What If? Lab
What if an ice age came back?
Long ago, thick ice covered lots of land, and the sea was lower. That time is called an ice age. What if one came back? Let's imagine it with Olly!
About 20,000 years ago, ice sheets more than 3,000 meters thick covered much of North America, and the sea was about 125 meters lower than today. Ice ages come and go over tens of thousands of years as Earth's orbit and tilt slowly change. Imagine an ice age returning, and follow what might change.
Glacial-interglacial cycles are paced by slow changes in Earth's orbit and axial tilt, the Milankovitch cycles, amplified by ice-sheet reflectivity and carbon dioxide. At the Last Glacial Maximum sea level stood about 125 m below today's. This thought experiment brings glacial conditions back and reasons about what might follow.
The big question
Today, the big ice sheets are far away near the poles.
Long ago, ice covered much more of the land. What if those times came back?
Earth's climate has swung between cold times, called ice ages or glacial periods, and milder times between them, called interglacials. Right now we live in an interglacial.
What if an ice age came back? In this thought experiment you can follow three paths: thick ice sheets spreading over the land, the sea dropping and uncovering new land, and the animals of the last ice age. Then find out why ice ages come and go.
NASA describes Earth today as being in an interglacial, a milder period between ice ages, and Princeton researchers note that ice-age cycles have repeated throughout the last 3 million years.
This thought experiment reinstates glacial conditions like those of the Last Glacial Maximum, about 20,000 years ago, and asks what might change for ice cover, coastlines and living things. It cannot say when, or whether, a new glaciation might begin; it reasons from evidence of the last one.
Explore the idea
Make a prediction
What slowly changes to help start and end ice ages?
What slowly changes over tens of thousands of years to help start and end ice ages?
According to NASA, the timing of glacial-interglacial cycles is driven by changes in…
- Earth's orbit and tilt
- The color of the Moon
- The number of rivers
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Established fact
Ice ages come and go very slowly.
Ice ages come and go over tens of thousands of years as Earth's orbit and tilt slowly change (NASA).
Milankovitch cycles drive the timing of glacial-interglacial cycles, with ice sheets and CO₂ amplifying the response (NASA).
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What the model shows
Scientists use computers to study how orbit changes affect climate.
Climate models show the orbital nudge is overwhelmed when carbon dioxide is above about 350 parts per million (NASA).
Climate models indicate Milankovitch forcing is overwhelmed above about 350 ppm CO₂ (NASA).
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An idea scientists discuss
An ice age coming back is pretend. We imagine what might happen.
What might happen if an ice age came back is an idea built from the last ice age.
Every consequence of renewed glaciation here is reasoned from the Last Glacial Maximum and might differ greatly in reality.
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Established fact
We live in a warmer time between ice ages.
Today we live in the most recent interglacial, a milder time that started about 10,000 years ago.
The present interglacial, the Holocene Epoch, began about 10,000 years ago (USGS).
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Established fact
Long ago, very thick ice covered much of North America.
In the last ice age, the Laurentide Ice Sheet grew more than 3,000 meters thick and covered almost all of central and eastern Canada and parts of the northern United States.
The Laurentide Ice Sheet exceeded 3,000 m and covered almost all of central and eastern Canada and the northern Great Plains, Midwest and Northeastern U.S. (University of Washington).
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Established fact
Ice reflects sunlight, so more ice makes things colder.
Cooler summers in the far north let snow and ice build up into big ice sheets, and the ice reflects sunlight back to space, cooling Earth even more.
Decreasing obliquity yields cooler high-latitude summers that allow ice-sheet growth; increasing ice cover reflects more solar energy, promoting further cooling (NASA).
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An idea scientists discuss
If an ice age came back, thick ice might spread over land again.
If an ice age came back, ice sheets might slowly grow over the far north again, over many thousands of years.
Renewed glaciation might rebuild Northern Hemisphere ice sheets over millennia; their extent and timing are beyond this experiment.
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Established fact
In the last ice age, the sea was much lower than today.
At the peak of the last ice age, about 20,000 years ago, sea level was about 125 meters lower than today.
Global sea level was about 125 m below today's at the Last Glacial Maximum about 20,000 years ago, as water was stored on the continents as ice (USGS).
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Established fact
A strip of dry land once joined Asia and Alaska.
Low seas uncovered Beringia, a huge area of land between Siberia and Alaska that included the Bering Land Bridge. Princeton scientists found the land bridge appeared only about 35,700 years ago.
