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I an working on a Si-Fi world, and am looking for ways that an area would, over a long timescale, lose the majority of its water. I understand how the lose of vegetative cover due to climate changes and overgrazing can lead towards desertification, but how does the dryness come about, and if you can answer it, how can I justify this on a more planetary scale?

I am also not looking for human-caused changes, but things that naturally came about.

Assume an Earth-like climate before the change.

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  • $\begingroup$ Do you really want to exclude all human-caused changes, or just the ones since the Industrial Revolution began? Otherwise we might run into whether all biological-based climate change such as The Great Oxygenation Event should be excluded. $\endgroup$
    – Spencer
    Apr 3, 2022 at 15:00
  • $\begingroup$ All, humans came to the planet probably millions to billions of years after it became dry. $\endgroup$
    – Zoey
    Apr 3, 2022 at 19:09

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The major reason for aridification is the decline and eventual loss of rainfall. The reasons for this are numerous:

Plate tectonics - continents moving from one location to another. An example of this is Australia. It was once attached to Antarctica. It was also lot lusher than it is today, but as it slowly moved northwards it moved into latitudes which had less rain and dried out.

Orogeny - which is the formation of mountains, particularly on the western side of continents, interrupts the flow of moist air from oceans over continents. In moving over the mountain ranges the air must rise resulting in rain zones on the oceanic side of the mountain ranges and rain shadow regions on the other side of the mountain ranges, The other result of this is the drying out of land on the rain shadow side of the mountain ranges. Examples of this are the Patagonian Desert, in Argentina, and the deserts of North America.

Excessive volcanic activity can lead to volcanoes emitting excessive amounts of carbon dioxide, which along with other events can tip the balance regarding global warming. Likewise, excessive volcanic inactivity can lead to an excessive decline in atmospheric carbon dioxide which would lead to global cooling and a loss of rainfall in some regions.

Changes in Ocean Currents - can also cause aridification. The changes can be changes in direction which can result in the movement of either warm or cold ocean water which disrupts what was previously there. If the Gulf Stream were to collapse one of the results would be a significant change in the rainfall patterns over Europe. This would be an example of a shutdown of thermohaline circulation.

Prolonged changes in El Niño and La Niña events in the Pacific Ocean can seriously disrupt rainfall creating either too much or too little rain various regions. The Indian Ocean has a similar system called the Indian Ocean Dipole. All this can have serious implications for east Africa, southern Asia, Australia and North and South America.

Land salinization occurs when salt from below the surface of the Earth is brought to the surface by excessive land clearing for agriculture and the water rises bringing with the subterranean salt, as has happened in many parts or the world that have cleared land for agriculture. The other cause of land salinization is the over use of irrigation for agriculture, as occurred in Mesopotamia. All agricultural "fresh" water contains small amounts of salt which accumulates in soil over time. Land salinization results in the loss of vegetation and land cover. Large areas of land salinization have resulted in desertification and aridification of regions.

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  • $\begingroup$ Thank you for your assistance Fred. It is most appreciated. $\endgroup$
    – Zoey
    Apr 3, 2022 at 19:10
  • $\begingroup$ You forgot glaciation. Also, land salinization is a problem even in wet environments, so not a cause of "aridification". e.g. cbc.ca/news/science/… $\endgroup$
    – Spencer
    Apr 3, 2022 at 19:16
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Glaciation

causes short-term (several thousand to a few million years) changes in rainfall in various regions on the Earth.

Regions that are normally dry can go through a "pluvial period", that is, one of increased rainfall, in which they green up. When the ice retreats, the rain goes away and these areas become dry again. The Sahara and the western US are great examples.

Similarly, areas that are wet in a warm period can become dry during an ice age.

Glaciation also causes a sea level drop, ans subsequent changes from meltwater and isostasy, and this makes ocean currents and littoral climate zones move around.

Finally, the ice sheets themselves can be as rainless as Antarctica, which has sometimes been called "the biggest and dryest desert on Earth". The Dry Valleys of Antarctica are the dryest ice-free areas on the planet.

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