Thousands of years ago, a little white and
yellow flower lived in the Arctic tundra, blooming under the feet of Ice Age giants
like mammoths and woolly rhinos. This little flower belongs to the genus known
as Dryas, and it’s still around today(!), thriving in cold climates like the Rocky Mountains,
Alaska, Iceland, and Sweden, although those giant megafauna, sadly, are not. But for a brief time, about 12,000 years ago,
these flowers could be found all over the Northern Hemisphere, from New England to central
Europe. So how did this little, cold-loving flower
wind up in places that we associate today with warmer climates, and for hundreds of
years? Well, at the time, those places weren’t
warm. After the northern glaciers reached their
maximum extent around 20,000 years ago, the planet started to warm up. But then … something happened. And it sent the world into a cold snap that
lasted around 1,200 years. For example, in the Channel Islands of Great
Britain, it may have gotten as cold as -20 celsius, whereas today it rarely drops below
9 degrees. This cold spell was so intense that geologists
have given it a name: The Younger Dryas, after those little flowers that flourished in the
newly cold climate. And yes, in case you were wondering, there
is also an Older Dryas, which was a shorter cold event around 14,000 years ago that only
lasted about 200 years. The thing is, during the Younger Dryas, it
didn’t just get cold. This chunk of time also coincided with one
of the largest mass extinctions in recent history. So for decades, scientists have been studying
the cause of this event, and trying to figure out if something like it could happen again. And it turns out that what caused the Younger
Dryas and the extinction of the megafauna are both the subjects of ... heated debate. In the late 19th and early 20th centuries,
experts started finding fossils of Dryas flowers in places where they didn’t seem to belong,
like Denmark and the southern UK. And they actually uncovered two layers of
these fossils, representing two separate cold spells -- the Older, and then the Younger,
Dryas periods. Now, you can’t find these flowers there
today, because it’s too warm for them. So, since the flowers could only have lived
where it was cold, the fossils suggested that this part of Europe had returned to a more
tundra-like state And in more recent decades, more evidence
has been found to support this. For example, in ice cores from Greenland,
experts have found chemical signatures that are consistent with a period of much colder
temperatures. And in the UK, scientists used fossils of
insects and plants to estimate a temperature range for the Channel Islands. And they found that the warmest month was
between 9 and 13°C, which is the same as today’s winter temperatures. But, while the evidence gave us a sense of
what happened - that there was a sudden cold event that allowed an arctic environment to
expand south - it doesn’t explain how it happened. And figuring out the cause of the Younger
Dryas has been … hard. There are lots of different hypotheses, ranging
from changes in ocean circulation to volcanic eruptions and meteorite impacts. One suspected culprit is the planet’s flow
of water between oceans. You can think of it like a conveyor belt. Oceanographers call it the Atlantic Meridional
Overturning Circulation. This flow of ocean water starts near the Equator. And when the warm water moves north, some
of it evaporates, which makes it saltier. It also cools down as it moves toward the
Arctic, releasing its heat around Western Europe. Both of these processes make the water more
dense, and density is what’s behind all the movement of the conveyor belt. If this circulation pattern slowed down, the
Northern Hemisphere would get a lot cooler. And that’s actually the premise of the movie
The Day After Tomorrow, if that helps you, where New York freezes over - albeit on a
much shorter timescale. But, what would make this system slow down? Well, some scientists think that the Younger
Dryas happened because a dam made of ice in Arctic Canada that held back a huge glacial
lake suddenly collapsed. In this scenario, A LOT of glacial water ended
up in the North Atlantic Ocean, and because that water was fresh, not salty, it was less
dense than the seawater. That density difference messed up the whole
system, because water sinking down to the deep Atlantic is a crucial step in the ocean
conveyor belt. So, when that part broke down, it made the
circulation slow down or even stop completely. Then, the warm water got stuck near the Equator,
so there was no heat transport to Europe. And experts have found evidence of that meltwater
-- in the form of a specific isotope of oxygen -- in sediments north of Alaska dating back
13,000 years - which is around the same time as the beginning of the Younger Dryas. Now, we know that the sudden flow of meltwater
made its way all the way to the Atlantic. So the fact that it originally came from Alaska
suggests that this wasn’t just a trickle: it was a huge amount of water. And what’s surprising is that this vast
amount of meltwater was probably there in the first place, because the ice sheet on
land was melting, due to a warming climate. So it’s possible that warming actually drove
the Earth into a mini ice age. Which sounds weird Now, another hypothesis is that the Younger
Dryas was caused by a big volcanic eruption in Germany, which we know happened around
the same time from radiocarbon-dating the deposits that it left behind. Now, the direct effects of such an eruption
on the climate would’ve only lasted a couple of years -- like, maybe some cooling caused
by sulfur particles in the atmosphere reflecting sunlight. But it could’ve had indirect impacts that
lasted much longer, by causing changes in ocean circulation or the expansion of sea-ice
- or both - that in turn cooled the North Atlantic. So far, experts think it’s possible that
an eruption triggered the Younger Dryas, but they’re still running computer models to
figure out how it all worked. And maybe it happened alongside the meltwater
event. Then, in 2007, another possible cause gained
traction: a major meteorite impact. This event could’ve unleashed that flood
of meltwater. In this scenario, sometimes called the Younger
Dryas impact hypothesis, an asteroid at least a kilometer wide hit the Earth around 12,900
years ago. Ow But, evidence of this impact remained elusive
until 2018, when geologists discovered a crater buried under Greenland's Hiawatha glacier. Early data show that it could be associated
with the Younger Dryas. And, in addition to the crater itself, researchers
have found chemical signatures of an impact all over the world. For example, research from South Africa has
found a spike in the element platinum in layers dating to before the Younger Dryas. Platinum is rare on Earth, but it’s more
