Climate Explorations: Combining Obliquity and Eccentricity

"True" color approximation of hemispherically mirrored Earth with 45° obliquity and 0.5 eccentricity

Hello, this is part of a series where I explore different possible climates with the ExoPlaSim model. As per usual, you can check the first post in the series for details on my approach and some of the limitations of the model. This will also be my first exploration using my new climate classification system, so you may want to take a quick look at that if you’re not familiar and compare how it looks with previous explorations.

We’ve already looked at the individual effects of different rotational obliquities on a world with zero eccentricity and different orbital eccentricities on a world with zero obliquity. For the most part, I want to go through each individual parameter in turn throughout this series before I start exploring their combinations, but in this case I couldn’t resist seeing how these two seasonal forcings work together.

To review, obliquity creates seasons through the shifting orientation of the planet relative to the star (due to the orbit; it doesn’t actually change in absolute terms), causing an offset seasonal cycle between hemispheres: summer solstice (the point of maximum heating and longest daylight at high latitudes) in one hemisphere coincides with winter solstice (minimum heating and daylight) in the other, with mild equinoxes (equivalent heating and daylight between hemispheres) in between. Higher obliquity brings more heat on average to the polar regions but also greater seasonal changes in heating, and at high obliquity the equatorial regions also experience their own twice-yearly seasonal cycle, with warm equinoxes and cold solstices.

Eccentricity creates seasons through shifting distance from the star, causing a simultaneous global seasonal cycle: the entire world experiences the same warm periapsis (closest approach to the star) and cold apoapsis (farthest point from the star). Higher eccentricity causes more intense changes in heating, but also affects the length of seasons; the planet moves faster through its orbit near periapsis and slower near apoapsis, so higher eccentricity creates shorter summers and longer winters, though in practice this seemed to manifest as a short spring warming just before periapsis and then gradual fall cooling thereafter.

Movement of planets at different eccentricities (0.0 to 0.8). Phoenix7777, Wikimedia

How seasons play out on a planet with both substantial obliquity and eccentricity will depend on exactly how these two cycles align. In this post, we’ll look at the particular case of periapsis aligning with northern summer solstice. This creates an asymmetrical seasonal pattern, where the northern hemisphere sees a greater range in heating from the sun (the sun is both highest in the sky and closest in space in summer and both low in the sky and distant in winter) but the southern seasons are dulled by an offset pattern (the sun is high but distant in summer and low but close in winter). But again, eccentricity also affects the lengths of seasons; obliquity causes seasons because of the shifting relative orientation of the planet’s rotation and the sun, and as the planet moves faster in its orbit near periapsis, that relative orientation shifts faster. Currently, Earth’s equinoxes are in March and September, at about the 21st in both months, such that each hemisphere gets about the same period of greater light. If periapsis coincided with northern summer solstice at June 21 (instead of falling in January as in reality), then at 0.1 eccentricity, the solstices would shift to April 2 and September 8; at 0.3 eccentricity, they’d be at April 25 and August 17; and at 0.5 eccentricity, they’d be at May 16 and July 26, with only 71 days between them in norther summer but 294 days through northern winter.

Movement of planet around the sun in this scenario (eccentricity exaggerated) and orientation of the planet relative to sunlight at these points (with the rotational axis shown, north pole on top).

Thus, the northern hemisphere experiences short, hot summers and long, cold winters, while the southern hemisphere experiences long, cool summers and short, mild winters. This all balances out such that the average heating across the whole year is still symmetrical across hemispheres. Higher eccentricity increases global average heating, but in proportional terms it doesn’t affect the distribution of average heating across latitudes, only how that heating is distributed throughout the year.

The tricky thing is that, even neglecting its slight eccentricity, Earth already has a somewhat asymmetric climate due to the different distribution of continents across the hemispheres, with the northern hemisphere generally having stronger seasons due to its greater land area (land warms and cools faster than oceans and so sees stronger seasons). To get a better sense of how combining obliquity and eccentricity can create asymmetric climates across the hemispheres, I want to work with a more symmetric baseline. The solution I’ve found to this is to take the topography of Earth’s northern hemisphere and simply flip it over into the south:

The greyscale heightmap used in this exploration, with seas marked in blue.

The result is admittedly a bit cursed and blatant northern hemisphere chauvinism (sorry Chile), but gives us a more neutral baseline to compare against. As a control, I ran this topography through the same conditions as my original baseline case, which includes 23.5° obliquity and no eccentricity.

