In part 1 of this three-part series, I wrote about how 800,000 years ago, a miraculous transition from a 41,000-year to an approximately 100,000-year ice age cycle took place—a change known as the Mid-Pleistocene Transition (MPT). If not for the exquisite fine-tuning and precise timing of this event, the high population and civilization we presently enjoy would have been impossible.
I explained why the most popular explanation for the MPT, variations in Earth’s orbital eccentricity, has proven inadequate. Here, in part 2 of the series I will describe eleven new explanations, besides the two I described in my book Improbable Planet,1 and their inherent designs that various science research teams have proposed to explain the MPT.
Scientific Explanations for the MPT
- In 2005, geophysicists Peter Huybers and Carl Wunsch argued that the 41,000-year obliquity cycle has always been the dominant driver of Earth’s ice age cycle, but that beginning 800,000 years ago only every second or third obliquity cycle triggers an ice age.2 They suggest that the recent 100,000-year periodicity in the ice age cycle is the result of averaging together cycles lasting 82,000 and 123,000 years. The Dome C ice core temperature record (see figure 1) roughly accords with their hypothesis. In 2009, geologist Peter Ditlevsen proposed that the glacial cycles of the past 800,000 years are due to a nonlinear response to the 41,000-year obliquity cycle, where the nonlinearity is driven first by internal stochastic (random) noise and later by deterministic responses to subtle changes in Earth’s orbit.
- In two papers published in 19954and 19975, physicists Richard Muller and Gordon MacDonald noted that while the approximately 100,000-year ice age cycle of the past 800,000 years is a poor match for insolation models (see part 1 of this blog series), variations in the inclination of Earth’s orbit about the Sun could account for the cycles of glaciation. In their model, the orbital inclination variations result in Earth’s passing through an interplanetary dust cloud approximately every 100,000 years where the dust cloud blocks some of the incoming solar radiation. Muller and MacDonald cited helium-3 measurements, which are sensitive to tiny extraterrestrial dust particles, in sediment cores that showed the 100,000-year cycle.6 They then hypothesized that one or more disruptive collisions in the Themis and/or Koronis families of asteroids occurred about a million years ago to create a new interplanetary dust cloud.7
- In 2016, environmentalist Carolyn Snyder noted that the average global mean temperature gradually cooled from 2.0–1.0 million years ago and that this cooling stalled from 1.0 million years ago until the present (see figure 1).8 She argued that this global cooling “may have been a precondition for, but probably not the sole causal mechanism of, the shift to quasi-100,000-year glacial cycles at the mid-Pleistocene transition.”9Figure 1: Global Mean Temperatures over the Past Two Million Years. Beginning at approximately 800,000 years ago, temperatures transitioned from 41,000-year to 100,000-year intervals. Image credit: Graph created from data published by C. W. Snyder, Nature 538 (2016): 226.
- In 2013, an international team of seven geoscientists demonstrated the significant role that North America plays in regulating the ice age cycle.11 Of all Earth’s continents, North America receives the greatest impact from the ice age cycle. It oscillates from 10 to 55 percent coverage by ice. (By comparison, the entirety of Earth’s surface oscillates from only 10 to 20 percent coverage by ice.) The team of seven showed that the larger the North American ice sheet “grows and extends to lower latitudes, the smaller is the insolation required to make the mass balance negative.”12 In other words, once a very large ice sheet is established on the continent, only a slight increase in insolation—such as what would be induced by small changes in the eccentricity of Earth’s orbit—is needed to trigger an almost complete retreat of the continental ice sheet within just a few thousand years. The team showed that the primary driver of this fast retreat is “delayed isostatic rebound.”13 (The weight of thousands of feet thickness of ice overlaying much of the continent pushes down the North American landmass elevation. There is a long delay between the melting of the ice and the rebound of the landmass to a higher elevation. The lower elevation resulting from this delay delivers an added warming effect.)
- In 2010, a geology research team led by David McGee analyzed dust flux records over the past million years.14 They noted that mineral dust emissions during glacial periods were 2–4 times higher than during interglacial periods. They showed that a wide range of data supports wind gustiness as the primary cause of the increase. They suggested that the higher dust depositions during glacial periods likely played a role in regulating the ice age cycle.
- In 2011, a team of six researchers led by Alfredo Martínez-García explained how enhanced dust deposition could modify the global climate.15 Reporting on a high-resolution record in marine sediment cores of dust emissions over the past four million years, Martínez-García’s team showed that this dust was rich in iron minerals. They then demonstrated that the deposition of these iron minerals on the oceans would stimulate the growth of phytoplankton. They calculated that this extra phytoplankton biomass would draw so much carbon dioxide from Earth’s atmosphere as to account for a 40 parts per million drop in the atmospheric carbon dioxide level. They demonstrated that such a large drop would significantly cool the global mean temperature. In a later study, Martínez-García and eight other researchers established (from marine sediment cores) that a tight correlation existed throughout the past 800,000 years between dust flux to the Southern Ocean and global mean temperatures.16 The greater the dust flux, the lower the global mean temperature. Finally, the teams led by Martínez-García noted that the marine sediment cores showed a sharp increase in dust deposition in the Southern Ocean that coincided with the date for the MPT.17 Therefore, they concluded that enhanced dust deposition may have played an important role in causing the MPT.
