Interglacial Comparisons

Most people don’t realize that the Earth is still in a long term ice age that started about three million years ago and has had many alternating cold glacial periods interspersed with warmer interglacial periods.  We are currently in an interglacial period where global temperatures have been near our modern “normal” for about 12,000 years now.  In addition to our current interglacial period, there have been four previous interglacial periods in the last 500,000 years.  Each one has been spaced about 100,000 years apart and lasted about 2,000 to 25,000 years with temperatures at or above our current modern “normal”.  These observations are based on the EPICA Antarctic ice core climate reconstruction using oxygen isotope ratios as a proxie for global temperature change.

The graph below shows the current and last four interglacial periods plotted together, normalized to the year where the estimated global temperature first reached the level of our modern “normal” climate. The approximate year where each interglacial episode first reached the modern “normal” temperature is shown in the legend.  Notice that all four previous interglacials had global temperatures reaching 2 to 4 degrees Centigrade higher than our current modern “normal” without any help from humans, based on this reconstruction.

Interglacial Period Comparison

This graph compares the current interglacial period with the previous four interglacials using the EPICA ice core climate reconstruction. Each interglacial period has been normalized to the time the global temperature departure first reached the current “normal” temperature. Click on graph to enlarge.

I find it amazing how abruptly and similarly each glacial period ended in about 5,000 years to start each following interglacial warm period.  In contrast, the duration of the interglacials has been much more variable. The most recent previous interglacial period that started about 130,000 years ago lasted about 14,000 years at temperatures at or above our current modern “normal”. The second previous interglacial lasted about as long as our current interglacial while the third previous was by far the shortest, lasting about 2,000 years, but arguably had somewhat of a double peak. However, the latter secondary peak did not quite reach the warmth of our current modern “normal”. The fourth previous interglacial which started about 418,000 years ago was by far the longest at about 25,000 years.

As repeatable as the glacial cycles have been over the last 500,000 years, I see little reason not to expect more of the same in the future. Using this interglacial comparison as a climate persistence forecast, we might expect about a 75% chance that the global average temperature will begin to drop dramatically sometime within the next few thousand years and about a 25% chance of staying warm for another 10,000 years or so … at most. Perhaps we need all the anthropogenic warming we can muster to stall or prevent the next glacial period?

Our understanding of what causes these glacial cycles, which are relatively recent on a geological scale, is still very limited although there are plenty of hypotheses. Our current climate models cannot predict them and therefore to me are somewhat useless. Until we can create climate models that can accurately track past glacial and interglacial periods I will not be too impressed and I certainly don’t believe our infant and untested climate models should be used to shape policy regarding “climate change”.

Update 2016 November

Below is a link to an interesting analysis of the causes of glacial cycles, along with conclusions made by the author, which seem reasonable to me.  The author hypothesizes that evidence suggests that the current interglacial period is likely to be only average in length and therefore should be ending soon, most likely sometime within the next two thousand years.

Nature Unbound I: The Glacial Cycle


1) Obliquity is the main factor driving the glacial-interglacial cycle. Precession, eccentricity and 65°N summer insolation play a secondary role. There is no 100 kyr cycle. Milankovitch Theory is incorrect.

2) The current pacing of interglacial periods is the consequence of the Earth being in a very cold state that prevents almost half of obliquity cycles from successfully emerging from glacial conditions. The rate for the past million years has been 72.7 kyr/interglacial, or 1.8 obliquity cycles between interglacials. This can be generally described as one interglacial every two obliquity cycles except when close to the 413 kyr eccentricity peaks, when interglacials take place at every obliquity cycle.

3) Glacial terminations require, in addition to rising obliquity, the existence of very strong feedback factors manifested as very low glacial maximum temperatures. High northern summer insolation at the second half of the rising obliquity period is a positive factor, and if high enough during eccentricity peaks can drive the termination process.

4) CO2 can only produce a minor effect in glacial terminations since the measured change in concentration (roughly a third of a doubling which represents half of the warming effect of a doubling) is too small to account for any important contribution to the large observed temperature changes.

5) Since the precession cycle has bottomed and the obliquity cycle is half way down we should expect the next glacial inception to take place within the next two millennia.

14 responses to “Interglacial Comparisons

  1. 75% chance of glaciation is a far more serious danger than a little bit of warming. On ice core Murry Salby incorporating diffusion.
    See also:
    By Pehr Björnbom A comparison of Gösta Pettersson’s carbon cycle model with observations

  2. Yes, a little bit of warming may not be that bad, especially considering that during the most recent previous interglacial period evidence indicates that temperatures were high enough to raise the tree line 2,500 feet (800 m) higher than today in the mountains of Colorado. The benefits of a little warming far outweigh the detriments of another glacial period – which would (will?) be disastrous.

    Colorado mastodon bones show ancient warmer Earth

  3. Thanks for that graph. I found it very interesting and I left a comment about it at CE.

  4. oz4c, far too much of the global warming debate is based on a few recent decades with little attention given to longer term data. Good to see your blog and posts at CE. Michael Cunningham, Brisbane, aka Faustino aka Genghis Cunn

  5. Yes, maybe a bit like focusing on just one tree to describe a forest. Most forests are very diverse. We need a broader perspective, especially if we are going to be making policy about the forest

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  14. Sorry to necro this article, however I would like to add to the conversation. This could be a peak “interglacial” we are entering as evidenced by such ice core samples. According to NOAA’s own website, they peak every 5 periods. Looking at the link below, it certainly looks like we are entering the warmest interglacial. Another interesting trend from their graph is that the preceeding “glacial” period was the worst.

    My belief is that the glacial and interglacial periods are driven by Earth’s orbit. As our star hurdles through the galaxy on its own orbit about the super massive blackhole, the Earth’s own orbit will obviously experience variability.

    That variability elongates from a circle into a elipses or oval. The more compressed oval the orbit is, the closer/further the sun will be to Earth. This would explain why we see a pattern of about 5 interglacial periods with one being the max. Or one of those periods being at the most compressed orbit, then they even out as Earth’s orbit stabilizes.

    The gasses on Earth are a finite resource that will always be present on the face of Earth. Humans will certainly have a very minute impression on these interactions, however our effect takes a bakeseat to inter/intrastellar orbtial physics. The sun is the engine and our orbit is the variability.

    The dinosaurs had an atmosphere that consisted of CO2 levels 5 times than those today. Why is it the Earth didn’t slip into a “greenhouse from hell” like many climate alarmist claim? The effects I describe are the obvious answer.

    I am not sure if everyone remembers when many theories about Mars and Venus were that thos planets suffered from a runaway greenhouse effects due to the high concentrations of Co2 detected. This is what alarmed them to the supposed “threat” of too much Co2 and started looking at our own climate.

    Turns out those theories were absolutely wrong. Mars and Venus suffers from a lack of a strong enough magnetic or ozone layer to protect their atmospheres. The solar wind from the Sun would strip their atmoshperes of oxygen over thousands of years, thus leaving the Co2 to cause the harsh atmospheres we see today.

    Their internal dynamos cooled down much faster than Earth’s which is a topic for another discussion. However, I think this is where some scientist went of the rails in fingering Co2 as the sole killer. Would it make sense for life on Earth to spawn how many species that exhale Co2, which will slowly cause our own doom?

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