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I have figured out a fair bit of processes for my fictional world but am having trouble trying to decide if this desert is a realistic size.

I have highlighted the area in yellow and a rainforest in green. The arrows show the air currents and the ocean currents are similar. The continent is about 152 million square kilometers(Afro-Eurasia is about 85 million kilometers). The desert is roughly 21 million square kilometers. For scale, Asia is around 44.6 million kilometers. The world has an average temperature of 18C.

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I know it is a bit wonky but I am traveling and don’t have access to a computer.

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  • $\begingroup$ The only way I know that desserts influence other climate zones at a distance is by the dust that gets blown around. Here in Europe we sometimes have deposits of Sahara sands. In China they have the dust of the Gobi desert. The influence is minor, but noticeable. The influence of the Gulf Stream is far more dominant here in Europe. The question is still a bit open ended though. $\endgroup$
    – Menno van Lavieren
    May 31 at 12:36
  • $\begingroup$ The size of the desert isn't your problem - it can be any size you want. The problems arise in rationalizing the desert. You don't explain the nature of your desert (there are many kinds of desert) and there's nothing stopping you from rationalizing a very large desert by using a combination of desert types. Once you've worked out the type(s), it's a matter of fixing the topography to rationalize them. Please remember not to get too wound up in the small details. ... (*Continued*) $\endgroup$
    – JBH
    May 31 at 12:47
  • $\begingroup$ ... We only have one data point to work with - Earth - so "realistic" has severe limitations. What you're really looking for is believable. $\endgroup$
    – JBH
    May 31 at 12:47
  • $\begingroup$ @JBH I guess it would be an interior rain shadow desert with a surround grassland/steppe mix. $\endgroup$
    – Martamo
    May 31 at 14:33
  • $\begingroup$ @Martamo Thanks, but never trust people to read through comments to find details. Please edit your post with that information. $\endgroup$
    – JBH
    Jun 1 at 6:23

2 Answers 2

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As per your comment on @TheDemonLord post, you're asking about how a desert about the size of North America could exist and how it would affect nearby biomes. First, it's important to know what type of desert you're talking about. There are two different types of desert I could see your desert being, and each forms differently.

Based of there being a nearby rainforest, your desert could be a subtropical hot desert. These deserts form along the Tropic of Cancer and Tropic of Capricorn, but since tropical rainforests form along the equator, you might need to change their positions or come up with another hand-wavy explanation for why the desert and rainforest are on the same parallel. If, though, your desert is a subtropical hot desert (the Sahara is an example of a subtropical desert), the surrounding land would most likely be tropical grasslands as tropical grasslands typically form around subtropical deserts and tropical rainforests. Your desert could get as big as it is through climate change. Desertification, the process of grassland turning to desert, is caused by deforestation, over-grazing of animals, and the earth heating up. If these processes occurred naturally (or unnaturally depending on when your story is set) then that could be an explanation for how your desert grew to be so large, however you should probably align it more along one of the Tropics so that it has a better chance of existing.

The other option is for your desert to be a rain shadow desert. These deserts form on the leeward slopes of mountain ranges (leeward meaning facing away from winds, so you would need to flip your wind direction). These deserts form when moist air collects of the wind-facing side of a mountain, causing the leeward side to get very dry. Death Valley in the USA is an example of a rain shadow desert. Real rain shadow deserts don't grow to be very big because they eventually reach too far away from the mountain that is preventing them from getting moisture, so you would need another hand-wavy explanation for that. The other side of the mountain, although being wet, would be cool and therefore your rainforest would more likely be temperate than tropical. Rain shadow deserts in the USA are bordered by temperate grasslands and temperate deciduous forests.

Those of the types of deserts I find most likely be yours. Both require a bit of hand-waving because of how big your desert is, but you can choose which one yours is based on what you want the surrounding land to be like and what characteristics you want your desert to have.

Here's some basic info on different types of deserts: https://education.nationalgeographic.org/resource/desert/

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    $\begingroup$ I will shrink the size of the desert and call it a rain shadow desert. I can make the surrounding land a grassland/steppe kinda of thing. I didn't know about the effect a rain shadow would have on the forest. It will probably be a temperate forest closer to the Lines of Cancer and Capricorn and then a rain forest closer to the coast and equator. $\endgroup$
    – Martamo
    May 31 at 13:35
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I mean, it's twice as big as the Sahara desert (which is the largest hot desert in the world) - but Frame Challenge time - Realistic by what metric?

Compared to earth, yeah it would be bigger than the biggest dessert (14 million Square Kilometres - the Antarctica Dessert).

But if this is a fictional world, with fictional world properties and you can justify it with reasons (like geography, air currents - which you are already doing) - then yeah, it's perfectly fine.

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  • $\begingroup$ I will add the science in a second. I’m not looking a a hand wave explanation for a desert nearly the size of North America. I am asking if this massive, arid blob can exist without screwing the climates around it. Also, what does “frame challenge time” mean? $\endgroup$
    – Martamo
    May 31 at 12:24
  • $\begingroup$ @Martamo It would affect the climates around it simply by sharing dust and possibly by heating the atmosphere. The central California desert affects San Franscico by pulling a lot of cold, damp air in during the summers. There will always be some effect on the climates around any land blob. $\endgroup$
    – David R
    Jun 1 at 14:03
  • $\begingroup$ @Martamo - a Frame Challenge on WB is where someone is questioning the premise of the question: For example - a person might ask 'why would X do Y' and an answerer will reply with 'Frame Challenge - X would do Z instead' - in your case - you asked if your desert was realistic - My point was - if it's a fictional world, you get to dictate what is realistic for that world. $\endgroup$
    – TheDemonLord
    Jun 1 at 21:45
  • $\begingroup$ @TheDemonLord That’s alright and dandy for a magical world. But one that is meant to be similar to Earth has more limitations. (Also, thanks for explaining that.) I also think I have got my answer. lost_not_found gave an excellent answer that wasn’t just, “It’s your world, you can do whatever”. $\endgroup$
    – Martamo
    Jun 1 at 22:03
  • $\begingroup$ @Martamo - exactly - similar can do an awful lot of heavy lifting before we get into the realm of Magical. The other answer ends with 'you have to do a bit of hand waving' - essentially the same point. $\endgroup$
    – TheDemonLord
    Jun 1 at 22:31

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