Happy Perihelion!

Xnn

Not quite yet I know, but as everyone else is wishing happy New Years, it seems to be the season to wish good will.

On or around the 4th of January we (everyone on earth that is) will be at our closest to the sun. That the calendar year just started is only a coincidence. Due to orbital variations, our date of closest approach varies a little bit over time. That is the time/season of perihelion will gradually shift over thousands of years. Earth's axis is slowly but continuously changing, with a cycle of approximately 25,765 years.

Currently the distance between the earth and sun varies between 98.3– 101.7% of its average distance. At its average distance sunlight amounts to about 1365.5 watts/m^2. Being at perihelion, the sun’s intensity is greater of course. However, since intensity varies by the inverse square of the distance, its intensity is now about 1412.3 watts/m^2. That is a 6.7% increase over where it was just last summer!

Ever wonder how our climate would be if perihelion occurred in June instead of January? It won’t happen for another 12,000 years or so, but when it does Northern hemisphere summers would be warmer and winters colder. Just the opposite will happen in the southern hemisphere and there is another difference too. It not just that most of us live in the north, but there is much more land than the south.

The extra land of the north provides a big platform for seasonal snow, and snow feeds back into the climate through the change in albedo. That is the amount of sunlight that is absorbed. Less snow means more sunlight is absorbed and more warmth. More snow leads to less absorption and cooler temperatures.

Andre

Although the precession cycle of the equinoxes amounts to 25,765 years, this precession cycle of the aphelion/perihelion is a mix of cycles between 19,000 years and 23,000 years. The difference between the two is caused mainly by Jupiters gravity, shifting the perihelion/aphelion.

http://www.emporia.edu/earthsci/stud...rd2/theory.htm

marcus

Sure we should have a Perihelion Party!
And there should be a lifesize papier mâché figure of Hans Kepler to sit at the table.
He is Mr. Perihelion of course because if it weren't for ellipses we wouldn't ever have a perihelion, it would just be dumb circles all the time.

I think you had a good idea to wish us a Happy Perihelion and I wish you one too, and fun and good fortune throughout the Orbit!

We need to be more aware of the sun. And the other stars as well.

Maybe to save trouble making the papier mâché figure I could dress up as Kepler. I always wanted to do this...

Mammo

How does this post relate to the 100-kyr eccentricity cycle? After reading 'The Two Mile Time Machine' by Richard Alley, I was left with the impression that the insolation difference of a varying Earth-Sun distance of around 6% was not enough to explain the cycle of the ice ages. Can someone please enlighten me if I'm simply being stupid.

Xnn

Mammo;

Hopefully, I can figure out how to include attachments here.

About 2.5 million years ago periodic ice ages began.
Originally, the warm periods were on 41,000 year cycles.
Over time, the warm periods degraded to 100,000 year cycles.

The earths orbit basically has 20,000, 41,000 and 100,000 year cyles to it.
There is not much differance between the 41,000 and 100,000 year cycles.
So, it has been a struggle to figure out why the earth did the transition.
The latest that I've heard is that CO2 levels were just getting too low and the earth appeared to be gradually slipping into a permanent ice age.

Here's a useful image and an good science article.

Attached Thumbnails

 

Attached Files

Integraded Solar Irradiance.pdf (345.7 KB, 4 views)

Andre

But it's a bit different.

The big main eccentricity cycle are 413,000 years and 100,000 years. (413 ka and 100ka)

The former is stronger but does not show up at the Maritieme Isotope Stages, the latter is weaker but seems to dominate the cycles. I have never heard a satisfactory explanation for that.

There are more events that could be tied to the switch from the 41ka world to the 100ka world, the magnetic reversal from Matuyama to Brunhes chron (730 Ka ago). the latter being much more variable than the former and the Stilostomella extinction during the Mid Pleistocene.

hypatia

I'm making a vast array of perihelion foods. Cooked outside on a solar collector while wearing sunglasses. And if I have any time left, I may carve a spirit stone, and line it up with my house and the angle of the sun. Just so the archeologist's have something to do, in a 1,000 years from now.

Xnn

Andrea;

The Huybers paper explains that glaciers/ice caps are sensitive to insolation integrated over the duration of the summer. And integrated summer insolation is primarily controlled by obliquity and not precession, which is on a 40K yr cycle.
However, as the earth cooled during the Pleistocene, the 40K cycle heat eventually wasn't always enough to triger glacial termination.

Sometimes it wasn't until the 2nd or 3rd cylces that the glacial sheets would be sufficiently melted. 80K and 120K averaged out to 100K.

So, there really is no pure 100K cycle. It just happens to be the average value over the last million years of 2 or 3 40K cycles.

Andre

You're Italian?

It would be nice to have a ref to see what they did to validate that hypothesis. For instance if we look at the over familiar See saw here:



We note pretty low conditions around 50-25ka. However if we compare that with other climate data, things look much different, take for instance North Siberia in Hubberten et al 2004



we see a distinct dry an warm period in the Middle Weichselian MW-I and MW- II("Karginian") warmer summers than at present, evidently the mid Weichselian glaciation was long forgotten

Xnn

No, just a typo. Sorry.

25-50ka NH glaciation was much greater than the present.
Globally dryer conditions existed; which should include North Siberia.

Centered around about 30 and 50ka, perihelions occured during the summer.
So, during those periods, one could expect Siberia to have somewhat warmer summers.
Not as warm as 10ka when perihelion and obliquity worked together, but warmer.

One the other hand, centered around about 41ka, perihelion occured in winter as it does now, but with differant eccentricity. Not sure how eccentricity would work out 41ka. Possibly it leads to warmer summers, but it depends somewhat on actual latitude. Siberia is a very large area. Not all of it is at 65N.

Also, as the Huyber paper points out, glaciers respond to intergrated insolation.
Short hot summers are not as important as longer warmer summers.
The Hubbertin paper may be focusing on fauna, which probably respond differantly.

baywax

Excellent thread!

Another excuse to celebrate. Perfect!

Andre

Note that the climate analysis is based on insect remains. Several insects are supposed to reside in well defined biotopes including climate.

For testing statements like that, perhaps we should also have a look in the Yukon territory

And more, Zazula et al 2006

Of course we all have the lonely mammoth on our retina's dragging itself through the snow in a howling blizzard, a pack of hungy wolves in his wake, but horses? and antelopes? Doesn't really suggest extensive glaciation, does it?

*Note that Paleontologist aways talk carbon dates So calibration makes it a few thousand years older

Mammo

Thanks Xnn for the excellent clarification. It's now a lot clearer.

Xnn

Are you of the belief that glaciation 25-50 Ka in the NH was less extensive than the present??

Andre

I try never to be "in belief". I merely test hypotheses and at stake currently is if the isotope see saw in the benthic stacks, indeed represents ice (sheet) volume according to the direct evidence and as demonstrated here, it does not look good so far.

Moreover the formulation "less extensive than the present" is a strawman, the statement was:

Perhaps it wasn't that 'much' enough to satisfy the isotope - ice sheet hypothesis

Xnn

Then are you a perpetual skeptic?

That is no beliefs, just criticisms?

Andre

It's irrelevant but see my signature what I stand for.

Just trying to validate theories like this this.

However this one was develloped while there was still notion of a large East Siberian ice sheet during the Last Glacial Maximum.