Executive board member, Society for Fishery Industry in the 21st Century

　

　

 In Japan the sea-warming phenomenon called the "Jomon transgression" peaked about 6,000 years ago. It is thought to be a cyclical phenomenon taking place in the nature, not a global warming phenomenon like what is caused today by the economic activities of mankind. Information on the Jomon transgression, however, is important because it allows us to predict what will actually happen if global warming continues to develop further on the earth today.

 It is thought that the glacial period ended and the interglacial period began about 20,000 years ago, and that about 6,000 years ago the temperature was about 3 degrees centigrade higher than the present one. It is also thought that although the rise in the sea level was different depending on locations, the Bay of Tokyo was greatly affected by the transgression and that it was much larger than it is today (see Figure).

 There are many shell mounds in the Kanto region, and information on the Jomon transgression is abundantly available. Shell mounds are distributed northward to Tatebayashi and Fujioka in Gunma Prefecture. It is thought that the transgression occurred in areas at the mouth of and along the Tone River and that the Boso Peninsula was almost an isolated island at that time.

 It is predicted that 100 years from now the temperature will increase 4 to 5 degrees centigrade and the sea level will rise about 0.5 m. Considering that the sea level was incredibly high in the Jomon Period, it is reasonably predicted that the sea level will rise a few meters 100 years from now, depending on locations.

 Fish and shellfish remains are distributed in locations from western to southern Japan. It is generally thought that in the Jomon Period they were distributed as far as the Tohoku region.

 The typical example of shells is the hai-gai shell. It is today distributed south of the Bay of Mikawa, while it was distributed as far as the northern part of Miyagi Prefecture, according to data obtained by examining shell mounds. A large number of fish bones are unearthed from the Sannai-Maruyama remains in Aomori Prefecture; the majority of them are the bones of warm-current fish, and the number of yellowtail bones was particularly large.

 With all this information put together, it is thought that the Kuroshio Current reached the sea off the coast of Hokkaido when the transgression peaked in the Jomon Period.

　

 The results of oceanographic surveys conducted in seas close to Japan show that the quantity of plankton in the Kuroshio Current is largely different from that in the Oyashio Current. Plankton net was dropped to a depth of 150 m to collect plankton samples, and it was found that the quantity of zooplankton in the sea of the Oyashio Current is ten times as large as that in the sea of the Kuroshio Current. Although it was thought that the same phenomenon might be observed as to phytoplankton, the number of phytoplankton samples collected was small, and therefore it was difficult to make a comparison. It should be noted, however, that phytoplankton is caught in nets in such a large quantity as to cause clogging and breaking of nets. This shows that phytoplankton is abundant in the North Sea.

　

　

 This abundance of plankton is thought to be attributed to the fact that in the cold North Sea, the seawater is stirred smoothly upward and downward, and nutritious substances in lower layers of the sea rise and come to the surface. Although phytoplankton is a main factor responsible for determining the primary productivity of the sea, it proliferates by photosynthesis, and therefore it grows only in surface layers of the sea where solar rays can reach. The temperature of the sea surface in the warm sea is high all the year round, and therefore nutrient-rich water does not rise to the surface. In the cold sea, the water around the sea surface is cold in winter, it sinks to the ocean depths, and in turn the water in the ocean depth rises to the surface, a phenomenon called upwelling.

　

 The Oyashio Current today runs southward from Kamchatka to the sea off Hokkaido and sometimes further southward to the sea off Fukushima Prefecture. If the Kuroshio Current reaches the sea off Hokkaido, as it did in the Jomon Period, the Oyashio Current will recede northward. As a result, the primary productivity in seas close to Japan will decrease greatly.