Arctic sediment nitrogen isotopes show the Bering Strait was flooded until about 35,700 years ago, far later than the 70,000 years previously assumed (Princeton University, 2022).
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An idea scientists discuss
If the ice came back, the sea might drop and new land might appear.
If an ice age came back, the sea might drop again, beaches might move far out, and new land bridges might appear.
Renewed glaciation might lower sea level and re-expose continental shelves such as Beringia; the amount depends on ice volume and is beyond this experiment.
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Established fact
Mammoths and giant ground sloths lived during the ice ages.
During the Pleistocene, the time of the most recent ice ages, mammoths, mastodons, long-horned bison and giant ground sloths lived in North America, Asia and Europe.
Pleistocene habitats in North America, Asia and Europe held mammoths, mastodons, long-horned bison, sabre-toothed cats and giant ground sloths (UC Museum of Paleontology).
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Established fact
Many plants and animals from the ice ages still live today.
Many kinds of Pleistocene conifers, mosses, flowering plants, insects, birds and mammals survive to this day.
Pleistocene biotas were extremely close to modern ones; many genera and species survive today (UCMP).
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An idea scientists discuss
If an ice age came back, plants and animals might move to new homes.
If an ice age came back, plants and animals might move to places where the climate suits them, and land bridges might let some travel between continents.
Renewed glaciation might shift species' ranges toward lower latitudes and open migration routes across exposed shelves; which species might move is beyond this experiment.
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Established fact
Earth's tilt slowly changes, about every 41,000 years.
Earth's tilt changes over about 41,000 years, the shape of its orbit over about 100,000 years, and its slow wobble, called precession, over about 23,000 years on average.
Obliquity varies between 22.1° and 24.5° over about 41,000 years; eccentricity cycles over about 100,000 years; combined precession averages about 23,000 years (NASA).
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Established fact
Ice ages used to come about every 41,000 years. Later, they came about every 100,000 years.
Between one and three million years ago, ice ages came about every 41,000 years. About 800,000 years ago the cycle lengthened to about 100,000 years, and scientists do not yet know why.
Glaciations recurred at 41,000-year intervals one to three million years ago; about 800,000 years ago the cycle lengthened to 100,000 years, matching eccentricity, for reasons not yet clear (NASA).
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What the model shows
Scientists use computers to study the slow changes in Earth's orbit.
Climate models show the slow nudge from Earth's orbit is overwhelmed when carbon dioxide in the air is above about 350 parts per million. In 2023 it was 421.
NASA reports that climate models indicate Milankovitch forcing is overwhelmed once CO₂ exceeds about 350 ppm; CO₂ was 421 ppm in 2023, up from about 280 ppm before industrialization.
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An idea scientists discuss
An ice age coming back soon is pretend. We imagine what might happen.
This experiment cannot say when, or whether, another ice age might come. Its changes are ideas built from what happened in the last one.
Every consequence here is reasoned from the last glaciation; the timing of any future glaciation depends on orbital cycles and carbon dioxide and is not forecast here.
In our world
We live in a warmer time between ice ages. It began about 10,000 years ago.
Today most of Earth's big ice is in Antarctica and Greenland.
USGS says today we live in the most recent interglacial, a time that started about 10,000 years ago. Geologists call it the Holocene. Most of Earth's land ice is now in the Antarctic and Greenland ice sheets.
It was very different during the last ice age. A University of Washington team describes two great ice sheets covering most of northern North America. The bigger one, the Laurentide Ice Sheet, grew more than 3,000 meters thick.
USGS dates the start of the present interglacial, the Holocene, to about 10,000 years ago, and places most of today's land ice in the Antarctic and Greenland ice sheets.
During the last glaciation (25,000 to 12,500 years ago in the University of Washington account), ice sheets spread across northern Eurasia and North America. The Laurentide Ice Sheet exceeded 3,000 m in thickness and covered almost all of central and eastern Canada and the northern Great Plains, Midwest and Northeastern United States; the Cordilleran Ice Sheet covered almost all of British Columbia.
Imagine the ice returning
Let's pretend the ice age comes back.
Thick ice might slowly spread over land. The sea might drop. Let's find out why.
Now imagine an ice age returning over many thousands of years. Snow that does not melt in summer might pile up, year after year, into ice sheets spreading over the far north.
