The Younger Dryas

Yesterday, in A Theory of Ice Ages, I argued that when the Earth gets covered with ice, the ice acts as a layer of insulation exactly like the clothing on a human body. The underlying rock heats up,  just like your skin warms up after you put on some clothes. And eventually the warm underlying rock may melt the ice above it. And when the layer of insulating ice is removed, the rock beneath starts to cool down again, just like your skin cools when you take your clothes off.

And so the cycle of ice ages is one of rock warming and cooling. During the ice age when the Earth is covered in ice, the rock beneath the ice heats up. And when the ice melts, the rock cools down again. And during the ice age there is a slow loss of heat from the rock, and during the interglacials between the ice ages, there is a high heat loss. And because the heat flows are so small, it takes a long time for the rock beneath the ice to warm up and melt the ice above it,  And it takes a long time for the rock to cool down again. In fact it takes thousands of years.

We’re currently living in an interglacial period that started about 12 thousand years ago. And you might imagine that the ice just melted  and hasn’t returned since.

But actually that wasn’t quite what happened. What actually happened was that the ice melted at the end of the last ice age, and then about a thousand years later it refroze. And it stayed frozen for another thousand years, and then melted again. And this brief return of the ice is called the Younger Dryas. (Dryas is an alpine flower, and I suppose that geologists must find dryas flowers in both the Younger Dryas and the Older Dryas.)

The Younger Dryas appears as a brief sharp dip in the temperature record below:

And it seems to be a bit of a mystery how the ice could melt and almost as quickly refreeze, before finally melting again. The chart above is taken from The Intriguing Problem of the Younger Dryas by Don Easterbrook, in which he writes:

The cause of these remarkably sudden climate changes has puzzled geologists and climatologists for decades and despite much effort to find the answer, can still only be considered enigmatic.

Is there any possible simple explanation of this in terms of heat flows like those I was considering yesterday? I think there might be.

In the first place, 15,000 years ago saw the end of an ice age that had lasted about 100,00o years. And that means that the rock beneath the ice had been slowly warming up for 100,000 years. And it had quite possibly got very hot. I’m not going to guess what temperature it reached, but I can well imagine that it could well have been hot enough not just to melt ice, but to boil water as well. There may well have been steam coming out of cracks and crevasses in the ice sheet.

These hot rocks would seem to have been melting the overlying ice pretty rapidly. And about 15,000 years ago, it seems they melted it all.

So what happened then? Well, the hot rock now became exposed to the cold air in the atmosphere, now that it had lost its layer of insulating ice. And the air temperature during the ice age seems to have been much colder than it is today. And so the hot rock would have started losing heat very rapidly to the cold air. And the effect of this was to warm up the air, and cool down the rock.

In fact the temperature of the rock could have plummeted very rapidly as it lost heat. So rapidly, that within a thousand years or so, it had fallen below the freezing point of water.

And when that happened, the ice started to rapidly build up again. The Younger Dryas had begun.

So now you’ve got cold rocks underlying the ice. And these rocks start to heat up again, now that they’ve got a blanketing layer of ice over them again.

But this time, instead of heating up slowly, the cold rock heats up rapidly. And it heats up rapidly because while the surface rock may have fallen below freezing, the rock beneath it was still very hot after those 100,000 years beneath the ice.

And so, in next to no time (about a thousand years), the ice that had just formed started melting again. And pretty soon it had all melted. And the Younger Dryas came to an end.

But, unlike at the end of the ice age, the rock that emerged into the upper air wasn’t quite as hot as it had been. It had lost a lot of heat during the first brief interglacial. And so when the the second interglacial period began, the rock was a lot cooler. And maybe the air in the atmosphere was warmer as well. And so this time, the rock lost heat rather more slowly to the atmosphere. And it didn’t cool down so rapidly as it had the first time.

And, crucially, it didn’t fall below freezing. So the ice didn’t start reforming again to create what would have been, I suppose, the Even Younger Dryas.