(For anyone who missed the last post, these plates show the uninterpolated climate map in my new bioclimate system in the top left, the interpolated map below with the key, and a Koppen-Geiger map in the upper right with the same color scheme as in my previous explorations and on Wikipedia.)

The result is about 3 °C cooler than actual Earth, likely because the higher land area (39% of the surface to actual Earth’s 29%) gives it a higher albedo, and it appears to have a somewhat stronger monsoon around the East Asian ocean, which is typically underestimated in ExoPlaSim models of actual Earth at this resolution.

Look closely and you’ll still see some discrepancies between hemispheres, which can be attributed to some combination of A, random noise that the 10-year sampling period didn’t completely filter out: B, ExoPlaSim’s default year is slightly offset from the periapsis-apoapsis cycle (to match Earth’s year, with periapsis in early January), which subtly influences how the monthly averages turn out; and C, perhaps some slight biases somewhere in the internal workings of ExoPlaSim—I’m not aware of any specifically, but they’re not unusual in models of this type and ExoPlaSim in particular has in past versions had issues with the ways data at different latitudes is split between CPU cores and combined back together. Regardless, any such bias looks to be small enough for this to serve as a decent baseline for the sort of asymmetric seasons we’ll be seeing today.

To cover the range of reasonable obliquities and eccentricities, I’ve picked out 3 levels of each to combine: 23.5°, 45°, and 90° obliquity and 0.1, 0.3, and 0.5 eccentricity.

One final note is that rather than altering CO2 levels to adjust temperature, as I have done in previous explorations, I’ve switched to holding CO2 at 300 ppm and altering stellar flux (the average level of sunlight, before adjusting for eccentricity). This is because I’ve found that part of ExoPlaSim’s vegetation growth model varies with the CO2 level. This does make some of the vegetation maps I’ve included with previous explorations a bit suspect, but it’s hard to say how much, especially for the cases with higher CO2—for what it’s worth, maximum vegetation growth is also limited by water availability and temperature in the model.

• 23.5° Obliquity
• 0.1 Eccentricity
• 0.3 Eccentricity
• 0.5 Eccentricity
• 45° Obliquity
• 0.1 Eccentricity
• 0.3 Eccentricity
• 0.5 Eccentricity
• 90° Obliquity
• 0.1 Eccentricity
• 0.3 Eccentricity
• 0.5 Eccentricity

Previous Exploration: Eccentricity

23.5° Obliquity

This is roughly the obliquity of modern Earth, so any shift in seasonality will be due largely to the influence of eccentricity. Earth’s slight orbital eccentricity of 0.0167 does actually have a subtle influence on our climate, with shifts in the alignment of the obliquity and eccentricity cycles particularly influencing the humidity of northern Africa (because alignment between periapsis and northern summer create stronger northern seasons which pull the ITCZ farther north in summer). But the eccentricities here will all be greater than any Earth has ever experienced, so far as we can tell.

0.1 Eccentricity

Prior Model: Symmetric Earth baseline
Stellar Flux Relative to Earth: 1.004
Average Temperature: 15.5 °C

To start off with, the asymmetry here is moderate but still obvious at a glance: the combination of periapsis with summer solstice in the north gives it more intense seasonal swings in temperature and precipitation, while the south sees more regular rain and less hostile extremes but also a shorter growing season at high latitudes.

In detail, large areas of the northern subtropics pass 40 or even 50 °C averages in summer, while their analogues in the south tend to remain a more moderate 20-35 °C. Winter temperatures are a bit more even but frost still spreads further in the north in winter, while only the south retains a permanent ice cap that survives through summer. For now, though, these different seasons still balance out to a roughly symmetrical average temperature profile.

Stronger northern seasons bring larger shifts in circulation: the ITCZ stretches far into Africa and Asia in northern summer, while the Ferrel and polar cells almost disappear over much of Eurasia, but they return with a vengeance come winter. But in the south, Africa barely sees a monsoon, though there is still one in East Asia.

The effect is that much of arid regions in the north actually see fairly regular summer rains (though they also get so hot in summer that the ground is unlikely to retain much moisture), whereas the southern deserts are more consistently dry. But that same northern monsoon also draws moisture away from much of the higher latitudes in summer, creating a more widespread Mediterranean climate band—though there’s also widespread pluvial steppe, a climate type we’ve seen before with eccentric worlds indicating relatively steady rainfall through summer but greatly increased evapotranspiration. Thus, the overall trend is that much of the northern hemisphere is dominated by variously semiarid climates, while the south is more cleanly divided into wet and dry regions.

