ITEM OF THE WEEK
THE CAUSE OF THE YOUNGER DRYAS COOLING EVENT AND PLUTO’S SO-CALLED DEMOTION
Two Case Studies of the Process of Scientific Inquiry
Originally appeared May 1, 2026
Geologists have made great strides in understanding how the Earth’s climate has changed over time, learning that many of the extinction events that mark the boundaries of geologic ages are due to singular events, such as asteroidal impact or major episodes of volcanism. The ability to link cause and effect naturally improves as a function of closeness of the event/change is to the present. One such change event was a period of dramatic cooling of the climate 12,870 to 11,700 years ago, known as the “Younger Dryas Event.”
Another issue dear to the public is the status of Pluto. Both the Younger Dryas and Dwarf Planethood are excellent examples of the process of scientific inquiry.
THE YOUNGER DRYAS COOLING EVENT
Geologists have been able to work out details of the Quaternary Period (2.59 million year before present) quite well in general, correlating evidence of the advance and retreat of glaciers in many areas. The use of radiometric dating, tree-ring studies, and the interrelationship between glacial deposits are common tools. Another source of valuable data are deep core samples of glacier ice, where the variation in ice characteristics with depth of burial can provide clues to climate parameters and their change with time.
Some of the details of the link between climate and glacial/ice cap dynamics are less-well understood. The “Ice Ages” were not a simple case of ice accumulating, advancing, and retreating in an orderly progression everywhere. The climate did not vary as a “sine curve” over the four main periods of ice advance; there were some periods of variability in the “regular” pattern.
One such period is known as the Younger Dryas Event, reliably dated from 12,9870 to 11,700 year ago, when the overall Earth climate cooled by 15° C. The geological evidence is quite compelling; the period of cooling did in fact occur not long after the ice retreated from the most recent glacial advance. As the climate warmed and rain patterns changed, what had been tundra or tundra-like terrain gave way to forests, but in some places the change reversed with the onset of the Younger Dryas cooling. What could have caused such a relatively-brief period of cooling amidst what had been a clear warming trend?
Competing Ideas
Three basic hypotheses arose to explain the cause of the Younger Dryas cooling. The first was related to the changes that had been occurring during the melting glaciers and polar ice at the end of the most recent Ice Age. That melting would have cause a lot of runoff into the world’s oceans, which could have disrupted prevailing ocean current patterns to produce the observed cooling. A competing view was that some sort of event caused changes in Earth’s atmosphere that would have reduced the amount of sunlight getting to the Earth’s surface, causing cooling. Two mechanisms are known that could block some sunlight, volcanic eruptions or asteroidal impact. Both could put particulate material high in the stratosphere where it could reside for decades or longer.
The effect of runoff water on ocean temperature on local/regional basis was certainly possible, but was more difficult to understand on a global basis, making unlikely that it was the sole cause of the Lower Dryas cooling. But the two mechanisms hypothesized as the cause of the cooling were still in play.
Large-scale volcanism was known to have affected global climate significantly in historical time. The gigantic eruption of the Laki volcanic complex on Iceland in 1783-4 killed 20% of the population of the island, and many more died of starvation as volcanic ash blocked much of the incoming sunlight, causing crop failure far beyond Iceland’s border. Fluorine emitted by Laki killed most livestock and poisoned agricultural fields. The devastating effects reached into Europe, killing thousands and causing damaging food shortages. In France, the sequence of extreme weather events included a failed harvest in 1785 that caused poverty for rural workers, as well as droughts, bad winters and summers. The winter of 1784-5 was bitterly cold, and the 1785 harvest was almost completely destroyed. The Laki-caused poverty and famine may even have contributed to the French Revolution in 1789! Similarly, the April, 1815, eruption of Mt. Tambora in the Dutch East Indies resulted in 1816 being referred to as the “Year without a Summer.” Crops failed not only in Europe, but in the United States as well (almost killing a very young Abraham Lincoln. India had it even worse, losing millions of people to climate-related causes. More recently, the 1991 eruption of Mt. Pinatubo in the Philippines caused measurable global cooling for several years. Fortunately, that eruption was too small to cause the death and destruction of Laki or Tambora.
