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In my Book Series, I want there to be a vaguely Earth-sized planet called Awal with an Earth-like atmosphere. However, climate-wise, I need the subtropical climate zones in each hemisphere to cover 40% of the planet's surface when combined, just as the tropics do. The polar zones would not exist and the remaining 20% of the planet's surface would be filled by the temperate zones. How could I scientifically justify this phenomenon?

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  • $\begingroup$ What climate did Earth have in the 60 million years long Carboniferous period? What about the 80 million years long Cretaceous period? $\endgroup$
    – AlexP
    Sep 6, 2020 at 7:49

4 Answers 4

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Increase the atmospheric CO2.

As others have mentioned, the cretaceous and paleocene are believed to have had a climate exactly as you mentioned. These are known as equable climates, where the average temperature does not change much from the equator to the poles. The driving force behind this are hadley cells. This page explains how the cretaceous may have developed an equable climate. I believe it is taken from this paper if you would like to read further but I have not read it myself.

As the air at the equator heats up, it rises and as it rises, it moves towards the poles. As it moves towards the poles, it gets closer to earth's axis but due to the preservation of angular momentum, the air starts to move eastwards (the coriolis effect). - Think of it like being on a spinning pole, if you lean into the pole, it makes you spin a lot faster!

Increasing the height of the tropopause offsets the coriolis effect on moving air. Because the air is travelling in a more direct line to the poles, it 'reaches' higher latititudes - extending the hadley cells. Because hot air holds more water than cold air, this is also what determines the horse latitudes on earth - which are around 30 and -30 degrees. This is also why you see large deserts at these latitudes (the sahara, australia).

Increasing the amount of greenhouses gasses such as CO2, increases the tropopause height and allows warm air to move more efficiently towards the poles. The page I linked will give you more precise figures but as a reference, CO2 levels in the cretaceous were above 1000 parts per million, compared to 400ppm today. So not a huge amount is needed.

Water vapor also plays a huge role in climate so the configuration of your oceans will effect this too. If your planet's landmass is similar to Earth then you shouldn't need to worry about this. Since I don't have a degree in meteorology, this is the best answer I can give, however I will leave these links here to fill in any gaps. Explanation of equable climates Short video on Hadley Cells

Just to note, if your planet has hadley cells extending to your poles then it wont have the rainforest >> desert >> temperate >> tundra pattern that you see on Earth today. At a guess, I would say it would move from rainforest to temperate forest over a gradient. The north and south poles themselves would still recieve little precipitation so expect either tundra, taiga, or boreal forests. The deserts on your regions would be more influenced by local factors such as large mountain ranges.

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A thicker, denser atmosphere.

From what I understand, you could solve this problem with a thicker atmosphere. The thinner an atmosphere is, the more extreme the temperature variation is. Particularly between light and dark areas. The extra air tends to have a certain thermal inertia.

Mars is a little chilly, but not unbearable at equatorial latitudes, but has frozen carbon dioxide (dry ice) at the poles. That's a massive temperature variation.

Venus hardly has any temperature variation across its surface.

Earth falls somewhere in between.

In addition, the greenhouse effect would trap heat and allow you to have temperate regions at the poles, and an overall higher global temperature.

P.S. You might not want to go much more than twice as dense if you want it to be breathable by humans though. 2.5x is borderline upper limit for breathable air pressure, so probably kind of hard to breathe.

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  • $\begingroup$ +1 good. You might be able to get a breathable atmosphere at higher pressure with other gas mixtures eg en.wikipedia.org/wiki/Hydreliox but not sure it would be required 2 atm probably sufficient $\endgroup$
    – Slarty
    Sep 6, 2020 at 7:32
  • $\begingroup$ en.wikipedia.org/wiki/… this is a good article to read. Oxygen toxicity is dependant on the partial pressure of oxygen in the air. If your air is thicker, you need relatively less oxygen. It should be noted that the safest way to increase atmospheric thickness is with noble gasses (nitrogen) but these do not contribute towards the greenhouse effect. You don't necessarily have to increase the atmospheric thickness, just increase the amount of co2. seas.harvard.edu/climate/eli/research/equable/hadley.html $\endgroup$
    – Zac Walton
    Sep 6, 2020 at 11:38
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The location of the today's climate zones are driven by the 3 cell Hadley-Ferrel-Polar atmospheric circulation system. It's not actually known when Earth first developed this 3 cell atmospheric circulation system.

If I recall correctly, there is a theory that the current 3 cell system came about when the Antarctic circumpolar current was established after the Drake Passage opened up. The 3 cells may have already been forming but this action solidified it into what we know today. Essentially by allowing the polar south to become surrounded by freezing cold water it allowed the cold current to become isolated and limit mixing with the warmer currents. This lowered temperatures at the South Pole and helped established the up/down convergence/divergence air movements. Previously the global temperature average was higher and there was less variation in the latitude temperatures. So you could have a larger subtropical zone. (At least how I remember the theory).

If you have a landmass sticking across the polar region preventing this isolating current from forming, you would probably have a justifiably warmer planet. For example when South America landmass was connected to Antarctica (before the Drake Passage opened). If the atmospheric circulation is 3cell or some other variation is still up for debate.

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The Holocene climatic optimum around 11-8 thousand years ago, average temperatures in the "Temperate zone" was 2-4 degrees warmer so the "Sub-tropical" zones of those areas naturally extended

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