Global temperatures began to soar as early as May this year, beginning with a 50ºC heat wave that hit India. Meanwhile parts of Europe endured sauna-like conditions, while North America and the Arctic saw temperature records melt like ice cream in summer. While weather and climate are two very different things, such severe warming is part of the extreme phenomena that climate scientists predict will become commonplace as atmospheric CO2 levels continue to rise around the world.

The effects of climate change on Earth’s surface temperatures are worrying enough, but equally disturbing is the impact of carbon emissions on ocean chemistry.  When CO2 is absorbed by seawater, it kicks off a chemical cascade that reduces its pH, i.e. acidification. Nor is this just the result of carbon emissions — humans directly affect oceanic pH levels as fertilizers and other chemicals wash off both urban and agricultural land.

The US National Oceanic and Atmospheric Administration (NOAA) estimates that since the Industrial Revolution, the pH of the ocean has decreased from a historical global average of around 8.16 to about 8.07 today. If current emissions trends continue, by 2100 that number could be as low as 7.67 — a five-fold increase in acidity from current levels. The NOAA goes on to state that such changes “have probably not been experienced on the planet for the past 21 million years.”

Two researchers, Diane Lavoie of Fisheries and Oceans Canada at the Maurice Lamontagne Institute in Mont-Joli, Que. and Helmuth Thomas, a specialist in chemical oceanography in the Department of Oceanography at Dalhousie University and at the Helmholtz Center for Coastal Research in Germany, weigh in on the short- and long-term impacts that climate change, greenhouse gas emissions, and freshwater and chemical runoff are having on the world’s oceans. 

A matter of equilibrium

Once atmospheric CO2 is absorbed by water, it creates carbonic acid (H2CO3), which then dissociates or splits into a hydrogen ion (H+) and a bicarbonate ion (HCO3). The bicarbonate can also split further, into carbonate (CO2-3) and another H+ ion. In seawater, bicarbonate, carbonate and H+ all exist in equilibrium, and it is from the dissolved carbonate proportion that organisms such as snails, clams and oysters extract the minerals they need to construct their shells.

For Thomas, changes to the state of equilibrium are key to the pernicious nature of ocean acidification. Once the CO2 is absorbed by the seawater, it is no longer a single entity but rather part of a complex three-way balancing act. “You cannot undo it,” he says. “Once it’s in the ocean, you cannot remove it anymore.”

Thomas looked in detail at such changes in both the Arctic and Antarctic seasonal cycles in “Vulnerability of polar oceans to anthropogenic acidification: comparison of Arctic and Antarctic seasonal cycles,” a 2013 paper published in Scientific Reports. Among his team’s conclusions was that the Arctic is more vulnerable to anthropogenic change than the Antarctic. This is a result of a variety of factors, but one of them is that the Antarctic waters contain excess nutrients, which marine microbes need to grow. The bodies of algae and phytoplankton are made of carbon-based molecules, so they have the potential to absorb some of the additional carbon that humans are introducing into the system. In contrast, the Arctic is already about as productive as its nutrient levels allow; since the growth of new organisms is limited by the availability of nutrients, additional carbon would go straight into lowering pH.

This finding seems to suggest that oceanic life itself may act as a buffer for rising carbon, but Thomas is quick to point out that it can just as easily exacerbate the problem. His current research looks at how pH is altered by eutrophication. This occurs when nutrient runoff — usually nitrates or phosphates from fertilizers, animal waste, or even human sewage — causes explosive growth in receiving waters. The result is a giant bloom of algae or other microbes, followed by a catastrophic bust as the rapidly growing organisms die and their decomposition sucks all of the dissolved oxygen out of the water. Other organisms, such as fish or invertebrates, suffocate and die, and the decaying bodies end up releasing CO2. Although the mechanism might be different, this carbon lowers pH just as effectively as that absorbed from the atmosphere.

“Farmers want to increase their yield, which is understandable, but you end up fertilizing the ocean as a side effect, which is not wanted,” says Thomas. “You have the existing ocean acidification problem, and then you have the effects of biology, which you trigger by human nutrients. In terms of pH, they both act on the acidity of water.”

Ocean acidification due to the atmospheric absorption of CO2 is a slow process, while the impact of land nutrients occurs more quickly. Thomas regards the latter as more reversible, since humans can undertake mitigation measures, such as using less fertilizer or treating runoff before it meets the ocean. But the two processes do interact in one other way, and that has to do with glaciers. These rivers of ice slowly flow across land over thousands of years. If they pass through sand, limestone or silicate stone, they can pick up some of the same growth-sustaining minerals mentioned in the two previous scenarios. If climate change continues to speed up glacier-melt and subsequently increase the transport of minerals to the ocean, they can then stimulate the right amount of growth and provide a buffer for excess CO2. However, Thomas sees the most significant effect of glacier melt as the addition of large amounts of freshwater, which lowers the carbonate ion concentration and raises sea level.

Making life more difficult

Whether fast or slow, decreases in pH can have significant negative impacts on coastal economies by affecting marine life. The availability of carbonate ions affects organisms such as oysters, mussels, and clams. Any change in pH will upset the carbonate equilibrium, but Thomas says this carbonate ion concentration problem possibly affects coastal creatures for only a short period of time — from four to six weeks of the year — during the critical life stage when the organisms are forming their shells. The effect on crabs, lobsters, coral reefs and even fish and algae can be significant.