The water to build that ice comes from the ocean, so the sea might drop and shallow sea floors might become dry land. Plants and animals might move to new places as the climate changes. These are ideas: we reason from what happened in the last ice age.
NASA explains that cooler summers at high latitudes let snow and ice accumulate into large ice sheets, and that growing ice reflects more sunlight, promoting further cooling. A return to glacial conditions might therefore build ice sheets over northern continents over millennia.
USGS describes the hydrologic shift: water evaporated from the oceans is stored on land as ice, lowering sea level. Coastlines might migrate seaward and continental shelves might emerge, as Beringia did. These are reasoned ideas, not predictions.
Why ice ages come and go
Earth moves around the Sun in a path that slowly changes. Earth's tilt slowly changes too.
These slow changes help start and end ice ages.
About a century ago, a Serbian scientist named Milutin Milankovitch suggested that slow changes in how Earth moves around the Sun start and end ice ages. NASA describes three cycles: the shape of Earth's orbit changes over about 100,000 years, the tilt of its axis over about 41,000 years, and Earth's slow wobble, called precession, over about 23,000 years on average.
When the tilt gets smaller, summers in the far north get cooler, so snow and ice can build up. Ice reflects sunlight, which cools things further.
The Milankovitch cycles are eccentricity (about 100,000 years), obliquity (22.1–24.5° over the last million years, about 41,000 years) and precession (about 23,000 years combined). Together they vary insolation at mid-latitudes by up to 25 percent.
NASA notes that ice ages came at 41,000-year intervals between one and three million years ago, then about 800,000 years ago the cycle lengthened to 100,000 years; scientists do not yet have a clear answer why. Ice-sheet reflectivity and carbon dioxide, which ranged from about 180 to 280 ppm in past glacial cycles, amplified the orbital changes.
Ice ages and us
Earth's tilt is slowly getting smaller.
But people have added lots of carbon dioxide to the air, and the world is getting warmer.
NASA says Earth's tilt, now 23.4 degrees, is slowly decreasing. Without people's influence, scientists say Earth's orbit today points to slow cooling, continuing a trend that began 6,000 years ago.
But people have added a lot of carbon dioxide to the air by burning fossil fuels, and Earth is warming instead. NASA reports carbon dioxide reached 421 parts per million in 2023. Climate models show the orbital nudge is overwhelmed when carbon dioxide is above about 350 parts per million.
Obliquity is 23.4° and slowly decreasing, with its minimum about 10,000 years from now (NASA). NASA states that without human influences, Earth's current orbital positions predict cooling, continuing a long-term trend that began 6,000 years ago.
Human emissions have raised atmospheric CO₂ from about 280 ppm to 421 ppm (2023). NASA reports that climate models indicate Milankovitch forcing is overwhelmed when CO₂ exceeds about 350 ppm, so orbital cycles cannot explain present warming, and a glaciation in our time is an idea for this experiment, not a forecast.
Amazing facts
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Today Earth's North Star is Polaris. Long ago, different stars had that job!
Because Earth's axis slowly wobbles, the North Stars change. Today they are Polaris and Polaris Australis; a couple of thousand years ago they were Kochab and Pherkad.
Axial precession, a cycle of about 25,771.5 years, changes the pole stars: today Polaris and Polaris Australis, a couple of thousand years ago Kochab and Pherkad (NASA).
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Earth is a little closer to the Sun in January than in July.
Earth is closest to the Sun around January 3 and farthest around July 4. The difference is about 5.1 million kilometers.
Perihelion (about January 3) and aphelion (about July 4) differ by about 5.1 million km, so about 6.8 percent more sunlight reaches Earth in January than in July (NASA).
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Wild horses and camels lived in North America in the ice ages.
Native horses and camels once roamed the plains of North America during the Pleistocene. The horses living there today descend from animals brought from Europe.
Native horses and camels lived in Pleistocene North America; today's North American horses descend from animals brought from Europe in historic times (UCMP).
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Scientists learn about old ice ages from mud at the bottom of the sea.
In 1976, scientists studying mud cores from the deep sea found that Milankovitch cycles matched big climate changes over the past 450,000 years.
A 1976 Science study by Hays et al. used deep-sea sediment cores to show Milankovitch cycles correspond with major climate changes over the past 450,000 years (NASA).
