• Metaphysician Undercover
    10.2k

    We can draw an example from weather forecasting. The accuracy of the long term forecast is highly dependent on the accuracy of the short term forecast. Consider a seven day forecast. If the first 24-48 hrs. is incorrect, by even a small factor which is somehow missed, this will likely make the later portion completely irrelevant so that it's not even close. But if the first 48 hrs. hold true to the forecast, the later period will likely just need small changes. What this indicates is that a very slight, unforeseen change, in the very near future will render any long term model which does not account for it, completely useless. The closer in time to the beginning point of the forecast period, the unforeseen factor is, the smaller it needs to be, to have a large affect on the accuracy of the long term. This could be like the "trigger point" you referred to earlier.
  • Tate
    865
    What this indicates is that a very slight, unforeseen change, in the very near future will render any long term model which does not account for it, completely useless.Metaphysician Undercover

    True. I want to review some articles on long-range forecasting. It's fascinating stuff. I was hinting earlier about why the latest models are better, because they include previous research into how to weight factors.

    Then I want to discuss one article on the 100,000 year problem and a startling implication of its proposed solution. If the 100,000 year cycle is a result of a change in ocean currents that further cooled the surface of the earth, this means reglaciation is now being triggered by a shutdown of the oceanic heat conveyor, the very same oceanic current that is slowing now due to global warming. No scientist is predicting a shutdown, but if it did, reglaciation would be triggered.

    If nothing else, it's a cautionary tale about being certain of the future of the climate.
  • Metaphysician Undercover
    10.2k

    The general principles involved in ocean currents can be described. First, the prevailing wind at the equator is from the east, due to the spin of the earth, the air doesn't spin along with the earth fast enough to keep up with the earth, so there is a constant wind from the east. This pushes the warm water at the equator, which is the most heated from the sun, toward the west. It naturally curves away from the equator, to the right in the north, and the left in the south, as the Coriolis effect. So in the northern hemisphere, surface water moves northward along the east coast of the large continents, and southward along the west coast This brings heat toward the poles in a way which is sort of like convection, but the principal source of the wind is the mechanical spinning of the earth, rather than a convective wind. This is important, the principal "weather maker" is the mechanics of a spinning body with oceans and an atmosphere, solar heating and convection are secondary.

    The surface currents of the oceans would be a simple process, except the surface water is forced away from the equator by the winds of the spinning body, and there is no such force to bring replacement surface water, from the north and south. That's why it's different from convection which is solely temperature driven. So much of the replacement water, at the equator, has to come from the depths. This creates flows in the depths, and these flows are not well understood. El Nino is known as a slowing of this flow of cold replacement water along the west coast of South America.

    You can see that the real cycle is not a flow of surface water, but the cycling of water from the surface to the bottom, and back to the surface. It is very complicated because there are many layers and the layers are not necessarily vertical, as it is a three dimensional activity. We can imagine principles similar to air movement, but on a slower scale, horizontal winds, vertical convections, and the movement of distinct "air masses" (water masses in this case). None of these activities are well understood.
  • Tate
    865
    Yes!

    It looks like this:

    currents1.jpg
  • Tate
    865

    There's a theory that once we came out of primarily Milankovitch forcing, the 40,000 year cycle, to an orbital forcing cycle of 100,000 years, shutdown of this oceanic heat conveyor became the trigger for reglaciation.

    This means the Northern Hemisphere summer insolation minimum isn't the main trigger, it just lowers the threshold. We're in that low threshold period now, and the ocean currents are slowing down due to global warming.

    This means the forecast could be worse than just global warming. We could be headed for a period of extreme volatility.
  • Tate
    865
    Long range climate modeling has been going on for a couple of decades. It's now made easier by increased computing capabilities and knowledge about the factors involved in reglaciation, but unknowns still abound. Any model starts with some basic assumptions and data from the climate of the past, then variables like pCO2 can be tweaked to give projections.

    A model known as CLIMBER-2 was created in 2005. It assumes that reglaciation is triggered by a minima in summer insolation in the Northern Hemisphere.

    This model predicts an unusually long interglacial even without an increase in pCO2 forcing. With a 5000 Gton increase in CO2, the climate comes out of the glacial/interglacial cycle completely for at least 500,000 years.

    One possible answer as to why the model is predicting such a long interglacial (50,000 y) even with baseline pCO2 might be that the so-called Anthropocene didn't start in the 1800s. It might have started with human agriculture as long as 6000 years ago. Here.
  • Metaphysician Undercover
    10.2k
    It looks like this:Tate

    That map is extremely vague, just showing some generalities. It's not at all useful for any attempt to determine anomalies. The whole concept of "oceanic heat conveyor", is equally vague, and overly generalized. The oceans are always going to convey heat, so long as the earth is spinning, and the sun is heating, that's what the oceans do. What is at issue, if you are talking about a potential trigger point, is minute peculiarities, and changes to how the oceans convey heat. Since the vast majority of oceanic flow is well below the surface, the information is not available to produce an adequate model. That's why El Nino cannot be accurately predicted, it is a feature of the upwelling of cold water, resulting in a change in surface temperature.

    We're in that low threshold period now, and the ocean currents are slowing down due to global warming.Tate

    I really see no reason why warming would cause ocean currents to slow down, because the earth would continue spinning. It could cause changes to them, perhaps even speed some up.
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