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The Premise

The Earth rotates on its own axis, which is tilted by about 23 degrees.

The setting for my fantasy novel is an earth like planet without the axial tilt, all other factors remaining constant in terms of distance from the sun etc. Some of you may have read my previous question on climate and technology, this is on similar lines but for plant life.

For those who haven't read the other question (or even for those who have), here's some background:

From general reading, it appears the largest impact of removing an axial tilt would be on seasonal change, in the sense that there wouldn't be any changes whatsoever. Every latitudinal belt would have its own set season all year round, with some minimal changes depending on the earth's distance from the sun in its usual orbit (ranging basically from 91.4 million miles in January compared to 91.5 million miles in July - which is less than 1%).

As a result, since there is no major climate change, most regions of the world will generally experience the same weather everyday. Therefore, the cold regions of the planets are likely to become even colder to the point of being uninhabitable. Likewise for the deserts and heat. Those regions receiving lots of rainfall will either be covered in forests or experience continuous top soil erosion.

The very nature of evolution is such that life will find a way to survive in most conditions. Any ecosystem survives primarily on transference of energy. That means you need a basic breakdown of three tiers in the ecosystem for any kind of a balance:

  1. Plant life to convert Sunlight to edible energy (food)

  2. Herbivores to consume said plants

  3. Carnivores to consume said herbivores

Let's take a simple example - the dry grassland.

  1. Lots and lots of grass that bathes in sunlight and needs minimal water to grow.

  2. Small/medium game that eats grass (some kinds of deer or wildebeests)

  3. Carnivores that hunt down this game (lions, cheetahs etc.)

The Question

Given the premise of no axial tilt, what would be the implication on plant and animal life? For the purpose of this question, I am more interested in regions that under normal conditions faced two or more extreme seasons leading to migratory behavior or adaptation of some kind. Continuing with the above example, the question would be as follows:

The Grasslands - Usually, these regions see two climates: Dry and Wet. The vegetation dries out in the heat, forcing the herbivores to literally seek out greener pastures, which in turn forces the carnivores to follow. Now, with only one constant climate type, wherever the new grasslands get formed, they will stay the same all year round. No starving or dehydration, therefore no migration. How will this affect plants and animals? With an unhindered supply of sun and water, how will the ecosystem that has adapted to a twin season cycle change?

I understand that no ecosystem is that simple, but I'm not looking for a full fledged, scientifically sound design - only for a logical and preferably fantastical alternative. My question currently only addresses grasslands, but if you have another type of region in mind (such as the tundra or rainforest) that may lead to the above premise playing out in interesting ways, do feel free to add on.

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    $\begingroup$ No time for a full answer, but things to consider: Animals probably wouldn't migrate. Ecosystems would support smaller populations of fauna, since they are under constant pressure. Deciduous plants wouldn't develop. There would be no spring melt to fill rivers. No growth rings on trees. $\endgroup$ – Turophile Mar 31 '15 at 11:59
  • $\begingroup$ It's worth mentioning that tropical climates on earth aren't really affected much by the axial tilt yet they still have seasons (albeit only two, the rainy season and the not rainy season). I don't know if these are created by the other 4 seasons in more temperate climates, but as long as your planet is still spinning you still have an atmosphere getting all mixed around. $\endgroup$ – Pyritie Mar 31 '15 at 15:15
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  • $\begingroup$ Wait for it, I'm working on an answer. You should see something later on today - 1800 UTC $\endgroup$ – ArtOfCode Apr 13 '15 at 8:21
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    $\begingroup$ You could still have very minor seasons depending on how circular your orbit is. $\endgroup$ – Michael Richardson Mar 23 '16 at 18:54
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There are a couple good answers here already but they're missing some of the detail I think you're looking for. As has been stated (in slightly different words), the climate would stabilize into "bands" around the planet, with the equatorial band being the hottest and the polar regions being the coldest. The lack of seasonal change would have some effects on plants and animals, notably the lack of migration or seasonal effects such as hibernation and growth.

Let's extrapolate from that a bit.

