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DISCLAIMER: This question was closed on Astronomy Stack Exchange as off topic, so I'll try my luck here.

I'm trying to understand the climatic effects of the far future scenario of an Earth-like planet with a reduced rotational speed caused by tidal locking with the moon (day-night period of 28 days, one hemisphere always facing the moon, the other never facing the moon).

I understand that observations of the moon's surface temperature have shown day temperatures of 120°C and freezing cold nights of -230°C.

How would the day and night temperatures of tidal-locked Earth vary?

I am also interested in climatic effects caused by the reduced tides, coriolis force, increased evaporation, but this may reasonably be outside the scope of this question and it's answers.

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This should be similar to planets that are completely tidally locked to their star, since the day would be very long. There is several articles that study their climate. For example Joshi 1997 or Joshi 2003, or Yang 2013. While the Earth has difference in average temperatures between equator and poles around 40 K, these articles predict temperature differences 40-70 K for tidally locked planets with Earth-like atmosphere.

Earth, however, would not be completely tidally locked, it would only have very long day. This could be paradoxically more dangerous for its habitability. Earth is very vulnerable to freezing into a snowball state. If completely tidally locked to Sun, the insulated side of the planet would probably never freeze. With very long day, however, the freezing of the night side might trigger the global freezing of the Earth. According to Linsenmeier 2014, planets with high obliquity are less vulnerable to freezing, while planets with low obliquity are quite vulnerable.

However, the day length after tidal locking with the Moon would be 46.8 days, not 28 days, since the Moon gets further from Earth as the Earth rotation slows down, which in turn slows its orbital period. At this point, the Moon would be 1.4x further than it is now.

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The wide temperature ranges on the moon are also to a large part caused by the lack of atmosphere. Atmosphere tends to soften such effects.

Take a look at polar regions on earth for a better comparison. Midwinter and midsummer there can give very long periods of continuous daylight and continuous darkness.

Reduced tides would not have a large climate effect (you would have small tides from the sun and none from the moon but tides don't have a huge effect on weather patterns), reduced coriolis force would result in different weather patterns though and reduce the tendency and strength of storm cells. My recent weather question might help you there:

Creating a realistic world map - Currents, Precipitation and Climate

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  • $\begingroup$ The problem with using our poles as an example is that, while they do get sunlight for months at a time, it's at a very low angle. Insolation is very low, even at noon during the day, above the arctic circle. $\endgroup$ – Ryan_L Jul 12 '18 at 23:02

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