What would Earth be like with a 45 degree tilt?
If Earth had a 45 degree tilt, everything north of 45 degrees (even Seattle, Washington) would technically be in the arctic circle, and everything south of 45 degrees would be in the antarctic circle (which surprisingly adds little more than the southern tips of south America and New Zealand). You could see the midnight sun in Paris on a summer day.
The weather at the poles would also be more extreme. The polar winters would be even harsher than they are now because for months at a time even more land and see would never see sunlight.
south of 45 degrees south, that is...
But you'll get a lot more sun at the poles on average and less in the tropics.
At the north pole you'll still get a half year of sun in the summer, but now it goes through an altitude of 45 degrees and not 23, so you'll get sin(45)/sin(23) = about 79% more sun on average.
That's also true. Arctic summers could be extremely warm because of all the extra sun, and there would be more seasonal variation in the tropics (the equator would be at its coldest in the summer and the winter, though this is true now too)
um, no, i don't think that is the case.
assuming that the Earth would continue to spin and continue to revolve around the sun, every spot on Earth would see the sun 50% of the time on average as each spot does now. this is not to say that some spots on Earth don't get a more direct hit from the sun at noon.
The angle of incidence is very important in terms of ground heating. The poles would experience a 45 degree angle of incidence at the summer solstice compared to the 23 degree maximum angle that actually occurs. Meanwhile the sub-solar point would dip to 45 N&S at the solstices meaning the angle of incidence at the equator at noon is 45 degrees at each solstice compared to the about 90-23= 67 degrees that actually occurs.
It's precession of the equinoxes would likely also be more extreme. Precession is one factor that has been attributed to the cause of ice ages. See Milankovich Cycles:
While this is true, I think a 45 degree tilt would work against an ice age. The key to continental glaciation is a perennial snow pack. With the amount of insolation over the polar regions due to the sun circling at high angles (peaking at 45 degrees at the solstice), it's unlikely that significant winter snow could survive though the summer even with the other conditions preceding the last ice age. Your reference pointed out the importance of temperatures in July at 65 degrees N. Note the 45th parallel would correspond not only to the Arctic circle, but also the Tropic of Cancer in the northern hemisphere. The sub-solar point would be only 20 degrees south of the of the 65th parallel at the solstice.
I can't say an ice age would not occur, but the conditions at the outset of the Pleistocene glaciations were just sufficient to get the glaciers started with a 23 degree tilt according to the article you posted.
Extraordinary climates of Earth-like planets: three-dimensional climate simulations at extreme obliquity goes into a lot of detail, but mainly for the extreme case of 85d tilt. It also discusses simulations for 54d and 70d.
So one has to interpolate for the 45d case.
High-latitude summer temperatures go up as one might expect, and there is likely less persistent ice. Low-latitude temperatures go down, since the Sun is close to the zenith for less of the year.
[0807.4180] Habitable Climates: The Influence of Obliquity
Climate of an Earth-like Aquaplanet at High Obliquity
but they are more difficult to interpret.
Seasonal differences would be much greater. There would be repercussions for very long cycles of light intensity.
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