0.3 Eccentricity

Prior Model: 23.5° obliquity, 0.1 eccentricity
Stellar Flux Relative to Earth: 0.96
Average Temperature: 14.4 °C

Here we see a more obviously asymmetrical climate, with a major ice cap in one hemisphere and only seasonal ice in the other (and this is another case where the Pasta bioclimate system more clearly shows how dramatically the extreme seasons are affecting the north compared to Koppen-Geiger). Again, both hemispheres receive the same total amount of light in a year, so this is due only to how sunlight is distributed across the year. In particular, the extent of ice is largely limited by summer melt: so long as winters remain below freezing, the exact temperature doesn’t much affect rates of ice accumulation, but summer temperatures do substantially affect rates of melt and so the overall balance of melt to accumulation; thus, even though the north has a longer and dimmer winter, the hot summers keep the ice at bay, while the south has gradually iced over, which raises its albedo and so does ultimately create an imbalance in total effective heating of each hemisphere. The northern polar regions are even balmy enough to see significant vegetation growth in their few summer months (though this is partially a consequence of the automatic global temperature adjustment; to maintain the same global average temperature, a colder southern hemisphere has to be counterbalanced with a warmer north).

At lower latitudes, the temperature patterns are more complex, as eccentricity is beginning to dominate over obliquity as a driver of seasons in many areas but not to the point of obviating the obliquity seasons. In the north, we see something like the same seasonal cycles we saw at high eccentricity: an intense heat wave in midsummer, then gradual cooling through the long winter, with April being the coldest month in many areas, and then rapid spring warming, with some areas spiking from well below freezing in May to over 70 °C just 2 months later.

Average temperature by latitude throughout the year.

In the south, meanwhile, many of the mid latitudes peak in temperature in August or September, what should be late winter or early spring for them going by solar declination, and then gradually cool off to a temperature low in January or February, which should be summer. But the exact timing doesn’t much matter because many of these regions are only seeing around 10 °C of variation in average temperatures anyway; with regards to temperature variation, much of the south is essentially seasonless, the eccentricity and obliquity seasons neatly canceling out.

Precipitation patterns in the north also resemble high-eccentricity climates at higher latitudes: a burst of early summer rain as warming begins, a dry spell through the intense midsummer heat, broad downpours shortly after, and then gradual drying through winter. Elsewhere, though, a number of different patterns dominate:

The rapid summer warming at high latitudes means the monsoon largely skips over the northern subtropical regions, leaving them dry aside from some mountainous regions that can catch moisture from the northbound winds.

The equatorial regions are wetter, but starting to see significant seasonality and their own monsoon cycle from a mix of obliquity- and eccentricity-related effects: near periapsis/northern summer, the ITCZ shifts north and the equator warms enough to widen the Hadley cell and generally intensify global winds, but not so much as to suppress rain, so heavy rains reach into much of north Africa; near apoapsis/northern winter, the ITCZ shifts back towards the equator and the Hadley cell narrows, keeping the equator wet but leaving north Africa dry.

In the south, for the most part weak thermal seasons come with correspondingly little precipitation variation, with the deserts largely staying dry and temperate regions staying wet, but there is still a substantial Mediterranean strip between the deserts and tundra; the same intensifying of winds near periapsis also draws moisture away from these areas, which are also hottest at around the same time, creating a dry-summer precipitation pattern.

This eclectic mix of seasonal patterns comes with a similar diverse distribution of vegetation, with the lushest regions being the consistently wet equator and southern subtropics, along with monsoon regions just north of the equator and in mountainous parts of the northern subtropics. Overall, despite perhaps appearing to be the harsher hemisphere at a glance due to its vast ice cap, the south may actually be more habitable here thanks to the north’s hellish summers and inconsistent rains.

0.5 Eccentricity

Prior Model: 23.5° obliquity, 0.3 eccentricity
Stellar Flux Relative to Earth: 0.821
Average Temperature: 14.4 °C

This case is in some ways a bit more symmetrical than the last case, because eccentricity is starting to dominate over obliquity in forcing seasons, though it’s fairly clear that the south still offers better prospects for survival despite the vast ice caps.