The notion that a large asteroid impact could harm Earth’s climate significantly came into focus when the cause of the “Death of the Dinosaurs” was determined. Geologists had often wondered as to why the dominant alpha predators had so abruptly disappeared. And it wasn’t just the dinosaurs; many, many species also went extinct at that same time.
Such mass extinction events evidenced in the geologic record had been well-known by geologists from the earliest days of the field, who used them to mark boundaries between geologic ages. But what calamity could be powerful enough to render much of the life on Earth extinct in a short amount of time? A larger-scale version of Laki, perhaps, but could there be something else?
The answer came from an unlikely duo, Luis Alverez, a physicist who had already won the Nobel Prize in Physics in 1968 for his work on elementary particle physics, and his geologist son, Walter. They had been wondering about an odd clay layer that was often present in geological sections that marked the time period of the dinosaur’s demise. Curious as to what could cause such a thin widespread layer to be sandwiched in between the limestones of a deep-water deposit, they did a chemical analysis of the clay.
The Alverez team found that the clay contained an abnormally-large quantity of the element, iridium, a rare metallic element that on Earth makes up only 0.001 parts-per-billion of the Earth’s crust. The clay layer contained 500 ppb! Worse, when the similar clay layer elsewhere in the world was examined, all of them showed a ridiculous overabundance of iridium. No earthly explanation could begin to explain such a high amount.
But the explanation wasn’t Earthly at all!
Iron meteorites also contain a high content of iridium, too. Was it possible that the clay layer was formed when a large iron asteroid hit the Earth?
The Second Hypothesis for the Younger Dryas
The “iridium anomaly” in the clay described above was excellent evidence of an asteroid impact. But all scientists seek corroborating lines of evidence, especially for a radically-new idea like this one. Geologists knew that impact events on Earth also produce distinctive damage to terrestrial minerals on the microscopic scale, especially quartz, and produce small glassy objects of debris melted by the heat of the impact and scattered widely, called tektites. The clay layer had both. Now the only thing missing was the crater, of necessity quite large, formed by the impact event. Geologists scoured the Earth far and wide, looking for it. Several candidates were considered, but rejected. Clues pointed to a region in the Americas, but nothing definitive was found. Meanwhile, geologists at Mexico’s government-run oil company, Pemex, were finding a weird sub-surface structure just off the Yucatan Peninsula; the closest town on shore nearby was a village called Chicxulub. The planetary geologists and the oil guys from Mexico didn’t often talk to each other much, but both groups did work with a good-old-boy reporter named Carols Byers in Houston, who covered the oilpatch and NASA beats for the Houston Chronicle. He catalyzed an interaction that put the last piece in place. The puzzling buried underwater structure was the impact crater of the asteroid that killed the dinosaurs! And that impact was large enough to distribute its ejecta worldwide, which would also have had the effect of screening incoming sunlight and cause a lowering of surface temperature worldwide.
Situation Review
A scientific question has come up: An observation in which we have a high degree of confidence has come to our attention regarding the cooling trend known as the “Younger Dryas” at the end of the most recent Ice Age. Two hypotheses are available to explain such a lowering of surface temperature worldwide, both involve putting fine particulate matter very high in the atmosphere, blocking some of the incoming solar heat: volcanism and impact. The next step in the process of scientific inquiry is to acquire more information so we can discriminate between the two hypotheses.
New Information
The use of analyzing core samples of thick layers of ice that formed during the most recent Ice Age was previously mentioned. “The Greenland Ice Sheet Project (GISP) was a decade-long project to drill ice cores in Greenland that involved scientists and funding agencies from Denmark, Switzerland and the United States. Besides the U.S. National Science Foundation (NSF), funding was provided by the Swiss National Science Foundation and the Danish Commission for Scientific Research in Greenland. The ice cores provide a proxy archive of temperature and atmospheric constituents that help to understand past climate variations” (source). Twenty ice cores were obtained over the ten years the project operated. Most of the cores were a few tens of meters in length, but the final drillhole of the project penetrated two kilometers of ice, all the way to bedrock. The U.S. conducted a follow-up program of drilling, GISP2, which also penetrated the thickest part of the Greenland ice layer all the way to bedrock.
