Ocean Acidification
In the era of climate change, our planet’s oceans provide an invaluable service: they absorb tremendous amounts of carbon dioxide, at least temporarily putting it out of action. Instead of causing atmospheric temperatures to rise further, the greenhouse gas dissolves in the water. It is then ingested by algae and other marine organisms. When these organisms die, at least part of the carbon in their bodies drifts down to the seafloor. Without this enormous carbon sink, the Earth’s climate would have changed far more drastically than it already has. But this service provided by the oceans comes at a price: when CO2 dissolves in water, it produces carbonic acid. Therefore, the more greenhouse gas the oceans remove from the atmosphere, the more acidic their waters become. Many marine organisms can’t cope with this change, not to mention the fact that the acidification is often accompanied by other stress factors like rising temperatures and hypoxia. As a result, some species could disappear, and entire ecosystems could suffer irreparable harm. This could also affect marine carbon sinks, reducing their effectiveness. As such, there are plenty of reasons for the experts at the Alfred Wegener Institute to take a closer look at the consequences of acidification.
How long have we known about the phenomenon and how was it discovered?
By 2000, it had long-since become common knowledge that too much carbon dioxide in the atmosphere was harmful to the climate. In response, the search intensified for ways to partly remove the greenhouse gas from the atmosphere. Could compressing it and storing it in deep-sea reservoirs be a solution? This idea was initially tested in a range of experiments focusing on the potential effects of such CO2 sequestration. And it soon became clear that a very similar but natural process was already well underway: especially in the shallow shelf seas, carbon dioxide from the atmosphere was dissolving in the water, making it increasingly acidic. Since the early 21st century, this process, sometimes referred to as “global warming’s evil twin”, has received more and more attention from the scientific community. In 2001, Wallace Broecker and Elizabeth Clark from Columbia University, USA, first coined the term “ocean acidification”.
How bad has the acidification become?
The amount of carbonic acid in the ocean varies from region to region. In this regard, the water temperature is an important factor, as are the activities of algae, which consume CO2 during photosynthesis. But normally, seawater isn’t acidic; it’s slightly alkaline, with an average pH value of just over 8. However, that value has dropped from 8.2 to 8.1 since the beginning of industrialisation. Since pH is calculated on a logarithmic scale, that’s already a substantial change: it tells us that the oceans have become roughly 30 percent more acidic over the past 200 years.
Facts and Figures
20-30
percent
According to data from the IPCC, since the 1980s the oceans have absorbed 20 to 30 percent of total anthropogenic CO2 emissions.
7.7
pH value
By the year 2100, today’s average pH value in the oceans could drop from 8.1 to ca. 7.7. If that happens, the oceans will be roughly 150 percent more acidic than today.
1
trillion US dollar
According to projections, the costs of ocean acidification for corals and bivalves alone.
FAQ
Are some regions affected more than others?
Carbon dioxide dissolves especially well in cold water. Accordingly, the acidification effect is particularly pronounced in the polar seas due to purely physical factors. In addition, most of the species living in them are adapted to consistently cold temperatures. When temperatures rise due to climate change, they often have a hard time adjusting. And when acidification is accompanied by other stress factors, these organisms don’t fare well.
Has this type of thing happened before? What’s different today?
The Earth has experienced several phases in which the oceans were far more acidic than today; they often led to mass extinctions. The last such event, roughly 56 million years ago, caused various species of coral to go extinct. But there were always survivors. The reason: back then, acidification was an extremely slow process that went on for millennia. As a result, on the one hand the organisms had time to adjust to the changed circumstances; on the other, the weathering of limestone, which is also a very gradual process, partially mitigated the acidification. Today, neither aspect applies. The pH value is dropping far more quickly than in the past, and neither evolution nor weathering can keep up.
What does the forecast look like?
Using computer models, the IPCC has tested various scenarios for the future. According to the simulations, if we could succeed in reducing our greenhouse gas emissions and limiting global warming to 1.5° C, the average pH value in the world’s oceans would still be slightly above 8 by 2100. In contrast, if humanity followed a “business-as-usual” approach, the value would drop to 7.75 or 7.74, which means the water would be 100 to 150 percent more acidic than today.
What marine organisms are hardest hit by acidification?
