1. Blue Carbon: Characteristics of the Ocean’s Sequestration and Storage Ability of Carbon Dioxide

The first life on Earth evolved in the ocean about 3.5 billion years ago. Photosynthetic organisms, which first appeared in the ocean, eventually changed the oxygen and carbon dioxide concentrations in the atmosphere to the current concentrations. This gaseous exchange was the first and can be considered as the most important ecosystem service provided by marine ecosystems. That service has been on-going from the first primitive photosynthetic organism to the present and reflects the ability of the oceans to absorb carbon dioxide. Nevertheless, recent discussions of sequestration of CO₂ have mainly promoted the concept of land-based green carbon sequestered by terrestrial ecosystems.

The Blue Carbon Report, which was released in 2009, has shown that more than 50% of the carbon dioxide absorbed by the plants on Earth is actually cycled into the ocean; the remainder of the carbon dioxide absorbed by plants is stored in terrestrial ecosystems. More than half of the carbon stored in the ocean has been sequestered by shallow coastal ecosystems, which account for only 0.5% or less of the total ocean area. In addition, there is good evidence that shallow coastal ecosystems have been greatly affected by human activities and continue to be seriously denuded. However, the importance of shallow coastal ecosystems has not yet emerged as common knowledge within society in general, and full comprehension of the role of shallow coastal ecosystems has not yet been applied to climate change mitigation and adaptation strategies. For example, shallow coastal ecosystems are not considered in the inventory of absorbed carbon dioxide. One reason is that the sequestration and storage processes of blue carbon are complex, and it is still difficult to determine what criteria are essential for calculating the relative efficacy of carbon sequestration in shallow coastal ecosystems versus terrestrial ecosystems. In this chapter, which introduces this book, we give an overview of the key points of the Blue Carbon Report. We then try to provide a better understanding of the blue carbon concept by explaining the important characteristics of blue carbon ecosystems. We use the carbon absorption process in seagrass meadows as an example to illustrate important concepts.