Tuesday, September 14, 2010

Eemian orbital forcing

Astronomical solutions for paleoclimates generates Earth's orbital parameters (eccentricity, tilt, climatic precession, and insolation) over a span of 50 million years in the past to 20 million years in the future. One of my slow learning projects has to been to use the output from this program to create illustrations of orbital configurations at key climatic transitions, such as the Eemian interglacial. When graphed, data of orbital parameters often look like this:

(Diagram redrawn from Wikipedia and IPCC 4th Assessment Report)

For a scientist, this is useful, but not being a scientist I looked for a display that I would find inituitive and useful for explaining what I'm learning to others. I'm not going for precision, but rather, for an illustration that can show the change and show when some orbit parameters would have a collective warming or cooling influence. For example, the following contains my exaggeration of orbit eccentricity and tilt:

Tilt is a stretching of the 4-season ring so that the points of summer and winter solstice are elongated. Elongated means tilt is high, making summers and winters more extreme. Eccentricity is the blue elipse, which is exaggerated in relation to Earth's real orbit so that one can easily see when a solstice occurs near perihelion or near aphelion, and then intuitively recognize that this can make a northern hemisphere summer or winter on top of tilt milder (winter near perihelion or summer near aphelion) or more extreme (winter near aphelion; summer near perihelion).

So here are three orbits from the Eemian, starting from right to left, 30,000, 27,000, and 24,000 years ago:

I think I captured the orbits correctly (according to my analogy) in the top half of the diagram. The bottom half is wrong, but I'm keeping it as a puzzle for me to ponder how to fix (the right Earth should be paired with the left orbit; and the left, with the right, but I still want the tilt oriented to the sun).

Using Astronomical Solutions, I gathered the mean monthly insolation for June at 60 degrees North latitude. I chose 1000-year intervals to minimize the numbers I have to chart. I included eccentricity, tilt, and climatic precession. Also, I generated the insolation for 60 degrees South latitude during December, so I could compare the Northern summer's solstice to the Southern summer's solstice. One can see a big difference in insolation as a result of the two summers occuring at opposite ends of Earth's orbit. I also throw in some Holocene values to compare astronomical forcing of the Eemian to today.

Next, I hope to get some data on CO2 increases from warming (which in turn would warm the Eemian further) and add some illustrations of possible arctic and ocean feedbacks that will have added to warming or would have contributed to the rise of atmospheric CO2.



Steve said...

Hi John, I'm Steve Brown and I'm on the author team at skepticalscience.com. I'm in the process of writing a series of posts there on the Eemian interglacial climate and I stumbled across your excellent orbital forcing and sea-level illustrations for the period. I also found that you've also posted at SkS too. I was wondering if it would be possible to have your kind permission to use some of your illustrations in my posts. I would of course give you full credit and link back to your blog/website.

If you still have access to the SkS author forum, you can find the draft of part 1 of my Eemian series in the Blog Posts area under "Last Interglacial Climate part 1". It's due to be published in the next week or so with 4/5 follow up parts in the coming weeks.

Many Thanks, Steve

jg said...

Hi Steve,
Help yourself to any illustrations you may find. I can also revise or create one suited to your article.
I'll take a look at your draft at SkS.