字幕列表 影片播放 列印英文字幕 Thank you to Draper and its Hack the Moon initiative for supporting PBS Digital Studios 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. PBS is bringing you the universe with Summer of Space, which includes six incredible new science and history shows streaming on pbs.org and the PBS video app, along with lots of Spacey episodes from PBS Digital Studios Creators. Follow me over to Reactions to check out their Summer of Space episode on the awe-inspiring aurora borealis, including the chemistry behind its spectacular colors. Thank you to Draper and its Hack the Moon Initiative for supporting PBS Digital Studios. You know the story of the astronauts who landed on the moon? Now you can log into wehackthemoon.com to discover the story of the male and female engineers who guided them there and back safely. Hack the Moon chronicles the engineers and technologies behind the Apollo missions. Brought to you by Draper, the site is full of images, videos, and stories about the people who hacked the moon. If you've made it this far then thanks for joining me today in the Konstantin Haase Studio, and thanks to Konstantin and this month's eontologists: Jake Hart, John Ivy, John Davison Ng, and Steve. If you'd like to help support what we do here, go to patreon.com/eons and pledge.