Abstract:Approximately one-half of the carbon dioxide (CO2) produced by fossil fuel burning and cement production as a result of human activities during the past 200 years is being taken up by the oceans. This absorption process is chemically changing the oceans, in particular, increasing its acidity. It is reported that the uptake of CO2 has led to a reduction of the pH value of surface seawater of 0.1 units, equivalent to a 30% increase in the concentration of hydrogen ions. If global emission of CO2 from human activities continues to rise at the current rates, the average pH value of the oceans could fall by 0.5 units (equivalent to a three fold increase in the concentration of hydrogen ions) by the year 2100. This pH value would be lower than that has been experienced for hundreds of millennia and, critically, this rate of change is probably hundred times greater than at any time over this period. The evidence suggests that acidification of seawater would damage the calcifying organisms but it is unclear how that will affect stability of ecosystem and human health. The increasing fragility and sensitivity of marine ecosystems need to be taken into consideration during the formulation of any policies about their conservation, sustainable use and exploitation, or the communities that depend on them. Actions need to be taken now to reduce global emissions of CO2 to the atmosphere to avoid the risk of irreversible damage to the oceans. No option that can make a significant contribution should be ignored.