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It said that the Lord Ruler moved the planet too close to the Sun and it became too hot. So he kicked the ashmounts awake and let the ash cover the atmosphere and cool the planet down to a habitable temperature.

But I was under the impression that a lot of ash would lead to a greenhouse effect and cook the people much faster. So how does it work actually?

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IRL, volcanoes make ash and block the sun, cooling it down. – Mithrandir Feb 21 at 13:20
The year without a summer – Joe L. Feb 21 at 23:54
You don't get a greenhouse effect from ash. The main atmospheric compounds that produce a greenhouse effect are methane, carbon dioxide, water vapor, ozone, nitrous oxide, and CFC's. Some of those (CO2, particularly) are typically present in volcanic eruptions, yes, but not generally in sufficient quantities to counteract the cooling effect that the ash itself provides. Also note that we're not talking about normal volcanic eruptions; we're talking about Rashek manipulating the power of creation in a deliberate attempt to cool the planet. Perhaps he called up 'clean' eruptions? – aroth Feb 22 at 4:50
up vote 16 down vote accepted

The sulfur dioxide that spewed from the mouth of the volcano created a haze far up in the stratosphere, the upper limits of our atmosphere. The haze reflected sunlight away from the Earth, cooling the planet by almost 1 degree Farenheit (half a degree Celsius) over the two years that followed.

-Muse magazine, April 2010

So this is based off of real-life science. Volcanoes can cool the world.

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Sanderson likes to use real science, or rather like lots of authors attempts to give his books some real-ish explanations for events, instead of Entirely fake. – Himarm Feb 21 at 16:27
@Himarm I wouldn't know, as I've never read them. :P – Mithrandir Feb 21 at 16:32

A thick layer of ash, like from a volcano, would block the sunlight. The greenhouse effect has to do with letting light, and heat, in but not out, through translucent gasses - but the eruption would cause a cloud of ash and dust, one that is opaque. It would reflect both light and heat, so the area beneath it would be cooler... like it is cooler to sit in the shadow instead of the sun, or cooler in the winter when we're getting less warmth from the sun because of the tilt. So the planet would keep getting cooler until the ash was all settled - which might take a while depending on how much dust was made, and how much keeps getting picked up by the wind.

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