Next Ice delayed?

A new paper led by Potsdam authors has received much publicity because it claims that human emissions will delay the next ice age for 100,000y. This is based on model simulations which predict another glacial period starting naturally only in 50,000y time.

Using an ensemble of simulations generated by an Earth system model of intermediate complexity constrained by palaeoclimatic data, we suggest that glacial inception was narrowly missed before the beginning of the Industrial Revolution. The missed inception can be accounted for by the combined effect of relatively high late-Holocene CO2 concentrations and the low orbital eccentricity of the Earth7. Additionally, our analysis suggests that even in the absence of human perturbations no substantial build-up of ice sheets would occur within the next several thousand years and that the current interglacial would probably last for another 50,000 years. However, moderate anthropogenic cumulative CO2 emissions of 1,000 to 1,500 gigatonnes of carbon will postpone the next glacial inception by at least 100,000 years. Our simulations demonstrate that under natural conditions alone the Earth system would be expected to remain in the present delicately balanced interglacial climate state, steering clear of both large-scale glaciation of the Northern Hemisphere and its complete deglaciation, for an unusually long time.

Currently the earth’s orbit is in a low eccentricity cycle which is similar to the one 400,000 years ago. We can therefore make an analogy with that interglacial to determine when this one will end. The growth of northern ice sheets begins when summer insolation is insufficient to completely melt back winter expansion, starting within the arctic circle. An expanding ice surface reduces albedo thereby leading to less absorption of solar energy. This ice albedo feedback effect accelerates the growth of ice sheets, reducing CO2 levels until another ice age begins. Something else is needed to end deep glaciations. A likely mechanism is that described in the previous post. Low CO2 levels lead to the death of boreal forests, soil erosion and dust storms which deposited on the ice sheets break the ice-albedo feedback loop.

We are now 15,000 years into the current interglacial (12,000 if you exclude Younger Dryas). What will cause it to end ? The answer is a large reduction in summer insolation inside the arctic circle. Currently the precession of the equinox is unfavorable for northern summers since it coincides with aphelion (largest distance from the sun). We are lucky that the earth’s orbit is close to circular and obliquity low so that the effect is rather small, although the earth has indeed cooled a little since 8000 years ago.  We narrowly avoided another glaciation a couple of hundred years ago. Now polar summer insolation is beginning to increase again.

NEXT-ICE-AGE

Figure 1

The northern hemisphere will naturally warm slightly for about another 4000 years before another dip in summer insolation will slide us back into another glaciation. We can see when this will happen by comparing the current interglacial to that 400,000 years ago. The earth eccentricity follows a 420,000 year ‘amplification’ cycle superimposed on the 100,000 year cycle. Ice ages tend to follow the 100,000 year cycle but their strength depends on the super-cycle. About 400,000 years ago we had a similar pattern of eccentricity to the current one leading to weaker precession peaks. Changes in obliquity are constant and simply increase/decrease net radiation at both poles and extend the arctic/antarctic circles.

The arrow in figure 1 shows the interglacial 400Ky ago compared to average June/July insolation at the north pole (blue curve). Note how similar it is to the Holocene.  It ended when summer insolation fell below 520 W/m2. Currently  summer insolation is ~540 W/m2. Another precession dip will occur in about 15,000 years time when summer insolation will again fall below 520 W/m2 reaching 515 W/m2. This is when the next ice age is most likely to begin. The Potsdam authors of the new paper seem to want to ignore this dip preferring instead to wait another 50,000 years for a slightly bigger drop of ~511 W/m2.  The aim of their paper though is to  claim this too will be delayed (in their model) because we have pumped too much CO2 into the atmosphere !

 

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8 Responses to Next Ice delayed?

  1. Bob Peckham says:

    I am very disappointed in this news. I had hoped to see the Alpine glaciers replenished during my lifetime. It now seems there is little hope…..

    More seriously, there is a subtle difference between your insolation plot and that of Ralph Ellis, in that he plots it at 65 degrees North while you plot the values at the North Pole. Maybe it makes more sense to plot it at 65 degrees North because that is nearer to where the crucial sensitive processes are happening ? That might give a different estimate of when the next ice age will happen.

  2. Clive Best says:

    Bob,

    Everyone plots insolation at 65N so I decided to plot it at the pole. It doesn’t actually make much difference as the shapes are almost the same, but in June the pole gets more sunshine than the equator let alone 65N. However I agree that melt back is more likely at lower latitudes.

    Don’t worry there is still lots of snow in the Alps and the Rhone Valley glacier is still very much there.

    Cheers
    Clive

  3. Javier says:

    I am surprised you did not include the obliquity cycle in your analysis. According to some researches, and I fully agree, the obliquity is the main determinant for glacial inception and termination. Prior to the Mid-Pleistocene transition, glacial cycles faithfully followed the obliquity cycle. Since then only one every two or three obliquity cycles is capable of getting the world out of a glaciation. We still have 11,000 more years of a reduction in obliquity that is reducing every year the total amount of insolation that high latitudes receive.

    Also the best astronomical analogue to the Holocene is MIS19, a short interglacial 777,000 years ago.

