(Commentary) Younger Dryas Period

The second explanatory session.

- ほうこうおんち

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Is it possible for the North Atlantic Current to stop, preventing heat from being transported to higher latitudes?

This has actually happened in the past, during a period known as the Younger Dryas, which took place from 12.900 to 11.500 years ago.

The term "Dryas" refers to a genus of high-altitude plants in the rose family, specifically the mountain avens (Dryas octopetala). These are typical tundra plants that inhabit polar regions and high-altitude cold areas, making them indicators of ice ages. While there is also an "Older Dryas," we will not discuss it here.

As the last ice age ended, the Earth warmed, and the Gulf Stream, a warm ocean current, extended as the North Atlantic Current to higher latitudes. The heat released by this current contributed to the warming of Europe, even at high latitudes. On the North American continent, the continental ice sheets were also retreating. The melted ice formed Lake Agassiz, which was much larger than the Great Lakes. At one point, a massive amount of freshwater from Lake Agassiz flowed into the North Atlantic via the St. Lawrence River. This enormous volume of freshwater, being less dense than seawater, spread across the surface of the North Atlantic and halted the northward flow of the North Atlantic Current. As a result, Europe experienced a return to colder conditions.

During this period, the summit of Greenland became about 15°C colder than it is today. In the UK, the average annual temperature dropped by approximately 5°C, leading to the formation of ice fields and glaciers in high-altitude areas. In Scandinavia, forests were replaced by glacial tundra (Dryas). These changes occurred within just a few decades and persisted for about 1.300 years.

In geological terms, 1.300 years is a blink of an eye. However, from a human historical perspective, it is enough time for an entire civilization to rise and fall.

By 13.000 years ago, modern humans already existed. As the ice age ended and warming began, natural food sources increased, leading to population growth. Alongside hunting and gathering, primitive agriculture began to emerge. However, the arrival of the Younger Dryas disrupted human activities that had been adapting to the warming climate. Large mammals went extinct in various regions, reducing the availability of game. The sudden cooling also hindered the growth of fruits and nuts. The increased population could no longer be sustained by relying solely on natural resources. Humans became more dependent on annual grasses and grains. Wild rice and beans, through cultivation, evolved from perennial to annual plants. High-yield varieties were selected, gradually giving rise to cultivated crops. As a result, rice farming flourished in the Yangtze River basin in China, while wheat farming became prominent in the Levant region of the Middle East.

Fragmented information about this cooling event had already been published by 1942.

The concept of an ice age was first proposed in the 19th century. Before that, in the 18th century, the presence of erratic boulders in the valleys of the Alps led to questions about whether glaciers had once been more extensive.

In 1824, Norwegian geologist Jens Esmark proposed the idea of an ice age in a paper, suggesting that glaciers had once extended as far as Scotland and Thuringia in Germany. This marked the beginning of ice age research. The term "ice age" was coined by German botanist Karl Friedrich Schimper.

In 1837, Louis Agassiz, an American geologist and biologist and a friend of Schimper, proposed a more extensive ice age, suggesting that not only Europe but the entire Northern Hemisphere had been covered by a massive ice sheet. At the time, many opposed this idea, arguing that such a vast area could not have been covered by ice. However, as the debate deepened, research into the extent of glaciers and ice sheets in North America began.

In 1879, Warren Upham investigated the topography of North America and identified a massive lake formed by glacial meltwater. The North American continent had once been covered by a vast ice sheet. As the ice age ended and the Earth warmed, the glaciers melted, creating enormous lakes. One such lake was named Lake Agassiz, in honor of Louis Agassiz. Lake Agassiz was larger than the combined area of the Great Lakes of North America, even surpassing the Caspian Sea in size, and was comparable to the Black Sea. The final drainage of Lake Agassiz is estimated to have raised global sea levels by 0,8 to 2,8 meters.

At this point, research had not yet discovered that the drainage of Lake Agassiz had covered the North Atlantic and disrupted the heat circulation. It was believed that the massive glacial meltwater lake in North America had drained into the Arctic Ocean. Therefore, it was not yet understood that the freshwater outflow from Lake Agassiz had once caused a mini ice age on Earth.

However, although the exact timing could not be determined, evidence of past climate changes began to emerge from fields other than geology. First, in archaeology. In 1928, Dorothy Garrod, an archaeologist invited by the British School of Archaeology in Jerusalem, excavated the Shuqba Cave and discovered prehistoric stone tools. This culture was later named the "Natufian culture." Upon further investigation of Natufian sites, stone sickles were discovered in 1931, along with evidence of earlier storage of wild grains. This period was identified as a transitional phase from primitive farming, involving gathering and transplanting wild plants, to planned agriculture. Dorothy Garrod estimated that the Natufian culture existed around 10.000 years ago, during the Mesolithic or Epipaleolithic period, bridging the Old and New Stone Ages.

Additionally, after 1916, advancements in pollen analysis techniques allowed palynologists to study pollen preserved in peat layers in Northern Europe. Although not entirely clear, they found anomalies where forest pollen disappeared, and pollen from mountain avens (Dryas) increased, only to be replaced by forest pollen again. This suggested significant vegetation changes during a certain period.

Fragmented evidence of various kinds was being uncovered. However, as of 1942, no one had yet recognized the Younger Dryas as a mini ice age. It was only a matter of time before someone would connect the dots between the current cooling due to changes in ocean current directions and the past cooling caused by the disruption of the Gulf Stream due to the drainage of Lake Agassiz.

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The Younger Dryas period ended 1.000 years later. The cooling caused glaciers to advance again, blocking the freshwater inflow into the North Atlantic. Similarly, the current cooling in Europe will eventually come to an end.

During the last ice age, the development of glaciers caused sea levels to drop. Some estimates suggest a drop of about 120 to 135 meters. The North American shelf is about 50 meters deep. Even if glaciers do not develop to the extent of the last ice age, a significant drop in sea levels could cause the re-emergence of landmasses. If this happens, the surface currents would once again be warmed in the Gulf of Mexico and flow northward into the North Atlantic. Europe would then recover.

The question is, how many years will it take for this to happen? Even with isostatic rebound causing continental uplift at a rate of 5 mm per year, it would take 10.000 years; at 5 cm per year, it would still take 1.000 years. Geologically speaking, even a "fast" rate is meaningless. The real issue is how to survive the next few years.

The cooling caused by the Younger Dryas and the North American disappearance event do not perfectly align. While the Younger Dryas event caused cooling across the entire Northern Hemisphere, this time, the Far East is experiencing warming. The ocean currents have not been blocked but have dynamically shifted. They are being warmed more extensively and cooled over longer periods. The northward flow of the currents has shifted significantly westward. While the Younger Dryas provides some predictive insights into cooling in Europe, it does not apply to the Far East or tropical regions. Past climate changes can serve as references, but they cannot be directly applied to the present.

So, the question remains: how will they survive?

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The next update will be on August 12 at 17:00.

(I'm quite busy during Obon, so I won't be able to do something like "posting 4 chapters at once in a day" as I did before...)

- ほうこうおんち