Even though it’s hard to see it, our planet is in a continuous state of change. Continents constantly shift and clash with each other. Volcanoes erupt, glaciers expand and recede, and life has to keep up with all of it. Throughout its existence, Earth has at various times been covered by miles-high polar ice sheets and alpine glaciers, in periods that lasted for millions of years. Generally characterized by a long-term cold climate and ice as far as the eye can see, these Ice Ages will be the topic of discussion in today’s list.
10. What is an Ice Age?
Believe it or not, defining an Ice Age is not as straightforward as some may think. Sure, we can characterize it as a period in which global temperatures were much lower than they are today, and where both hemispheres are covered in huge sheets of ice that extend for thousands of miles towards the Equator. The problem with this definition, however, is that it analyzes any given Ice Age from today’s perspective, and doesn’t actually take the entire planetary history into account. Who’s to say, then, that we’re not actually living in a cooler period than the overall average? In which case, we would actually be in an Ice Age right now. Well, some scientists, who’ve dedicated their lives to the study of these sorts of phenomena, can say. And yes, we’re actually living in an Ice Age, but we’ll get to that in a moment.
A better description of an ice age would be that it’s a long stretch of time in which both the atmosphere and the planet’s surface have a low temperature, resulting in the presence of polar ice sheets and mountainous glaciers. These can last for several million years, during which time there are also periods of glaciation, characterized by ice sheet and glacier expansion over the face of the planet, and interglacial periods, where we would have an interval of several thousand years of warmer temperatures and receding ice. So, in other words, what we know as “the last Ice Age” is, in fact, one such glaciation stage, part of the larger Pleistocene Ice Age, and we’re currently in an interglacial period known as the Holocene, which began some 11,700 years ago.
9. What causes an Ice Age?
At first glance, an Ice Age would seem to be like some sort of global warming in reverse. But while this is true to a certain extent, there are several other factors that can initiate and contribute to one. It’s important to note that the study of Ice Ages is not that old, nor is our understanding complete. Nevertheless, there is some scientific consensus on several factors that do contribute to the onset of an Ice Age. One obvious element is the level of greenhouse gases in the atmosphere. There is consistent evidence that the concentration of these gases in the air rises and falls with the retreat and advance of ice sheets. But some argue that these gases don’t necessarily kick start every Ice Age, and only influence their severity.
Another key factor that plays a part here are tectonic plates. Geological records point to a correlation between the position of the continents and the onset of an Ice Age. This means that, in certain positions, continents can obstruct the so-called Oceanic Conveyor Belt, a global-scale system of currents that bring cold water from the poles down to the Equator and vice versa. Continents can also sit right on top of a pole, as Antarctica does today, or can make a polar body of water become completely or semi-landlocked, similar to the Arctic Ocean. Both of these favor ice formation. Continents can also bulk up around the Equator, blocking the oceanic current – leading to an Ice Age. This happened during the Cryogenian period when the supercontinent Rodinia covered most of the Equator. Some specialists go even as far as saying that the Himalayas played a major role in the the current Ice Age. They say that after these mountains began forming some 70 million years ago, they increased the amount of global rainfall, which in turn led to a steady decrease of CO2 from the air.
Lastly, we have the Earth’s orbits. These also partially account for the glacial and interglacial periods within any given Ice Age. Known as the Milankovitch Cycles, the Earth experiences a series of periodic changes while circumnavigating the Sun. The first of these cycles is Earth’s eccentricity, which is characterized by the shape of our planet’s orbit around the Sun. Every 100,000 years or so, Earth’s orbit becomes more or less elliptical, meaning that it will receive more or less of the Sun’s rays. The second of these cycles is the axial tilt of the planet, which changes by several degrees every 41,000 years, on average. This tilt accounts for the Earth’s seasons and the difference in solar radiation between the poles and the equator. Thirdly, we have Earth’s precession, which translates to a wobble as Earth spins on its axis. This happens roughly every 23,000 years, and will cause winter in the Northern Hemisphere to happen when Earth is farthest away from the Sun, and summer when it’s closest. When this happens, the difference in severity between seasons will be greater than it is today. Besides these major factors, we also have the occasional lack of solar spots, large meteor impacts, huge volcanic eruptions, or nuclear wars, among other things, that can potentially lead to an Ice Age.
