Did you know that for thousands of years there were barely any earthworms across a large swath
of North America? The animals all but vanished from the northern part of the continent around
10,000 years ago, and it wasn’t because they were gobbled up by some sort of prehistoric
early bird. Glaciers caused their demise. During Earth’s most recent glaciation period,
which peaked around 20,000 years ago, glaciers crept across the northern latitudes, grinding and
scouring everything in their path. Soft, squishy earthworms were no match for the heavy ice.
For thousands of years, much of the continent’s soil was without earthworms—that is,
until European colonists inadvertently introduced hordes of the creepy crawlies to the landscape.
Hi, I’m Erin McCarthy, and this is The List Show. A virtual worm wipeout is just the
first fact about the ice age I’ll share with you today, from Antarctic palm trees to a theory for
how carbon dioxide can actually cool the planet, under the right circumstances. Let’s get started.
Before we go any further, let’s take a second to clarify what an ice age actually is.
The Utah Geological Survey defines an ice age as “ … a long interval of time…when
global temperatures are relatively cold and large areas of the Earth are covered by continental ice
sheets and alpine glaciers.” Basically, a weirdly cold winter or five wouldn’t be an ice age,
but millions of years of chilly temperatures with ice sheets and glaciers would.
Over its 4.5-billion year history, Earth has swung between periods of extreme heat
and extreme cold. Crocodile-like reptiles once lurked within the lakes of the North Pole
and palm trees swayed in Antarctic breezes. There are a number of factors that might help
plunge the planet into an ice age, and some explanations aren’t particularly intuitive.
You probably know that increased carbon dioxide levels in the atmosphere can warm the planet,
but they might also have the opposite effect, given the right set of conditions.
Around 460 million years ago, the volcanic eruptions that helped create parts of the
northern Appalachian Mountains dumped record-breaking amounts of CO2 into
the atmosphere. It’s believed that all that carbon created acid rain.
As geochemist Lee Kump explained to the Earth science publication EOS, when acid
rain “‘ … attacks silicate rocks like granites and basalts,’ a chemical reaction incorporates
the CO2 into limestone, which removes the molecule from the atmosphere.” More carbon in rocks and
ocean floor sediment and less in the atmosphere might have contributed to the Earth’s cooling.
That volcano example is a somewhat speedier version of what’s sometimes called the slow
carbon cycle, in which carbon moves between rocks, soil, the ocean, and the atmosphere
over hundreds of millions of years. Because rocks can sequester carbon—that
is, remove it from the atmosphere and store it in a form that doesn’t contribute to the
greenhouse effect—scientists are interested in what factors might slow down or accelerate the
weathering processes that break down rocks and lead to this sequestration. The formation of
large mountain ranges, like the Himalayas, is often at the center of these investigations.
Some believe that the formation of these mountains causes increased weathering,
which could then allow more carbon from the air to get stored in rocks.
Newer research complicates this picture—it may be that mountain formation doesn’t lead to an
increase in weathering, overall, but instead exposes rocky material that is more reactive,
and therefore more efficient at weathering rocks and sequestering carbon.
Anything that exposes new rocky material would therefore lead to increased carbon sequestration.
This is an area of intense interest for geologists and other climate scientists.
At minimum, the slow carbon cycle could give us insights into how the planet regulates its
temperature. More ambitiously, some believe that human beings could eventually harness
certain rocks’ carbon sequestering powers to help combat climate change.
The planet’s orbit and the tilt of its axis are not as constant as you might think, and variations
in them can contribute to ice ages. These changes happen regularly over the course of hundreds
of thousands of years, affecting the amount of sunlight different latitudes on Earth receive,
and by extension, the planet’s temperature. When the angle of Earth’s tilt decreases, summers cool
down, allowing snow to accumulate. As layers of snow condense into glaciers and ice sheets,
they reflect more sunlight—and therefore heat—causing global temperatures to drop.
Each of the major ice ages themselves went through cycles of freezing and thawing,
called glacial and interglacial periods. During an interglacial period, glaciers recede toward
the poles but don’t completely disappear. This ebb and flow of ice and heat is a long process
that plays out over a span of tens of thousands of years, so you can probably cross off an instant
planetary freeze, à la 2004’s The Day After Tomorrow, from your list of climate concerns.
The first of Earth’s five major ice ages was the Huronian glaciation, which kicked off
about 2 billion years ago, possibly helped by a 250-million year pause in volcanic activity.
That ice age was so intense that the entire planet froze over to form the first of a few
“snowball Earths.” (Though many geologists think “slushball earth” would be a more accurate label,
as the planet may not have been entirely frozen.) It’s believed land plants caused the Karoo,
or Late Paleozoic, ice age, which started around 360 million years ago.
As plants covered the planet, they sucked carbon dioxide from the atmosphere
and released oxygen into the air, which caused temperatures to once again plummet.
When people talk about the ice age, they aren’t talking about any random glaciation period.
They’re referring to the most recent glaciation during the Pleistocene epoch of the Quaternary
period. (An epoch is a timeframe within a period; a period is a longer timeframe within an era.) So
when I say the ice age, going forward, I’m talking about a specific timeframe that
lasted from about 120,000 to 11,500 years ago. It’s believed tectonics played a big role in
triggering the Quaternary Glaciation. More specifically, the formation of the Isthmus
of Panama, the strip of land that links North and South America. The land bridge divides the
Atlantic and Pacific Oceans. When it formed, it drastically changed both oceans’ currents,
as the warm, tropical water could no longer flow between them. The warmer water went north, causing
more precipitation in the Northern Hemisphere’s high latitudes. This precipitation—which,
at high latitudes, fell as snow—kept piling up and freezing to form glaciers and ice sheets.