“The seasonality of carbonate ion concentration (and pH) and changes thereof need to be well understood,” he argues. In addition, if there is early and copious rainfall at the same time, this freshwater runoff further decreases carbonate ion concentration levels. This can have economic consequences for aquaculturists like oyster farmers, who face unpredictable seasonal changes in ocean waters, which makes them wonder if and when they should seed oyster beds.

There is another reason that the Arctic is more vulnerable than the Antarctic: baseline carbonate ion concentration is already close to saturation level. If the pH changes, so too will the amount of carbonate that can dissolve in the water.

“Even small changes will have an effect,” says Thomas. “The impacts of climate change will worsen this massively.” He adds that the concentration may reach the point of no return in 10 or 15 years and when it does, it will be irreversible.

“The atmospheric CO2 will bring ocean waters to this line, but slowly and steadily,” he notes, “while the influx of fresh water from melting glaciers will do this very fast.” 

Thomas is also researching how sediments and bacteria impact seawater pH. He is undertaking work in Nova Scotia along the Atlantic coastline in the mudflats of the Bay of Fundy and the St. Lawrence close to Prince Edward Island, after spending the last few years studying similar geographical areas in Germany. More specifically he is investigating how oxygen-depleted conditions affect the way bacteria in sediments use nitrogen. These interactions can in fact increase pH, effectively acting as a buffer against ocean acidification. He hopes this will help others find ways to “prevent further damage” environmentally. Simply put, atmospheric CO2 levels must decrease and the key way of doing this is to “stop using fossil fuel for energy production,” he says.

Modelling the future

In addition to the impact of fluctuating levels of carbon, nitrogen, phosphorus, and iron, ocean currents can also have a profound impact on acidification, especially at a regional scale. This is the focus of researchers such as Diane Lavoie, a climate modeller and research scientist in the Pelagic and Ecosystem Science Branch at Fisheries and Oceans Canada (DFO). Lavoie specializes in the effect of climate change on planktonic ecosystems and biogeochemical cycles in Arctic and Subarctic seas. In 2011, DFO began looking into the impact of climate change on the department’s mandate, and Lavoie was tasked with undertaking a risk assessment by researching past, current, and future climate trends using global models available from the Coupled Model Intercomparison Project phase 5 (CMIP5) and the United Nations Intergovernmental Panel on Climate Change (IPCC). These models offer projections of environmental changes 50 years into the future, allowing biologists to evaluate the impact on biological organisms. Lavoie summarized her own evaluation in “Projections of Future Trends in Biogeochemical Conditions in the Northwest Atlantic Using CMIP5 Earth System Models,” a 2019 paper published in the journal Atmosphere-Ocean.

Her findings included details of how oceanic conditions in the Northwest Atlantic are influenced by deep convection in the Labrador Sea. The phenomenon starts in winter, when cold, dry air, flowing eastward over land in northern and eastern Canada, encounters ocean for the first time. There it cools the surface of the ocean water, which grows denser, sinks, and eventually joins up with other deep-water currents circulating throughout the North Atlantic — one of the few places in the world where huge volumes of surface water get drawn downward in this way.

Deep convection pulls whatever gases are dissolved in the surface water down into the depths of the ocean, where they can remain for years. In the past, these dissolved gases have included lots of oxygen, which is vital for nurturing underwater food chains. But more recently, they have also included higher and higher amounts of anthropogenic CO2.

Another new influence is a dynamic similar to eutrophication. Runoff from land masses in eastern Canada can contain elevated levels of nutrients such as nitrogen. While they may not be high enough to cause huge blooms of algae or phytoplankton, they do increase the population of these organisms in the water. As the organisms die and are consumed by bacteria, they use up the vital oxygen that the sinking waters are bringing to the deep and produce more dissolved CO2.

Finally, rising air temperatures around the world are reducing the contrasts between the cold and warm water that give rise to the deep convection in the first place. In other words, not only is the water lower in oxygen and higher in CO2 than it once was, it is also less well mixed as a result of stronger stratification.

“It’s very complicated,” says Lavoie, who notes that it is virtually impossible to provide exact long-term projections to stakeholders such as the fishing industry and governments. She and her colleagues aim to reduce the range of uncertainty that comes with these projections; they have just published a report that includes information on changing habitat for key commercial fish species, as well as shifting temperature changes, dissolved O2 and pH changes, and the calcium carbonate saturation levels so critical to shell species.

“We have deoxygenation occurring, we have acidification occurring and these trends are greater in many coastal areas,” says Lavoie, who regards this information as critical to the creation of marine-protected areas. “Should we protect a certain area if the organisms we are trying to protect are going to leave it anyway, because it’s going to be so bad for them?”

Despite the difficulty in predicting with certainty what changes anthropogenic forces are having on the oceans, she does not accept this challenge as an excuse for inaction.

“Some people say, ‘Oh, it doesn’t matter what I do; it’s not going to change anything,’” she concludes. “Personally, I think that if everybody would start changing little things, then, in the end, it will improve the bigger things.”