8. Why do they last so long?
We know that Ice Ages usually last for millions of years at a time. The reasons behind this can be explained through a phenomenon known as albedo. This is the reflectivity of the Earth’s surface when it comes to the Sun’s shortwave radiation. In other words, the more our planet is covered in white ice and snow, the more of the Sun’s radiation is reflected back into space, and the colder it gets. This leads to more ice and more reflectivity – in a positive feedback cycle that lasts for millions of years. This is one of the reasons why it’s so important for Greenland’s ice to remain where it is. Because if it doesn’t, the island’s reflectivity will decrease, adding to the overall global temperature increase.
Nevertheless, Ice Ages do eventually come to an end, and so do their glacial periods. As the air becomes colder, it can no longer hold as much moisture as it did before, leading, in turn, to less snowfall and the eventual impossibility for the ice to expand or even replenish itself. This starts a negative feedback cycle that marks the beginning of an interglacial period. By this logic, a theory was proposed back in 1956 which hypothesized that an ice-free Arctic Ocean would actually cause more snowfall at higher latitudes, above and below the Arctic Circle. This snow may eventually be in such great quantities that it will not melt during the summer months, increasing Earth’s albedo and reducing the overall temperature. In time, this will allow ice to form at lower altitudes and mid-latitudes – kick starting a glaciation event in the process.
7. But how do we really know Ice Ages even exist?
The reason people began thinking about Ice Ages in the first place was because of some large boulders located seemingly in the middle of nowhere, and with no explanation as to how they got there. The study of glaciation started during the mid-18th century, when Swiss engineer and geographer Pierre Martel began documenting the erratic dispersal of rock formations inside an Alpine valley, and downhill from a glacier. The locals told to him that those huge boulders were pushed there by the glacier that once extended much farther down the mountain. Over the decades, many other similar features were documented around the world, forming the basis for the theory of Ice Ages. Since then, other forms of evidence have been taken into account. The geological features, among which are the previously mentioned rock formations, also contain moraines, carved valleys such as fjords, glacial lakes, and various other forms of land scarring. The problem with these, however, is that they’re extremely hard to date, and successive glaciations can distort, or even completely erase the previous geological formations.
More accurate evidence comes in the form of paleontology – or the study of fossils. Though not without its own series of drawbacks and inaccuracies, paleontology talks about the history of Ice Ages by showing us the distribution of cold-adapted organisms once living in lower latitudes, with organisms that usually thrive in warmer climates either being restricted closer to the equator or going extinct altogether. The most accurate evidence, however, is in the form of isotopes. Variations in the ratios of isotopes between fossils, sedimentary rocks, and ocean sediment cores can tell us much about the environment in which they formed. When talking about the current Ice Age, we also have access to ice cores taken from Antarctica and Greenland – and which are the most reliable form of evidence to date. When formulating their theories and predictions, scientists rely on a combination of these, wherever possible.
6. The Big Ice Ages
At the moment, scientists are confident that there were five major Ice Ages throughout Earth’s long history. The first of them, known as the Huronian glaciation, happened roughly 2.4 billion years ago and lasted for about 300 million years, and is considered the longest. The Cryogenian Ice Age happened around 720 million years ago, and lasted until 630 million years ago. This one is considered to be the most severe. The third massive glaciation took place about 450 million years ago and lasted some 30 million years. It’s known as the Andean-Saharan Ice Age, and caused the second largest mass extinction in Earth’s history, after the so-called Great Dying. Lasting for 100 million years, the Karoo Ice Age happened between 360 and 260 million years ago, and was caused by the appearance of land plants, whose remains we now use as fossil fuels.
