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I’m trying to refine the worldbuilding for the planet featured in my story “Aleyara’s Descent” from the May/June 2023 Analog Science Fiction and Fact. The planet is 86% the diameter of Earth with 82% gravity, and has a Jurassic-like climate, warm and wet with maybe 25% oxygen or more, supporting a lush biosphere with abundant megafauna. It rotates a little slower than Earth, in 27 hours, 23 minutes, and has a moon 84% the size of Luna but a bit further out.

The most notable feature is the rainforest, consisting of gigantic trees more than 150 meters high with a canopy so densely grown together that it’s almost like solid ground. The trees share water and nutrients through their branches, becoming a single superorganism; they also secrete a fire-resistant resin to counter the high-oxygen atmosphere. This much is already established in the published story, so it should be treated as a given.

My intent is that the rainforest covers nearly 60% of the land surface, because most of the land is in the tropics or subtropics, and the planet’s 9.6-degree axial tilt and current geology result in year-round steady rainfall for a broad region around the equator. A lower axial tilt would technically mean a narrower tropical zone, but I’m assuming it would result in less seasonal variation in the weather. High mountains to the north trap moisture on their southern side and enhance the wetness of the rainforest.

I created a rough map some years back by editing random fractal planetscapes generated by the Terranova screensaver (so this isn’t a flat projection, just two hemisphere views). It’s not entirely accurate, and the ratio of water to land should probably be greater.

Approximate map of planet Rulenau

Roughly, the dark green is the super-rainforest, the lighter green is more conventional forest, etc. The big northern mountain range starts at the northeastern peninsula of the supercontinent and extends west-southwest; I assume the yellow-green spur extending southwest from the northern coast is a steppe or desert in the rain shadow of the mountains. The planet’s too warm for permanent icecaps, but the northernmost lands consist of tundra, desert, and glaciated mountains, so they’re inhospitable.

The main problem here, as I understand it, is that the middle of that single vast supercontinent should be very dry, because it’s far from the oceans. So the question is, how do I get enough moisture there to sustain the rainforest? It is narrowest there, but is it plausible for it to be there at all? There are several factors that could help. There’s the northern mountain range trapping moisture to the south. There’s the interconnected superorganism of the trees sharing moisture and nutrients throughout, and the transpiration of moisture from their leaves would itself increase the humidity. Since the forest is mostly in the intertropical convergence zone, I presume there’s no fixed wind direction, so winds from the southern inland seas might blow north and east to help supply moisture, at least in monsoon seasons.

Does this work well enough as is, or do I need to make some changes to the geography? Are there problems I’m missing? Are there solutions I’m missing? Would the lower gravity or slower rotation have an effect, for better or worse?

EDIT:

All right, here's a rough attempt at an improved map:

Rulenau version 2

I didn't want to get entirely rid of the land in the far north -- is this good enough to get a reasonable amount of moisture continent-wide?

Also, about how much of an axial tilt would be preferable? Since Hadley cells help distribute moisture and heat, should I go for a larger tilt than Earth's for larger cells? Will I have to accept more severe seasons as a tradeoff?

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It's not, you have created a mostly desert continent.

You have done nearly everything possible to ensure as much desert as you can.

Low axial tilt means smaller Hadley cells, which means mess moisture in the tropical zone and a smaller tropical zone. Most of the continent is in the subtropical highs, it will be mostly desert just due to that.

You have too much land, somewhere between 2-3 times as much as Earth, and not enough ocean so your planet is a lot drier than Earth. There are no monsoons on this planet. There might not even be hurricanes.

The huge continent makes it even drier so a lot of your tropics will also be dry because they don't have anywhere from which to get moisture.

These are such dominant forces that there is not much you can do about them. You have to change them. Slowing the rotation a lot will help but only if you have a lot more ocean. That way you have a single set of large circulation cells instead of 3. This does mean your moisture would be coming from the north and south more than the east.

