# Are there "eccentric seasons", where do they occur, and why don't we hear more about them?

The earth is on an eccentric orbit. It's a small eccentricity (let's say e=0.01 https://www.wikiwand.com/en/Orbital_eccentricity) but it's there.

As it moves closer and farther from the sun the amount of energy being absorbed from the sun's light will change, and thus there will be a temperature shift dependant on earth's eccentricity (an "eccentric season")

The flux on earth and thus the rate it which it absorbs the sun's energy goes as r^-2, where r is the orbital radius. Thus the fractional change in flux is proportional to 4e. (taking into account the factor of 2 from the orbit ranging from a(1-e) to a(1+e) where a is the semi-major axis)

The effective temperature (assuming a blackbody) goes as (P)^0.25, where P is the incident power (= flux on earth * cross sectional area of earth).

Thus, for small e the fractional change in temperature should be ~ e.

For Earth, with a temperature of roughly 300 K, this gives a 3 degree temperature shift over an orbit.

I'm not sure I've ever heard anyone talk about this, and it doesn't seem negligible (especially given that there are parts of the world where seasonal temperature changes only by about 10 degrees http://ggweather.com/sf/narrative.html).

So my three questions are:

a) Is the above logic correct, are there (small) "eccentric seasons"

b) Where would they occur, specifically what latitude if any has a boosted seasonal change?

c) Is this something talked about that has just passed me by (wouldn't be the first time)?

• you should give a link. The seasons we have , summer autumn winter spring are dependent on the orbit, so I cannot understand what you are asking about. Feb 9, 2018 at 14:28
• sorry if that was unclear, I've renamed them "eccentric seasons" to better distinguish them Feb 9, 2018 at 14:38
• Maybe this will help , I will read it later aa.usno.navy.mil/faq/docs/seasons_orbit.php Feb 9, 2018 at 14:43
• Are you aware that in the northern emisphere winter occurs when the earth is closer to the sun? The relevant effect is the angle of incidence of light. This is a coincidence: the perihelion shifts over time. So are you asking if the perihelion shift will have an effect on seasons? Feb 9, 2018 at 15:00
• Yes, this is dealt with. The climate model I work with for instance has parameters for orbital eccentricity and uses this to calculate the effective solar constant which, as you say, varies over an orbit. I think it does not deal with Milankovitch cycles (ie, the parameters which vary over those cycles are treated as constants). The same model is used for weather forecasting so the eccentricity is also taken into into account for that. I don't have specific answers as to how great the effects are (hence this is a comment, not an answer).
– user107153
Feb 9, 2018 at 16:32

Eccentricity plays but a minor role in the seasons. Eccentricity would lead one to think that the Earth as a whole should be warmest in early January when the Earth is near perihelion, coolest in early July when the Earth is near aphelion. The reality is that exactly the opposite occurs, at least currently. The Earth as a whole is at its coolest in January and its warmest in July.

The key driver in the seasons is the Earth's axial tilt. This is why the seasons run counter to one another in the northern and southern hemispheres. Next in line is the very different distribution of land in the northern and southern hemispheres. That the southern hemisphere is predominantly water moderates its seasons. That the northern hemisphere has much more land mass makes its seasons more extreme.

Eccentricity does play a role in the seasons in the long term. Glaciations ("ice ages" colloquially, but that term is incorrect; the Earth remains in an ice age to this day) generally start when northern hemisphere summers are mild and end when northern hemisphere summers are hot. The long but mild northern hemisphere summers that result when summer in the northern hemisphere coincides with aphelion don't get warm enough to melt winter snow in far north latitudes. Those mild summers means winter snow at 65° N latitude can remain on the ground all summer long. The snow builds up over the centuries to form sheets of ice that are multiple kilometers thick . On the flip side, those multiple kilometer thick accumulations of snow and ice melt in the face of the short but fierce summers that occur when northern hemisphere summer coincides with perihelion.

• A note on the glaciations paragraph. Changes in the orbit aren't enough to explain the temperature changes on their own, although I think they may pretty much explain why they start and end. There are amplifying factors which at least include albedo changes (snow has higher albedo than what it covers in almost all cases, so once things start they tend to run away somewhat) and greenhouse-gas changes. (I am sure you know this: not all readers will though.)
– user107153
Feb 9, 2018 at 17:05
• @tfb - That is correct. Apparently slightly elevated $CO_2$ levels helped the Earth dodge a glaciation bullet half a millennia or so ago, when conditions were close to but not quite ideal for the onset of a glaciation from a Milankovitch cycle perspective. Amplifying factors such as albedo also play a very important role. And so does climate. Whether snow falls predominantly on Siberia vs North America is one of the proposed solutions to the 100000 year problem. Feb 9, 2018 at 17:17
• this is a great answer, but I can't work out if you're saying that the 3 degree shift (from my back of the envelope calculation above) is not valid for some reason, or just that there are bigger fish to fry? Feb 10, 2018 at 0:09
• @Zephyr: I think that your back-of-the-envelope calculation is right, but that the climate system is this enormously complicated thing where it's very hard to disentangle what is going on in any easy way. As well as the factors mentioned in the answer there are at least two elephants in the room: the atmosphere and the oceans. Both of these move energy geographically around the system in ways which are just extremely complicated (the ocean moves much more, but more slowly). I think your 'there are bigger fish to fry' comment is pretty much right.
– user107153
Feb 12, 2018 at 22:16