Excess CO2 threatens our oceans and life on Earth

Excess CO2 is altering the ocean chemistry with potential disastrous consequences for our planet. We need to reduce human emissions now before the changes are irreversible.

By Babatunde Adeleke and Dr Deborah Robertson-Andersson, University of KwaZulu-Natal

Rising atmospheric carbon dioxide (CO2) is not just a critical factor in global climate change because its consequences are globally persistent and irreversible on ecological timescales as reported by Intergovernmental Panel on Climate Change, but also because of its effects on a planetary scale. The amount of CO2 released into the atmosphere in the last century has increased hundred times faster than any observed in the course of the past 650,000 years. Historically, there has been a balance between CO2 being generated, and CO2 being taken in. When tiny aquatic plants (phytoplankton) photosynthesize, they convert atmospheric CO2 into their cell walls and when they die, a proportion of that carbon sinks into the deep ocean where it remains isolated from the atmosphere for 100 to 1000 years. The problem now is that CO2 is being created faster than it can be absorbed through natural processes. It is excess carbon dioxide that is the problem.

Many people don’t realise that the oceans absorb a third of CO2 each year
and that the effects of climate change would be a lot worse if it wasn’t for the oceans.

Excess CO2 is altering the ocean chemistry with potential disastrous consequences for our planet.

The direct effects of rising atmospheric CO2 on the oceans are increasing ocean temperatures and ocean acidity. Ocean acidity is affected because of the interactions of CO2 with water molecules. The more CO2 dissolving into seawater, the more carbonic acid (H2CO3), bicarbonate ions (HCO3) and hydrogen ions (H+), are produced. It is hydrogen ions which are responsible for pH. The more you have of them, the more acidic something is. This relationship is illustrated below:

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You don’t need to be a rocket scientist to understand this equation. All you need to do is think of a sea-saw (seesaw). This equation is reversible, meaning that it can move in either direction depending on what you add to it.

So think of CO2 as a person sitting on the left hand side of the equation: if we add another person (increase the CO2 in the oceans), the right hand side of the will rise because of more weight on the left of the seesaw. Because pH is the negative logarithm (–log) of H+ ions concentration, the more H+ ions we have, the lower the pH and thus the more acidic something is. In the oceans, the production of H+ ions decreases pH thereby resulting in a phenomenon known as ocean acidification. The pH level of the oceans has already fallen over the past 150 years from 8.2 to 8.1 (on a scale of 0 to 14). This may not sound like much, but on a logarithmic scale it equates to a 30 % increase in the concentration of hydrogen ions.

Studies have revealed that the uptake of anthropogenic CO2 by the ocean, and the resulting changes in the chemistry of seawater have adverse effects on many calcifying marine organisms (think anything with a shell). This is because elevated concentration of CO2 in seawater reduces calcium carbonate saturation (the stuff shells are made from), resulting in low calcification rates (i.e. accumulation of calcium salt in body tissues). Studies have shown that many marine invertebrate species such as corals and clams are threatened by ocean acidification, hindering their shell and exoskeleton formation ability and therefore would require a higher energy budget for calcification. Some species such as blue crabs, lobsters and shrimps were found by Justin Ries, a marine scientist at the University of North Carolina, to have grown thicker shells with increasing acidity which could make them more resistant to predators, but this is probably costing them in terms of energy for reproduction and growth. Therefore, the responses to ocean acidification by marine species could either be favourable or unfavourable depending on the physiology and adaptive mechanisms of species.

Closer to home researchers at the MACE Lab at UKZN were comparing photographs of abalone from farms 17 years ago and those taken last year. What they noticed was startling: abalone grown on South African farms 17 years ago had much higher numbers of Spirorbid worms (a genus of very small (2 – 5 mm) polychaete worms, usually with a white coiled shell) on their shells.

When looking at the pH records on the farms, they noticed that some farms were experiencing pH of 7.9, a figure that was projected for South Africa for 2100. Animals in experiments grown in lower pH actually lost shell weight, which meant the shells were dissolving under the more acidic conditions, or they weren’t generating new shell as quickly as they would have under normal conditions. It is possible that the fact that there are very few Spirobid worms on the shells of abalone mean that the shells have simply dissolved. The impact of ocean acidification on species like Spirobids and abalone is not just an issue for aquaculture farmers. These molluscs play important roles in the marine ecosystem and provide food for species above them in the food chain.  Researchers think that ocean acidification effects are ocean-wide and that just because you don’t have a shell won’t mean that you won’t be affected by ocean acidification. The abalone industry in South Africa employs more than 2,500 people and is a significant earner of foreign income for this country.

Can we slow or stop the process? There are ideas to lime the ocean, i.e. put more carbonate into the sea, but the scale is just massive. Imagine breaking up the White Cliffs of Dover and tossing them around the ocean. The better solution is to reduce our carbon emissions.

How critical is this issue? Well, if all the life on terrestrial Earth ceased tomorrow, earth as a planet would still continue. But if the ocean chemistry is fundamentally altered, then Earth as a planet would cease to exist.