This is a sequel of sorts to my previous post. During my research I found information which didn’t quite fit that post’s topic but was relevant to the broader topic of climate change, plus it was just interesting. The ice age comes up fairly often when discussing our current situation with a changing climate and biosphere as well as any discussion of early human history. So to put these topics into context, let’s talk about what an ice age is and what it can tell us about anthropogenic climate change.
The Late Cenozoic Ice Age
One difficulty I found when writing this was that there are two definitions of an ice age, one scientific and one more colloquial, and one is a subdivision of the other. I will go through both definitions and I’ll work to keep the difference clear.
The scientific term of an Ice Age, also known as a Glaciation or an Icehouse Period, is a long period of Earth’s history defined by reduced temperature that results in the formation of polar ice sheets. You might notice that since there are currently ice sheets at both poles, we are still technically in an ice age. This current one, the Late Cenozoic Ice Age, started around 34 million years ago and is the fifth Ice Age in Earth’s history. (If you’re curious, this video from Atlas Pro talks about the previous ice ages. Also, from this point forward, I’m going to use the term Icehouse Period when using this definition of an ice age just to make the difference clear.) Having permanent ice at the poles has historically been the exception instead of the rule, with polar ice being present for less than 10% of Earth’s history. The factors that lead to this abnormal drop in global temperatures are too complicated to be the result of any one factor and scientists debate which factors were most important in causing the current icehouse period, but they definitely include changes to atmospheric composition and the positioning of continents, which in turn leads to changes in oceanic currents and the regulation of the planet’s carbon cycle. To give a more hands-on description, I’ll go over some theories for what contributed to our current icehouse period;
The isolation of the Arctic Ocean. If you look at a map of the Earth, you’ll notice the Arctic Ocean is almost completely enclosed by North America and Eurasia. Normally, warm water would be moved by ocean currents from the equator to the poles, increasing the temperature of the polar waters. But with the land’s current configuration, the Arctic Ocean is largely cut off from these warm currents and is thus much colder than it would be otherwise. Add to this the amount of land in the Arctic Circle (ice sheets can only form on land), and one has the perfect conditions for creating ice sheets.
The isolation of the Southern Ocean. The Antarctic is similarly cut off from warmer water, but for the opposite reason. Ocean currents flow in a west-east direction, driven by atmospheric circulation which is driven by the Earth’s rotation. Currents flow in a north-south direction only when they meet continents which divert their flow. Currently, there’s only one band of latitudes where a current can flow undiverted continuously, where one can sail in a complete circle west-to-east and never touch land, and that is around Antarctica. Currents become stronger the longer they go without being diverted, so the Antarctic Circumpolar Current is the strongest ocean current by a very wide margin. This current pulls in warmer water from other currents outside the circle and diverts it away from the circle’s interior, keeping the waters of the Southern Ocean more hydrologically isolated and thus colder than they otherwise would be. Combine that with a continent 40% larger than Europe inside this cold circle, and one once again has the perfect conditions for creating ice sheets.
Himalayan weathering. Tectonic activity can add carbon dioxide to the atmosphere via volcanic eruptions, but it can remove CO2 from the atmosphere as well. The short explanation is that when rain falls on newly formed or exposed rock, it triggers chemical reactions with CO2 in the air that pulls CO2 out of the atmosphere to create carbonate rock such as limestone. For a longer explanation, watch this video from HHMI. This is known as the Inorganic Carbon Cycle and it plays a major role in regulating Earth’s climate over the long-term. Geologic evidence shows that periods of significant rock formation and weathering are associated with lowering global temperatures. The Himalayan Plateau is relatively new, only forming in the past 40 to 50 million years, with half of its growth believed to have happened in the past 10 million years. There is evidence that the weathering of the Himelayas played a role in cooling the planet, though how significant a role it played is subject to debate. The Andes and Rockies are also relatively new mountain ranges that have been suggested to be significant carbon sinks.
Formation of the Isthmus of Panama. The Isthmus of Panama is arguably the most recently formed feature on the world map, forming only three million years ago. Before then, the Atlantic and Pacific Oceans were connected via a seaway that allowed tropical ocean currents to flow directly from the Atlantic to the Pacific. This seaway was closed as the Caribbean and Cocos Plates collided with each other, raising the seafloor and creating volcanic islands which would eventually grow and merge to create the Isthmus. With the two oceans now disconnected, the warm Atlantic currents that used to flow to the Pacific were rerouted northward toward Greenland, creating the modern AMOC (Atlantic Meridional Overturning Circulation). While sending warm water to the Arctic Ocean is believed to have temporarily warmed the polar region, this warm water produced evaporation which produced precipitation. This added snowfall on the polar region sped up the formation of polar ice sheets. This probably wasn’t so much a cause of the ice age as it was a trigger, but the Isthmus’ formation occurred at roughly the same time as the Arctic Ice sheet’s formation.
