We have conducted a quantitative environmental reconstruction of Mutsu Bay, spanning a period of several thousand years. In the quantitative environmental reconstruction, the temperature is reconstructed digitally such as 3.4 degrees C, rather than using expression like warm or cold, and it aims at reconstructing the environment so the values can be used for numerical modeling in the future. It is a research method which has became dominant in the field of paleoclimate/paleoenvironment since around 1990.

Since 1992, remains of a huge village and large-scale pillar-supported structures have been unearthed during the excavation research at the Sannai-Maruyama site in Aomori prefecture, which drastically changed our view of the Jomon period and attracted a great deal of interest. Then, why did such a huge village disappear at some point in time? The method we employed to solve this mystery was to verify the climate around the site back in those days in a quantitative, time-continuous manner, by using the environmental reconstruction method.

The result was just what we had imagined. Around 5,000 years ago when the Sannai-Maruyama site was flourishing, the air temperature around that area was about 2.0 degrees C warmer than today, and we found out that a lot of chestnuts were growing by examining pollen fossils. However, it seems that people abandoned the village and dispersed somewhere because the air/water temperature suddenly decreased (by 2.0 degrees C) 4,200 years ago. Quite interestingly, East Asian civilizations have also declined during the same period. They must have been related to the climate change.

From these research results, it is possible to estimate what the warmer earth will look like, which is a matter of concern. The temperature is expected to increase around 2 degrees C by the end of this century, and the agriculture, forestry and fishing industries could be heavily affected.

Characteristics of the Sannai-Maruyama site

The Sannai-Maruyama site located in Aomori city is one of the largest Jomon village remains in Japan, and it is characterized by the high level of culture such as high-rise buildings built from huge trees and its wide area of trade. It has attracted attention because it changed the conventional view of Jomon people’s lifestyle, which was thought to be a nomadic life based on the needs for hunting and gathering. According to the excavation research, the village was established 5,900 years ago and its size grew since then. However, the reason why people abandoned the village around 4,200 years ago is unknown.

A trend that is consistent with all of Japan

The total population of Japan was about 20,000 around the beginning of the Jomon period (12,000 years ago), reached a maximum (about 260,000 people) around the middle of the Jomon period when this village was in existence, and decreased (to about 80,000 people) around the end of the period (Koyama/Sugito, 1984; Koyama, 1984). Since the trend of growth and decay of the Sannai-Maruyama site is consistent with the trend of the whole country, it has been said that they must have been related to some common environmental factor, but there has been almost no reconstruction of a continuous and quantitative environmental record.

The big advantage of environmental reconstruction in the sea near the remains

An environmental reconstruction was carried out by collecting sediments from Mutsu Bay/Aomori prefecture (at 41 degrees north latitude, 140 degrees 46 minutes east longitude, depth of 61 meters), located only 20 kilometers away from the Sannai-Maruyama site. Unlike land samples, ocean samples record the environment in a continuous manner, and they have an advantage of enabling an accurate determination of the era and water temperature. It is possible to find out the water temperature of the era when the sediments were deposited, by taking the ocean-living planktons called alkenone within the sediment and examining the organic compounds that they produced. Since the positive correlation between the air and water temperatures in Aomori city is quite high, it is possible to determine the variation of air temperature once the water temperature is found.

The reason why the village was established

As a result, it was confirmed that the air temperature of the land around the Sannai-Maruyama site suddenly rose around 5,900 years ago, and it became easier for people to live. In particular, it is significant that acorns and chestnuts thrived and people were able to enjoy the crops. It was presumed from the various fish bones unearthed from the remains that seafood production also increased, and this can be explained from the water temperature as well.

The reason why the village declined

Then, is it possible to derive the reason why the huge village disappeared from such research results?

It has found that a sudden cooling occurred 4,200 years ago. In particular, seawater showed a temperature drop of 2.0 degrees C. A difference of 2 degrees C in air/water temperature is equivalent to a distance of about 230 kilometers in the latitudinal direction (the distance between Aomori and Sendai, or Aomori and Sakata). Since chestnut forests that produce large fruits (for commercial purpose) can only be found in Yamagata prefecture or in areas further south of southern Miyagi prefecture at the present day, we can tell that there were chestnut forests that produced large fruits in Aomori prefecture during the mid-Jomon period, and food was abundant for people in the Sannai-Maruyama village. However, production of food on the land, such as chestnuts must have decreased sharply because of the sudden cooling. It is thought that land-dwelling animals decreased as well, having a seriously impact on the people in the village to obtain food, and this brought the decline of the village. This cooling of the climate occurred all over Japan, and it is highly possible that this was a significant cause of the decrease in the population of Jomon people.

Link to the growth and decay of civilizations around the world

Civilizations in other parts of the world, such as those around Chang Jiang in China or Mesopotamia in East Asia have also been reported to have declined around almost the same period (4,000-4,300 years ago). The reason why civilizations in the mid-latitude region in Asia declined nearly simultaneously may have been due to the effect of various factors, such as the cooling of Asian monsoons or aridification.

In the current global warming, it has been estimated that the average air temperature of the world will increase about 2 degrees C within this century, even if all countries around the world tried to change the social structure on a global scale to that which can achieve a balance between environmental preservation and economic development. Our research results show undeniably that a change of 2 degrees C in the average annual temperature, and in addition, a rapid change, has a high possibility of bringing a huge impact to communities that are driven by primary industries.

This means that we cannot afford to have any delay in our efforts toward realizing a low-carbon society.

Figure 1 Transition of water temperature and air temperature (pollen distribution) at the Sannai-Maruyama site. It shows that water temperature decreased sharply 4,200 years ago.

Figure 2 A difference of 2 degrees C in water and air temperature is equivalent to 230 kilometers in the latitudinal direction. This indicates that there were chestnut forests that grew large fruits even in Aomori prefecture during the mid-Jomon period.

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