 Based on the results of oceanographic surveys conducted, I compared the temperature of water 100 meters deep in the sea and that of water around the surface of the sea, and examined the area in which the inversion of water temperature (the temperature of the water deep in the sea becomes higher than that around the surface) occurs. In seas off the Tohoku region and Hokkaido (north of north latitude 38 degrees), the inversion of water temperature was actually observed in about a quarter of the sea area surveyed (during the period from February to March 1960). Simulations were performed in which the temperature of the surface water of the sea was increased, based on certain data, and the following state of temperature distribution was obtained:

 This result is based on the data acquired at the time global warming was not considered as seriously as today, and therefore a different result may be obtained if the latest data is used.

　

 The ecosystem in the sea functions based on the food chains of various sea animals. At shallow depths, seaweed does not grow and sea animals play the key role in the ecosystem. Because the number of large herbivores is smaller than on land, the decrease of plankton due to warming is estimated to lead directly to the decrease of animal resources. The high fish yield in the sea off the Tohoku region is supported greatly by the explosive proliferation of phytoplankton. If the productivity of the sea area of the Oyashio Current decreases to the level of productivity of the Kuroshio sea area, i.e., a tenth of the present productivity, the overall productivity in seas close to Japan will decrease to about a half.

 If this occurs, the effects are serious. The catch of fish and shellfish will be seriously affected. If this phenomenon is viewed from a wider perspective, the global productivity of the sea will drop, and there is the possibility that Japan will become unable to import marine products. It is estimated, according to this scenario, that many countries in the world may suffer a shortage of food.

　

 Another concern is that global warming is occurring in a much shorter time span than it did in the Jomon Period (a difference between one hundred years and several thousand years). I wonder if the oceanic life has a capacity for adapting itself to the change in the surrounding environment. In Seto Inland Sea, the phytoplankton called heterocapsa proliferated in abnormally large quantities, and bivalves are most seriously affected by the resultant red tide. In Japan this heterocapsa used to be a very weak existence that could hardly over winter. The over propagation of heterocapsa may be associated with global warming. I also note coral bleaching and other phenomena and am afraid that the effects of global warming will continue spreading.

　

　

Manager, Osaka Branch Office, National Maritime Research Institute

　

　

 Carbon dioxide, or CO2, is generated from the combustion of fossil fuels, typically petroleum and natural gas. CO2 is a compound that has one carbon atom (C) and two oxygen atoms (O) combined. Because the oxygen atom is 1.33 times heavier than the carbon atom, the weight of CO2 generated by combustion is about three times as heavy as the unburnt fuel. Japan imports 800,000 tons of crude oil and other fossil fuels every day. This means that four oil tankers, each 200,000 tons in capacity, are required to transport them, and that to transport the CO2 that these fossil fuels discharge to the atmospheric air when combusted, ten oil tankers, each 200,000 tons in capacity, are required if the CO2 is to be loaded on these tankers in the form of liquid. If 5% of the CO2 can be recovered, it fits into one 200,000-ton oil tanker in every two days. Considering that the CO2 discharged in Japan is less than 5% of the total CO2 being discharged worldwide, we can understand how difficult it is to solve the global warming problem when viewed only from a technical standpoint. The situation seems more aggravating if we consider the facts: China and other developing countries that consume enormous energies now and in the future are not members in the Kyoto Protocol, and the population explosion is happening in India and Africa.

　

　

 The number of molecules at the average depth of 3,795 m in the sea is 430 times as large as in atmospheric air. In 1978 American scientists proposed the idea for controlling the climate by disposing of CO2 in the depths of the ocean. This idea did not attract much attention. However, it was reconsidered after a meteorologist presented evidence on global warming in the U.S. Upper House in 1988. In Japan a deep-sea CO2 disposal project began in 1990: the first official research project of this kind in the world. In December 1997 the Third Conference of Parties of the United Nations Framework Convention on Climate Change (COP3) was held in Kyoto. In 1998 the deep-sea disposal of CO2 was added to the Outline of the Battle against Global Warming as one innovative technique to mitigate the global warming.

