During the last ice age, how long would it have taken for the ice sheets to form?

Submitted by CDNEmpire t3_11jm4sd in askscience

I’ve read that during the last ice age, ice thousands of meters thick covered Canada and most of the United States.

  1. How long would it have taken for those ice sheets to form? Like a decade? A century?
  • And what is it that happens exactly? Snow falls in the winter, doesn’t quite melt in the summer, and more snow falls on it next winter?
  • Or do we completely lose temperature variants between seasons, and it just stays cold all the time?
  1. What was going on in central and South America? Or Africa? Was it just business as usual for life down there? The regular flow of seasons and such, unaffected by the ice sheets?
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ParatusLetum t1_jb3xrdx wrote

I was briefly a Geology Major long ago so some info may not be up to date.

1a. A quick look at that last glacial maximum shows time frames on 10’s of thousands of years. So the ice would be accumulating for around ~10-15 thousand years then declining to where we see it today possibly.

1b. Sounds about right. The “warm season” is not sufficient to erase the previous winters snow fall. A lot of these cycles seem to work on positive feedback loops. More ice > more light reflected back to space > colder weather > more ice etc. the condition will accelerate its effects until another factor steps in like natural variation in the earths tilt and orbit, or volcanic activity etc. which may cause a warming cycle to begin causing the ice age to diminish. Technically I believe we are still in an Ice Age as we have ice caps where as in the distant past earth did not have such caps year round possibly.

1c. Less significant variations than you may see today across the globe seems to most plausible.

  1. Weirdly most glaciation happens on land and land is mostly in the Northern Hemispheres. The glaciation would of had effects globally. Lowered sea levels had caused England to have more landmass and possibly even connected Australia to New Zealand by a land bridge now submerged. They would of had a wildly different ecosystem than they have now would be my guess. But they may be the reason they have such unique animals today.

All very cool questions. Keep on digging.

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Dan19_82 t1_jb4k80z wrote

The one thing I've never understood is, isn't Antarctica considered a desert because of complete lack of snowfall / precipitation. I've always wondered how anything builds up unless it's incredibly slow.

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PigHillJimster t1_jb4rigr wrote

A dessert is defined as a place where there is less than 25 cm of precipitation per year, so yes, Antarctica is considered a desert.

There are many other regions that fit this description that are very cold.

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Busterwasmycat t1_jb5dqbn wrote

rate doesn't affect the fact that accumulation is happening, what matters is that addition exceeds removal over time. How long it would take to make a thick pile, though, that does matter on rate, or more particularly the size of the difference between addition and removal (high addition rates combined with high removal rates would still result in slow accumulation, same as if both were low). The Antarctic has been accumulating ice for maybe as much as 25 million years, or about there, and it probably wasn't such a desert for all that time.

The earth did not go from "Antarctica is a temperate forest and grassland" to "Antarctica is a frozen desert of ice" overnight.

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7eggert t1_jb4s224 wrote

As I understand, it's rarely snowing because it doesn't evaporate. Off cause a little bit of water vapor will also come from the outside and - because cold air can't contain it - fall down sooner or later. In the other direction there are glaciers transporting the snow / ice back to the ocean.

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BobbyP27 t1_jb4sp2t wrote

>Lowered sea levels had caused England to have more landmass

My understanding is the entire North Sea was basically dry land (Doggerland) during the last ice age, with the Thames joining the Rhine as it flowed down what is now the English Channel to the Atlantic. Apparently fishermen in the North Sea occasionally dig up terrestrial animal bones and the like from time to time.

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KhaelaMensha t1_jb4pemb wrote

Loved your answer! Concise, easy to understand! And then at the end you go full ham and use "would of" instead of "would have". Twice! :D

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mkculs t1_jb4kphh wrote

Thanks you. I love this topic. Op had good questions and you had answers a layperson could understand.

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CDNEmpire OP t1_jb530sz wrote

So if we are still in an ice age, then the gradual increase in global temperatures is all part of that.

So where do scientists draw the line between the global warming caused by humans, and the global warming that’s part of the natural cycle of things?

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Syfer2x t1_jb5d0sr wrote

I think it’s actually fairly easy to do when you correlate the rise in temperature to a rise in human industrial activity. As a previous comment mentioned, these processes are slooooow, like reeeeaally slow. The insane spike in global temperatures since the beginning of the industrial age is far too sharp to be attributed to normal interglacial activity, which I believe technically we should be exiting anyways. We’re becoming overdue for another glacial period but are staving it off and then some just with our own activities.

