Its normally said that deserts cool planets because they reflect more radiation back to space. According to this logic if all the land mass in the world was desert the world should be colder. Is it true that the average temperature of a desert is higher than green land in a similar climate? If so wouldnt the average surface temperature be higher if all the world was a desert? And if so how can we still claim deserts cool the earth? Isnt the metric of whether we have global warming, the average surface temperature?
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$\begingroup$ The Sahara is a net heat loss. The Earth loses heat through the Sahara. I haven't modelled it, but if all the Earth was a desert, I think it would be a pretty cold place with very little Greenhouse Effect for lack of water vapour. $\endgroup$– gerrit ♦Jan 30 at 7:57
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1$\begingroup$ many of the questions you ask here is answered @ en.wikipedia.org/wiki/Albedo $\endgroup$– trond hansenJan 30 at 7:57
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Water vapor is a powerful greenhouse gas. The key characteristic of a desert is lack of precipitation, and that is typically correlated with a low humidity. That reduced humidity means more thermal infrared radiation going from the Earth's surface to space.
Desert doesn't necessarily mean hot. A place is qualified as a desert if it has very reduced precipitation. The Atacama Desert, one of the (if not the) driest places on the planet, can get quite cold. Almost all of Antarctica is technically a desert, and it is very cold. At the height of the last glaciation, much of the northern portion of the Northern Hemisphere was technically a desert, and it too was very cold.
On the flip side, a billion years or so from now, the Earth is predicted to go through a warm greenhouse phase due to the Sun getting more luminous. The oceans will evaporate, some water will be split into hydrogen and oxygen, and eventually the hydrogen will escape into space. At that point, the entire Earth will technically be a desert as there will be no water anywhere, and it probably will be hot.
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$\begingroup$ If the atacama desert was regreened would it be colder or warmer? $\endgroup$– mudpuppyJan 30 at 12:21
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$\begingroup$ If you say if the entire earth is desert a billion years from now because of runaway greenhouse, then according to conventional climate science shouldnt it be cold because deserts escape more heat? Why do you say it is hot? $\endgroup$– mudpuppyJan 30 at 12:24
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$\begingroup$ @mudpuppy Because the Sun will be hotter and presumably the CO2 levels will increase. The predicted increase in free oxygen due to photodecomposition of water will oxygenate whatever material is available, and since hydrogen won't be available, it will combine with carbon. The Earth will eventually become Venus 2.0. $\endgroup$ Jan 30 at 12:46
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$\begingroup$ @mudpuppy Geoengineering has lots of unintended consequences. Besides, why would we want to regreen the Atacama? It's one of the best places (if not the best place) on the planet for astronomy. $\endgroup$ Jan 30 at 12:53
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$\begingroup$ The question is more about if the atacama was naturally green what would be the effect. Climate scientists say that regreening deserts will heat it up, not cool it down. climatefeedback.org/claimreview/… . But I think this may be wrong... $\endgroup$– mudpuppyJan 31 at 13:33
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Blisteringly hot at day, freezing cold at night
Assuming that "deserts" here mean sandy places with extremely dry air, then it will get really hot at day, but freeze over at night.
Deserts, which are mostly made up of sand (excluding a few places of bare rock and stuff), tend to act like mirrors. Sand has an extremely low specific heat capacity i.e. it heats up quickly with much less energy, but also cools down really quickly. Sand also has a really high albedo i.e. it is very reflective, so it tends to reflect away sunlight. Dry sand also has the nasty habit of reflecting away infrared (wet sand, on the other hand absorbs IR). This gets even worse, as desert air is generally extremely dry, with no water. If it had been humid air, there would atleast have been a chance of water absorbing this heat and thus reducing the temperature (Water has a extremely high specific heat, second to only hydrogen). But with no water vapour, there is nothing to absorb the heat.
Assuming that we switched Earth's entire surface with just sand and a few areas of exposed rock, Earth would look like Mercury on steroids. The dayside would get ridiculously hot, perhaps hundreds of degrees, as the sand reflects infrared and visible light back into the air, heating it up rapidly. However, since sand has an extremely low specific heat capacity, it will rapidly cool, once daytime has ended. At night, the temperatures may plummet to below freezing, as the sand quickly releases its heat away.
TlDr Blisteringly hot at day, but freezing cold at night
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1$\begingroup$ Antarctica is technically a desert. It does not get anywhere close to "blistering hot" at day, and its day lasts for up to six months. $\endgroup$ Jan 30 at 11:22
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2$\begingroup$ @SteveSaban It is not twice as cold. The lowest temperature at the North Pole is not the lowest temperature in the Northern Hemisphere. (That honor currently goes to a spot in Greenland.) Moreover, the only way one can legitimately say "twice as cold" (and then it would be better to say "half as warm") is to use an absolute temperature scale such as the Kelvin scale. While the South Pole is definitely colder than is the North Pole, it is not "twice as cold." $\endgroup$ Jan 30 at 20:07