line.

If CO2 emissions continue unabated, by the end of the century, ocean acidification is expected to reduce harvests of U.S. shellfish. It’s estimated that by the end of the century annual supplies of clams could decrease by 35 percent, oysters supplies could fall by 50 percent, and scallops could see a decline of 55 percent. Overall, the shellfish industry could experience cumulative consumer losses of $230 million. In this same scenario, ocean acidification paired with warming could cost $140 billion in today’s dollars in lost recreational benefits associated with coral reefs, and the US coral reef recreation industry could decline in value by more than 90 percent by 2100.

The most effective way to limit ocean acidification is to act on climate change, implementing solutions to dramatically reduce the use of fossil fuels. If we dramatically cut our global warming emissions, and we limit future warming, we can significantly reduce the harm to marine ecosystems.

  • The most recent National Climate Assessment projects that by taking action now we could avoid steep declines in fish catch potential, thus reducing harm to fisheries.
  • The IPCC report highlights that with significant emissions reductions, 30% of coral reefs would be spared from extinction.

We also need to ensure that resources reach those communities that will be most affected by ocean acidification. At present, taxpayers foot the bill for climate damages and adaptation costs. However, climate change negatively impacts local economies and stymies these communities’ ability to adapt.

Courts are beginning to consider holding fossil fuel producers accountable for damage they knew their products were causing because they chose to misinform investors and the public about those risks instead of acting to mitigate them. Making a case for these companies’ responsibility, Henry Shue, professor of politics and international relations at the University of Oxford, argues “Companies knowingly violated the most basic moral principle of 'do no harm,' and now they must remedy the harm they caused by paying damages and their proportion of adaptation costs.” Scientific findings that show the extent of the damage caused by carbon pollution can inform those efforts.

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