28 - The bottom line
And the winner in the category "Worst Late Pleistocene Catastrophe" is ....
There is no “consensus” version of how or why the Last Glacial Period (LGP) started or ended, though there are numerous theories, almost all of which fit some but not all of the observations. The previous posts have analyzed most of the unusual events that occurred around the end of the LGP using explanations that were deemed likely (p > 66%), very likely (p>90%) or almost certain (p>99%), whereby the estimated probability p is a function of how well the observations are explained by the model or theory. This post provides a final accounting.
The explanations have been mostly uniformitarian - the present is the key to the past - with only the 2 megafaunal extinctions, a couple of Miyake Events, and the Lake Missoula megafloods being truly unusual and catastrophic in nature, and therefore “interesting” in a clickbait, Black Sea Deluge manner. Most of the other explanations invoke a more gradual, evolutionary mechanism: time passed, ice melted, sea levels rose, currents weakened, climate changed.
The explanations have benefitted from taking a holistic approach: geographical and temporal contexts are important. For example, many researchers (and Wikipedia) treat the Younger Dryas (YD) as a global cooling event, when in fact the cooling mainly occurred around the North Atlantic. The YD was the longest and most severe cooler, dryer climate event in a series of North Atlantic cooler, dryer climate events that started during the Bølling-Allerød (BA; 14.1, 13.3, 13.0 ka events) and ended during the early Holocene (10.6, 10.2, 9.6, 9.2, 8.8 and 8.2 ka events), so any analysis is unnecessarily hobbled by attempts to treat it as a one-off, separate event. A holistic approach focuses on the evolving set of conditions and processes that caused the unusual events that accompanied the end of the LGP.
A 120 m sea-level rise between 20,000-7,000 years ago
Earth’s ice sheets largely melted causing global sea levels to rise. Among the first to melt were the circum-Antarctic sea ice cover (~20 ka) and the Patagonian Ice Sheet (~17.5-17.15 ka) in the Southern Hemisphere, in response to an increase in geothermal heat. Similar geothermal increases in Europe and North America caused ice sheet disappearance in central Europe and most of the UK, as well as the melting of an interglacial corridor in central Canada. Global sea levels only rose a modest 10-20 m over the initial melting period (20 - 14.7 ka) despite a large area becoming ice-free.
Sea level rise accelerated during the BA (~30 m) and the YD (~20 m), mainly due to increased solar energy absorption due to surface albedo changes caused by the retreating North American ice sheets. The start of the Holocene marks the halfway mark (~60m total sea level rise). The ~60 m sea level subsequent rise during the early Holocene concurred with the ever-increasing North Atlantic temperatures in the run-up to the Holocene climate optimum (~8 ka)
15 °C and 10 °C temperature increases in central Greenland and Antarctica resp.
A small part of the fairly large temperature increases observed in ice core proxies can be attributed to global temperature increases, although the bulk of the increase can be attributed to the gradual (Antarctica) or abrupt (Greenland) retreat of neighboring sea ice cover during the Oldest Dryas (Antarctica) and the BA (Greenland).
A volcanically active Mount St. Helens during the "Cougar Stage" (28,000–18,000 years ago), and the "Swift Creek Stage" (13,000–10,500 years ago)
Under the integrated geomagnetic-geothermal-surface temperature model (IGGT model) periods of renewed geothermal activity occur during periods of Outer Core heating as the result of:
Earth’s decreasing orbital inclination since 32 ka that is causing an ever-increasing amount of solar wind energy to heat the Outer Core
Episodic incidences of very large solar particle events
Solar cosmic ray intensity was 50 times higher than present around 16,000 years ago, declining to 15 times higher by 12,000 years ago
Solar electromagnetic radiation and/or solar wind strength may have previously been significantly higher than what has been historically recorded, though more likely causes of increases in “solar cosmic ray intensity” proxies in lunar microcraters are:
the heliomagnetic field likely did not deflect as many Galactic Cosmic Ray (GCR) particles away from Earth between 16 - 12 ka: Earth’s orbital inclination angle was maximal around 32 ka, after which heliomagnetic field protection has been increasing.
Several extremely large Solar Particle Events/Miyake Events almost certainly hit Earth during the 14,000+ years the ice sheets were melting, especially between 15 - 10 ka.
The Lake Missoula Floods: over 40 catastrophic floods that occurred in the Northwestern USA between 20,000 - 14,000 years ago
An increase in geothermal heat flux during the Oldest Dryas very likely both generated the floodwaters of the Lake Missoula megafloods and created the dam that blocked their path into the Pacific Ocean. The basal parts of the Cordilleran (CIS) and Laurentide (LIS) ice sheets melted, which caused:
the Purcell Trench Lobe to advance to the south, thereby periodically damming Lake Missoula
huge volumes of CIS and LIS meltwaters to flow to Lake Missoula
An abrupt doubling of atmospheric radiocarbon concentration around 12,840 years ago, that is around the start of the Younger Dryas.
