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The conclusion of my question Is it possible to have a planet unsuitable for agriculture? was that the most plausible reason for preventing agriculture from developing (beside nomadic herding) was having arid planet with erratic rainfall.

Is arid planet where climate is very unpredictable possible?

The effect I want is that farmer never knows when and how much rainfall there gonna be on his fields, it might be too much, just right, or none at all, and every year is a gamble.

If that kind of planet is possible how would it look like (moon(s), oceans, landmass, axial tilt, atmosphere, placement in the star habitable zone).

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    $\begingroup$ Re "farmer never knows", you might explain just how this is different from most of Earth. $\endgroup$ – jamesqf Oct 14 '16 at 16:40
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Possibilities:

  • Supercontinents like Pangaea magnify climactic variation causing large swings in temperature sand precipitation seasonally.
  • High air pressure results in high partial pressure for water vapor, and means more vapor is needed to cause rainfall. If solar insolation, and thus evaporation potential, stays the same then rain is less likely to fall.
  • Low insolation can result in a cool dry world, covered in steppes and tundra like Northern Asia. These areas have low rainfall, large temperature swings, and even today are marginal for agriculture.
  • Lack of topographic variation (i.e. no mountains) will reduce rainfall from orographic lift, and prevent cold mountains and their snow-caps from acting as 'water-towers' for surrounding dry lands.

Combine any/all of these factors as needed.

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  • $\begingroup$ So world with a supercontinent, dense atmosphere, on the outer edge of habitable zone which is quite flat. $\endgroup$ – Platypus Oct 14 '16 at 16:19
  • $\begingroup$ Yup you also want as little vegetation cover as you can, vegetation supports a regular rain cycle, less vegetation will reduce rainfall as well as less rainfall reducing plant growth. $\endgroup$ – Ash Aug 7 '17 at 15:13
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I lived in Iceland for two years, which has a pretty erratic climate (to the point that Icelanders deny that they have a climate, but instead have random samples of weather).

What you need is a planet where the land exists only at places where two large-scale weather-driving systems come into conflict, such that the border between the two systems moves back and forth over the space in question.

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One possibility would be an erratic orbit around a variable star. The warming and cooling due to the two interacting systems would result in very erratic weather systems and while the results of the erratic orbit would be reasonably predictable those due to the variability of the star would not.

Be aware that increases in luminosity tend to also be accompanied by massive doses of gamma ray and x-ray radiation. The atmosphere may mitigate this effect but it's still going to make life difficult.

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This would depend somewhat on the type of crop.

If you were importing Earth species, then the erratic climate would, indeed, make it impossible to grow them. However, the indigenous species would have evolved with the variability, and would have developed ways of coping.

This is likely to be an irregular life-cycle, which is triggered by the environment. For example, plants which can live for years in semi-hibernation, as something like a bulb. When the rains come, everything bursts into life, flowers frantically, and tries to set seeds before it gets frozen back to just a root bulb. Sometimes, the good times last long enough for a half dozen cycles, and sometimes they don't even make it through one cycle. The pollinators would also need to hibernate indefinitely.

Since flowering is a high-risk operation, plants would probably also reproduce asexually by budding from the root bulb.

If humans could eat the indigenous plants, this would mean long periods of living on roots, and greens being a luxury. Assuming your characters are human, of course.

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