Climate Letter #1008

Something interesting that we learn from Milankovitch cycles—

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Human civilization as we know it developed over the last 10,000 years, constituting an interglacial period known as the Holocene.  This has been a time of mild and stable climate conditions, generally comfortable and quite favorable to the development of agriculture.  Why have we been so fortunate?  It’s all because of a most unusual pattern in the sequence of changes due to the way the three Milankovitch cycles were combining.  Let’s take a look, using one of the handy gadgets I gave you yesterday:  https://cimss.ssec.wisc.edu/wxfest/Milankovitch/earthorbit.html
On the vertical right panel move the cursor down 10,000 years and leave it there.  Click on the Vostok Ice Core data and notice how unusual and well-positioned the Holocene formation is compared with the other 400,000 years.  Now click just the Tilt (Obliquity) button, showing a position at full warmth and peaking at this time.  Now look at Precession by itself, which was already falling away from a peak that had been relatively low.  Add back Tilt and the combination of these two gives you a nice flatly rounded sort of peak that is not too warm and exactly matches the position of the Holocene.  Then add Eccentricity and you get the same shape of peak but a bit cooler, just right for our needs to thrive in.
If everything had been left alone, you can see that the next 10,000 years and more would have been much different, with a new pattern of reduced polar insolation in the North favoring a renewed period of expanding permafrost and glaciation.  Because that will not happen global temperatures will not be affected by any of their customary feedbacks related to greenhouse gas and albedo.  The same amount of sunlight will hit the Earth but will be redistributed more toward warming of the South.  Temperature changes will now be left largely in the hands of greenhouse gas that humans are adding and all the different forms of albedo that remain when ice sheet growth is missing.
Looking back, the Milankovitch cycles came into their heyday during the Pliocene when global temperatures got cool enough to allow patches of glaciation to start forming in the North.  A cool part of the cycle would then give such patches an opportunity to grow and start producing the powerful feedbacks that enabled them to grow even more, until eventually stopped by a cycle that had the right amount of extra warmth.  Without the occurrence of that sequence the role of Milankovitch cycles will be greatly diminished.  We can still thank them for accidentally giving us 10,000 years of largely benign climate stability.
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One more thought.  When we came out of the last major glacial period, starting about 20,000 years ago, the temperatures became warm enough to melt all of the lowland continental glaciers but only about half of the permafrost that lay beneath them, or to the side, which was the case in Siberia.  It never quite got warm enough for the remainder farther to the north, thanks in large part to the merely modest warming contribution from Precession and Eccentricity cycles that we observed in the exercise above.  The other half of permafrost land is still there, covering about 15% of Earth’s land surface, having settled in place for the entire Holocene, waiting for some extra heat to come along so it could finish melting.  What surprises does it have in store for us when that occurs, the beginnings of which we now see happening?
Carl

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