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What stopped global cooling?

  1. Mar 29, 2009 #1
    Approx. 14,000 years ago, a large percentage of the globe was covered in ice. Estimates vary from one third to one half of the globe covered. This was the result of a several thousand year build-up. The passing of each winter/century/decade/millenium, would have seen an increase in the ice cover which, in turn, would reflect more of the sun's heat away, resulting in an ever greater rate of ice coverage This process is self perpetuating and a reasonable expectation would be that it would have continued, after all, it had, up to that point. But it didn't! It stopped. With no input from humans, the process started to reverse. Why and how?
     
    Last edited: Mar 29, 2009
  2. jcsd
  3. Mar 29, 2009 #2

    sylas

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    Actually, there have been cycles of glaciation and retreat, called the ice ages, for millions of years. (Though not indefinitely. It seems to be a feature of the present epoch.)

    A few comparatively straightforward calculations show that the process of ice build up is not self-perpetuating in the way you propose. Rather, there is a natural equilibrium level of ice cover for a given external insolation. What you describe is a feedback process. Less absorbed sunlight means more ice; more ice means less asborbed sunlight. This is a positive feedback loop, and you can describe it with a diagnostic quantity called the "gain" of the loop, to amplify the base level of ice cover change expected from an external change in insolation.

    If the gain is greater than one, then you have a "runaway" feedback, which would lead to complete ice cover. That's what you have described... but it doesn't apply here. The natural "gain" is indeed positive, but it is less than 1, which means you still get a bounded equilibrium level of ice cover... not a runaway build up.

    Glaciation and retreat over the last several million years align with changes in Earth's orbit: eccentricity, obliquity and precession. These are called the Milankovitch cycles. With orbital changes, there are also changes in sunlight, and this is almost certainly the initial stimulus for ice ages, and for the intervening interglacials. The interaction is subtle. Over the last million years or so the alignment has been strongest with orbital eccentricty, and for a couple of million years before that there was an alignment with obliquity.

    In any case, the short answer to your question is that changes in Earth's orbit are the stimulus to take us into and out of the ice ages; or at least that is the strong implication of available evidence and theory.

    The ice cover feedback process you describe basically gives some amplification to the small insolation stimulus from the orbital changes. This feedback loop is an important part of explaining the magnitude of temperature change involved.

    In more detail, this all turns out to be a very useful test of the physics of climate in general. Earth's climate is a complex system with many interacting variables; much more than just the ice cover feedback. The Milankovitch cycles basically perturb this system, and the response is useful data on how the variables interact, and on the sensitivity of the whole. Studies of paleontological climate changes triggered by insolation changes tell us a lot about the climate system, which in turn is important for determining responses to other stimuli than merely the Milankovich cycles.

    Cheers -- Sylas
     
  4. Mar 29, 2009 #3
    I believe this happened once. The whole earth was covered in ice and the oceans were frozen down to 1 km deep in some places. It is believed that it was reversed by the release of CO2 and other greenhouse gasses from volcanoes. With little or no vegetation to use the CO2, it slowly built up in the atmosphere until the planet warmed up.
     
  5. Mar 29, 2009 #4

    sylas

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    Quite so! Great example. This is colloquially called "snowball earth", and the most recent possible occurrence is some 635 million years ago. To put that in context, the so-called Cambrian explosion of animal life was about 530 million years ago.

    The snowball model is not universally accepted; but my understanding is that it is becoming more and more a recognized feature of Earth's past. The conventional explanation for how it occurs is indeed a runaway feedback effect.

    Sometime I'll try to put together a post with a highly simplified model of an idealized planet which has some of the features described so far in the thread. That is, of runaway albedo effect leading to total ice cover in some cases (the snowball hypothesis) and with other cases where a sub-critical positive feedback merely amplifies the effects of any perturbation (the current situation).

    As a bit of a foretaste, here is a description of the effects.

    Earth currently intercepts about 342 W/m^2 from the Sun. The albedo is 0.31, which means 0.3*342 = 106 W/m^2 is reflected, and 236 is absorbed. Our surface temperature is, averaged out, about 15C. We radiate the energy from the Sun as thermal infrared radiation, most of which arises high in the atmosphere, where temperatures are much cooler. The characteristic temperature of our thermal radiation is -18C, which corresponds to high in the troposphere.

    Now the albedo of ice and snow is very high. It reflects a lot of light. For example, over the Antarctic the albedo is about 0.8.

    Imagine what would happen if a mad scientist managed to snap freeze the entire Earth, overnight, even right through the tropics. That would be bad... But we'd hope that that the Sun might melt all the ice, starting with the equator and back to current conditions.