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CDNEmpire OP t1_jb5m1p5 wrote

That’s fair. I mean we definitely can be helping things by speeding up, or even completely changing the natural process.

The natural balance will eventually be restored. Just a question if we’re still around to witness it

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Jewnadian t1_jb5mw2b wrote

That's not really a guarantee, it's entirely possible to push a planet into runaway greenhouse mode and it never recover to where carbon/DNA based life is possible again.

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GumboDiplomacy t1_jb5s6gl wrote

This is a good visual representation of climate throughout earth’s geological history. Notice how slow most of these changes are, a matter of a couple of Celsius change over millions of years. Then look at the change over the last 200 years.

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SgtExo t1_jb5yz1d wrote

Check out this helpful xkcd timeline, it is a bit old by now and we should be on the optimistic path, if not even a bit better depending on if green tech keeps getting adopted faster. But it is a good way of seeing how stable temperatures have been for the last 20 000 years.

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[deleted] t1_jb5hn6u wrote

Generally the SPEED at which the climate is changing is the problem compared to geological records from past ages similar to our own.

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Busterwasmycat t1_jb5ia6z wrote

Yeah, well, there is the nub of the argument, and a good part of the reason why most scientists accept that humans are affecting climate but many disagree about the importance of the human role. It is also where a lot of uncertainty appears in predicting into the future.

The problem is that there is no "signature" we can measure directly and say "this 23 % is from humans (pick any number), and the other 77% is what nature does".

What we have are climate models. The climate models are an attempt to imitate the natural system behavior, and seeing how different changes to input conditions cause changes to output. The system is complicated, and so are the models.

There has been a lot of work on trying to figure out how much change has been "forced" by human activities versus how much can be explained by natural changes. They can compare models with, or without, changes to various important parameters over time as an input, and see how the results differ, and then interpret what is really important and what is not so important for what is actually happening. Fairly complicated work in actual practice.

It is from playing around with the various parameters that can affect climate, and finding mismatch between observation and the changes that ought to have happened if nature alone is the cause, that a lot of climate scientists have concluded that CO2 is the main problem.

Are they right? We are finding out. Not sure it is wise to perform this experiment in real time, but we are, even if not purposefully.

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forams__galorams t1_jb9cmi2 wrote

> So if we are still in an ice age, then the gradual increase in global temperatures is all part of that

For the last 10,000ish years the Earth has been in an interglacial, ie. the least cold stage of an ice-age. Glacial-interglacial cycles have been occurring for the last 2 million years or so, which is the Quaternary Ice Age.

The increasing global mean temperature in very recent times is on top of being in the warmest part of an ice age, it cannot be attributed to part of the natural cycle. The natural cycle would be due to start cooling sometime in the next few thousand years and transition back to a glacial episode. Anthropogenic warming has eliminated that possibility and the fear is that the Earth could exit Ice Age mode entirely and switch into hot-house mode with virtually no ice at the poles at all, correspondingly higher global sea levels and a lot more energy in the climate system for extreme weather events to become a regularity.

> So where do scientists draw the line between the global warming caused by humans, and the global warming that’s part of the natural cycle of things?

Somewhere around the year 1900. Take your pick, determining an exact date is a bit of a moot point by now.

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forams__galorams t1_jb7rb6y wrote

> 1a. A quick look at that last glacial maximum shows time frames on 10’s of thousands of years. So the ice would be accumulating for around ~10-15 thousand years then declining to where we see it today possibly.

A fair bit too quick for the ice accumulation phases. 10,000-15,000 years sounds more like the length of an interglacial, ie. what we’re in right now. It has always taken several times the length of an interglacial to transition to the next glacial maximum (though transitions back into interglacial are typically more abrupt). Maybe you are reading the graphs the wrong way around? 15,000 years is about the length of the phases in the last million years when going from glacial to interglacial.

Even before the Mid-Pleistocene Transition — before the glacial-interglacial cycle started turning from a 40,000 year cycle into the current 100,000 year cycle — it was taking at least 20,000 years to get from minimum to maximum ice volume.

For the last million years or so we are within the territory of the 100,000 year cycle, with much more drawn out changes (at least going from interglacials to glacials). For instance the previous interglacial ended about 115,000 years ago and the last glacial maximum began about 30,000 years ago.