The rock temperatures at various depths below the surface are shown below (a bit roughly) with dark red for cold rock, light red for hot rock:

During period A the ice age is coming to an end, and there are very hot rocks under the ice. During period B, the ice has vanished, and the underlying rock is cooling as it loses heat to the atmosphere. Eventually the surface rocks fall below freezing point, and the ice reforms at the start of period C, the Younger Dryas. And then the underlying surface rock rapidly heats up again during the Younger Dryas, because there are still very hot rocks lying deeper beneath it. The ice then melts at the end of the Younger Dryas, but not quite as quickly as it did at the end of the ice age, because the rocks beneath the ice are cooler than they were back then. And we then enter the current modern (Holocene) period D, where the surface rocks are warmer than they were in the first interglacial B, but they are slowly cooling.

In summary: At the end of the ice age, the underlying rocks were very hot, and the atmosphere very cold, and this made for very large heat flows, and very rapid accompanying temperature changes – from freezing to warm to freezing and finally warm again. But by the end of it all, the hot rocks had cooled a lot, and the heat flows were smaller, and temperatures changed more slowly.

Anyway, that’s my guess. And it’s just a guess. I haven’t tried to build a heat flow model of it yet.

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18 Responses to The Younger Dryas

  1. Fred says:

    Interesting idea. Have you any knowledge of geology / geography? It seems logical that at the height of an ice age, when the insulated interior is hottest, convection currents in the magma would increase, speeding up the movement of tectonic plates that has been relatively sluggish.

    I have no idea what tools one needs to investigate a potential correleation, continental drift being measured in hundreds of millions, not thousands, of years – but maybe you can think of something?

  2. Rose says:

    I don’t know if this is of any interest but last night, I caught a bit on TV showing how this video had been filmed. The thing that particularly struck me, having read about your hot rocks and insulating ice that day, was that while they were filming they noted that it was -20 above the ice and 4 degrees underneath it.

    None of this is in the finished video but here it is anyway.

    Risky dinner: Foraging for food under the frozen sea ice
    “For a few days each year, Inuits in northern Canada risk their lives to gather mussels under the frozen sea ice.”

  3. Peter Carter says:

    Are you telling me that if I half fill a polystyrene vessel with concrete, say, with a thermostatically controlled heating element embedded in it (the thermostat monitoring the element’s temperature to simulate the earth’s core being at a constant temperature), and then put a layer of water on top of it and stick it in a (thermostatically regulated?) freezer, I will see the ice periodically thawing out and then freezing? My gut expectation is that it would just reach an equilibrium, between ice thawed and frozen depending on the heating element temperature. What is the ‘magic’ that makes it oscillate?

    • Frank Davis says:

      if I half fill a polystyrene vessel with concrete, say, with a thermostatically controlled heating element embedded in it (the thermostat monitoring the element’s temperature to simulate the earth’s core being at a constant temperature), and then put a layer of water on top of it and stick it in a (thermostatically regulated?) freezer, I will

      ,,,get a very nasty electric shock?

      I don’t understand your experiment at all,

      • Peter Carter says:

        “I don’t understand your experiment at all”

        Earth = ball of rock with hot core with enough gravity to hold liquid water all around it.
        Bucket made of polystyrene full of concrete with heater in middle = ‘earth’, but with just a section of surface exposed. Water on surface simulates a layer of ocean. (If I could make a ball surrounded by water clinging to it all the way around, I would).

        Earth surrounded by layer of cold air.
        Bucket in freezer surrounded by cold air.

        If this doesn’t resemble your model, where am I going wrong?

        • Frank Davis says:

          an equilibrium, between ice thawed and frozen

          I don’t think there is such an equilibrium. Water is either in one state or it’s in the other. It’s either liquid (water) or solid (ice). Depending on whether it’s gaining or losing heat it will change from one state to the other.

          So if you have a drop of water sitting on a surface which is at -1 degree C, it will turn ibto ice. And conversely, if you have a lump of ice sitting on surface at +1 degree C, it will turn to water.

          In the case of the hot rock, the layers of ice above it turn to water, one by one, and the water runs out from underneath it in subglacial streams, and plays no further part in matters. Only the still-frozen ice remains.

  4. Peter Carter says:

    For this to happen, the ice effectively has to have some sort of hysteresis effect..? I think it’s that aspect that I’m still not clear about.

