There is little doubt among archaeologists that the Younger Dryas, a cold snap following the initial retreat of Ice Age conditions some 11,000 years ago, had a major impact on human history. It seems that humans are highly motivated to return the impact to the Younger Dryas. Two times in recent years, evidence of an impact, a celestial object whacking into the Earth, has been suggested as the cause of the famous climatic “two step.” As sexy as impacts are, however, it is very unlikely that the Younger Dryas was caused by one.
I cut my climate teeth on the Younger Dryas. I was studying under Glynn Isaac when a number of palaeoclimate related data and observations rather suddenly came together, and the likely relevance of Milankovitch orbital geometry emerged. This is the subtle and regular pattern of the way the Earth goes around the Sun, resulting in periods of time — thousands of years long — when the Summer Solstice (in Late June, these days) is also a time of year when the total amount of Sun’s energy falling on the Earth is at a periodic minimum. In other words, the potential for having a really cold summer is high. Or a few of them in a row. This in and of itself does not cause an Ice Age. It probably takes a handful of different things being true all at once for the planet to plunge into a cold phase, including the position of continents and mountains, behavior of sea currents, and atmospheric dust caused by large volcanic eruptions or meteor impacts. But during these Milankovitch set-ups, an Ice Age, or more properly termed, a glacial period, is reasonably likely all else being slightly colder than equal.
The periodic and orbit-determined nature of glacial periods was proven from work done during the 1960s, 1970s, and then assembled into something that made sense in the 1980s. Fort this to happen, oceanography had to be invented. Stable isotope chemistry had to develop. The ability to raise cores from the deep sea needed to develop, and then, a bunch of such cores had to be raised and studied. Then it all had to be put together. The key “moment” was the development of deep-time core sequences, first from deep sea sediments, then ice cores, covering over 100,000 years (and eventually, 800,000 years) of time, showing changes in the isotopic composition of sea water at a scale approaching year-to-year variation.
I got to see that happen, and I was enthralled, while being Glynn’s student. He knew all the coring and isotope people, and they were in and out of Isaac’s “Stone Age Lab.” Not long into this, however, Glynn died, and the Stone Age Lab was taken over by Ofer Bar-Yosef. Ofer was working on the origin of plant agriculture in the Near East, and was very interested in the fine tuned time scales provided by dipsy cores, er, deep sea cores. (Ofer’s Israeli accent confused many an undergrad. “I love this course, but he keeps showing all these ocean cores but he’s talking about dipsy this and dipsy that!!!”) These cores, with their detailed isotopic records, showed the coming and going of major ice ages very clearly.
The cores contain the remains of the tiny skeletal bits of microscopic organisms that made part of their hard parts using Oxygen from the sea in which they lived. Short lived organisms, they contributed on their deaths to the sediment at the bottom of the sea, leaving an ongoing record of that Oxygen’s isotopic makeup. Specifically, you can measure, with some fancy machines and some necessary and logical adjustments to the data, the ratio of heavier vs. lighter isotopes of Oxygen in the ocean at the time the organisms lived. Water made with the lighter oxygen literally jumps out of the ocean, in the process of evaporation, at a slightly higher rate than the heavy Oxygen based water.
Therefore, rainwater is isotopically light.
Therefore, glacial ice is isotopically light.
Therefore, when a lot of the Earth’s rainwater is trapped as glacial ice, the oceans are isotopically heavy.
Therefore the organisms that use that oxygen are isotopcialy heavy or light depending on global climate, and this signal is preserved in the dipsy, er, deep sea, cores.
And so, we were able to see the “Younger Dryas” pretty clearly.
But what is the Younger Dryas? What is a Dryas? And what ever happened to the Older Dryas?
A Dryas is a cold loving cute little flowering plant that is abundant enough, when it is abundant, to produce sufficient pollen that is readily identifiable as to be counted in ancient pollen records, taken from fresh water swamps and lakes and such.