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pistolpete0406 t1_jbbf63s wrote

Yea but they also melted within a damn decade . Greenland icecore samples for reference . Alot of craziness was occuring . I get it though . Little lag and than above freezing they disappear .

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thatmikeguy t1_jb5j3hd wrote

Is it a coincidence that ice has been close to the magnetic north and south poles? I searched for information, but only found that the ice caps are made up of fresh water, which has a lower electrical conductivity than salt water or rock, and act as a barrier to the flow of electrical currents in the Earth's crust, and this can affect the magnetic field in their vicinity.

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Busterwasmycat t1_jb5prnl wrote

I'm going to take a stab at this. Hope I don't get too complex even though the question is really several questions.

Glacial ice is mostly the result of past snowfall. Ice is what you get when you compact the snow and get rid of all the open space. It is pretty much the same idea as the conversion of mud into rock. The snow undergoes metamorphism in a away, it recrystallizes from pressure (weight) perhaps with some melting, flow of water down into open space, refreezing, as well.

As you know, we live on a seasonal planet. The result is that there is seasonality in precipitation and temperature. These variations cause ice to form distinct layers for each year, very much like how tree rings form and the thickness of the rings reflects what went on with the weather during that year. Some rings (some layers) can be fairly thick, and some are almost non-existent depending on what happened over the course of the year. Dry year, not much snow=thin layer.

Loose snow can be 80-90 percent air, or to put it another way, you get about 10 inches of snow for 1 inch of rain. The conversion is ballpark, not precise. I am sure you have heard something along those lines, and the point being made is that water, liquid water, has a density of about 1 (g per cubic centimeter) but snow has a density of about 0.1. Only 10 percent. When converted to ice, the ice will have a density slightly less than water (why ice floats in water).

So, you lose about 90 percent of the volume even if no melting or sublimation (evaporation but without passing through a liquid state, from solid to vapor directly). Apparently about half that density change from snow to ice happens in the first year. (makes stuff called firn, snow from previous winters that has not converted into solid ice) and the rest happens over several years or more.

That is how we go from snow to glacier. How much snow? Well, that really varies a lot. Some places will only see maybe a meter of snow per year, and other places maybe 10 meters or more, with most places having permanently winter conditions getting something in between.

Lots of places still see some above-freezing or near freezing temperatures so sunlight heats the snow to melting, during part of the year (like say on high mountains) so part of the snowpack gets lost by melting and evaporation/sublimation. So in many regions, the amount of original snowfall that makes it down into ice might only be a small percent of the original, perhaps 10-20 percent. So, if we started with a meter of snow per year, we might only get about 5 cm of ice. Still, even at that slow rate, you could get 5 meters in a century and 500 meters in 10,000 years.

So, how fast does it accumulate? You can get some pretty important changes to ice extent and thickness over the period of a millennium (1,000 years), even when there is not a lot of snowfall, but generally it will be slower even if there is a lot of snow (because of loss through evaporation/sublimation and melt runoff).

Many thousands of years is generally needed to make a good glacier and tens of thousands of years to make an ice sheet (massive glacier). Some ice sheets, like in Greenland, have been drilled and cores removed, going back 100,000 years, and even further back, several hundred thousand years, in the Antarctica ice sheet.

Africa has largely missed on continental glaciation because it is too close to the equator, or too far from the poles. Mountain glaciers and flow onto nearby lowlands is indicated, but no massive ice sheets. Same idea with Australia. South America had a pretty well-glaciated spine almost up to the equator (the Andes mountains are so high that glaciers could be formed even near the equator), but much of the continent did not get glacial cover.

Two reasons why the northern hemisphere got a lot more glacial cover than the southern hemisphere: 1) the Arctic is an ocean so lots of moisture can be transferred to nearby land, and 2) a lot more land in the northern hemisphere is near and in the polar regions than in the southern hemisphere.

The ice sheets in the northern hemisphere mostly never got further south than about 45 degrees N. Most of Australia, Africa, and SOuth America, is further north than 45 degrees south. Basically southern lands a lot further from the south pole than northern lands from the north pole. Apart from Antarctica, of course. Frozen pretty solid down there.

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Sylvurphlame t1_jb6cnan wrote

Nah. I saw that movie. Once the Ice Age starts, it only takes until The Day After Tomorrow. :)

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