    Would my freezer-based model ‘work’?

    • Peter Carter says:

      Or am I missing the effect of the ice completely melting away but then re-appearing due to ‘precipitation’ rather than just sitting there as a layer of water with ice on the outside? Would my simulation work if the outer surface was a mixture of heights with some water and some ‘land’, perhaps with a sheet of cellophane over the top?

    • Frank Davis says:

      I think you’re conducting an extremely complex thought experiment (I take it that you haven’t actually constructed this apparatus). I think it’s far too complicated for me to have any idea what would happen.

      Of course I construct my own thought experiments, and get to convince myself of one silly idea or other that way. But in the end, I usually have to admit to myself that I haven’t a clue what’s likely to happen. And that’s when I’ll usually build a model (usually a computer simulation model, sometimes a physical model), and get that to tell me what’s going to happen. You put in all the forces or currents or heat flows, and you watch what happens when it starts working.

      That’s what I did with my asteroid model. After thinking about it, and not knowing what would happen, I built a computer simulation model, put in the heat flow physics, got hold of some plausible data, and set the thing running, and watched what happened. And learned from it. And then described what had happened in my blog.

      And I think I learned a lot. I could see where I’d gone wrong with my thought experiment, and where I’d got it right

      My advice to anybody who wants to look at this stuff is: build a model. Even a very simple one.

      • Peter Carter says:

        I’m sure you’re right (and no, I haven’t built the model – but I’d love to see it work!). It doesn’t ring true to me that it should work at a global scale, but not at desktop scale.

        If I were to build an electronic SPICE model, it might resemble a relaxation oscillator of some sort. Basically a voltage source charging up a capacitor and then some component that starts discharging it once a certain threshold is reached and doesn’t switch back until a lower threshold is reached. It is that component that interests me. How does ice/water/air perform that function?

        • Frank Davis says:

          40 years ago I was working with an electric analogue heat flow model. It can be done! Because I used to do it.

          You use voltage as a proxy for temperature, resistance as a proxy for thermal resistance, capacitance as a proxy for thermal capacity, and current as a proxy for heat flow. You could build your own earth with a chain of resistors and capacitors, with one end at a voltage representing the temperature of the Earth, and the other end at ground representing outer space, and you could add in resistors with a switch to represent ice sheets, and remove them when they rose above whatever 273K is in voltage terms, in order to represent them melting and running away. And if you had a storage oscilloscope, you could watch what happens to temperatures in the Earth. And you could add in solar heat gains at the surface of the Earth (or glaciers) And you’d have a real dynamic simulation model, that would tell you how the Earth behaves. And it would be faster than my digital simulation model.

          I’ve drawn the circuit diagram:

          It would probably cost just a few pennies.

          Here’s a pdf paper on analogue heat flow models. There are lots of them.

  5. garyk30 says:

    Wonder if the compression of the rock by the weight of the ice above it would have had some impact.

    As the rock was compressed it would gain in temperature; thus, adding to the ambient heat level.

    • Frank Davis says:

      I haven’t allowed anything for the pressure in rocks (although I can calculate it if I want to). As far as I know, the physical characteristics of most materials don’t change very much with increasing or decreasing pressure (although this is not true with gases).

      As for a gain in temperature due to compression, when materials are compressed they gain in stored internal energy, but if this energy is released as heat, it will only be while energy is being added. So I can imagine that while a tall building is being constructed, and the brick walls are being compressed, they might heat up a bit, but this heat would be rapidly conducted away. The walls wouldn’t stay hot. If the subsurface rocks in the crust off the Earth are warmer than those at the surface, it’s not because they’re compressed, but because there’s a heat flow from the hot core of the Earth.

    • smokingscot says:

      @ Gary

      You have a real and very valid point. Various areas of Norway, Sweden and Iceland are experiencing a part of the consequences of gigantic weights from glaciers that used to lie upon rhe land.

      Land levels are rising!

      This article gives the cause and effect,

      Whether this resulted in an increase in temperature at the surface is not discussed, however – as a lay person – my guess is there are bound to be hot spots as the earth’s crust is not uniform.

  6. Pingback: An Electrical Analogue Model of Ice Ages | Frank Davis

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