There are two fairly recent periods when Dryas, always rare, peaks, in pollen profiles observed in the Middle East and parts of Europe. An older time (27K to 24K years ago) and a more recent time (12,900 to 11,700 years ago). The more recent time is the Younger Dryas. Since the plant shows up during cold periods, the Younger Dryas was considered to be a brief return of ice age conditions following the initial de-glaciation from the last major Ice Age.
People figured out that the Younger Dryas was actually visible in other places, not just where those pollen profiles showed up. In fact, everywhere where there is a record of ice age glacial activity that is sufficiently detailed, it is there. In North America, it is called the “Lexingtonian Readvance” because of a lobe of glacial ice re-grew near Lexington Massachusetts, indicating a rapid and short turn return to glacial conditions. Every major region glaciated at the end of the last Ice Age has these re-advance lobed, generally locally named. It is pretty clear that the Younger Dryas was global, and significant enough to leave a mark.
The Younger Dryas is absolutely clear in the ancient record of ocean Oxygen isotopes. It looks like this:
This is a portion of a graph you can see in Wikipedia. The squiggly lines represent Oxygen isotopes, but in ice rather than deep sea sediments. You can see a trend from lower (colder) to higher (warmer) that is followed by a long period of warm conditions (that’s us, now, on the far left). The arrow points to a clear reversal in the warming trend, showing this as the Younger Dryas.
Ofer Bar-Yosef has proposed that the Younger Dryas was linked to the origin of cereal agriculture in the Middle East. He was probably right. How did that work? People living in the Levant at the time were pretty successful foragers. They relied on a wide range of foods, including hunted gazelle and a range of plant products. But they were increasingly exploiting cereal grains, likely wild barley and wheat. Foraging was so successful that many settlements formed that became permanently occupied. People may have moved around a lot, but somebody was staying in these small villages year round.
As climate got better and better, over a period of just a few human generations, this pattern developed. But then the cold shift occurred. Bar-Yosef suggests that this was the trigger to increase exploitation of certain resources, shift away from some resources that became more rare, but mainly, to start or accelerate an ongoing process of tending the grains that were already being harvested. The morphological indicators that distinguish wild from domestic grain show up in various species of edible grasses starting at this time. Humans invent agriculture in the Middle East.
I hasten to add that humans invented agriculture (horticulture and/or animal husbandry) in many locations around the world, starting at or near the beginning of the warm period shown in the graph above. But the situation in the middle east was a bit different, probably earlier than all or most other cases, and very sudden owing to the invention, or as Bar-Yosef puts it, revolution, of agriculture.
So, the Younger Dryas, which you now know all about, was very impactful for humans, in a way you now also know about. But what about the reverse? Did impacts cause the Younger Dryas?
I ask this now because there are news stories everywhere (see this) about a giant crater found in Greenland, suggesting that this meteor strike caused the Younger Dryas.
A meteor impact could cause a cooling trend. If the meteor hit a major Greenland glacier, it could cause melt-water to alter sea currents, thus causing the Younger Dryas. Indeed, the idea of glacial melt-water associated with the simple melting of glaciers following an ice age was at one time thought to be sufficient to trigger a re-glaciation, short lived.
Unfortunately, the dating of this impact is very very iffy, and the chances of it having actually happened at the exact right time to be implicated with the Younger Dryas is approximately zero. Also, any melting of ice causing fresh water to alter Atlantic sea currents would possibly cause a climate shift like the Younger Dryas; there is evidence of melt water changing currents around that time; that evidence has already been examined closely and the timing of that event does not fit with the Younger Dryas.
Mark Boslough, on Twitter, and Stefan Rahmstorf, in several publications, agree. Rahmstorf has documented a 1470 year period abrupt glacial event phenomenon, of which the Younger Dryas looks like just another one. During these glacial periods, either full on glacial events or as the planet is pulling out of or falling into one, there is a lot of wild swinging around of the climate. The Younger Dryas is probably just one of those swings, and may well be one that fits into a periodic schedule unrelated to meteors, as sexy as meteors are.
Expect more on this over the next few weeks. Keep an eye on RealClimate blog, I’ll bet they discuss it there.