Most of the planet is now warmest near periapsis, and even those polar regions that aren’t reach their peak temperature in September or October, when light starts reaching the south pole, rather than nearer the summer solstice, when the sun is highest in the sky but also reaches its greatest distance from the planet. The equatorial regions are coldest around March and the north pole in May, before temperatures shoot up to over 80 °C across much of the continental interiors.

All this should be fairly familiar from our previous exploration with eccentricity, but here the southern hemisphere offers some respite: for those who’d rather not boil alive in summer, the most habitable part of the planet is a thin strip between about 40° and 60° south latitude, where temperatures still swing significantly between a July/August high and April/May low, but not to anything like the same extremes.

High eccentricity precipitation patterns now reach well into the southern hemisphere. The equator manages to retain some rain year-round, but north Africa has dried again, and the subtropical dry strip “skipped over” by the summer monsoon in Asia has grown as well, forming the strip of hyperarid desert between the equator and the seasonally wet northern semideserts.

Wind patterns are largely just further exaggerated versions of what we saw last time, with some impressive prevailing winds of over 100 km/hour in northern hemisphere Asia at periapsis. Note again that even though the ITCZ moves north in northern summer, the Hadley cells still expand enough to push their southern border south at the same time, pulling moisture from the mid latitudes and forming an even broader dry-summer Mediterranean belt.

The overall result is another entry in our collection of marginally habitable worlds, with a region of fairly hospitable climates trapped between extremes. Though, because the alignment of the eccentricity and obliquity cycles tends to shift over time (on Earth, over a roughly 21,000-year cycle), the durability of this habitable region is questionable.

45° Obliquity

If you’ve been following my previous explorations, you may remember that 45° obliquity represented a somewhat interesting intermediate state, with the tilt high enough to show a high obliquity circulation pattern of predominantly equator-crossing winds, creating equatorial deserts and a mid-latitude monsoon belt; yet low enough that the equator remains the warmest part of the planet on average. I figured then that it would be worth seeing how this responds to asymmetrical seasons as well as the low- and high-obliquity cases.

0.1 Eccentricity

Prior Model: 23.5° obliquity, 0.1 eccentricity
Stellar Flux Relative to Earth: 0.998
Average Temperature: 15.3 °C

To start out with, much as with the first 23.5° case, the differences in temperature are modest at first, with the average temperature profile still largely symmetrical, but there are some notable differences.

The north has the same sweltering summers we’ve come to expect from high obliquity, but in the south they’re a bit tamer, with fatal heat constrained mostly to the deep interiors of the near-polar landmasses (and also coming a tad later in the season, as the planet’s slower motion near apoapsis brings a slower decline in polar heating towards fall). The winter freeze, too, covers slightly less of the southern hemisphere and ends slightly sooner.

The more obvious difference is in precipitation patterns: the north experiences the expected parched summers flanked by bursts of rain, but the south only sees some scattered interruptions in rain and so has more like actual Earth’s continental climates with moderately wet summers.

But by the same token, equator-crossing winds are weaker in the southern summer and tend to converge more directly on the largest landmasses, and so much of the lower latitudes see less summer monsoon rains than their northern counterparts.

0.3 Eccentricity

Prior Model: 45° obliquity, 0.1 eccentricity

Stellar Flux Relative to Earth: 0.915
Average Temperature: 16.0 °C

Again as with the 23.5° cases, this intermediate eccentricity shows some of the clearest asymmetries.

There is again a portion of the low southern latitudes where the obliquity and eccentricity cycles roughly cancel out to give fairly steady temperatures, with the hottest month tending to fall in southern spring when the northern hemisphere and equator haven’t yet cooled much from their periapsis summer and the south is starting to warm at the start of its long summer.

At higher latitudes, much of the south, even the deep continental interiors, experiences remarkably moderate summers under the high but not-too-bright sun. Winters, too, are milder: the winter freeze that lasts over half the year across much of the northern continents is only around 3-4 months in the south.

Thus, while circulation in the north shifts closer to the archetypal high-obliquity climate (though some of the monsoon belt is giving way to desert as the same tendency we saw before for the summer monsoon rains to bypass the lower latitudes during the rapid spring warming manifests here as well), the south almost begins to resemble a low-obliquity climate, though without much of a temperature decline towards the pole.

There is a slightly more confused patchwork of climates at lower latitudes where some areas return to the subtropical deserts typical of low obliquity, but others experience that same sort of Mediterranean-like climate with a hot but dry periapsis summer we saw at lower obliquity. Africa also experiences something like the same pattern we saw at 23.5° with a northern summer monsoon that stretches to the equator but leaves the south parched.