    Alas, not so. With the whole Earth having an average albedo of 0.8, the amount of absorbed energy would be 0.2*342, or about 65 W/m^2. The characteristic temperature would be way down below -85C. Even allowing for a significant warming effect from the atmosphere, this would still be well below freezing at the surface, even up to the equator.

    Put another way; given the current sunlight levels, there are two stable equilibria for ice cover. There's what we have now. And there's a snowball.

    Don't panic. A feature of this kind of bistable situation is that the "gain" of the feedback (presently stable, obviously) increases with ice cover. If you can just push the ice cover past the point where the gain goes critical, you set up the runaway effect. It turns out that for the Earth and our present atmosphere, this edge of stablility is way way colder than the ice ages or anything else for hundreds of million of years. So we're safe from the snowball.

    Conversely, once in a snowball state, something has to push past the edge in the other direction. Basically, you just need to heat things up enough to start melting ice in the tropics. That suddenly starts a reverse runaway, which melts all the ice right up to the poles; a huge and sudden shift that dwarfs anything else we can see in the records of paleoclimate. This is indeed what is thought to happen at the end of the snowball periods. It ends abruptly, with the Earth suddenly converted to very warm throughout.

    In control theory, this is called hysteresis.

    Cheers -- Sylas
     
    Last edited: Mar 29, 2009
  6. Mar 29, 2009 #5

    Xnn

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    As the earth cools, the oceans also cool and the level of precipitation decreases. Globally, during the height of an ice age, the earth is much drier than it is now. Eventually the ice sheets cease to grow because they don’t accumulate enough water to offset the amount that which melts and runs off or sublimates.

    However, there are also variation in earth orbit over thousands of year which change the amount total amount of solar heating . This heating can in turn releases CO2 from the oceans and together, these work together to shift the earth’s climate enough to melt the ice to present levels.
     
  7. Mar 29, 2009 #6
    It is not so much the total amount of insolation as it is the distribution. Most of the Earth's land is in the NH. When the SH receives the majority of solar insolation ice sheets begin to grow on the continents in the NH, precipitating an albedo flip that amplifies the negative feedback. The opposite occurs at termination.

    Lack of large land masses in the SH means there is no place for glaciers to grow and therefore no large shift in albedo.
     
  8. Mar 30, 2009 #7
    Thank you for your replies. The natural follow-up is, "What causes the earth's orbit to alter?" Isn't there a law, or rule, regarding inertia that states that with the lack of any external influence, things will pretty much remain the same? And wouldn't this particularly apply to our orbit around the sun?
     
  9. Mar 30, 2009 #8

    sylas

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    There are conservation laws, like conservation of momentum and angular momentum and energy. All of those are properly conserved in the complex motions of a spinning asymmetrical body in an elliptical orbit.

    Over time, the angle of the polar axis, the direction in the sky of the polar axis, the eccentricity of the orbit and the direction of the major axis all shift in regular cycles. All the conservation laws continue to hold, because the Sun is involved as well; and other bodies as well for more subtle effects. See Milankovitch cycles at wikipedia for a good introduction to the major cycles involved for Earth.

    Cheers -- Sylas
     
  10. Mar 30, 2009 #9
    There is external influence. The Earth interacts gravitationally with the Moon, Sun and the other planets.
     
  11. Mar 30, 2009 #10
    Jupiter is the big player in altering the Earth's orbit from circular to elliptical and back again.
     
  12. Mar 30, 2009 #11
    Could planetry alignment be behind such orbital shifts? There must be occasions when we and they all line up on one side, giving us a slight outward tug, and, conversely, when they are all on the opposite side from us, a slight inward pull. Or is this to simplistic? Scientists do seem to prefer the complex. Axial shift effects would appear to be neutral as longer winter days are countered by longer summer days
     
  13. Mar 30, 2009 #12

    Andrew Mason

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    Why do you need land masses to grow ice? Ice floats, after all. The north polar ice cap is floating on the Arctic Ocean.

    AM
     
  14. Mar 30, 2009 #13

    Xnn

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    True.

    During the last glacial maximum, beside the Arctic Ocean, there was year long sea ice in the North Atlantic and Pacific. However, it did not go as far south as that on land. Sea ice tends to break up and float away (towards the equator). So, it is more difficult to establish year long ice at the same latitude as on land.

    Another factor is elevation. For a given amount of ice, the elevation of ice is about 10 times higher when it is on land. When ice forms on water, 90% of it is under the surface. So, land ice is also colder simply because the top of it is at a higher elevation.
     
  15. Mar 30, 2009 #14
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