All of these are contributing factors to a broader trend of lower global temperatures and increased ice formation that has been the case for millions of years. Starting about 55 million years ago, the global climate began cooling from its highest point in hundreds of millions of years. (For comparison, the dinosaurs went extinct only 65 mya) India started colliding with Eurasia about 40 to 50 mya, creating the Himalayas and pulling carbon out of the atmosphere.* Global temperatures would slowly fall over the next few million years. South America was originally connected to Antarctica, but they would begin splitting apart about 43 mya to create a narrow passage for a new Antarctic Circumpolar Current. Permanent ice sheets would form in Antarctica roughly 34 mya, marking the official beginning of an icehouse period. But the north pole would remain ice free until about 3 mya, around the same time the Isthmus of Panama formed and rerouted AMOC northward. The ice covering enormous swaths of land increased the albedo of the regions; ice is shiny so sunlight gets reflected back into space instead of heating the planet. This increased albedo creates a feedback loop, cooling surrounding regions and allowing for more snow. As more snow falls than melts, it builds up and gets crushed under its own weight into ice sheets. With North America and Eurasia offering such large tracts of land for the expanding ice sheets, they extend all the way down to the Great Lakes.
Greatest extent of ice sheets shown in gray, current extent shown in black
Last Glacial Maximum
If ice ages are just defined by there being ice, then what differentiates the present from that time in the past when there were mammoths and saber toothed cats? When the term ‘ice age’ is used colloquially, it’s usually describing what scientists call glacial maximums. While icehouse periods are defined by a climate colder than Earth’s overall average, just how cold can vary a lot over millions of years. And because there is so much land around the Arctic Circle for ice sheets to spread to, the difference between ‘somewhat cold’ and ‘very cold’ can mean whether or not whole continents are covered in ice. Evidence shows that ever since the Arctic ice sheets formed, there has been a roughly 100,000 year cycle of global temperatures rising and falling, leading to the arctic ice sheets† expanding and contracting. Earth is currently in an interglacial period, meaning the glaciers have retreated to their minimum area and the climate is at its warmest for an ice age. (For clarity’s sake, from this point forward I’m going to refer to glacial and interglacial periods as freeze and thaw stages respectively. These are not scientific terms, but you may find them more intuitive.) But we know from geologic data that there have been about 30 cycles of glacial growth and retreat going back at least 2.5 million years. During the glacial (freeze) parts of the cycle, global temperatures drop 6°C below what they are today. Ice sheets expand well into what is currently the temperate region and sea levels drop as water gets locked up in ice sheets. During interglacial (thaw) parts of these cycles, ice sheets retreat to the northernmost latitudes, sea levels rise as this ice melts, and the planet warms to what it is today if not warmer.
Just like with icehouse periods, our understanding of what causes this freeze-thaw cycle is complicated, but the primary mechanism appears to be Milankovitch cycles. You see, Earth’s orbit around the sun is continuously tugged on by the gravitational pulls of the Sun, Moon, and other planets in the solar system, which affects the tilt of the Earth’s axis, the direction this axis points, and how close Earth gets to the Sun. This results in a 25% difference in how much sunlight the Earth’s middle latitudes receive. These three astronomical cycles play off of each other to create a roughly 100,000 year cycle between these highest and lowest points, lining up with the freeze-thaw cycle. This video from PBS Eons gives an in-depth description of what these cycles are and I would highly recommend it.
Milankovitch cycles are what drives this freeze-thaw cycle, but more or less sunlight doesn't account for the entire difference between the coldest and warmest parts of the cycle. More or less sunlight might kick off a warming or cooling period, but there need to be some feedback loops at play that turn a slight change in sunlight into a massive change in climate. As stated above, changing ice sheets changes the planet’s albedo; more ice means more reflected sunlight means colder climate means more ice, and vice versa. We also know that the oceans can absorb CO2, but they become worse at it when they’re warmer. Warmer climate means less CO2 getting absorbed by the oceans means more CO2 in the air and vice versa. Colder climate also leads to air and ocean currents becoming more sluggish, so less heat from the tropics gets transferred to the poles. Which of these feedback loops contribute most to changing the global climate is still being studied, but I hope this shows how many factors go into determining how one change in the amount of energy the planet receives turns into a complex series of effects on the whole planet.
