Acting against acidification

September 12, 2014

Acidification is one of the biggest challenges facing the world’s oceans, largely the result of human carbon dioxide emissions. Professor Alex Rogers, Professor in Conservation Biology at the University of Oxford, explains the problems caused by dropping pH, and how we can tackle it.

If we look back to past extinction events, most of them are associated with disturbances to the carbon cycle. Three symptoms appear again and again in the geological record, ocean warming, decreases in the oxygen content of seawater and ocean acidification. Warming has already caused mass mortality of marine life, such as reef-forming corals, and polewards shifts in the distribution of marine organisms globally.

Expansion of the natural low oxygen zones of the oceans, which generally occur between 200 and 1000m depth, has also been detected and is influencing the distribution of marine species such as marlins (these fast swimming fish have a high oxygen demand). Ocean acidification is occurring because the oceans are absorbing a portion of the CO2 humans are causing to be released into the atmosphere. The CO2 is converted to carbonic acid and lowers the pH of seawater, making it less alkaline. So far the pH of the ocean has only declined by 0.1 units but this is a logarithmic scale and so this represents an increase of 30% in acidity.

Why is this significant? Swimmers in the ocean will not suddenly feel a burning sensation, the ocean is still officially alkaline. However, these changes fundamentally alter the chemistry of seawater. One of the main effects is to reduce the amount of calcium carbonate available for marine animals to build skeletons or shells. Already, snails that swim in the surface waters of the Southern Ocean have been found with corroded shells as a result of this effect combining with naturally acidified seawater rising up from the deep. The growth of corals has decreased in areas such as the barrier reef, likely a result of acidification. We are also seeing unexpected effects of acidification. Experiments have shown, for example, that the chemical sensory perception of marine fish is affected by seawater with a lowered pH so they fail to detect their habitat or predators or prey.

Overall the problem of acidification is very serious. We know it has the potential, combined with warming, to seriously impact coral reefs and may rapidly lead to the demise of these ecosystems. It may impact parts of polar marine ecosystems as these are likely to become corrosive to calcium carbonate within decades. However, the potential to impact other aspects of physiology of marine life is only just coming to light. The only solution is an immediate and significant programme in reduction of CO2 emissions globally.