The sinking pH value spells trouble for all species that form shells or skeletons from calcium carbonate. In acidic waters, these structures become fragile; in some cases, they crumble, or can’t even be formed to begin with. This problem affects e.g. bivalves, snails and corals. Further, some microscopically small animals among the plankton form calcium carbonate exoskeletons to protect themselves. If they’re no longer able to do so, they may initially survive, but will have no way of defending themselves from predators. A second group of acidification victims: the eggs and larvae of various marine fauna, whose metabolisms are normally geared toward rapid growth and can’t invest much energy into regulating their bodies’ acid-base homeostasis. Consequently, the young of fish, sea urchins and the like are more sensitive to such changes than their adult counterparts.
Could this change entire ecosystems?
That’s not only possible but highly probable. First of all, those species that can easily adapt to lower pH values will have an edge on the competition. As a result, the composition of many biotic communities is likely to change. Secondly, acidification could create serious problems for organisms that are normally hardy survivors: in the complex food webs of the oceans, all it takes is for a few species to disappear; then their predator species suddenly have no more food. And this in turn has consequences for other organisms.
Could acidification worsen climate change?
It’s quite possible that the oceans may not be as effective carbon sinks in the future, as this ecosystem service depends on tiny algae that protect themselves using calcium carbonate exoskeletons. When they die, their bodies drift down to the seafloor – where they deposit the carbon stored in their shells. But when these calcium carbonate structures are weakened by acidification, they become thinner and lighter, and don’t sink as well. This could mean that less carbon is transported to the seafloor. Experts at the AWI and many other research institutes are currently investigating the potential scale of this aspect.
What about the coral reefs?
Today, the most biodiverse marine ecosystems are already massively suffering from climate change. In response to climbing temperatures, more and more coral stocks are bleaching out and dying off. And that’s not the only way that acidification is attacking the reefs; it can also weaken their calcium carbonate frames, making them brittle. As a result, more and more of the “buildings” in these once-vibrant underwater cities are in danger of collapsing. When this happens, countless species lose a vital habitat whose diverse structures offered them a safe haven from predators and a nursery for their young.
How are fish responding to the lower pH values?
Unlike their larvae, mature fish can regulate their acid-base homeostasis. But doing so costs them quite a bit of energy, which they then don’t have for other purposes. For instance, they might grow more slowly, swim more sluggishly or have difficulties spawning. This can become especially problematic when they’re already under stress due to rising water temperatures. Moreover, a low pH value can affect fish’s nervous systems, changing their behaviour. Experiments have shown that, in such cases, clownfish, sea bass and other species can suffer from impaired olfactory senses in acidic water. They either fail to detect smells or react to them abnormally.
Can marine organisms adjust to the new conditions?
Some species are coping with the acidification and warming of the water far better than others. For instance, a European plaice in the North Sea can react fairly flexibly. After all, it’s used to the carbon dioxide content in the Wadden Sea varying over the course of the day and the year. As a result, it can more easily adjust to these changes than fish from the polar regions that are used to unchanging environmental conditions. But even within the same species, not all animals are equally sensitive. When environmental conditions change, young fish can develop somewhat differently, allowing them to more easily cope with the new circumstances. But in some cases, this can also produce deformities or higher mortality in the next generation. As such, how well various species will actually adapt to these new challenges remains unclear.
Are there any organisms that will actually benefit from the new conditions?
As long as the seawater doesn’t turn into pure acid, the high carbon dioxide concentrations are particularly advantageous for marine flora: algae and seagrasses use CO2 to produce energy through photosynthesis. Further, to a certain extent they can regulate their pH value by themselves. This can especially be seen in algae blooms: wherever countless tiny algae float in the water, they consume so much carbon dioxide that the pH can rise to significantly more alkaline values of 9 or higher.
How can ocean acidification affect human beings and economies?
Fishing and aquaculture will likely be the hardest-hit sectors. In the future, economically interesting fish species may no longer grow as well, or may be less able to evade predators. Oyster breeding operations on the West Coast of the USA are already confronted with the effects of acidification. And according to model-based calculations, red king crab fisheries in the Bering Sea may suffer massive losses by 2100. Moreover, the damage done to coral reefs will have dramatic consequences for fishing, tourism and coastal defence. Today, nearly 400 million people around the globe depend on these ecosystems for food and benefit from intact reefs’ function as wave-breakers.
How can we combat ocean acidification?
The only effective strategy consists in attacking the root of the problem by reducing carbon dioxide emissions. But even if we succeeded in reducing emissions to zero, it would take millennia for the oceans to fully recover from anthropogenic changes. As such, the fateful trio of warming, acidification and hypoxia will continue to plague marine ecosystems for many years to come.