Climate Zones

First let's look at the climates. Obviously there would be some variation between the equator and poles no matter what, but does that mean the equator would be a desert and the poles would be ice? Not necessarily. Depending on the planet's distance from its star(s) the overall climate could be hotter or cooler than Earth's. You could be slightly farther away which would result in the equator being more like a jungle than a desert, because it wouldn't be hot enough to dissuade condensation. The result of this would be colder poles, however. On the flip side, you could be slightly closer to the star(s) and have little to no polar ice, but a very hot and dry equator. For the record this is based on a planet with roughly the same water content as Earth.

Geology

If your planet is geologically active you still have the potential for larger mountain ranges. These will be independent of seasons and will be able to occur in any practically part of the environment. Geology becomes even more important when you consider...

Weather

With a nearly-flat planet (geologically inactive for enough time to erode mountains) and no seasons, you'd have massive wind storms that would make life very very difficult. With geology you have the ability to create smaller climate zones inside the greater bands. Large mountains can direct rainfall to keep part of the equatorial band wetter, for example. The more open the space, the more grassland-like it will be (hence the prairies in the middle of the US, for example). Another thing to note is that rivers and water sources wouldn't necessarily behave exactly the same, because you wouldn't have seasonal thaws in the mountains that melt snow and flood mountain lakes. In all likelihood this wouldn't affect your story too much but it would slightly change the geography that you depict depending on where you are. Higher-elevation lakes would still be an important source of water propagation to the rest of the world but they would be fueled almost exclusively by rainfall. I don't know for sure but I'm guessing this would make them only occur in select high-rainfall zones and they would probably be quite large and deep, otherwise they would be prone to evaporating during a dry spell and then never completely re-forming.

An important point to note is that if your bodies of water include ocean-like areas, and those oceans cross climate bands, you would end up with some interesting weather effects. The star would heat the equatorial ocean more, but much of that heat would transfer to upper/lower climate bands of the ocean. It wouldn't be perfectly even, but it would probably be within a few degrees. This would mean life around the oceans would have some interesting adaptations compared to the rest of life in its latitude. A temperate-area ocean that crosses the equator would be warmer in the temperate zone than you'd see here on Earth, which could create more violent storm systems due to the extra evaporation. Life around it would also be able to benefit from the warm water and be a bit different from land-locked life (even if near a water source) but wouldn't be able to spread inland for lack of warm water. It would give you an opportunity to create yet another highly unique biome.

Plant Life

This would probably make one of the biggest differences but not for the reason you might expect. Plants would simply be adapted specifically for whatever climate zone they are in, much as they are today, but with less emphasis on growing seasons. What this means is you wouldn't see changes in color throughout the year, and they would be producing fruit (seed) year-round. This would pose an interesting problem for farming because you likely wouldn't be able harvest in the same manner as you're used to here on Earth. Unless some sort of technical trickery can be figured out, my guess is the majority of harvesting would have to be done by hand or in some sort of very controlled fashion to make sure that an entire field is planted at the exact same time, watered the exact same amount, and harvested at the same time while dropping no seeds to grow unintentionally (or those would have to be regularly weeded out). Preventing animals and wind from transferring seed into your field unintentionally (which is a huge part of plant propagation normally) you'd likely have to do all of that indoors somehow, or just stick to hand-picking, which could be inefficient for wheat-like produce due to the low calorie content per plant yield. If you can't cultivate such produce in a very controlled or automated fashion your people would likely resort to exclusively farming high-value foods instead, or you'd have to imagine some type of fantasy grain that's far removed from what we're familiar with. Your farming communities would likely look much different from ours. Another possibility is that plants would diversify into two main groups, those who bear fruit year-round and those who sort themselves through some internal clock into budding seasons. Things like grasses that have a much higher volume of fruit would likely bud year-round, while higher-value ones like fruit trees would probably coordinate their pollination to conserve energy. It's also fathomable that those high-value plants could be in a state of flux where some buds are available for pollination while others are already fruiting, but I don't know what the internal biological implications of that are. They would possibly work like a colony of individual plants growing on the same root foundation, similar to coral.

Animals

This part wouldn't operate too much differently. Of course seasonal things like migration are out of the question, but once you have your plant base established, the animals just feed off of it and then each other. The herbivores would forage much as they do on Earth, and carnivores would stalk and eat them in a virtually identical fashion. Hunting done by humanoids would be very similar. On another note, animal populations would still likely default to some sort of "mating season" so as to have the highest chance of finding a ready mate, though it would be regulated by some other timing than seasons, like "when the current generation leaves the nest." These "seasons" would be unique to each population, and could to some extent influence their predators.