Scientists analyzing the GISP2 core in 2013 found “platinum concentrations that were well above normal levels. The ratio of platinum to an element called iridium was also unusual because space rocks usually have high levels of iridium, while the ice core spike does not. The ice core signature was very different from anything seen in known meteorites or volcanic rocks.”
Platinum, not iridium. Hmmm. The observed platinum layers did not immediately discredit the impact hypothesis, since some iron meteorites contain small amounts of platinum, but two additional problems arose. A detailed examination of the GISP2 core showed that the platinum layer began 45 years after the Younger Dryas started, so the putative platinum profile could not be related to the cause of the temperature drop. Another feature of the platinum-rich layer of the core was that it represented 14 years of accumulation, another feature that had to be explained.
Meanwhile, geologists were considering a volcanic origin for the lower temperatures of the Younger Dryas, one consistent with its platinum concentration and 14-year duration of emplacement. A nearby 13,000-year-old volcanic crater in Germany, the Laacher See (for the lake now occupying the crater), was a possible candidate. It was of the right age, but a detailed examination of the material it produced contains virtually no platinum.
Further comparison of “the ice core’s chemical signature with various other geological samples and found the closest match was with volcanic gas condensates (the products formed when gases released from a volcano cool from a gas to a liquid or solid state) particularly from submarine volcanoes.” (source of this quote and those below)
Maybe neither iridium nor platinum was the key to the puzzle.
“Because of the chronological mismatch, whatever mechanism was responsible for the platinum spike didn’t trigger the Younger Dryas. Our research does, however, highlight previous results showing a massive volcanic sulphate spike in multiple ice cores coinciding precisely with the onset of cooling 12,870 years ago.
“This eruption, whether from the Laacher See eruption or an unknown volcano, injected enough sulfur into the atmosphere to rival the largest eruptions in recorded history. Volcanic eruptions can trigger cooling by releasing sulfur into the stratosphere, reflecting incoming sunlight and potentially setting off a cascade of positive feedbacks including sea ice expansion, changed wind patterns and disruption of ocean currents, though future research needs to explore this further.
“The substantial volcanic forcing around the Younger Dryas onset – a time when climate was already sitting between a glacial and an interglacial (the periods between cold snaps) – may have provided the nudge that tipped Earth’s climate back into a cold state.
“It is important to note that our research focused on the platinum spike and did not consider other evidence, such as spherules (spherical fragments of melted rock) and black mats (mysterious dark layers in soil), for an extraterrestrial impact. That said, based on our analysis of the new results and existing data, a large northern hemispheric volcanic eruption seems to be the most straightforward explanation for the Younger Dryas Event.”
(Unsatisfying) Conclusion
Geologists cannot definitively state whether it was a volcanic or impact event that caused the lower temperatures of the Younger Dryas Event. Both hypotheses have attractive features and both have problems. Scientific inquiry does not operate on a one-hour basis, complete with commercial breaks, like you see on TV shows and video reels about “Science.”
PLUTO’S “DEMOTION”
Public interest in poor little downtrodden Pluto and its “demotion” gives science educators/popularizers the IDEAL CHANCE to showcase the proper process of scientific inquiry!
We all know about the “demotion” of Pluto from planetary status, even if we do not agree about it. Personally, I believe that making such changes in nomenclature is a necessary and proper reflection of our expanding knowledge of the nature of the Solar System, and I like to use as an analogy the actions taken during a spring cleaning of my garage (as long-time A+StW readers will recall).