The overall effect is a substantially lusher southern hemisphere, benefiting from a fairly uniform moderate climate with little variation across most of the mid to high latitudes.

0.5 Eccentricity

Prior Model: 45° obliquity, 0.3 eccentricity

Stellar Flux Relative to Earth: 0.82
Average Temperature: 15.7 °C

As with 23.5°, the high eccentricity begins to dominate seasons at low latitude and so make them more symmetrical, but the bulk of the southern hemisphere manages to remain moderate as the north gives way to sweltering desert; after all, anywhere beyond 45° south simply gets no light in midwinter no matter how close the sun is.

In the north, the climate is much as we’ve come to expect in these cases, with vast stretches of Eurasia and North America climbing past 100 °C near periapsis and surface temperatures peaking as high as 170 °C. Even coastlines and islands are at best barely survivable at higher latitudes—and of course in many areas this alternates with a 6-month freeze. Though at lower latitudes, smaller landmasses fare notably better here than at lower obliquity due to the tilt saving them from direct light.

In the south, there is a heat wave in the lower latitudes in August when the planet rapidly speeds through equinox while still close to the sun. But at higher latitudes, this rapid winter is a boon, lasting just long enough to save the surface from the intense periapsis sun but quickly passing to give prolonged periods of summer sun. The winter freeze only lasts about a month, and recall that, given the timing of the equinoxes, even the south pole only has to endure about 70 days of winter night and then passes the rest of the year in continuous light.

The sun still gets a bit more distant towards apoapsis, which gives many of these areas curious double-seasons: Warming from the brief winter freeze in July to an initial post-periapsis high of 10-25 °C in September, then gradual cooling back down to a ~5-10 °C low in February with widespread mountain frosts, and then warming again as the sun grows closer to another high in May before the rapid onset of winter.

The prevailing winds appear somewhat indecisive: at periapsis one massive equator-crossing convection cell dominates much of the planet, but this quickly fragments afterwards and a northern Ferrel cell soon reemerges. Then there’s a curious period where the winds over the Indian and Atlantic appear to settle into paired converging Hadley cells, but those over the Pacific don’t and at points blow directly east along the equator for reasons I can’t quite work out—perhaps because the east Pacific becomes one of the warmest parts of the planet as the landmasses both cool towards apoapsis. Let this be a lesson, if one is needed, that a planet’s wind patterns can’t always be summarized as a set number of convection cells with latitudinal boundaries.

Some areas of northern hemisphere Europe and North America actually see fairly steady rains through much of the year, so might be a bit better off than their classification as semidesert implies, but it comes mostly when they’re too cold or dimly lit to take much advantage of it; any growth would have to come in brief spurts in early and late summer.

The south sees some large deserts at low latitudes and a broad Mediterranean belt denied summer moisture by the strong periapsis winds, but beyond that the high latitudes enjoy steady rains, which in combination with long summer days could make much of them quite productive for vegetation or agriculture.

90° Obliquity

Of course, I had to see how eccentricity would combine with extreme obliquity. For these cases I have presumed that people are particularly interested in the potential for such worlds to develop equatorial ice belts, so rather than 15 °C I aimed to balance these cases at -5 °C, the highest average temperature where we found a stable ice belt in our previous exploration of high-obliquity worlds.

0.1 Eccentricity

Prior Model: None

Stellar Flux Relative to Earth: 0.998
Average Temperature: -5.0 °C

Previously it seemed like -5 °C made for a happy medium for high-obliquity worlds, not so hot to be completely desiccated by the summer heat but not so cold that too little water evaporates to produce much rain, but in this case that hasn’t worked out quite as well and much of the continental interiors in both hemispheres are fairly inhospitable.

I won’t reiterate much of what I’ve already said about the extreme conditions of high-obliquity worlds; what we’re mostly interested in here is the asymmetries. Extreme summer heat is notably more widespread in the north, and the north also tends to see a somewhat longer winter freeze, but the south has more permanently frozen area.

The somewhat less extreme southern summers seem to allow for a bit more rain in midsummer, particularly along long coastlines.

The south also has less stiff summer winds, though the glaciers over the south Himalayas get 60 km/hour winds through much of the year.

