Increasing Acidity

According to the UN-sponsored Intergovernmental Panel on Climate Change (the IPCC for short,) the balance of scientific opinion is that mankind is warming the planet. While climate changes are complex, it does seem that emissions of greenhouse gases, especially carbon dioxide, are a significant factor in this process. The impact of global warming will be felt across the globe, but perhaps no region will be hit harder than the oceans.

One obvious change is that as the polar icecaps melt, sea levels will rise. This is already leading to increased flooding in low-lying coastal areas and is expected worsen in the years ahead. As a result, much fertile land will be lost to the sea and millions of people will be forced to relocate inland.

A second issue is increased stratification of seawater. Stratification refers to the way in which the oceans separate into layers that mix very little. As reported by The Royal Society, “Ocean acidification due to increasing atmospheric carbon dioxide,” (published in July 2005) surface temperatures will rise, resulting in a steeper transition from top to bottom. This will further reduce mixing of the layers and will probably increase both the frequency and severity of tropical storms. However, a far bigger threat comes from the impact on the oceans ability to absorb carbon dioxide.

Carbon dioxide (CO2) dissolves in water (H2O), creating first carbonic acid (H2CO3), and then carbonate (HCO3) along with free hydrogen ions (H+). Some of this carbonate is taken up by plants and animals in the sea while the balance returns to the atmosphere through evaporation. These processes, discussed in detail in a FAQ on the web site of Woods Hole Oceanographic Institute, have been in balance for millennia. However, in the last two centuries that has begun to change as atmospheric CO2 has risen from 280 ppm (parts per million) to 380 ppm today. (Source: National Oceanic & Atmospheric Administration (NOAA).)

The solubility of CO2 in water is dictated by Henry’s Law. This says that the quantity of gas that can dissolve in the liquid is proportional to the pressure of the gas. Thus, as the amount of CO2 in the atmosphere increases, more dissolves in the oceans, which creates more carbonate and free hydrogen ions. To put this in perspective, NOAA states that almost half of all the CO2 emitted by man’s activities has been absorbed by the seas.

Rising CO2 leads to more hydrogen ions in the oceans, and this is slowly raising their acidity. This has many consequences for sea life, the most significant of which may be a change in the calcium carbonate saturation horizon. This technical term describes the depth at which shells and skeletons composed calcium carbonate will dissolve. One result of increased acidity is to bring this depth closer to the surface. Indeed, it is estimated that by 2050 the calcium carbonate saturation horizon will be at the surface in much of the Southern Ocean. This will make it impossible for plants and animals that build skeletons and shells from Calcium Carbonate (CaCO3), especially corals and sea snails, to survive.

The effects of the disappearance of these species will be felt both up the food chain and economically. Those creatures that depend on coral for their habitat will probably go into steep decline, as will the predators that live on sea snails and similar creatures. This in turn will affect both fisheries and tourism, no doubt causing severe hardship to many.

Fish will also suffer directly from ocean acidification. While their bodies can adjust to temporary shifts in pH, in the longer term studies suggest their fertility and reproductive success will be greatly diminished. On the other hand though, many plants will probably thrive under conditions of increased CO2, so while fish and corals will decline seaweeds and grasses could grow to choke river estuaries and clog harbors.

Global warming is changing the oceans in many ways. Sea levels are rising and surface waters are becoming warmer, both of which will create problems for mankind. The bigger problem however seems to ocean acidification, which will probably destroy habits and bring irreversible changes to the marine biosystem.