Implications
When discussing the history of ice ages and climate fluctuations, some find it tempting to use this information to dismiss concerns about man-made climate change. This is wrong; human carbon emissions are by far the biggest and fastest driver of global climate in the modern day regardless of what drove the global climate in the past. All the processes I’ve spoken of here work over the course of hundreds of thousands if not millions of years. For example, due to rock weathering and other natural carbon sinks, the concentration of CO2 in Earth’s atmosphere has dropped from 1600 ppm about 50 mya to 280 ppm before the industrial revolution. That would mean that the atmosphere lost an average of roughly 0.003 ppm of carbon dioxide every century between this thermal maximum and the pre-industrial age. Today, Earth’s atmosphere is at 420 ppm of CO2, meaning we’ve gained roughly 210 ppm per century since the industrial revolution. While there are geological or astronomical events that could theoretically release this much carbon in a relatively short period of time, they would have been very noticeable. The Permian extinction I talked about last time released its CO2 far slower than our current emissions rate and it covered a continent with lava. Volcanoes are not known for their subtlety. The scientists that observe volcanic eruptions and discovered the effect Milankovich cycles have on our climate are in the same field of study as those who study climate change. If there was something else contributing to our warming climate, they’d be the first to notice. Studying climate change in the past is useful to understanding climate change in the present because it tells us what a warmer world will look like on a practical scale. But we can safely say the root cause of our current climate crisis is very different from those of the past.
CO2 levels over the past 800,000 years to compare to current levels. Note the colloquial use of the word ice age to refer to a glacial period.
What affects anthropogenic climate change will have on the glacial-interglacial cycle is still being researched. Based on how much CO2 has been added to the atmosphere and the rate at which it is being absorbed by chemical weathering, it could take between 100,000 and 500,000 years for all of our carbon to be reabsorbed by the planet, depending on how much we continue to add to the atmosphere. Because of this, scientists have predicted we might skip the next freeze period because by the time the Milankovitch cycle reduces our sunlight again, the planet will still be too warm to freeze properly. How warm this extended thaw period will get could vary depending on our actions over the next century, whether we transition to new sources of energy quickly or burn up all the remaining fossil fuels first. If we move quickly and significantly curb our carbon emissions, the following 100,000 years will be warmer, but still in line with previous thaw periods. If we fail to do so, we could see temperatures that haven’t been seen since the current ice age began. Bear in mind that no matter how long it takes for the Earth to reset itself to normal, the highest temperatures will be experienced in the next century within most of our lifetimes. What such a warm planet would look like practically is difficult to determine as predicting weather patterns from climate data is near impossible. But considering there are already parts of the tropics today becoming too hot and humid for humans to survive long-term (a topic for another time), a greenhouse Earth would certainly not be one we would fare well on. Homo Sapiens as we would recognize them emerged roughly 250,000 years ago, during a freeze period. Our ancestors left Africa in part because it grew too warm and arid during the previous thaw period. They spread across the world during the last freeze period. Everything we call human civilization has occurred within the current thaw period, only 12,000 years compared to the hundreds of thousands of years for even the smallest natural climatic shifts to occur. Our species has spent its entire existence within an icehouse. It’s hard to tell how we would fare outside of one. Once our carbon has been reabsorbed, the Cenozoic Ice Age will continue in the cycle it has for the past million years until the continents eventually shift enough to warm the planet permanently. But if there is one takeaway both these entries should leave you with, it's that we are not trying to save the planet, we are trying to save ourselves.
For More Details
*How significant the Himalayas have been for removing CO2 from the atmosphere is subject to debate. Some very recent studies have found that the Himalayan weathering was only a fraction of what scientists originally thought it was, so there has to be some other mechanism or mechanisms removing carbon from the atmosphere. What these mechanisms are is still being determined. I include the Himalayas here because they are the most well-researched carbon sink and they are certainly A major carbon sink even if they aren’t THE carbon sink.
†It should be noted that it’s only the Northern ice sheets expanding and contracting with this cycle. The Antarctic ice sheet remained roughly the same regardless of the cycle since there’s no land for the ice to expand to. Scientists do differentiate between the Late Cenozoic Ice Age, when Antarctica froze and the ice age officially stated, and the Quaternary Glaciation, when the Arctic froze and this cycle of freezing and thawing began. When we talk about these cycles, know that only applies to the past 3 million years.
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