This presentation discusses the onset of the Younger Dryas and the conditions that caused it to last 1,300 years. Three different hypotheses are discussed.
Transcript:
The Younger Dryas cooling event. Why did it last 1,300 years? This presentation discusses the onset of the Younger Dryas and the conditions that caused it to last 1,300 years. The Younger Dryas was a period of glacial conditions in the late Pleistocene from 12,800 to 11,500 years before the present that suddenly interrupted a warming trend at the end of the last Ice Age. The glacial conditions lasted for about 1,300 years, and then they ended as abruptly as they started. The end of the Younger Dryas was very rapid. In Greenland, temperatures rose 10 degrees Celsius, or 18 degrees Fahrenheit, in just 10 years.
The Younger Dryas Impact Hypothesis proposes that one or more extraterrestrial impacts caused the onset of the Younger Dryas. There are three proposed impact locations that fit within the time frame of the onset of the Younger Dryas. The Greenland crater may have formed between about 12,000 and 3 million years ago. The Saginaw Bay impact has been proposed as the cause of the North American megafauna extinction and the emplacement of the Carolina Bays 12,900 years ago. The Iturralde crater in Bolivia may have formed during an air burst by a low density extraterrestrial body between 11,000 and 30,000 years ago.
In 2018, Wendy Wolbach and several co-authors published two papers about the biomass burning triggered by the Younger Dryas cosmic impact. The first paper states that "extensive impact-related biomass burning triggered the abrupt onset of an impact winter, which led, through climatic feedbacks, to the anomalous Younger Dryas climate episode." The second paper states that "the Younger Dryas impact caused extensive atmospheric soot and dust loading that triggered an impact winter. This, in turn, triggered abrupt Younger Dryas cooling and other climate changes, reinforced by climatic feedback mechanisms, including Arctic sea ice expansion, rerouting of North American continental runoff, and subsequent ocean circulation changes." These papers do not discuss the mechanism that restored normal climatic conditions and brought an end to the cooling event.
Robert Schoch proposed a hypothesis that the onset of the Younger Dryas was due to reduced solar activity, and that the end of the Younger Dryas was brought about by solar outbursts and coronal mass ejections from the Sun. In his words: "Plasma hitting the surface of Earth could heat and fuse rock, incinerate flammable materials, melt ice caps, vaporize shallow bodies of water creating an extended deluge of rain, and send the climate into a warming spell."
Dr. Schoch's explanation has no extraterrestrial impact, and his hypothesis of the plasma burst states that "Plasma and electrical discharges hitting the surface of Earth may have been responsible for the vitrification (melting into crude glass) of ancient stone structures seen in some parts of the world, such as various hill forts in Scotland". The problem with this explanation is that vitrification is more likely to be the result of ground fires than of solar flares. This is specially true around a fort where defenders would be pouring burning oil over the attackers, and the attackers would be setting fires to overcome the fort's defenses.
The Glacier Ice Impact Hypothesis proposes that an extraterrestrial object impacted the Laurentide Ice Sheet and ejected chunks of glacier ice that formed the Carolina Bays and caused the extinction of the megafauna in North America. The extraterrestrial impact on the ice sheet would have ejected water in its three phases: ice, water and steam.
Above 35 kilometers from the Earth's surface, the atmospheric pressure is below the triple point of water, and water cannot exist in the liquid state. Any water ejected above the atmosphere would have boiled vigorously and produced clouds of ice crystals. This is a common physical phenomenon documented by many astronauts. The ice crystals can gain additional speed and go into low Earth orbit when water evaporates from their surface.
In addition, if the ejected chunks of glacier ice carried any water, the water would form ice crystals along the ballistic trajectories at heights from 150 to 370 km above the Earth's surface.
The ice crystals in low Earth orbit would have blocked the light of the Sun and triggered a global winter. The book "Solving the Mystery of the Carolina Bays" estimates the amount of water produced by the extraterrestrial impact by assuming that the duration of the Younger Dryas cooling event is related to the quantity of water ejected above the atmosphere. The amount of ice particles in low Earth orbit is estimated using the rate of sublimation of ice and the 1300-year duration of the Younger Dryas event. Once the mass of ice particles is known, the amount of water ejected and the energy to melt the glacier ice can be calculated. The size of the extraterrestrial object derived from these calculations can be used to constrain the results to a reasonable estimate.
It is possible that the Younger Dryas cooling event was caused in part by the smoke of biomass burning, but the smoke in the atmosphere would be washed out by rain relatively soon. However, the orbiting ice crystals would not be affected by rain or other atmospheric effects, and they would remain in orbit until they sublimated. The length of the Younger Dryas then corresponds to the time required to sublimate the ice crystals blocking the light of the Sun, without the need to invoke any climatic feedback mechanisms.
We conclude with the question of what caused the end of the Younger Dryas. From Wolbach's description of biomass burning, we could say that the restoration of normal climatic mechanisms ended the Younger Dryas, even though we don't know what those mechanisms were. If we are to believe Robert Schoch, a solar flare sent the climate into a warming spell. And finally, if the Younger Dryas was caused by water ejected above the atmosphere by an impact on the Laurentide Ice Sheet, then the end of the Younger Dryas was caused by the sublimation of ice crystals in low earth orbit that had blocked the light of the Sun, and without the shield of ice crystals, the Earth could again bask in sunshine.