All that ice reflected more light and absorbed less heat than the darker oceans it now covered,
creating a positive feedback cycle that further decreased the planet’s temperature.
The ice age reached its height, a period called the Last Glacial Maximum, around 20,000 years
ago. Back then, 8 percent of the Earth’s surface, and a quarter of the planet’s total land area,
was buried beneath ice. Ice sheets up to a mile thick covered North America—including
a whopping 97 percent of Canada—as well as large parts of Northern Europe, Asia, and Patagonia.
Other parts of Northern Europe were basically transformed into a tundra, wiping out the warm
weather plants that had previously thrived there. Steppes, which are flat, unforested grasslands,
expanded across the planet and sub-Saharan Africa became more arid. Because of all the water locked
in frozen ice sheets at the time, the sea level was 400 feet lower than it currently is.
During the Last Glacial Maximum, on average, the planet’s temperature hovered around 46
degrees Fahrenheit. That’s 11 degrees colder than the 20th century’s average global temperature.
And average Arctic temperatures back then were 25 degrees cooler than today’s.
Despite the cooler temperatures, wildlife thrived. Animals we still see today, like shrews,
mice, and lemmings, survived despite the changing landscape.
Megafauna reigned supreme during the ice age. We don’t know for sure why prehistoric animals
were so big. One theory says that bigger prey animals were less likely to be hunted.
Another theory, which follows a contested principle known as Bergmann’s Rule,
suggests that larger animals are more likely to be found in colder climates and higher latitudes.
Whatever the cause, there were some big animals walking around in those cold climes.
Wooly mammoths stood up to 11 feet tall at the shoulder. The largest saber-tooth cats,
commonly (though incorrectly) called saber-tooth tigers, reached nearly 1000 pounds—more than
double the size of modern tigers. Bear-sized beavers roamed North America, and 12-foot-tall
giant ground sloths burrowed around South America. There was also the Irish elk,
one of the largest deer species ever recorded. They stood 7-feet tall at the shoulder and had
antlers that spanned up to 12-feet—that’s double the size of a moose’s antlers.
A lot of the iconic ice age megafauna went extinct after the ice age ended.
There are multiple theories as to why. One suggests that when the last glacial period ended
and the climate shifted, it caused the vegetation that certain animals depended on to change. This
was bad news for the large herbivores—and bad news for the large carnivores that depended on them.
Another theory suggests an intense cold snap was the cause. One far-out idea claims meteorites or
comets slammed into Earth, shifted the climate, and annihilated various species. And of course,
there’s a popular theory that says a particularly powerful species of hunter was
responsible for these mass extinctions: humans. Humans thrived during the ice age. Modern Homo
sapiens evolved around 300,000 years ago. It was during the last glaciation period
that Homo sapiens began leaving Africa en masse and spread across the globe.
Modern humans were already in Europe, mixing with Neanderthals and creating detailed cave art,
tens of thousands of years before the ice age hit its peak. It’s believed that our unusually
big brains—and the tools and weapons Homo sapiens had spent millennia advancing—allowed people to
survive and continue expanding their range during the harsh conditions of the Last Glacial Maximum.
The last of the other human species scattered around the world went extinct during the ice age.
The latest known evidence of Homo erectus dates to between 117,000 and 108,000 years
ago. Neanderthals disappeared roughly 30,000 years ago, and some think the cooling climate
in the years before the Last Glacial Maximum may be to blame. Though the Neanderthals had
spent hundreds of thousands of years adapting to Europe’s chillier climate, the changing
landscape did not suit their hunting styles. ​​ The last glacial period ended over 10,000 years
ago. Earth’s orbit changed, and the planet’s angle of tilt increased, leaving the Northern Hemisphere
exposed to longer, more intense summers. Ice melted and the sea level rose. All that exposed
water absorbed rather than reflected light, causing the planet’s temperatures to increase.
You can still see traces of the ice age today. Head to Southern California’s La Brea Tar Pits,
where you’ll find a bubbling, tarry sludge full of ice age animal bones,
including dire wolves and saber-tooth cats. Or visit Kerry, Ireland, where there’s a single
lake that’s home to the only population of the Killarney shad, a type of fish whose ancestors
split their time between the ocean and freshwater but who became trapped in an Irish lake after
ice sheets altered the landscape. On Canada’s Calvert Island, you’ll find human footprints
from 13,000 years ago, left behind by the people who migrated to North America during the ice age.
When ice sheets moved across the land, they left deep scars in their wake.
Melting glaciers then filled those holes, creating famous bodies of water like the Great Lakes and
Scotland’s Loch Ness. Norway’s famous fjords are valleys carved by glaciers, which later filled
with sea water. But you don’t have to travel to some far-flung location to find hints of the ice
age. If you’re ever hiking in the woods and find a large, scratched up rock, you’re likely looking at
glacial striations caused by debris being dragged against the rock by an unrelenting chunk of ice.
Or, if you’ve ever seen a massive boulder seemingly dropped out of nowhere, it’s most
likely a glacial erratic, which is basically just a rock left behind by a moving glacier.
We’ll leave you with a fact about the current ice age. The ice age ended a long time ago.
But we’re actually still in an ice age. We’re currently in the Holocene interglacial period
of the Quaternary ice age. Large portions of the Northern Hemisphere are no longer covered by ice,
but we do still have some glaciers and ice sheets. Despite rapidly increasing global temperatures,
our glaciers and the Antarctic and Greenland ice sheets are still hanging around—for now.
Our next episode is all about the history of television. Drop your favorite show in the
comments. All answers are acceptable, as long as it’s either Buffy, Lost,
or What We Do In the Shadows. I’m regular human host Erin McCarthy—thanks for watching.