Variation

What all this allows for, which you may be happy about in a fantasy setting, is less inter-mixing of species, plant and animal, throughout different regions. What that means is whenever you have a mountain or a river that is uncrossable by most animal populations, on the other side of it could be a completely different ecosystem, much more-so than we see here on Earth. The closest analogue we have is the crazy differences between Australia and the rest of the world, because Australia is so isolated that species developed in completely different ways. With no migration and just a simple physical barrier, you could get that same kind of diversity in even land-locked regions.

The Two-World Possibility

Here's what I consider the most exciting part. If your equator was for some reason extremely difficult to pass, whether due to being a massive, harsh desert or a hazardous, labyrinthine rain forest, you would essentially have two sides of the world isolated from each other in a way we've never been here on Earth. The oceans were once a barrier, but ultimately we crossed them without an enormous amount of trouble once we figured out how to pack enough food and keep from getting too terribly ill on the voyage. If a physical region is actually out to kill you in some way, with deadly heat/dehydration or with myriad diseases/venomous living creatures and no sense of direction, it could be much much harder to get through. But when you do eventually get through, you might as well be standing in an alternate universe. It would be the same temperature as yours, but the plants and animals would potentially be completely different, having never inter-mixed in any way with the ones where you came from. You could make a whole story just from that alone.

If I missed any points, feel free to ask for clarification. For the record I don't have any sources on this, it's just some applied natural science knowledge using as much logic as I can muster up. If anyone has suggestions for improvement feel free to comment.

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    $\begingroup$ As a reinforcement of the "Two-World Possibility". Until people had a concept of the world as a globe, natural law for those in the north would be "the farther south, the hotter it gets". With the evidence of the equatorial desert band, it would likely be some kind of accidental drift voyage passing over the equator that told of better climate on the other side of the equatorial hell. $\endgroup$ – Michael Richardson Mar 23 '16 at 18:53
  • $\begingroup$ Wouldn't constantly growing polar ice caps accumulate more and more water from other regions and at some point turn the whole world into an ice ball? $\endgroup$ – Sebastian Dec 6 '17 at 14:11
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    $\begingroup$ Why would the polar caps keep growing, though? $\endgroup$ – thanby Dec 6 '17 at 14:26
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One of my favorite authors, Isaac Asimov, addresses this very question in one of his essays. He got to thinking about its use in Paradise Lost, book X. The plot is that Earth has no tilt in the "Eden" stage and had an eternal spring climate, and that as punishment it was tilted, so that mankind would have to suffer seasons.

It was in Oblique the Centric Globe as collected in Quasar, Quasar, Burning Bright (1978).

Wouldn't it be cool to have the legendary Dr. Asimov, Grand Master of Science Fiction, answer the Question?

cover

Later

Thanks to the encouragement, I've acquired a copy of the Book Club edition from 1978, to look up the actual quotation. It notes that Oblique the Centric Globe was originally in the August 1977 issue of The Magazine of Fantasy and Science Fiction, and that Isaac Asimov has published “Almost 200 books … and counting.”

I carefully transcribed it with the exact punctuation and nuances.

The essay continues from the previous month (as Chapter 7), which covered Ice Age cycles, and recaps that the axial tilt and the eccentricity combine to give mild winters and cool summers in the northern hemisphere and cold winters and hot summers in the southern hemisphere. It picks up the story by digging into precession (a 25,780 year cycle) and the musings of Milutin Malankovich.

How does this affect the Earth’s weather? —Not the way most people seem to think.

⋯ If there were no axial tilt at all, there would be days and nights equal in length over all the world. The situation would be permanently what it is now at the equinoxes.

It seems natural, then, to have the idea that if only the Earth’s axis were not tilted, there would be an eternal spring everywhere on Earth.

This idea finds expression in Paradise Lost by John Milton (who was great on poetry but weak on astronomy). Milton felt that before the Fall, when man still liven in Eden, there was no axial tilt and there was a world-wide and eternal spring. It was only after the Fall that the tilt was imposed.