Lastly, we have the Pleistocene Ice Age, also known as the Pliocene-Quaternary glaciation. It began roughly 2.58 million years ago and has since gone through several glacial and interglacial periods, roughly 40,000 to 100,000 years apart. Over the past 250,000 years, however, the climate changed more frequently and abruptly, with the previous interglacial period being interrupted by numerous cold spells that lasted for several centuries at a time. The current interglacial that began roughly 11,000 years ago is atypical because of the relatively stable climate it has had up until this point. It’s somewhat safe to say that humans may have not been able to discover agriculture and develop its current level of civilization if it wasn’t for this unusual period of temperature stability.
“Wait, what?” We know that’s what you’re thinking when you see that header in this list. But let us explain…
For a period of several centuries, beginning sometime around 1300 and ending around 1850, the world went through a period known as the Little Ice Age. Several factors worked together to lower the overall temperature, particularly in the Northern Hemisphere, allowing many alpine glaciers to expand, rivers to freeze over, and crops to fail. Several villages in Switzerland were completely destroyed by the encroaching glaciers during the mid-17th century, and in 1622, even the southern section of the Bosporus Strait, around Istanbul, had completely frozen over. Things got worse in 1645 and lasted for the following 75 years, in a period known to scientists today as the Maunder Minimum.
During that time, the Sun was going through a period with little to no sunspots. These sunspots are regions on the surface of the Sun that are much lower in temperature. They are caused by concentrations in our star’s magnetic field flux. By themselves, these spots would probably be able of lower Earth’s temperature, but they’re also surrounded by some intensely-bright regions, known as faculae. These have a significantly higher radiation output that far outweighs the reduction caused by sunspots. So, a spot-free Sun actually has a lower radiation output than usual. During the 17th century, it’s estimated that the Sun dimmed by 0.2 percent – something which partially accounted for this Little Ice Age. Over 17 volcanic eruptions took place across the world during that time, dimming the sun’s rays even further.
Economic adversity brought on by this several-century-long cold spell had an incredible psychological impact on people. Frequent crop failures and firewood shortages led many from Salem, Massachusetts to suffer from a severe case of mass hysteria. In the winter of 1692, twenty people – fourteen of which were women – were hung on accusations that they were witches and to blame for everyone’s hardships. Five other people – two children included – later died in prison for the same thing. Because of unfavorable weather, some people in places like Africa occasionally accuse each other of being witches, even to this day. In other places, however, gay people are the scapegoats for the effects of global warming.
4. Snowball Earth
Earth’s first Ice Age was also its longest. As we mentioned earlier, it lasted a whopping 300 million years. Known as the Huronian Glaciation, this incredibly long and freezing epoch happened some 2.4 billion years ago, in a time when only single-celled organisms roamed the Earth. The landscape would have looked completely different than today, even before the ice took over. A series of events, however, happened that would eventually lead to an apocalyptic event of global proportions, engulfing much of the planet in a thick sheet of ice. Life prior to the Huronian Glaciation was dominated by anaerobic organisms that didn’t require oxygen to live. Oxygen was, in fact, poisonous to them, and extremely rare in the air at the time, making up just 0.02% of the atmospheric composition. But at some point, a different form of life evolved – the Cyanobacteria.
This tiny bacterium was the first being to ever make use of photosynthesis as a means of generating its food. A byproduct of this process is oxygen. As these tiny creatures thrived in the world’s oceans, they pumped millions upon millions of tons of oxygen, raising its concentration in the atmosphere to 21%, and almost driving the entire anaerobic life into extinction. This event is known as The Great Oxygenation Event. The air was also full of methane, and in contact with oxygen it turns into CO2 and water. Methane, however, is 25 times more potent as a greenhouse gas than CO2, meaning that this transformation led to a drop in overall temperatures – which, in turn, began the Huronian Glaciation and the first mass extinction on Earth. The occasional volcano added further CO2 into the air, resulting in periodic interglacials.
3. Baked Alaska
If its name wasn’t clear enough, the Cryogenian Ice Age was the coldest period in Earth’s long history. It’s also the subject of much scientific controversy today. One topic of debate is whether the Earth was completely covered in ice, or a band of open water still remained around the equator – a Snowball, or Slushball Earth, as some call the two scenarios. The Cryogenian lasted from roughly 720 to 635 million years ago, and can be divided into two major glaciation events known as the Sturtian (720 to 680 Ma) and the Marinoan (approximately 650 to 635 Ma). It’s important to note that there were no forms of multicellular life at that point, and some speculate that one such Snowball or Slushball Earth scenario was an early catalyst for their evolution during the so-called Cambrian explosion.