    In a few thousand years glacial inception is likely to take place. I understand some scientists are going to publish the more popular opposite view and get away with it. I don’t think they will be cited as wrong in a few thousand years, or that they would care.

    • Clive Best says:

      Obliquity is included in the calculation of the insolation at the north pole (65N).

      I agree that changes in obliquity is the main driver of glacial cycles from 3 million years ago until about 800,000 years ago. At large obliquity the poles are tilted more towards the sun in summer months leading to larger incident solar radiation at the poles. However, it also means less insolation in winter, because the arctic circle increases in size. So the changes in seasons are exaggerated.

      If the earth’s orbit was circular (eccentricity= zero) then the precession term would have no effect. Eccentricity is important because it modulates the 23,000 precession cycle. Northern summer insolation is greatest when the summer equinox coincides with perihelion of the earth-sun orbit and maximum obliquity. You then expect the ice sheets to melt back for an interglacial. Except this does not always happen and that is the mystery.

      The last glaciation terminated when CO2 levels reached as low as 180 ppm so that plant life can barely survive. There also was extremely low annual snow fall so the build up of dust was a multi-annual process, especially at the edges of the ice-sheets. This reduces albedo increasing thermal heating in summer.

      • Javier says:

        Clive,

        Occam’s razor states that the simplest explanation is the most probable, and if you already have an explanation for 1.8 million years of Ice Age, that explanation is probably true also for the remainder 0.8 million years.

        This figure here is a Gabor fourier transformation of the 65°N summer insolation calculation side by side with a Gabor fourier transformation of the temperature data from Epica Dome C for the last 800,000 years:

        As you can see precession changes in insolation do not translate well into temperature changes, while the signal is a lot stronger for the obliquity band.

        Science of Doom also has a very nice article on the issue, where you should check figure 2, that shows how total energy is shifted on an annual basis from the poles to the tropics and from the tropics to the poles during the obliquity cycle.
        http://scienceofdoom.com/2014/01/30/ghosts-of-climates-past-fourteen-concepts-hd-data/

        Peter Huybers has several articles explaining his theory of obliquity driving glacial cycles.

        Huybers, P. 2007. Glacial variability over the last two million years: An extended depth-derived agemodel, continuous obliquity pacing, and the Pleistocene progression. Quat. Sci. Rev. 26 37-55.

        Huybers, P. and Wunsch, C. 2005. Obliquity pacing of the late Pleistocene glacial terminations. Nature 434 491-494.

        To me there is very little doubt that it is indeed the case, as obliquity solves three outstanding problems with 65°N summer insolation current explanation:
        – It solves the 100 kyr problem. There is no 100 kyr periodicity, just multiples of 41 kyr.
        – It solves MIS5 termination problem, as obliquity agrees with Devil’s hole and with Bermuda’s sea levels.
        – It solves the anti symmetry problem. Obliquity is symmetrical, just like the glacial cycle.

        • Clive Best says:

          Javier,

          The problem is that obliquity alone cannot explain the last 800,000 years. I once made a fit to all 5 million years of Benthic d18O data and indeed the 41 ky term dominates long term

          However for the last 900,000 years there is no escaping the fact that eccentricity now plays a major role. In reality it is not eccentricity itself that matters but rather the amplification of the precession term.

          You always need a maximum northern summer insolation at maximum obliquity AND closest distance to the sun. Yet you still need a catalyst, especially now and 400,000 years ago, therefore dust makes a lot of sense.

          • Javier says:

            As long as you ignore the little details that don’t match, Clive. For example at termination II (MIS5), where the glacial termination was already well underway before the rise in northern summer insolation.

            According to insolation, the Eemian or MIS 5 should have started at the earliest 135 Kyr ago, however data from crystals in a Nevada cave named Devils Hole in 1992 indicated that by that date glacial termination was essentially finished (Winograd et al., 1992; Ludwig et al., 1992. Glacial termination is defined as the mid point in sea level between glacial and interglacial). A great controversy erupted over that data in the literature and has not abated since. But Devils Hole data is not alone, as similar data has been uncovered from coral reefs in the Bahamas (Gallup et al. 2002), Barbados and Papua New Guinea, and from Iberian-margin sediments and Italian cave speleothems (Drysdale et al. 2009), and all of it indicates that termination was essentially completed by 135 Kya, a date when 65°N summer insolation was still below the levels of 70% of the previous 100 kyr. Additional data indicates that MIS 5 may not be the only glacial termination where the effect appears to precede the cause. The problem is further complicated because summer insolation has been used as a defining criteria to date the start and end of glaciations in the sediments in the officially UN sponsored SPECMAP series. This causes circular reasoning arguments when supporting that insolation sets the glaciations and terminations when it has been used to date them.

            Drysdale, R.N. et al. 2009. Evidence for Obliquity Forcing of Glacial Termination II. Science 325 1527-1531.
            Gallup, C.D. et al. 2002. Direct determination of the timing of sea level change during Termination II. Science 295 310-313.
            Ludwig, K.R. et al. 1992. Mass-Spectrometric 230Th-234U-238U Dating of the Devils Hole Calcite Vein. Science 258 284-287.
            Winograd I.J. et al. 1992. Continuous 500,000-Year Climate Record from Vein Calcite in Devils Hole, Nevada. Science 258 255-260.

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