19 July 2004
World's Oceans Becoming More Acidic
by Kate Melville

The oceans of the world are absorbing larger amounts of carbon dioxide (CO2) which is increasing their acidity and possibly threatening the survival of many marine species, particularly calcifying organisms such as corals, shellfish and phytoplankton. The research, presented at a UNESCO symposium, suggests that marine food chains could be disrupted thus altering ocean chemistry in ways that are not yet understood or predictable.

The symposium brought together scientists from the world's leading oceanographic institutions to discuss how the ocean might be affected by higher levels of atmospheric carbon dioxide. They were also asked to discuss potential environmental consequences of proposals to use the ocean to sequester excess atmospheric CO2. A report on the meeting's conclusions points out that the ocean is one of the Earth's largest natural reservoirs of carbon and each year absorbs approximately one third of the carbon dioxide emitted by human activities.

According to research led by Christopher Sabine of the National Oceanographic and Atmospheric Administration, the ocean has taken up approximately 120 billion metric tons of carbon generated by human activities since 1800. The IOC reports that some 20-25 million tons of CO2 are being added to the oceans each day.

The absorption of carbon dioxide by the oceans is considered a beneficial process that reduces the concentration of CO2 in the atmosphere and mitigates its impact on global temperatures. However there is growing concern over the price of this assistance. For the symposium participants, it is now well established that by the middle of this century, the accumulating burden of CO2 entering the ocean will lead to changes in pH or acidity of the upper layers that are three times greater in magnitude and 100 times faster than those experienced between ice ages. Such dramatic changes in the CO2 system in open-ocean surface waters have not been observed for more than 20 million years of earth's history.

The initial findings indicate that in a high CO2 world:

• the ocean would be more acidic globally, and would also be more stratifed in the high latitudes. In addition, nutrient concentrations in surface waters of high-latitude regions would be lower, subsurface waters would be less oxygenated, and phytoplankton would experience increased exposure to sunlight. These changes would affect many species and change the composition of biological communities in ways that are not yet understood or predictable.
• many calcifying organisms, including plankton and corals, and also non-calcifying organisms, would be unable to grow and reproduce effectively at higher CO2 and lower pH levels. Rising temperatures - combined with elevated CO2 and decreasing pH - pose a serious threat to coral reefs, possibly leading to the elimination of some reefs by the end of this century.

Participants at the symposium said that although the impact of climate change on the ocean has been much debated, the direct impact of CO2 itself has largely been neglected. They concluded however, that changes are clearly underway and their effects may be large and may seriously destabilize marine ecosystems.