Effects of Global Warming on Aquatic Resources

The Jomon period forewarns of global warming to a certain extent. Even then the conditions were not easy, and when global warming progresses in a short time, what will happen remains in the realm of the unknown. Then, the productivity of the sea will decrease for certain.

1. Information on the marine transgression in the Jomon Period

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.

1) Expansion of the Bay of Tokyo due to the Jomon transgression
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.
2) Change in the aquatic resources shown by the distribution and conditions of shell mounds and other remains
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.

2. Decrease in the primary productivity and its effects

Sea in the Jomon Period

1) Abundance of plankton in the Oyashio Current
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.
2) Upwelling stops if the temperature increases 4 or more degrees
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:

• The temperature increased 1 degree in 42% of the sea area.
• It increased 2 degrees in 8%.
• It increased 3 degrees in 3%.
• A 4-degree temperature increase was not observed in the sea area.

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.
3) Effects on the food chains of sea animals
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.

3. Adaptability of oceanic life to the effects of global warming

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.