 There is an increasing awareness that the deep-sea disposal of CO2 must be studied as one of the few techniques that enable us to dispose of environment-disrupting substances in huge quantities. Research and experiments concerning the deep-sea CO2 disposal are now expanding worldwide as the U.S., Norway and Canada have begun to do research and conduct experiments.

 Various methods of disposing of CO2 in the sea have until now been proposed. They are broadly classified into a dissolution method and a storage method. With the dissolution method, recovered CO2 in liquid or gas form is dissolved and diffused at a depth of 2,000 m or shallower in the sea; this method takes advantage of the immense capacity of the ocean. Its working principle is that the recovered CO2 is integrated into the natural circulation process in which part of the excess CO2 in atmospheric air dissolves in the sea. With the storage method, recovered CO2 in liquid form is stored in sunken places 3,500 m or deeper on the ocean floor where CO2 becomes heavier than the seawater with CO2 dissolved. This method aims to keep the effects of stored CO2 on the oceanic environment to a minimum. Because both methods are technically viable, the effects on the marine environment and the ecosystem must be evaluated before either method is adopted for disposing of CO2. Table 1 shows a summary of the features and differences of the two methods. Although the dissolution method is superior to the storage method in technical viability and cost because of the shallow disposal depth, the storage method gains an overwhelming advantage over the dissolution method if we consider the storage period and reversibility (whether the situation can be reversed to the original situation), as well as the precision and ease of environmental impact assessment.

 With all this considered, I conclude that the storage method should be used to dispose of CO2 in the deep sea and that the technical development, including the assessment of impact on the ecosystem in and around the CO2 storage site, should be undertaken.

　

 We at the National Maritime Research Institute (former Ship Research Institute) have conducted research on the deep-sea disposal of CO2 for the past 11 years, assuming that the storage method is adopted. We clarified the properties of the CO2 hydrate forming at depths 500 m or deeper (at a point in the northern sea of the Pacific Ocean) and 900 m (at a point in the northern sea of the Atlantic Ocean) and proposed the COSMOS (CO2 Sending Method for Ocean Storage) method, which is designed to overcome the drawbacks of the storage method: the difficulty and high cost in transporting CO2 to the depths of the sea. (The CO2 hydrate is a metastable crystalline compound with properties identical to those of ice, and it turns into a sherbet when it mixes with water.)

 The net cost of deep-sea storage of CO2 becomes nearly equal to that of the dissolution method if the COSMOS is used, specifically it is estimated to be about 20% of overall power generation cost. Considering that Japan must reduce CO2 as specified in the Kyoto Protocol (minus 6% of the result accomplished in 1990) and implement the reduction activities specified in the Energy Saving Law and other laws and regulations, the amount of CO2 to be reduced by the deep-sea CO2 disposal is estimated to be about 5% at most. Therefore, the cost of the deep-sea storage of CO2 is about 1% of the overall power generation cost, which is at an allowable level.

　

　

 Figure 1 shows the concept of the COSMOS. CO2 to be transported by a tanker is cooled to around minus 55 Celsius, where it is just about to turn into dry ice, so that the pressure on the tank can be reduced as much as possible. This low-temperature CO2 becomes sufficiently heavier than seawater at a depth of 500 m. If it can be discharged as a large droplet of one meter or larger in diameter, it will sink by its own weight to the storage site 3,500 m or deeper in the depths of the sea without being affected by the heat from seawater or the buoyant force from ice layers which cover the liquid bubble. Figure 2 shows a low-temperature CO2 slurry of 8 cm in diameter sinking by its own weight toward a storage site at the speed of 0.3 m/s around the depth of 530 m in the sea, which was photographed in the field experiment jointly conducted with the Monterey Bay Aquarium Research Institute (MBARI). The National Maritime Research Institute with the cooperation of the MBARI plans to begin the work of the COSMOS development using a large high-pressure tank for operations at a depth of 6,000 m. This tank will be completed soon.