What can we do to prevent SETONAIKAI SEA level rise ?

osamu tsukamoto

Osamu Tsukamoto

Department of Earth Sciences, Faculty of Science Okayama University

Born 1949 in Fukuyama-city, Hiroshima Prefecture. Mr. Tsukamoto completed his doctoral course at Kyoto University, Graduate School of Science. He specializes in Meteorology. His studies focus on local meteorology and on-site observation of heat and material transportation on the surface of the earth. His current major study theme is: "How much carbon dioxide resulting from global warming is absorbed by the ocean."

───How is the sea level of the inland sea "Setonaikai" changing ?

There is data taken from 8 points of the Setonaikai gathered by a student of Okayama University of Science. According to this, there has been an approximate rise of 30 centimeters at each point over the last 50 years. The sea level is rising at a pace of 6 centimeters each decade. Interviews reveal that local residents acknowledge a greater rise has occurred than the measurements suggest. This is probably due to the fact that the local residents' impression is of the sea level at full tide which appears higher. The data is the average height taken from the sea.

───Could we say from the data that the sea level is rising in Setonaikai ?

Yes. While there are effects of subsidence in some of the areas, the data had been taken from a pretty wide area of the Setonaikai and Japanese coastal areas, not limited to the Setonaikai, which are experiencing about an 8 centimeters rise in the sea level over the last two decades. Therefore, the increase in the sea level is greater here in Setonaikai.

───What is causing this rise in sea level ?

We are familiar with the theory about the effect of ice melting as a result of global warming. This is, however, often misunderstood. The melting of icebergs does not affect the ocean's volume. It is the continental ice that is the main cause of concern. For example, Antarctica's land height reaches 4,000 meters at its peak, and it consists mostly of ice. If a 4,000 meters ice sheet melts and flows into the ocean, it will significantly increase the volume of the ocean.


───Intergovernmental Panel on Climate Change (IPCC) has reported in its Fourth Assessment Report that the sea level will rise by a maximum of 59 centimeters by the end of this century.

Their data has calculated the melting of ice on continents, but they have not taken into account the secondary sea level change. Another 30 centimeters can be added to their figure when this is added into the calculation. Therefore, there is a possibility of a total rise of up to 80 centimeters.

───Are there other factors causing the rise ?

Yes, there is another major factor. It is not widely acknowledged, but water expands when heated. I believe you are familiar with the thermal expansion of air? While expansion of water is smaller than air, it is considered as the same mechanism. The rise in sea temperature increases the volume of sea water. Thermal expansion cannot be detected in small amounts of water such as in glasses or pans. However, the average depth of the Pacific Ocean is 4,000 meters. If even 1,000 meters from the surface is warmed, it has a significant effect that can raise the sea level as much as 1 meter.

───Setonaikai is an ocean, yet shallow.

It certainly is. The depth at the deepest point only reaches 20 meters. Therefore, the effect of thermal expansion would be minor. However, Setonaikai is connected to the Pacific Ocean. The rise in the Pacific Ocean's water temperature raises the sea level of Setonaikai. This is a great factor when considering the effects of "sinking island countries" of the world.

───Is there a geographical make-up of Setonaikai that makes it vulnerable to sea level change ?

The Kuroshio current is linked to rise of sea level in Setonaikai. This warm current flowing along the coasts of Shikoku Island does not always run the same course. It changes annually. Change in the flow, affects sea level. There is data to show that the sea level increases when the flow is close to land and the sea level decreases when the flow moves away from land. The sea level rises between 20 to 30 centimeters when Kuroshio current approaches the coast and is often referred to as an 'Abnormal Tide.' The mechanism of change in tides is not yet fully understood. Perhaps global warming itself has an effect on the tides.

───What would happen if the sea level continues to rise ?

Should the sea level continue to rise at its current pace, there is the possibility of flood damage at places that were not prone to such disasters before. The danger is sure to increase. This is true throughout the nation, and Setonaikai is no exception. Setonaikai originally has little experience of natural disasters and people's awareness is very low. In order not to repeat damage like that suffered from the typhoon in 2004, I believe there is a need to call people's attention to disaster prevention on a daily basis.

───What can we do to prevent sea level rise ?

We need to prevent global warming. While it is very difficult in reality, I think slowing down its speed is possible with effort. If left alone, the pace of global warming can accelerate bringing about what was estimated to occur in 100 years in just 50. It will be too late, then. Global protocol against depletion of the ozone layer which was called to our attention a while ago was implemented earlier than expected. As a result, chlorofluoro carbons have remarkably decreased from our atmosphere, and some even believe that the ozone layer can be restored in another 50 to 60 years. People's efforts can produce results. Dealing with CO2 may not be as easy as the ozone, but it is important to do what we can. That is the best prevention against global warming.