Milton, who wanted to cling to the Ptolemaic theory but reluctantly recognized the fact that astronomers were, by the time he was writing, virtually all Copernicans, wasn’t sure whether to say the tilt came about by tipping the Earth or tipping the Sun—so he waffled. In Book X of his epic, he writes:

Some say he [God] bid his Angels turn askance The Poles of Earth twice ten degrees and more From the Sun’s Axle; they with labour push’d Oblique the Centric Globe: Some say the Sun Was bid turn Reins from th’ Equnoctial Road Like distant breadth …

Milton was wrong, however, in thinking of the tilt (imposed either Copernically of Ptolemaically) as a punishment.

Suppose the axis were tilted less than it now is. In that case, the unevenness in length of day and night in the regions about the solstices would be less. The summer wouldn’t be so hot or the winters so cold. there would be a mild-winter–cool-summer for both hemispheres. The less the axis was tilted, the milder the winter and the cooler the summer for both hemispheres.

However, as I explained in Chapter 7, a mild winter tends to produce more snow and a cool summer to melt less snow. A smaller tilt to the axis encourages and Ice Age in both hemispheres, therefore, and if the axis were not tilted at all, the Ice Age would be permanent, north and south.

So tilting the axis was a reward, in that it unfroze the world.

In fact, one could argue this way. As long as Adam and Eve were in the Garden, which we might picture as in a tropical clime, a seasonless year was beneficial. After the Fall when human beings were going to multiply and spread out over the world, the Temperate Zones would have to be made habitable for them and hence the tilt was imposed. Had Milton been able to advance this explanation, he could have illustrated God’s loving-kindness rather than His vengeance—which means he would probably not have talked of the tilt at all, for pious people, in my experience, are more interested in vengeance.


Note ※: I used em-dashes where they are seen in the text, without adjacent spaces where there is none in the original. Whether there is "no space" or some "hair space" is really dependent on the font and rendering. I used curled apostrophes as seen in the book. seasonless is indeed shown as one word. I used an en-dash in the middle of “mild-winter–cool-summer” though it not obvious that it has a longer length in the printed copy. I used the ellipses code point where ellipses were used in the original, and how it appears depends on the font. The original appears rather wide, being the same length as the word “from” immediately above. So, I can't tell if there is a space before the ellipses (as I took it) or the ellipses unit has space before the first visible dot or it's just formatting. Really though on the dead-tree medium there are no code points and exact details of whitespace will vary as does the font and formatting on the window. I'm sure Asimov didn't type these niceties anyway, but it was done by the magazine people. The point is to preserve the vintage formatting and punctuation as the way we read it back in the day.

My own added ellipses use a centered-dot form (⋯).

Note that excessive footnotes getting all meta is also my own homage to the late Doctor Asimov. In some cases I recall conversions between the author and editor or "discourse" from an imagined Gentle Reader. He really anticipated the future media of comments adding useful content to a blog post, if you think about it.

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  • $\begingroup$ From the upvotes, I'm motivated to purchase a vintage copy through Amazon (I read it originally in a school library, and don't own it). In a week or so I'll re-read the essay after some 30 years. I'll relate what it states. $\endgroup$ – JDługosz Apr 14 '15 at 0:16
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    $\begingroup$ Hi JDlugosz - any input you can give would be greatly appreciated! I know this is a late response but the premise of what you're saying seems pretty interesting! $\endgroup$ – Ambarish Sathianathan Nov 18 '15 at 9:25
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I'll do a quick explanation of the impacts of Earth's axial tilt to start off:

As Earth orbits the sun, it's tilted at 23.5 degrees. This varies between 21 and 25 degrees, but I'll ignore that because the changes are slightly irrelevant here. With the tilt, our orbit looks like this:

earth with axial tilt

In this case, on the left point X is tilted away from the sun and is in winter, and point Y is tilted towards the sun and is in summer. On the right this is reversed.

If we had no axial tilt, orbit would look like this:

earth with no axial tilt

Now, point X and point Y are equidistant from the sun at all points of the year. There are no longer any seasons.


Since the equatorial grasslands are mentioned in the question, I'll answer about them. With no axial tilt, the grasslands are always the same distance from the sun. Since they're on the equator, they are always the hottest parts of the earth, with little to no rainfall. Instead of having a dry and a wet season, they would be dry all year round, becoming deserts. You would find all the animals we currently associate with this area would no longer be able to live there. Only desert animals like camels and scorpions would live there.