All in all, the south hemisphere still probably offers the best prospects for habitability, but it’s a narrow margin

0.3 Eccentricity

Prior Model: 90° obliquity, 0.1 eccentricity

Stellar Flux Relative to Earth: 0.926
Average Temperature: -4.7 °C

As per usual, now we start to see more significant asymmetry, and again the south appears to be the big winner in terms of habitability, but there’s some benefit to the lower latitudes in the north as well.

The story in the north is much as you expect, with some areas leaping from just above freezing the month before to over 60 °C at periapsis. But most of the north spends over half the year frozen, and even at sea only the Arctic stays ice-free year-round.

The south, on the other hand, sees a more gradual temperature climb over its long summer to a blistering but more survivable summer peak in March, and then typically a 3-4 month freeze.

All this is fairly familiar by now, but there different patterns near the equator: as usual the ice belt remains cold near the solstices, but during the northern fall equinox, increased sunlight to the equator combined with heat circulating out of the still-warm north manages to thaw much of the equatorial landmasses, and a gap even briefly opens in the sea ice on the west coast of Africa.

Sea ice cover through the year

This more dynamic ice belt seems to offer some intriguing possibilities for trade between the hemispheres and settlement at lower latitudes, though it seems to favor the north, with the largest land glaciers remaining in the south.

Wind patterns are much as we expect near the respective summer peaks, dominated by a single equator-crossing convection cell, but in northern fall there’s an odd transitional state where westward winds at the equator diverge towards both hemispheres, such that either hemisphere is largely dominated by its own equator–pole convection cell. But in northern spring, equator-crossing convection switches direction too quickly to see this.

The result is that however warm it might get, the equator remains quite dry, flanked by Mediterranean-like climates that receive heavy rain only when the equator-crossing wings converge on the continents near the solstices.

The other highlight here is of course that the precipitation patterns in the north show the typical dry summer and fall downpour, but the south’s more moderate warming allows it to retain rains through summer, with the only dry periods in winter when the hemisphere cools and moisture is all pulled north.

0.5 Eccentricity

Prior Model: 90° obliquity, 0.3 eccentricity

Stellar Flux Relative to Earth: 0.799
Average Temperature: -5.0 °C

For this case, I couldn’t get a stable permanent ice belt to form, but I kept to a -5 °C average for comparison.

The combination of constant direct light in summer and high eccentricity isn’t doing the north pole any favors; surface temperature in northern Eurasia can peak as high as 190 °C. But this period is brief, and most of the northern continents spend over 7 months of the year frozen. The overall temperature pattern resembles what we saw for high-eccentricity planets before: a sharp rise to periapsis and then gradual decline to a low just a few months before the next periapsis.

The south, of course, sees much the opposite pattern, with a brief winter freeze and then quick thaw followed by a gradual slight warming through its long summer, never reaching much more than 25 °C. A few islands even have tropical climates by the Pasta bioclimate definition.

Near the equator, an equatorial belt of sea ice does appear for most of the year, but thaws shortly after periapsis and reforms 3-4 months later, with a similar but slightly earlier summer on land.

The wind patterns are a bit inconsistent again. Near periapsis and towards the end of the southern summer—i.e., when the temperature contrast between hemispheres is greatest—we see the equator-crossing winds we’re used to from high-obliquity planets. But in the long northern fall/southern early summer, we get something like a reversed version of Earth’s circulation patterns with diverging equatorial winds, though it’s hard to pin down an exact number of convection cells.

Thus we get a fairly dry equator most of the year and a northern monsoon belt paired with a southern mediterranean belt due to the cross-equatorial winds at periapsis. At higher latitudes the rains stay fairly steady the rest of the year; even the north gets a fair bit of winter snow, but this melts fairly quickly near periapsis and is far from sufficient to keep the ground wet for long.

The south of course flourishes in its moderate climate, with vast forests near the pole, which is pretty good considering what you’d expect from combining such extreme conditions. But again, shifts in the planet’s orbit and alignment between its seasonal cycles over time may make this a short-lived Eden as each hemisphere alternates between more extreme and moderate climates. We may perhaps imagine some kind of regular cycle of life in each hemisphere having to retreat into highland or equatorial refuges as the climate turns hostile and then waiting out the thousands of years until they can spread out again.

That’ll do for this exploration. This is, of course, just one possible alignment for eccentricity and obliquity seasons, and at some point I may want to look at what happens If periapsis coincides with an equinox rather than solstice. But we’ve still got a fair few individual parameters still to explore, so next time we’ll be looking at varying surface atmospheric pressure. See you then.

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