Words to know
- ice age
- a long cold time when ice sheets and glaciers cover much more of Earth
- interglacial
- a milder time between ice ages, like the one we live in now
- ice sheet
- a huge, thick layer of ice that covers a large area of land
- Milankovitch cycles
- slow changes in the shape of Earth's orbit, the tilt of its axis and the slow wobble of its axis (precession)
- land bridge
- a strip of land that joins two places that are now separated by sea
- Pleistocene
- the time in Earth's history when the most recent ice ages happened
Discovery challenge
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At the coldest time of the last ice age, how much lower was the sea?
About 20,000 years ago, at the peak of the last ice age, how much lower was sea level than today?
USGS gives global sea level at the Last Glacial Maximum as how far below today's?
- About 125 meters
- About 1 meter
- About 10 kilometers
Show the answer
About 125 meters
About 125 meters lower!
About 125 meters, because so much water was locked up in ice on land.
About 125 m, as ocean water was stored on the continents as ice (USGS).
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How long does Earth's tilt take to change and change back?
About how long is the cycle of change in the tilt of Earth's axis?
The obliquity cycle spans about…
- About 41,000 years
- About 41 years
- About 41 million years
Show the answer
About 41,000 years
About 41,000 years. Very slow!
About 41,000 years. The tilt is now 23.4 degrees and slowly decreasing.
About 41,000 years, between 22.1° and 24.5° (NASA).
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What land joined Asia and Alaska in the last ice age?
Which land joined Siberia and Alaska when the sea was low in the last ice age?
Low glacial sea level exposed which land connection between Asia and North America?
- The Bering Land Bridge
- The Isthmus of Panama
- The Sinai Peninsula
Show the answer
The Bering Land Bridge
The Bering Land Bridge!
The Bering Land Bridge, part of a huge area called Beringia.
The Bering Land Bridge within Beringia, emerging about 35,700 years ago (Princeton, 2022).
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Which big animal lived during the ice ages?
Which large mammal lived during the Pleistocene, the time of the most recent ice ages?
Which of these was part of the Pleistocene megafauna?
- Mammoth
- Tyrannosaurus rex
- Stegosaurus
Show the answer
Mammoth
The mammoth!
The mammoth. The other two are dinosaurs, not Pleistocene mammals.
Mammoths, alongside mastodons and giant ground sloths (UCMP).
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What kind of time do we live in now?
What does NASA say Earth is in right now?
In glacial terms, NASA describes the present as…
- A milder time between ice ages
- The coldest part of an ice age
- A time with no ice anywhere
Show the answer
A milder time between ice ages
A warmer time between ice ages.
An interglacial, a milder time between ice ages, which began about 10,000 years ago.
An interglacial period (NASA); USGS dates the Holocene interglacial from about 10,000 years ago.
Experiments to try
- Watch Try it What If? Lab What if Earth were covered in ice? White ice bounces sunlight back to space, so an icy planet soaks up less of the Sun's warmth. In our energy-balance model, making Earth reflect 60 percent of its sunlight instead of 30 percent drops its average temperature from about 15 °C to about −23 °C.
- Watch What If? Lab What if the continents joined together again? Earth's outer shell is broken into plates that move a few centimeters a year. Long ago the continents were joined in one supercontinent called Pangaea, which USGS says began to break up about 225 to 200 million years ago. Imagine the continents joining together again, and follow what might change for land, climate and animals.
- Watch What If? Lab What if the air were full of dust? Every year, winds lift millions of tons of dust from the Sahara Desert and carry it across the Atlantic Ocean. Dust and other tiny particles in the air can color sunsets red, make the sky hazy and get into our lungs. Imagine air that was always full of dust, and find out why clean air matters.
- Watch Try it What If? Lab What if Earth’s tilt changed? Earth's axis leans 23.4 degrees, and that lean is what gives us seasons. Straighten Earth up in our sunlight model and the Sun climbs to the same height every noon, every day lasts 12 hours, and summer and winter disappear.
- Watch Try it What If? Lab What if glaciers melted? Glaciers are huge, slow-moving masses of ice on land. When land ice melts, its water runs into the ocean and raises the sea. In our model, melting every mountain glacier on Earth would lift the sea by about 40 centimeters; scientists' best estimate is a little less, about 32.
- Watch What If? Lab What if the Sahara turned green? Thousands of years ago, the Sahara Desert was a land of grass, wetlands and lakes, where cave paintings show hippos in watering holes. A slow wobble of Earth on its axis brought stronger monsoon rains. Find out why the Sahara was green, why it dried, and what might change if it turned green again.