The major impact on plant and animal life would be location. Since there are no seasonal temperature changes, the average temperature would be higher and therefore plants and animals would move further north or south to escape the heat.

The other major effect I can think of, although not plant or animal based, is an effect on the Arctic and Antarctic circles. At the moment, the seasons mean that half the year these areas see no sun and the other half they see constant sun. With no axial tilt, these areas would be in a state of constant dusk/twilight.

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I agree with your synopsis of the impact as stated in the question. I have no expert knowledge to impart, but here are some thoughts to consider.

  • Animals probably wouldn't migrate.
  • Ecosystems would support smaller populations of fauna, since they are under constant pressure.
  • Deciduous plants wouldn't develop.
  • No growth rings on trees.
  • Animals wouldn't moult and change colour.
  • No winter hibernations. No food storage for winter.
  • Babies could be born at any time of the year, not to coincide with plentiful food.
  • Animals would probably specialise a little more because they aren't dealing with as much environmental change. This may (to a very small extent) also slow evolution.
  • No spring melt to fill rivers and lakes.
  • More glaciers since snow would build up without warmer seasons to melt it.
  • No tropical monsoons/wet season bringing bulk rainfall to some locations.
  • As you mentioned, colder in cold places, and hotter in hot places. Hotter probably also means drier.
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What a fascinating topic! I am a very late-comer to this discussion, just signed up for this site today. There have been a few quality answers already, so just a few thoughts as a biologist with a keen interest in geology and astronomy. Forgive me if I am repeating some other comments.

First of all, I think that the only thing we can say with near-certainty about an earthlike planet without axial tilt, is that there would be no real seasonality, as all others also noted. That's about it, other than that we cannot say things with as much certainty as some others suggest, for the following reasons, let me elaborate a bit further;

Without axial tilt and its resulting strong seasonality, other factors would become (even) much more relevant in determining climate than they are now, in particular:

1) Solar insolation, determined by the type of star and proximity to it. I understand we are here assuming a similar sun and earth combination.

2) Certain large-scale characteristics of the earthlike planet, such as in particular magnetic field and rotational speed (day-length!). Again, I understand we assume earthlike here.

3) Atmospheric composition: we can see in our Earth's past that for instance varying CO2 levels had enormous impact on climates. Higher levels, warmer climates and globally more rainfall; lower levels, colder climates and globally less rainfall.

4) Distribution of land (continents) and sea (oceans), I mean both size and location of continents: obviously very great impact on local solar heating and hence also on winds and rainfall.

5) Mountain ranges: need I elaborate?! Tremendous impact on winds and rainfall.

6) Volcanism: in particular the amount of it, adding CO2, sulphur and many other elements to the atmosphere and biosphere.

7) Life itself: ecosystems, such as large forest areas, have an impact on their own local and even regional climate. For instance, the Amazon rainforest creates part of its own rainfall by a recycling of water.

Summarizing, geographical/spatial differences would become much more determining for climates, by lack of temporal differences, other than day and night. Wind and rainfall would mainly be created by differential (solar) heating as a result of spatial differences, such as land-sea, high-low, north-south, ...

Winds, which are mainly the results of these spatial heating (and resulting air pressure) differences, plus the Earth rotation, would still be blowing, such as the trade winds (though in a somewhat modified form), the equatorial rising airflows, the 'westerlies' and related winds resulting from colder sea / warmer land.

So, yes, there would definitely still be wind, and those bringing rain. We know this, because a lot, if not most wind and rainfall of our present Earth are also caused by relatively short-term differences in heating. But typical seasonal winds, such as monsoons, would be absent.

With regard to life and ecosystems:

What I find remarkable is that the lushest life and the greatest biodiversity on Earth are found in regions with the least seasonality (tropical rainforests and related ecosystems)! Tropical rainforests are climatically defined as places where the day-night temperature differences are greater than the annual/seasonal differences. So, it is absolutely clear that life on Earth does not require an axial tilt and seasons, it is just adapted to it.

At the same time, the equatorial regions also show us how important geographical differences are for creating ecosystems: at the same latitude in Africa and South America we find rainforests, wet and dry savannahs and even semi-arid places.

Therefore, when creating your world, first create the continents and mountain ranges and then look at the major differences in temperature. That should give you an impression of prevailing winds, and rainfall. And with that, give you a start for your ecosystems.

Success! And please let us know some results.

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