The amount of carbon dioxide (CO2) in the atmosphere is rising, and this is increasing the acidity of the oceans. The way this happens is that atmospheric CO2 dissolves in water to produce bicarbonate (chemically-speaking, HCO3) and hydrogen ions. The more hydrogen ions that are present in a solution, the greater its acidity.
Acidity is measured on the fourteen point pH scale, with 0 being extremely acidic and 14 the most basic or alkali. Pure water is considered neutral with a pH of 7, although for the last 400,000 years or so the oceans have had an average pH of around 8.2, meaning that seawater is in fact slightly alkaline.
Current research, as reported by The Royal Society (“Ocean acidification due to increasing atmospheric carbon dioxide” published June 2005,) shows that over the last two hundred years the pH of the oceans has dropped by about 0.1. As the pH scale is logarithmic this relatively small change means the concentration of hydrogen ions in seawater has gone up 30%. Furthermore, as atmospheric CO2 keeps rising, the oceans will continue to grow steadily more acidic.
What effect will this acidification have on the plants and animals that inhabit the oceans, and how will that impact the human race? The Royal Society report, along with work carried out by other august bodies such as Woods Hole Oceanographic Institute, explores in great detail the implications for life on our planet.
The most immediate impact will be on the creatures that build their shells and skeletons from the bicarbonate in seawater, principally corals and sea snails. These hard structures can only exist near the surface because at depths of 100 meters or more the chemistry of the water changes, which means they dissolve. The problem is that as seawater becomes more acidic, the depth at which shells dissolve gets nearer to the surface. In some parts of the globe, most notably the Southern Ocean, (the waters below 50 degrees south,) there is already very little depth of water remaining in which corals and sea snails can survive.
Coral reefs generate significant tourist revenues for those who live in their proximity and without the reefs their livelihoods will disappear. Perhaps more importantly, corals are also home to vast numbers of fish and other marine life, providing an entire ecosystem within their environs. Without the corals the species will either die or be forced to find other habitats and prey.
Sea snail numbers are also expected to decline dramatically as a result of acidification, and as they are at or near the bottom of the oceanic food chain this will have an impact on the predators that feed on them. As with the decline of corals, the knock-on consequences will be significant.
Fish are also very sensitive to ocean acidity. While most have a short term ability to deal with changes – for a few hours only – in the longer term it is likely that many species will suffer severe declines in population. As discussed in Scientific American, this will be exacerbated by an associated decline in reproductive success.
A further issue is that increased acidity will alter the levels of metals and nutrients in the water. In particular it is thought that there will be more ammonium and less nitrates. This will change marine habitats and the availability of prey will change – some species will thrive while many will decline – making survival more challenging for animals higher up the food chain.
However, not all ocean life will die out. In particular, as noted by the U.S. Geological Survey, many seaweeds and sea grasses are expected to thrive under conditions of increased CO2, as is the case with many land-based plant forms. This may well have other impacts in terms of animal habitats and coastal erosion.
There has been speculation as to whether marine life will evolve to deal with increasing acidity. Again, referring to the report of The Royal Society, the balance of opinion is that this is unlikely. While in prehistoric times the oceans have undoubtedly been more acidic than they are now, the changes happened over thousands of years, giving life forms time to adapt. Unfortunately the current wave of acidification is taking place over just a few hundred years and few, if any, creatures can be expected to evolve fast enough to deal with this shift.
Ocean acidification is underway and its effects will be felt across the planet. Employment in fisheries and marine tourism will decline and changed patterns of erosion will reshape our coasts while coral reefs will disappear and many species of aquatic plant and animal will become extinct.