common in meteorites. So the rapid increase in platinum could be
the chemical footprint of some celestial impact. But the case isn’t closed just yet. To figure out if a giant impact really caused
the Younger Dryas, we need to confirm how old that crater in Greenland is. Also much of the chemical evidence is still
heavily debated. One thing scientists are sure about is that
there were major changes to ecosystems taking place during the Younger Dryas. At the same time as this cold spell, an extinction
event wiped out many large vertebrates in the Americas, Europe, Africa, and Asia. By the end of this period, 36 genera of megafauna
went extinct in North America, including ground sloths, saber-tooth cats, and other iconic
animals of the Ice Age. Now, most of these extinctions happened at
the same time as the cold event, but that doesn’t mean the cold was the only factor
to blame. There are a few different ideas about how
- and why - so many vertebrates disappeared. Some researchers thought the most important
factor was hunting by humans. This model was developed in the 1960s and
is called the overkill hypothesis. But archaeologists weren’t convinced - why
would people around the world start overhunting megafauna all at the same time? There is some limited evidence from one beach
in Canada that people were hunting horses and camels some 13,300 years ago. In addition to the bones, archaeologists also
found a nearby hunting trail with footprints of mammoth, bison, and other game animals. And there’s evidence that people hunted
four other kinds of megafauna at different sites in North America. But the timing isn’t quite right: some genera
of megafauna went extinct before humans even arrived in North America. So, even though there are some signs that
point to hunting so long ago, the more popular analysis of the extinctions says that they
were caused by multiple factors, including humans and climate. And while a lot of research has been focused
on the extinction of the megafauna, there’s plenty of other evidence that shows that lots
of big changes were taking place. For example, fossils in Halls Cave, Texas
show that the diversity of animals and plants there dropped during the Younger Dryas. And even though Texas is a long way from Greenland,
cooling occurred simultaneously in both places. The landscape in Texas changed from forest
to grasslands, and some big herbivores - like bison and two species of horse - disappeared
for good. But other animals just shifted their range. Halls Cave shows that warm-adapted hares were
present before and after the Younger Dryas, but not during the cooling period itself. And hares were part of the story on the other
side of the Atlantic, too, where members of the Natufian culture were living in the southern
Levant. These people hunted animals like hares, gazelles,
deer, tortoises, and partridges. And based on isotopes found in deep sea sediment
cores, it took only 20 years for this region to cool significantly, which had major impacts
on both the people and the environment. And in response to the colder temperatures,
human society became more mobile. People had to move around more because they
couldn’t rely on the habitats that used to sustain them. Finally, after about 1,200 years of cold,
the Younger Dryas period ended. And because there’s no clear answer to what
caused the Younger Dryas, it’s hard to know exactly why it ended. But based, again, on sediment cores, we know
that the ocean conveyor belt started to strengthen again about 400 years before the period ended. And it’s still unclear why there was such
a delay between changes in ocean circulation and the climate of the Northern Hemisphere. Despite this lag, it took less than a hundred
years for this weird episode to finally end, which is really fast by climate standards. In fact, we know that temperatures in Greenland
rose by up to 10 degrees C in just a few decades or less! Now, the events of the Younger Dryas might
seem dramatic to us, or even unique, but this was actually just the most recent event of
its kind. There were many climate changes before the
Younger Dryas that were just as abrupt. This one has gotten more attention because
it happened at the same time as these other, big, biological changes, like the extinction
of megafauna and the movement of human populations. As temperatures warmed, the Dryas flowers
retreated back to their current habitats, and much of the world went back to its previous
state -- but many of the megafauna were gone for good. Ultimately, what we’re still learning about
the Younger Dryas might help us better understand the larger history of Earth’s climate and
of living things. To me, it’s nothing short of astonishing
how much we can learn from a simple little flower. Ok. Wanna know more about what happens when an
ice dam catastrophically fails on a glacial lake? Check out our episode How 7,000 Years of Epic
Floods Changed the World. Also thanks to this month’s super chill
Eontologists: Sean Dennis, Jake Hart, Annie & Eric Higgins, John Davison Ng, and Patrick
Seifert! Support us at patreon.com/eons for sweet benefits
like submitting a joke for us to read! Like this one from Lulu. I love the La Brea tar pits. There’s some beautiful spots there that
can just hold you forever! And as always thanks for joining me in the
Konstantin Haase studio. Subscribe at youtube.com/eons for more adventures
in deep time.
North American megafauna had plenty of experience with the shifts between glacial and interglacial periods, so this was likely nothing new for them. I think the effects of the melting of the ice sheets had a greater impact on the environment, and this combined with human hunting could have been enough to finish them off. Keep in mind that megafauna have relatively low birth rates, so any harmful changes would have a huge impact on their populations.
I just don't think the Younger Dryas would have as big of an impact as these other factors.
I find it odd that a cooling event would've caused so many cold-adapted, Holocene animals to fare worse.
There's also contentious evidence that says humans could've arrived in the Americas as early as 40,000 years ago, which I wished they mentioned in the video.
Undoubtedly humans had a negative impact on megafauna, it would be foolish to argue otherwise. as seen in Australia (still debated but most scholars point to humans as the cause of the megafaunal extinctions in Aussieland), New Zealand and Madagascar, with the sole reason for megafaunal extinctions in those areas being humans.
But North American megafauna seemed to coexist with humans longer than other megafauna, while of course some were already in decline, could it have been this sudden cold snap, which would have required drastic change to the ranges of megafauna, after centuries of human hunting, the nail in the coffin for North American megafauna?