How would the earth be different if the axis were perpendicular to the orbital plane? I understand the equator would be hotter, but would there be an equatorial desert encircling the globe? Would the poles be colder or would the lack of seasons cancel the extremes? Would there be two habitable zones, one north and one south? How about oceanic and air currents? And would there be any effect on tectonic plate movement? And assuming the moon is on the same plane, how would tides be affected? And how about migration patterns and species’ extinctions?
Interesting question. I wonder if anyone’s run climate models on this. Of course, most serious models would probably have seasons baked in, but who knows.
My suspicion is that it would change the climate the least at the equator. However, the lack of seasons elsewhere would circumvent tropical wet/dry seasons – if we still had them, I bet they’d be for different reasons and with different timing (possibly not annual, so they wouldn’t really be “seasons”).
We probably wouldn’t have deciduous plants! A lot of ecologies would be very different.
We would still have some minor seasonality. The Earth’s orbit isn’t a perfect circle, so we’re closer to the sun at some times. And the sun’s output varies a little over time. However, these cycles are tiny compared to what we see now - we can be at perihelion during winter and not even notice. It would be more like comparing a cool summer to a warm summer than comparing summer to winter.
You would definitely see some extremes of temperature and rainfall in some areas because there’d be no seasons to even it out over a year. That would certainly be a problem for almost all existing species, but might not be bad if you had an ecosystem adapted for that in the first place. Without seasons, animals could reproduce any time of the year, and have a constant food supply throughout the year.
I don’t see any real changes to the major atmospheric patterns like the trade winds. These are driven more by the Earth’s rotation, land masses, solar radiation, etc.
I also wouldn’t expect any changes to plate tectonics. You’d change how solar and lunar tides affect the plates, but these are not major drivers in plate tectonics.
Am I correct that there would be northern and southern temperate zones, separated by a desert equatorial zone? And how close to the poles would plants be able to grow, bearing in mind that the poles would have a roughly constant climate?
Why would the equatorial zone need to be desert? Earth’s largest hot deserts are well off the Equator, on the latitude of northern Africa; Australia’s largely desert despite being in the southern tropical to sub-tropical zone. At the Equator, warm air rises and cooler air is drawn in to replace it, and where this travels over sea it will bring water with it.
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that lower axial tilt (obliquity) favors ice ages. See the whole article at Milankovitch cycles - Wikipedia for an interesting discussion of the engines that drive ice ages. What it suggests to me is that the planet might uninhabitable for humans outside the subtropical zones, although doubtless life forms would evolve for the tropics. Or humans would have evolved to be more heat tolerant.
If humans developed on such a world (and whether or not humans could is a question in itself), human cultures might be radically different. One of the most obvious differences would be a lack of seasonal cycles and holidays. No winter solstice festival, no vernal equinox festival, no harvest festival, nothing. No Halloween, no Christmas as we know it, etc. Anything inspired by the seasons is right out unless it somehow comes about by some other process.