Explore next
- Watch Try it What If? Lab What if Earth were covered in ice? White ice bounces sunlight back to space, so an icy planet soaks up less of the Sun's warmth. In our energy-balance model, making Earth reflect 60 percent of its sunlight instead of 30 percent drops its average temperature from about 15 °C to about −23 °C.
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Watch
Animal
Muskox
Muskoxen are shaggy Arctic grazers with long coats that nearly reach the ground and a super-warm underwool called qiviut. Herds protect their calves by standing in a ring with their horns facing out. Despite the name, they are more closely related to goats and sheep than to oxen.
- Watch What If? Lab What if the continents joined together again? Earth's outer shell is broken into plates that move a few centimeters a year. Long ago the continents were joined in one supercontinent called Pangaea, which USGS says began to break up about 225 to 200 million years ago. Imagine the continents joining together again, and follow what might change for land, climate and animals.
- Watch What If? Lab What if the air were full of dust? Every year, winds lift millions of tons of dust from the Sahara Desert and carry it across the Atlantic Ocean. Dust and other tiny particles in the air can color sunsets red, make the sky hazy and get into our lungs. Imagine air that was always full of dust, and find out why clean air matters.
- Watch Try it What If? Lab What if Earth’s tilt changed? Earth's axis leans 23.4 degrees, and that lean is what gives us seasons. Straighten Earth up in our sunlight model and the Sun climbs to the same height every noon, every day lasts 12 hours, and summer and winter disappear.
- Watch Try it What If? Lab What if glaciers melted? Glaciers are huge, slow-moving masses of ice on land. When land ice melts, its water runs into the ocean and raises the sea. In our model, melting every mountain glacier on Earth would lift the sea by about 40 centimeters; scientists' best estimate is a little less, about 32.
- Watch What If? Lab What if the Sahara turned green? Thousands of years ago, the Sahara Desert was a land of grass, wetlands and lakes, where cave paintings show hippos in watering holes. A slow wobble of Earth on its axis brought stronger monsoon rains. Find out why the Sahara was green, why it dried, and what might change if it turned green again.
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Watch
Natural wonder
Niagara Falls
Niagara Falls is a set of three waterfalls on the Niagara River, which flows from Lake Erie to Lake Ontario across the border between Canada and the United States. About 500 waterfalls in the world are taller, but Niagara is famous for its height together with the enormous amount of water that pours over it.
For parents & teachers
What children will learn
- Ice ages come and go over tens of thousands of years, paced by slow changes in Earth's orbit and tilt.
- In the last ice age, thick ice sheets covered much of North America and sea level was about 125 meters lower.
- Low seas uncovered land bridges such as Beringia, between Asia and Alaska.
- Today, human carbon dioxide outweighs the slow orbital nudge toward cooling.
Recommended ages: 9 to 17 · About 15 minutes
Talk about it
- Why does water locked in ice on land make the sea lower?
- How do you think scientists can learn about an ice age that happened 20,000 years ago?
- Why can this experiment not say when the next ice age might come?
Try at home or in class
- Try NASA's land ice versus sea ice activity (My NASA Data): in two clear tubs, press clay into one side as land, add water, put ice cubes on the clay in one tub and in the water in the other, and mark the water levels as the ice melts. Talk about the reverse: in an ice age, water leaves the sea to become ice on land.
- Make a paper-strip timeline at 1 centimeter for every 1,000 years: mark the Bering Land Bridge appearing (about 35,700 years ago, Princeton), the coldest part of the last ice age (about 20,000 years ago, USGS) and the start of our warmer time (about 10,000 years ago, USGS).
Key vocabulary: ice age, interglacial, ice sheet, Milankovitch cycles, land bridge, Pleistocene
Where these facts come from
- Milankovitch (Orbital) Cycles and Their Role in Earth's Climate , NASA Science
- Why Milankovitch (Orbital) Cycles Can't Explain Earth's Current Warming , NASA Science
- Sea Level and Climate (Fact Sheet 002-00) , U.S. Geological Survey
- Washington Glacial Erratics: Science , University of Washington, Department of Earth and Space Sciences
- Research offers unexpected insights on the emergence of the Bering Land Bridge , Princeton University
- The Pleistocene , University of California Museum of Paleontology
- What's Causing Sea-Level Rise? Land Ice Vs. Sea Ice , NASA My NASA Data
Checked, not yet human-reviewed. Last updated October 24, 2026. Spotted a mistake? Tell us.