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  • 700,000 years ago a rust-colored Rhino roamed the vast open Highlands of Siberia and Central Asia.

  • This ginger beast is better known as the Woolly Rhino and it made its living foraging in the cold dry Tundra steppes.

  • Siberia in the Pleistocene might sound cold to you, but it suited the woolly rhino just fine.

  • And 700,000 years ago the world was only a few degrees cooler than it is now,

  • which is why at the time the range of the woolly rhinos was restricted to the cold wilds of Siberia.

  • But not for long!

  • By about four hundred and fifty thousand years ago, global temperatures had dropped by about six degrees Celsius,

  • and stayed there for thousands of years.

  • Glaciers crept out of their mountain ranges and down to lower elevations.

  • Tundra spread to other parts of Asia, and so did animals that were adapted to the cold,

  • including the woolly rhino, the mammoth, and the Saiga antelope.

  • After thousands of years of being confined to Asia, the woolly rhino

  • finally stepped foot into Europe, but that too didn't last long.

  • Four hundred thousand years ago,

  • the climate warmed back up, and the Rhino and its Tundra were forced back into the highlands.

  • This whole cycle happened again and again,

  • which is why the Ice Age is more accurately known as the Ice Ages.

  • Over the rest of the Pleistocene epoch, the Rhino's range continued to grow and shrink in sync with global climate.

  • During warm periods, most of the rhino population retreated to cold places like Siberia,

  • but small populations found themselves stranded in places like the Pyrenees Mountains in Spain and France.

  • And then about 12,000 years ago, they finally went extinct.

  • So what caused the climate, and the range of the woolly rhino, to cycle back and forth between such extremes?

  • And what caused the woolly Rhino after so many years to go extinct?

  • Basically: space.

  • More specifically, Earth's position in space, like where it is in its orbit around the Sun,

  • how far it's tilted over its axis,

  • and what direction is that axis pointing?

  • These factors, and the way they change through time, have caused our climate to change a

  • tremendous amount over the eons.

  • And it's only been within the last century or so that we've begun to figure out that all of these factors change in cycles,

  • and those cycles can coincide or counteract each other which makes the history of our climate incredibly complex.

  • But when you put all of the pieces of the climate puzzle in front of you, you can start to understand some

  • chapters of our deep past, like the fate of the woolly rhino.

  • We've known that the Ice Ages happened for a pretty long time.

  • But what actually caused them was largely unknown until the early 1900s.

  • The man who solved the mystery was a Serbian mathematician and astronomer named Milutin Milankovic.

  • So today these cycles are known as the Milankovitch cycles.

  • Milankovic was obsessed with Ice Ages, both on our planet and on Mars, and he became

  • convinced that small changes in the angle of sunlight could be responsible for starting and ending those Ice Ages.

  • He already knew that parts of the earth that

  • received more direct sunlight from overhead are warmer,

  • like at the equator, which gives overhead sunlight year-round. And after years of study

  • Milankovic concluded that there were three main things that changed the angle of sunlight

  • in the northern and southern hemispheres.

  • The first and most important is axial tilt, also known as obliquity.

  • This is the angle at which Earth's axis leans either to or away from the Sun.

  • Milankovic thought that this had the biggest effect on climate because it has the most extreme influence on the angle of sunlight.

  • After all, the tilt of the earth is why we have seasons.

  • Right now the axis of our planet is tilted at about 23 and 1/2 degrees,

  • so for those of us who are further away from the equator

  • sunlight strikes the surface at a higher angle when our hemisphere is leaning towards the Sun,

  • aka summer.

  • And when we're leaning away from the Sun, sunlight strikes at a more shallow angle, like in the winter.

  • How exactly our earth got knocked over is still a bit of a debate,

  • but one popular theory is that earth collided about four-and-a-half billion years ago with a huge

  • planetary body that went on to form the moon.

  • And that impact sent us spinning like a top.

  • And just like a spinning top, the amount of earth's tilt changes, between about 22 and

  • 24 degrees over the course of about 41,000 years.

  • Since a steeper tilt creates stronger extremes in temperature,

  • Milankovic was pretty sure that once someone figured out the precise timing of the ice ages,

  • they'd show that most major climate changes took place every forty-one thousand years.