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  • $\begingroup$ Okay, so smaller continent and larger axial tilt -- easy enough to fix. I already said the map was only an approximation that probably exaggerated the land/water ratio, so don't take its proportions literally. I'm more concerned with the geography of the continent. Assuming a proportionally smaller continent of the same shape to start with, what else do I need? $\endgroup$
    – Christopher Bennett
    Commented Mar 22 at 0:19
  • $\begingroup$ get rid of the northwestern peninsula so moisture can come in from every side. that should take care of most of it. $\endgroup$
    – John
    Commented Mar 22 at 0:36
  • $\begingroup$ On the question of where to get moisture... what about groundwater? What if the central portion of the continent were low enough that it breached the water table and turned into wetlands? Online maps suggest that a subsurface aquifer can span a major part of a continent, though the locations don't necessarily correspond to wet areas, due to their depth: researchgate.net/figure/… $\endgroup$
    – Christopher Bennett
    Commented Mar 25 at 12:47
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    $\begingroup$ @ChristopherBennett groundwater needs to come from somewhere if plants have access to it it will disappear quickly unless it it replaced just as quickly. the height of the water table is controlled by rainfall, death valley is lower than sea level. You may need to create a new question just about Hadley cells, $\endgroup$
    – John
    Commented Mar 25 at 19:52
  • $\begingroup$ I know, which is why I brought up the issue of how large an aquifer can be. If it spans a wide enough part of the continent, it might bring water from a rainier area into a drier area. The largest aquifers on the map I linked to are thousands of kilometers wide. And since my planet has a hot, wet climate like the Eocene, with no ice caps to take water out of circulation, it could have a considerably greater surplus of water than present-day Earth and thus have larger or deeper underground aquifers. $\endgroup$
    – Christopher Bennett
    Commented Mar 25 at 20:37
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Rainforest can generate, at least at some scales, their own weather. See "A rainforest-initiated wet season onset over the southern Amazon" in PNAS. While this may not be able to solve everything, it's a realistic mechanism by which the forest recycles moisture. As stated by "Forest-rainfall cascades buffer against drought across the Amazon", "We estimate that one-third of Amazon rainfall originates within its own basin" Whether this phenomena could occur on a super continent is probably another issue.

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  • $\begingroup$ Thanks, that helps. Between that and the wet, hothouse climate, it might work. I likened the climate to the Jurassic, but a thread about a similar question suggested an analogy with the Eocene, when most of North America and Eurasia were covered in forest. $\endgroup$
    – Christopher Bennett
    Commented Mar 25 at 11:25
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Make the center of the supercontinent a depression below sea level, with the edges of the elevated slightly to form a rain trap, and all the inland aquifers, rivers and streams going towards it.

Basically, imagine there is a giant sweetwater lake in the middle of the continent, but the plant growth had all but covered it, and siphoned the water to the sides, resulting in a sea-sized, rainforest covered swamp.

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  • $\begingroup$ Great, that sounds like a more plausible way to get a midcontinent wetland. Although what gives me pause about the swamp idea is whether it would be a strong enough base to support my super-redwood rainforest trees. But the roots would have to be very wide and deep anyway to support all that weight, so a few meters of water on the ground might not make that much difference. It could be an interesting environment to use in a story. $\endgroup$
    – Christopher Bennett
    Commented Mar 26 at 10:54
  • $\begingroup$ @ChristopherBennett why does it need to enough base to support super-redwood trees? Let some, or even MOST of them collapse, sprout new roots, mangrove-like and grow back upwards anyway. Rather than a giant rainforest of straight wooden towers, you would have a cyclopean maze of gnarled, crooked, almost tentacle-like trees, each simultaneously toppling over, rotting, rooting, and springing back up at various levels, like an undead plant abomination. Like some kind of unholy mixture of redwood, mangrove, willow and baobab that just never dies and never gives up. $\endgroup$
    – Going Durden
    Commented Apr 2 at 7:52
  • $\begingroup$ The fundamental question is whether it could plausibly support the trees. If so, then it presumably would. The whole idea is that the trees are so successful that they've spread over as much of the continent as the climate allows. Their roots are already similar to mangrove roots, with wide skirts of thick buttresses extending out from the sides to support their weight, so it seems possible. They do occasionally collapse, of course, but new ones eventually take their place. $\endgroup$
    – Christopher Bennett
    Commented Apr 2 at 12:06
  • $\begingroup$ @ChristopherBennett One extra thing would be to not allow the fungi and bacteria that rot wood to exist in your world, or make them far less efficient. This would mean that even a dead tree can still stand for centuries, or at least until some kind of hurricane hits it. $\endgroup$
    – Going Durden
    Commented Apr 3 at 9:58
  • $\begingroup$ I don't see the value of that. Individual trees die all the time in a forest, but the forest still thrives overall, and the rotting of dead trees helps feed the growth of their replacements. Also, the decay of the smaller epiphytic trees and plants that live on the branches of the giant trees is the main source of nutrients for the giant trees, because the ground in a rainforest is less fertile than the canopy. So I want efficient decay. $\endgroup$
    – Christopher Bennett
    Commented Apr 3 at 13:56
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All right, here's a rough attempt at an improved map: Rulenau version 2

I didn't want to get entirely rid of the land in the far north -- is this good enough to get a reasonable amount of moisture continent-wide?

Also, about how much of an axial tilt would be preferable? Since Hadley cells help distribute moisture and heat, should I go for a larger tilt than Earth's for larger cells? Will I have to accept more severe seasons as a tradeoff?

Improve this answer
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  • $\begingroup$ Just make this an edit not an answer. $\endgroup$
    – John
    Commented Mar 22 at 19:54
  • $\begingroup$ Okay, I've done that. Thanks for letting me know. $\endgroup$
    – Christopher Bennett
    Commented Mar 22 at 21:36

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