Ancient astronomers knew the five major planets quite well, and could predict their locations quite accurately, enough so to forecast eclipses and other astronomical events/movements. But occasionally a strange interloper would make a temporary appearance, causing consternation all around. Most of these objects received the name, “comet,” or “hairy stars,” based on their appearance. And Galileo showed that at least some planets have smaller objects, moons, orbiting them. In 1801, yet another type of Solar System body was discovered, much smaller than the other planets. They were called asteroids (“little stars”), an unfortunate choice since they were not stars at all. Therefore, the nomenclature had to expand from “stars and planets” in the sky to “stars, planets, moons, asteroids, and comets” in the sky.
The objects we then called “asteroids” reflect another aspect of the nomenclature changes that are a natural attendant of learning more about our Solar System. Thousands of them have been discovered and had their orbits calculated in considerable detail. Most of them are confined to the “Main Belt” between the orbits of Mars and Jupiter; the few outliers were initially considered inconsequential oddballs.
Astronomers also knew that something strange was going on in the Solar System’s outer reaches, even if they at first didn’t know enough to “clean out the garage.” Uranus’ obliquity was unlike any other planet’s, Neptune’s large moon, Triton, has a retrograde orbit that is decaying, and Pluto’s orbit is more elliptical than the orbits of the planets and is inclined to the Plane of the Ecliptic much more than any of the planets.
The discovery of Kuiper Belt objects “muddied the crick” considerably. So did the recognition of “active asteroids” that blur the line between “asteroids” and “comets.” So did the recognition that Jupiter’s gravity has an enormous effect on the evolution of the Solar System. But that’s a good thing, too, because it reflects an increase in our understanding of the nature of the Solar System as a whole.
Astronomers had to consider a more comprehensive classification scheme. No scientist likes a “special case” classification such as calling Pluto a “planet.” It was no longer possible for Pluto to retain that designation by itself, since many of the trans-Neptunian objects are more like Pluto, Charon, Arrokoth, and other distant objects (KBOs). Either all of them have to be “planets,” or yet another new term is needed. Planets, moon, asteroids, can comets just don’t fit as sole categories. The International Astronomical Union, the group solely responsible for all “official” Solar system nomenclature, voted to add “dwarf planet” as a new category that would fit Pluto, the new discoveries, and recognize the unique nature of the largest asteroid, Ceres. For more on this matter, see: https://science.nasa.gov/dwarf-planets and https://en.wikipedia.org/wiki/IAU_definition_of_planet.
No reasonable scientific definition would allow Pluto to remain a “planet” while disallowing the claim of Ceres and several of the trans-Neptunian bodies already discovered. It could never be only nine planets again; only eight or many. Sorry, Administrator Isaacman, it’s not NASA’s call!
Trans-Neptunian objects are rich in volatile materials because they have never been heated by the Sun to any extent. Comet C/2014 UN271 Bernardinelli-Berntein may be another example. Those that do approach the Sun have highly-elliptical orbits, at least at first.
Blame Jupiter. Its gravity can either eject first-timers (as Comet B-B likely will be) or make their orbits much less elliptical, exposing their surfaces to periodic solar heating and devolatilization. Some will eventually lose so much of their volatiles that they are no longer comets, but rather more asteroidal in nature.
Jupiter’s gravity tends to force shorter-period comets more and more toward the Main Belt. By the time that happens, they are comets no more but asteroids, some still capable of shedding meteoroids, some not.
Another problem with the “planets, moons, asteroids, comets, dwarf planet” definitions is that the borders between some of them are blurred. For example, some objects show characteristics of both comets and asteroids. They are rare because the overall time taken in transition is short compared to the age of the Solar System. Now called “Centaurs,” their discovery played an important role in astronomers figuring out this evolutionary process.
Most meteor showers we see today are the result of comets shedding rocky debris as they devolatilize, a process that could continue over many orbits. Two bodies very near the end of their activity are the parent bodies for two different showers: the Geminids last month are debris from the asteroid 3200 Phaeton, and the Quadrantids appearing now are debris from the near-Earth asteroid 2004 EH.