The Earth has gone through several ice ages and interglacial periods over the last 2.6 million years. This geological period is known as the Quaternary.
We are now in an interglacial period.
20,000 years ago, ice covered all of Norway, up to 3,000 metres thick. The glacier extended all the way to Germany. Most of the UK was also under ice.
This was not the only time the ice settled heavily over Norway and glaciers carved their way through the landscape.
"It's the glaciers of the ice ages that have shaped the landscape. Without them, we wouldn't have the fjords, lakes, and deep valleys that characterise the Norwegian landscape," says Jan Mangerud, professor emeritus at the University of Bergen's Department of Earth Science.
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So how many ice ages has Norway actually experienced?
Ice ages occurred long before this as well, hundreds of millions of years ago, but here the focus is on the past 2.6 million years. The last one ended 11,000 years ago, opening Norway to humans.
Jan Mangerud on fieldwork at Kapp Ekholm on Svalbard in 2002.(Photo: Private)
Earlier, people knew of three
Mangerud has researched Quaternary geology for over 60 years.
If he had been asked as a young researcher, he would have answered that there had been "at least three" ice ages.
Back then, ice ages were counted by looking for physical traces of the glaciers' extent. Such traces are found, among other places, in Germany.
Deposits from three ice ages and interglacial periods were found in northern Germany. These layers consisted of moraines with stones and boulders from Norway, Sweden, and Finland, Mangerud writes in the article.
This shows that glaciers stretched across all of Scandinavia and all the way down to Germany.
A comparison of the last ice age’s ice coverage with today’s can be viewed on the slider below.
An archive at the bottom of the sea
But there have been far more ice ages during the past 2.6 million years.
The key to assembling a timeline wasn't hidden in old moraine layers on land, but on the ocean floor.
In the deep sea, a little sand and clay sink each year along with remains of plants and small animals, settling at the bottom. There are few disturbances down there.
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"One layer ends up lying on top of the other," says Mangerud
It becomes like an archive that stores information. The deeper you drill, the further back in time you can see.
Oxygen in the sea
Within these layers on the seafloor lie answers about how much water was stored in the massive ice caps during past ice ages.
The ratio between oxygen isotopes in the layers on the ocean floor reveals this, Mangerud explains in the article.
When water evaporates from the sea, a bit more of the lighter 16O isotope evaporates than the heavier 18O.
When the water falls as rain from a cloud, the raindrops contain more of the heavier 18O than the vapour remaining in the cloud. The cloud thus becomes richer in 16O as it moves inland or over an ice cap.
"Today, that rain returns to the sea through streams and rivers over days, weeks, or years. The water is constantly cycling back," says Mangerud.
Because of this, the ratio between isotopes remains stable.
But during an ice age, the water does not return to the sea. Precipitation accumulates on land and builds enormous ice caps.
During the last ice age 20,000 years ago, the sea level was 120 to 130 metres lower than today because so much water was held in the glaciers.
These glaciers were enriched in 16O, and thus the ocean became richer in 18O.
Reconstructing the past
Within the seabed layers, researchers can determine past ocean isotope levels by studying the remains of tiny marine animals – calcareous algae called foraminifera.
They absorbed oxygen with roughly the same isotope ratio that the seawater had at the time they lived.
With the help of core samples from the seafloor, researchers can reconstruct how much 18O he ocean once held and trace the pattern of ice ages back through time.
The result is a graph like the one shown below.
The deep dips mark ice ages with large ice caps. The peaks represent warm interglacial periods.
Jan Mangerud has added three horizontal lines indicating the thresholds for large ice caps, small ice caps, and interglacial periods.
The ocean's concentration of 18O through time, shown as an average derived from numerous core samples taken from the seafloor in different parts of the world. The time axis is given in 100,000-year intervals, so the graph shows 1.8 million years into the past.(Graph: Lisiecki & Raymo, 2005, modified by Jan Mangerud)
A turning point one million years ago
During the last one million years, ice ages have occurred roughly every 100,000 years.
We can see on the graph that the ice ages marked 2, 6, 10, 12, and 16 appear to have been especially intense, with large ice caps.
If we go further back than one million years, the pattern changes. Ice ages came at intervals of about 41,000 years, and the ice caps did not grow as large.
"The earlier ice sheets were probably mostly confined to Norway, Sweden, and Finland and did not extend as far south as Germany," says Mangerud.
"Was Norway still covered by ice during these less dramatic ice ages?"
"That's almost certain. But perhaps the ice front at that time lay within the Norwegian coastline. This was a long period – 1.6 million years – so it may have been then that many of the fjords were carved out by the glacier," he says.
He notes that fjords can be several hundred metres deep but tend to be shallower at the mouth. One possible explanation for this is that the glacier ended there for long periods of time.
Long cycles
The timing of ice ages is governed by changes in Earth's orbit and the tilt of its rotational axis, which follow cycles of about 100,000, 41,000, and 17,000 – 21,000 years. These are called Milanković cycles.
These slow cycles cause small changes in solar radiation.
This is then amplified by feedback mechanisms in the climate system. For instance, white surfaces reflect more solar radiation and help lower temperatures.
Ocean circulation and the storage of CO2 in the sea are also affected. During ice ages, large amounts of CO2 are stored in the deep sea, and the content in the atmosphere becomes lower, according to the Great Norwegian Encyclopedia.
No big surprises expected
"Is it still uncertain how many ice ages there have been?"
Not really, says Mangerud, though the answer depends on how an ice age is defined.
"When it comes to isotope data, measurements have been taken in so many places that no major new surprises regarding the number of ice ages are likely," he says.
A Norwegian professor cracked the mystery
The Norwegian professor Jens Esmark was the first to realise that there must once have been a glacier so large that it covered all of Norway. He wrote this in 1824.
In 1840, Swiss researcher Louis Agassiz also proposed the idea that ice ages had occurred.
After the glacial theories were established, geologists began identifying moraine deposits from the ice age along the entire Norwegian coast, Thorbjørn Kaland wrote in an op-ed on forskning.no marking the 200th anniversary of the discovery of the ice ages.
Deposits from ice ages were also found in other parts of Northern Europe and in North America.
Then it was discovered that the outer boundary of the ice age in northern Germany, Poland, and Russia had different ages. This meant there must have been several ice ages, each with its own maximum extent of glaciers.
'When marine geologists began comparing seafloor sediments in the North Sea with ice age deposits on land, they discovered a long series of ice ages during the most recent geological period called the Quaternary (the last 2.6 million years),' wrote Thorbjørn Kaland.