- In 2013, a team of seven researchers led by Swiss geologist Samuel Jaccard and including Alfredo Martínez-García identified a second cause of increased Southern Ocean productivity during glacial periods.18 In addition to iron fertilization of Southern Ocean phytoplankton, they demonstrated that the export of organic carbon from surface waters in the Southern Ocean to the atmosphere decreased during the last ice age. They suggested that this decrease, combined with the iron fertilization, could account for the decrease in the atmospheric carbon dioxide level of 80–100 parts per million that scientists observed during the glacial periods of the past 800,000 years. Such a large drop in atmospheric carbon dioxide, they pointed out, would extend the duration of global cooling and, thus, play a major role in causing the MPT.
- In 2018, two climatologists, Gary Shaffer and Fabrice Lambert, used Earth system climate model simulations to show two things: (1) a second way that the 2–4 times higher mineral dust emissions during glacial periods lower the global mean temperature, and (2) why mineral dust emissions rose so dramatically at the MPT.19Shaffer and Lambert pointed out that the aerosols produced by mineral dust emissions “cool Earth directly by scattering incoming solar radiation”20 and indirectly by participating in cloud formation. They cited previous works21 to show that the cooling and drying of Earth’s atmosphere that Carolyn Snyder had demonstrated to occur from 2.0 to 1.0 million years ago (see figure 1) would have “increased the number and strength of dust sources.”22 This increase would have been a direct consequence of reduced vegetation cover23 and reduced washout of suspended particles24 that would have resulted from the atmospheric cooling and drying.
- In 2017, an international team of fourteen researchers led by Thomas Chalk used boron isotope data to show that the glacial to interglacial difference in atmospheric carbon dioxide levels increased from 43 parts per million before the MPT to 75 parts per million after the MPT.25 They then demonstrated that this increase arose from the lower glacial atmospheric carbon dioxide levels that occurred after the MPT. However, they argued that these changes by themselves are insufficient to explain the MPT. In their model it takes a combination of these changes plus dust fertilization of the Southern Ocean and a major change in ice sheet dynamics (that includes the formation of larger ice sheets, especially in North America) to fully explain the MPT.
- In 2018, mathematician Hermann Burchard revived and revised the interplanetary dust cloud explanation of Muller and MacDonald.26 Burchard claims that “this cloud actually is a belt of Australasian tektites ejected into space at super-orbital velocities that Earth encounters about every 100 ka.”27 (Tektites are millimeters- to centimeters-sized beads of black, green, brown, or gray natural glass that forms when terrestrial debris is heated and ejected during large meteoritic impacts.) Indeed, a team of ten German, Austrian, and American geophysicists used high-resolution argon-40/argon-39 isotope ratios to date Australasian and Western Canadian impact tektites at 785±7 thousand years ago and Belize impact tektites at 769±16 thousand years ago.28 The ten geophysicists concluded that a major impact event gave rise to the Australasian and Western Canadian tektites while a subsequent minor impact event produced the Belize tektites.29 The 785,000-year-ago date is simultaneous or nearly simultaneous with the 786,000-year-ago date for the Brunhes-Matuyama magnetic reversal.30 A team of four geophysicists led by Aaron Cavosie analyzed a large and diverse sample of the Australasian tektites.31 Cavosie’s team concluded that the impact crater must be 40–100 kilometers in diameter and located somewhere in Southeast Asia. Burchard claims to have identified the impact crater as the atolls of the Spratly Islands archipelago in the South China Sea. He produced calculations that were consistent with a comet “obliquely coming from the SW at an extremely shallow angle, striking the Sunda shelf yet unexploded with the shock of its compressed bow wave, and causing the continental shelf and slope to collapse.”32 He then showed how this collapse could produce the fault system observed in the area, the Brunhes-Matuyama magnetic reversal, the atolls of the Spratlys, and a powerful ejection of tektites.33 In Burchard’s model, about once every 100,000 years Earth encounters the belt of Australasian tektites that were ejected into interplanetary space, and he claims that these regular encounters cool Earth sufficiently to explain the MPT and the current 100,000-year ice age cycle.
In part 3 I will describe which of these eleven explanations plus the two I mentioned in Improbable Planet most likely explains the MPT and I discuss the event’s physical and spiritual implications.