An abrupt increase in historic atmospheric radiocarbon is known as a Miyake Event. The abrupt atmospheric 14C increase around the start of the Younger Dryas was almost certainly caused by an increase in Earth-incident space radiation, most likely due to a large solar particle event, but possibly due to a sudden large increase in GCRs
Abnormally low geomagnetic field strengths around 28,000-20,000, 16,000-10,000 years ago.
Under the integrated geomagnetic-geothermal-surface temperature model (IGGT model) periods of decreasing geomagnetic field strength occur during periods of increasing Outer Core heat, which can be the result of:
Earth’s decreasing orbital inclination that since 32 ka is on average causing an ever-increasing amount of solar wind energy to heat the Outer Core
Episodic incidences of large solar particle events (SPEs)
The Late Pleistocene megafaunal Extinction that mainly occurred in North and South America, where it killed off almost 80% of the large to medium-sized mammals, and that peaked during the Younger Dryas.
Two episodes of Late Pleistocene megafaunal extinction can be recognized: one around ~40 ka in Australia and one between ~14.5-11.4 ka in the Americas. These extinctions were almost certainly not mainly caused by “overkill” (human hunters), “overchill” (climate change) or “overgrill” (cosmic impact). A good temporal and geographical correlation exists between megafaunal extinction events, and periods & areas of low geomagnetic field strength, which inadequately protected these areas from space radiation. The megafaunal extinction events were very likely triggered by very large Miyake Events (SPEs) that caused increases in primary and secondary cosmic radiation, as well as large increases in ultraviolet radiation (UVR) due to atmospheric ozone depletion. This space radiation could directly cause any number of radiation-related illnesses or indirectly cause higher mutation rates among the flora and fauna as well as pathogens.
Göbekli Tepe, a large, Neolithic temple complex in southwest Asia that was founded around 11,500 years ago, and that was abandoned and purposely buried roughly 10,000 years ago.
The rise and fall of the Taş Tepeler hunter-collector civilization is inextricably linked to climate change:
from the early Holocene warming around 11.7 ka, when temperatures, rainfall and therefore food supplies increased,
to the founding of Göbekli Tepe around 11.5 ka, when the Taş Tepeler people had enough leisure time to be able to construct this impressive monumental complex,
to the colder, dryer climates around 10.6 and 10.2 ka, that very likely caused the collapse of their hunter-collector civilization, when low food supplies forced a significant part of the Taş Tepeler population to migrate away from the Göbekli Tepe area, perhaps to southeastern Europe and/or the Fertile Crescent, to pursue lives of agriculture.
Catastrophe? What catastrophe?
Numerous “catastrophes” happened during the last glacial period, many of which aren’t even mentioned in the list above. For example, Mother Nature wiped out most of the trees in Canada. But the trees literally and figuratively recovered.
Catastrophes are largely in the eye of the beholder. A wedding where the groom doesn’t show up, a starlet’s (lack of) dress sense, a stock market crash, etc. all indicate an objective definition likely doesn’t exist. And every cloud has a silver lining: one species’ catastrophe is another’s opportunity. The 14.5 - 11.4 ka extinction “catastrophe” wiped out most of the North American megafaunal predators, and while it would be intellectually fun to imagine sabre-tooth cats roaming Florida, it would be a tragic catastrophe if a prehistoric predator mauled a child at Disney World. Perhaps the sabre-tooth cat extinction wasn’t such a catastrophe after all.
We tend to view catastrophes through a lens of human suffering. A Black Sea Deluge that catastrophically raises sea levels by 100 m is entertaining, but one that wipes out the 10,000 inhabitants in the coastal villages is a tragedy. It’s hard to see the silver lining in what is arguably the largest tragedy in the list above: the collapse of the Taş Tepeler hunter-collector civilization. Humankind had very likely taken its first steps up the civilization ladder when its inhabitants learned - the hard way - that the hunter-collector life-style was unsustainable during colder, dryer climate periods: climate change very likely knocked humankind back into the stone age for ~3000 years. The silver lining was that it prompted humankind to develop and rely on agriculture to survive, thus paving the way to more advanced civilizations. Such catastrophes are however very unsatisfying from an entertainment point of view: “the weather turned bad, and the inhabitants left the big city to become farmers” will not get many clicks. From an entertainment viewpoint, a comet strike would have been so much more gratifying.
Summary
This sub-stack series has largely dealt with the shifting physical conditions and mechanisms that caused the last glacial period to end, but also the impact of these changes had on humankind’s road to its earliest civilizations.