  • But keep in mind our axis isn't just flipping back and forth.

  • It's also moving in a circle. Again, like that spinning top.

  • So in addition to the angle of its tilt, we also have to consider which

  • direction our axis is pointing at any given point in history.

  • This is known as axial precession or just axial wobble and our axis completes a full

  • circle about every twenty three thousand years.

  • And this affects the climate because it changes where in Earth's orbit each season happens,

  • because the Sun isn't in the exact center of Earth's orbit.

  • There are periods when we're closer to the Sun in our orbit, and periods when we're farther away.

  • Right now, based on the direction of Earth's axis, winter occurs in the northern hemisphere.

  • when Earth happens to be closest to the Sun in its orbit.

  • And summer occurs when it happens to be farther away.

  • But when precession is at the other end of the cycle and our axis is pointing in the opposite direction,

  • winters in the northern hemisphere occur

  • when we're *farthest* from the Sun,

  • And summers when we're closest.

  • This creates more extreme seasons than the northern hemisphere has now.

  • Finally, the third part of the Milankovitch cycle is a feature known as eccentricity.

  • This is a change in the shape of Earth's orbit, from being roughly circular to being ever so slightly more eccentric or oval-shaped.

  • And earth's orbit changes from being more circular to less circular,

  • and then back again over the course of about a hundred thousand years.

  • But rather than changing the angle of sunlight,

  • the main effect of eccentricity is changing the lengths of the seasons.

  • Think of it this way:

  • A circular orbit creates seasons of equal lengths,

  • but slightly less circular orbit stretches out some seasons while compressing others.

  • So during periods with a highly eccentric orbit,

  • there may be long summers but also long winters.

  • In the end, his extensive calculations led Milankovic to conclude

  • that changes in the tilt of Earth's axis were the main factor that could cause enough cooling to make ice

  • expand on the planet's surface.

  • As a result, he predicted that the most significant ice ages

  • would have happened every 41 thousand years or so,

  • falling in line with the tilt cycle.

  • And he was right! ...Pretty much.

  • If we look deep into the geological record,

  • we can see changes in climate that line up roughly with the cycle of earth's tilt,

  • about 41,000 years.

  • One such record is from the colorful

  • 25 million year old paleo cells of the John Day formation in Oregon.

  • There, scientists have found changes in carbon and oxygen isotopes in rock layers,

  • showing that rainfall patterns changed every 41,000 years or so

  • during the late Oligocene epoch.

  • During dry periods, this region got about 350 mm of rain per year,

  • but in wet periods that went up to nearly 500 mm of rain,

  • an increase of more than 40%.

  • That change in rainfall transformed the environment

  • from sagebrush to wooded grasslands and back again.

  • And with different environments came different animals,

  • so fossils from the wet periods at John Day contain more large mammals like rhinos,

  • while drier periods feature lots of tortoises, gophers, and rabbits.

  • And scientists can trace this climate cycling pattern back even farther.

  • In the Midland Basin of West Texas,

  • studies of the rock layers have revealed fluctuations in the amount of atmospheric dust during the late Carboniferous period,

  • about 300 million years ago.

  • These changes relate to dry and wet cycles that again match up with the Milankovitch cycles,

  • with changes happening about every 36 thousand years.

  • And yes, that's thirty six thousand years, even though the cycle of the axial tilt is about

  • 41,000 years.

  • That's because, to make things even more complicated,

  • Milankovitch cycles used to be a little faster than they are today.

  • The cycles of precession and axial tilt are set by the

  • gravitational interaction between the earth and the moon,

  • and the moon has steadily been moving away from Earth ever since it formed 4.5 billion years ago.

  • And Earth's rotation has slowed as well.

  • So both of these things mean that precession and tilt are slower now than they were in the past.

  • So if you look deep into the geological record,

  • you'll see that the biggest changes in climate line up pretty well with the cycle of our planet's axial tilt.

  • Which is why, when scientists began pulling up

  • ice cores from Greenland going back about 400,000 years,

  • they expected to find evidence that the biggest swings in climate happened about every 41,000 years or so.

