Penultimate Glacial Period

Much like the last glacial period, the penultimate glacial period was caused by a great orbital eccentricity of Earth.[5] This eccentricity causes greater seasonal impacts than normal because it limits the amount of sunlight that reaches the earth's surface, lowering the temperature.[6] Due to this, northern insolation (the amount of sunlight that reaches the surface) is reduced, this means when it's summer, less heat is exposed to the snows of the winter, which don't completely melt.[2] This buildup of ice and snow over thousands of years eventually leads to residual ice sheets, which would also reflect light away from the earth, further cooling the earth.[7] A decrease in greenhouse gas concentrations such as CO₂, are a result of the expanded ice sheets.[5] This is because as the Earth cooled, and ice sheets expanded, the ocean waters became colder, which then could absorb more CO₂ from the atmosphere.[8] These factors all feedback into each other: as the ice sheets extended, more CO₂ was absorbed, and more light was reflected off the ice sheets, furthermore expanding the ice sheets, this self-reinforcing cooling setting the world into a glacial period.[8]

In northern Europe, the biggest expansion of glaciation of the last 400,000 years covered the northern region in a thick ice sheet, which caused a drastic reduction of vegetation.[8] In the Mediterranean, polar winds from the now extended ice sheets brought cooler and wetter conditions that caused a significant reduction in large vegetation such as trees.[5] Pollen sequences found from MIS 6 indicated that early in the glacial period, tree abundance fluctuated heavily.[9]

Later in the glacial period, extreme conditions were followed by a mainly treeless landscape all across Europe.[9] This rendered Europe a polar desert just south of the now expanded ice sheets, and the rest of Europe was left with a sporadic herb based plant cover.[9] Europe north of the Alps was a tundra-steppe of predominantly grasses, sedges, and chenopods,[9] while land south of the Alps featured discontinuous steppe vegetation patterns.[9] There were some refugia in the sheltered areas of the mountainous Alps and the western Balkans where tree populations survived.[9] This was due to temperature variations not being extreme in these locations, as well as precipitation still being sufficient.[9] This is unlike the rest of Europe, such as in France where pollen samples revealed a precipitation decrease of almost 60% compared to the modern day.[8] The drastic changes in the climate also resulted in increased storms in the North Atlantic, affecting Europe as well as North America.[2]

Isotope dating was conducted in Hulu Cave, eastern China, and found that the penultimate glacial period's presence was felt in central China.[10] The dating showed an increased presence of oxygen-18, an isotope that reflects the meteoric water and cave temperature, as well as precipitation of the penultimate glacial period.[10] This data led to the confirmation of intense monsoons that impacted most of south-east Asia, and up to modern day Xi’an China.[10] The increase in the intensity of monsoons was due to the orbital shifting of the planet, but was amplified by the ice sheets that formed for the same reason.[10] These factors combined then affected the atmospheric hydrological cycle, creating more intense seasonal winds that led to increased precipitation over south-east Asia.[10]

In contrast to Europe, there is no geological evidence to support a similarly sized ice sheet in North America.[2] Ice-rafted debris from the Hudson area indicates that during MIS6, there were far fewer icebergs in the North Atlantic than in the last glacial period.[1] Simulations testing the extent of an ice sheet in North America have shown that a smaller ice sheet is probable, as the simulation produced weather data that is consistent with hypothesized temperatures at the time.[1] This simulation showed the precipitation rates over North America doubled during MIS6, which would have been a result of the icy winds expanding southward further into the continent, as well as the increased storms.[1]

Several studies have suggested that Homo sapiens went through a genetic bottleneck during the Penultimate Glacial Period which reduced numbers to a low level, but a 2012 analysis of three modern African populations finds no evidence for a bottleneck at this time.[3] Oceanic cores, taken from western Africa, show the deserts expanded, pushing the savannah and the tropical rainforests downward, and oak trees occupying the Mediterranean coast, disappeared.[9] This is thought to have occurred due to southward migration of the subtropical, and high pressure zone of the Mediterranean.[9] This dislocation of vegetation was thought to have displaced Homo sapiens.[9] However, studies have shown that the region which the early humans occupied was very lightly disturbed, and a bottleneck due to the Penultimate Glacial Period is unlikely.[3]