A particularly interesting study was published back in 2009, focusing on the Marinoan glaciation in particular. According to the analysis, Earth’s atmosphere was relatively warm, while its surface was covered in a thick layer of ice. This can only be possible if the planet was entirely, or almost entirely, covered in ice. They compared the phenomenon to a Baked Alaska dessert – where the ice cream doesn’t immediately melt when it’s placed in the oven. It turns out that the atmosphere had plenty of greenhouse gases in its composition, but that didn’t stop or mediate the Ice Age as we would expect. These gasses were present in such great quantities because of increased volcanic activity due to the breakup of the Rodinia supercontinent. This long volcanism is also thought to have helped start the Ice Age.
The science team warned us, however, that something similar could happen again if the atmosphere reflected too much of the Sun’s rays back into space. One such process could be triggered by a massive volcanic eruption, nuclear war, or our future attempts at mitigating the effects of global warming by spraying the atmosphere with too many sulphate aerosols.
2. Flood Myths
When the glacial ice began to melt some 14,500 years ago, the water didn’t flow to the ocean in a uniform pattern across the globe. In some places like North America, a huge proglacial lake began to form. These lakes are a result of damming, either by a moraine or an ice wall. In 1,600 years’ time, Lake Agassiz covered an estimated area of 170,000 sq. miles – larger than any lake currently in existence. It formed over parts of North Dakota, Minnesota, Manitoba, Saskatchewan, and Ontario. When the dam finally gave in, fresh water flooded into the Arctic Ocean via the Mackenzie River Valley. This great influx of fresh water weakened the oceanic current by up to 30%, plunging the planet into a 1,200-year-long period of glaciation known as the Younger Dryas. This unfortunate turn of events is suspected to have killed off the Clovis culture and the North American megafauna. Records also show that this cold spell came to an abrupt end some 11,500 years ago, with temperatures in Greenland rising by 18 degrees F in a mere decade.
During the Younger Dryas, the glacial ice replenished itself, and when the planet began to warm up again, Lake Agassiz also reappeared. This time, however, it joined with an equally large lake, known as Ojibway. Shortly after their merger, a new drainage took place, but this time in the Hudson Bay. Another cold spell happened 8,200 years ago, known as the 8.2 kiloyear event. Though cold temperatures lasted for only 150 years, this incident was able to raise sea levels by 13 feet. Interestingly, historians were able to link the origins of many flood myths from around the world to this exact time period. This sudden rise in sea levels also caused the Mediterranean to punch its way through the Bosporus Strait and flood the Black Sea, which at the time was only a freshwater lake.
1. Martian Ice Age
Influenced by forces beyond our control, Ice Ages are naturally occurring events that aren’t confined to Earth alone. Like our own planet, Mars also goes through periodical changes in its orbit and axial tilt. But unlike Earth, where an Ice Age implies polar ice caps growing in size, Mars experiences a different process. Because its axial tilt is more pronounced than Earth’s, and the poles receive more sunlight, a Martian Ice Age means that polar ice caps actually recede, while glaciers at the mid-latitude expand. This process is reversed during interglacial periods.
For the past 370,000 years, Mars has been slowly coming out of its own ice age and entering an interglacial period. Scientists estimate that roughly 20,900 cubic miles of ice has been accumulating at the poles since, most of it being in the Northern Hemisphere. Computer models have also shown that Mars has the capacity of being totally enveloped in ice during a glaciation event. This research is in its early stages, however, and given the fact that we’re still a long way away from fully understanding Earth’s own Ice Ages, we can’t logically expect to know everything that’s happening on Mars. Nevertheless, this research can prove useful, given our future plans for the Red Planet. It also helps us a great deal here on Earth. “Mars serves as a simplified laboratory for testing climate models and scenarios, without oceans and biology, which we can then use to better understand Earth systems,” said planetary scientist Isaac Smith.