  • But they didn't. Instead, the ice cores showed that while there was an influence of tilt,

  • the biggest ice ages were separated by a hundred thousand years.

  • This is what some scientists have called the Hundred Thousand Year Problem.

  • Basically during the whole Pleistocene epoch,

  • the biggest climate cycles didn't line up with the axial tilt cycle,

  • and it's only been in the last few years we've figured out why.

  • The reason that climate cycles changed from 41k years to 100k years during the Ice Ages

  • Involved a fourth factor that drives our climate:

  • ice itself.

  • When large amounts of ice form,

  • it makes a huge difference in Earth's climate. It's light in color,

  • so it reflects more sunlight which can help cool down the planet even further.

  • This phenomenon is called albedo.

  • When the climate becomes cold enough for ice to form quickly,

  • then the albedo effect causes the planet to cool down even more,

  • and the type of ice that forms the fastest is sea ice.

  • The Pleistocene epoch wasn't the first time earth had a lot of sea ice,

  • but it was one of the first times when one

  • Hemisphere made a lot more sea ice than the other, and that's still going on today.

  • Even though the North Pole is covered in water and the South Pole is land,

  • the southern hemisphere actually produces more sea ice than the north.

  • And this is important because, at least since the Ice Ages,

  • it has thrown off the balance between the poles.

  • The two hemispheres of our planet

  • haven't been heating up and cooling down at the same rate.

  • Instead, sea ice has been forming faster in the southern hemisphere,

  • faster than the hot summers in the northern hemisphere can counteract.

  • This means that in annual cumulative terms, southern sea ice has been able to create an overall cooling effect on the planet.

  • Now what really made the ice ages of the Pleistocene unique was the interaction of sea ice with our

  • Planetary cycles.

  • When Earth's orbit has been more elliptical, and winter in the southern hemisphere has occurred when earth was farthest from the Sun,

  • sea ice crew quickly and dramatically cooled the planet.

  • And those exact conditions only happened about

  • every hundred thousand years, so that's when the peak cold periods happened during the ice ages.

  • So remember when woolly rhinos were finally able to enter Europe 450k years ago

  • when the average global temperature dropped about 6 degrees Celsius?

  • Climate models suggest that this happened because of an eccentric orbit

  • and a procession that aligned just right to make the southern hemisphere's winter happen furthest from the sun.

  • This made sea ice in the southern hemisphere cool more rapidly,

  • which then went on to cool the rest of the planet, which in turn created more ice,

  • which in turn cooled the planet even more.

  • And during these periods of extreme cold, the woolly rhinos were still able to spread into Europe,

  • until the climate abruptly warmed at the end of the Pleistocene.

  • This marked the beginning of our current epoch: the Holocene.

  • It's not clear why this warming period was the last one for the Rhinos.

  • Other animals, like the Saiga and the caribou, were able to adapt to the new warmth,

  • but woolly rhinos couldn't.

  • So Milankovitch cycles can explain most long-term climate variations in deep time,

  • but there's also a complicated mix of other factors

  • I haven't even mentioned yet,

  • like the position of the continents, levels of greenhouse gases, and volcanic activity.

  • And what about solar activity?

  • Well, light from the Sun has actually gotten stronger over time, but only by about 6% in the last billion years,

  • so it's had a pretty minor effect.

  • All of these factors make it hard, but not impossible, to predict where we're headed.

  • We do know that about 26,000 years ago,

  • earth reached its last glacial maximum, the peak of the hundred thousand year cycle.

  • So in approximately 74,000 years,

  • eccentricity, precession, and sea ice should all align to make it very cold again.

  • But what makes it difficult to predict future temperatures

  • is the fact that humans are producing a lot of greenhouse gases.

  • In the last 300 years, the carbon dioxide content of the atmosphere

  • has increased by about 45%, and as a result temperatures have risen steadily for the last century,

  • almost 1 degrees Celsius, independent of the Milankovitch cycles.

  • The effects of human activity are essentially overpowering some of the cooling effects of sea ice.

  • So our climate is incredibly complicated, but understanding how it used to behave and how it might behave in

  • the future, is important for understanding the changes that are happening right now.

  • The more we try to understand, the more likely we can avoid the fate of the woolly rhino.

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