What if Earth had no axial tilt?

[Eternal_Freedom]

This is a question that came up over dinner with my parents today. What if Earth had never been impacted by that big old rock billions of years ago (and the Moon was instead a captured object rather than coalesced debris) and as a result never had an axial tilt?

The most obvious effect would be no seasons, but what effect would this have had on life and civilization?

[Guardsman Bass]

Ice Ages might be non-existent, since axial tilt changes are a big part of what causes them (Milankovitch Cycles).

Life on such an Earth would be very different. Little seasonality means that you'd have life much more adapted and specialized for its particular climate "band" of latitudes, but the overall habitability might be less since the poles and far north/south would be so consistently cold (in the absence of atmospheric and oceanic heat circulation). That type of consistently higher temperature gradient between the poles and equator might generate stronger storms and weather patterns, although I don't know enough about meteorology to confirm it.

If humans somehow did evolve, the stability of climates would be a boon for agriculture, at least in the areas where temperatures are such that you can grow crops. As long as temperatures stayed consistently above freezing, you could grow crops year-round.

[Brother-Captain Gaius]

Astronomy might be a bit more consistent, thanks to no axial wobbling. The appearance of the stars changes something around every 13,000 years (IIRC, it takes Earth about 26,000 years to do a complete 'wobble' on its axis). So the stars of the ancient Sumerians and our own might line up a bit better.

[Eternal_Freedom]

Astronomy might be a bit more consistent, thanks to no axial wobbling. The appearance of the stars changes something around every 13,000 years (IIRC, it takes Earth about 26,000 years to do a complete 'wobble' on its axis). So the stars of the ancient Sumerians and our own might line up a bit better.

This is true, and despite being an astronomer it hadn't occurred to me at all. In basic terms, Polaris wouldn't be the Pole Star anymore which would make navigating by the stars harder (unless there was a corresponding bright star at the new north celestial pole, but I don't know offhand if there is one close enough.

[Esquire]

There wouldn't be any migratory birds, right? That might do some goofy things to plant distribution.*

*Or not. Anybody know either way?

[Jerry the Vampire]

I doubt any kind of complex life would evolve.

[Borgholio]

I doubt having seasons is the only reason complex life formed.

[Guardsman Bass]

I think it would make it more likely that complex life would evolve. Having to adapt to changing seasonal conditions imposes "costs" on wildlife and plants, and is probably part of the reason why you see the greatest biodiversity in warm, wet areas with relatively stable climates (AKA the tropics). But in No-Tilt Earth, climates would be relatively stable in particular climate "bands" around the Earth barring the effects of oceanic and atmospheric circulation. It would be much easier for life to adapt to particular conditions in that situation, and go into greater specialization if the niches are there for it.

[Ziggy Stardust]

On the other hand, a major theory among evolutionary scientists is that the reason complex life evolved at all is because of the cost imposed by changing conditions. Without this force, there isn't as strong of an impetus for adaptation at all.

For example, the tides caused by the moon allowed for these coastal intertidal zones that would be flooded at some parts of the day and exposed at others. It is thought that the only reason life was able to move from the oceans to the land was because of creatures becoming stranded in these intertidal zones and forced to adapt. So over time some creatures learned to live when partially exposed to air.

EDIT: Also, IIRC, the reason for rainforest biodiversity isn't so much that they are stable climates, but rather that they are geographically old environments. In addition, the rainforests are ecologically diverse: although the tropics band stretches across the globe, there are immense local changes caused by mountains, volcanos, rivers, oceans, soil types, altitude, etc.. This allows for diversity because there are different pressures in different parts of the tropics, not because they are one large homogenous environment. In fact, you could argue the rainforests aren't stable - although they aren't subject to the extreme hot/cold seasonal shifts, for many forms of life the difference between the wet and dry seasons is even more taxing. The seasonal flooding of the Amazon River completely changes the way the entire region looks - from a forest to a massive swamp. Certainly, the climactic conditions are a PART of the story of tropical biodiversity, but I don't think you can say confidently that it is the only or major reason.

[PainRack]

Would warm/cold wind and currents like the Gulf stream still be present on such a world, and if so, would it still be strong enough to affect the climate?

One of the most strangest thing to me was learning that Iceland is relatively warm due to the currents and other geographical reasons

[Alyrium Denryle]

Would warm/cold wind and currents like the Gulf stream still be present on such a world, and if so, would it still be strong enough to affect the climate?

One of the most strangest thing to me was learning that Iceland is relatively warm due to the currents and other geographical reasons

They would exist but they would be significantly different.

You have to remember that the prevailing winds would change massively. The earth's axial tilt causes them to change. At equinox they look like this.



At summer solstice (Northern hemisphere) they are shifted by ~23 degrees so that the equatorial trough exists along the tropic of cancer. This is, for example, why certain places have wet and dry seasons. These seasonal changes are very important for how life functions on this particular planet. Without them, the leeward side of any given continent would never change, and only major storm systems would ever bring in enough moisture for rain. You would end up not only with very stark temperature gradients, but stark east-west and coastal-inland moisture gradients as well, that alternate based on latitude.

[Irbis]

For example, the tides caused by the moon allowed for these coastal intertidal zones that would be flooded at some parts of the day and exposed at others. It is thought that the only reason life was able to move from the oceans to the land was because of creatures becoming stranded in these intertidal zones and forced to adapt. So over time some creatures learned to live when partially exposed to air.

Um... How would no tilt affect tides? If there is Moon, there will be tides, tilt or no tilt.

Speaking of Earth, there could be equatorial band of lethal WBT where little in the way of larger animals could exist, making potentially 2 very different branches of life to evolve (think Australia, but on much greater scale), at least until humans wouldn't spead and screw things up for everything like they did in real life, possibly.

[Ziggy Stardust]

Um... How would no tilt affect tides? If there is Moon, there will be tides, tilt or no tilt.

I misread the OP, for some reason I thought it said that "no Moon" was part of the scenario.

[Borgholio]

Yeah if there were no moon and no axial tilt, the planet would be a far quieter place than it currently is. There'd still be weather patterns due to geography but no major seasonal changes.

[madd0ct0r]

For example, the tides caused by the moon allowed for these coastal intertidal zones that would be flooded at some parts of the day and exposed at others. It is thought that the only reason life was able to move from the oceans to the land was because of creatures becoming stranded in these intertidal zones and forced to adapt. So over time some creatures learned to live when partially exposed to air.

Um... How would no tilt affect tides? If there is Moon, there will be tides, tilt or no tilt.

Speaking of Earth, there could be equatorial band of lethal WBT where little in the way of larger animals could exist, making potentially 2 very different branches of life to evolve (think Australia, but on much greater scale), at least until humans wouldn't spead and screw things up for everything like they did in real life, possibly.

It would change how tides vary in the long term. Since the Earth’s axis is tilted 23.5 degrees with respect to the plane of its revolution around the sun, the combined effect is that the Moon’s declination varies from 28.5 degrees to 18.5 degrees in a cycle lasting 18.6 years.
An important lunar cycle, called the nodal period or Metonic cycle (after Greek philosopher Meton, fifth
century BC, who discovered the phenomenon) is 18.6 years (usually expressed in round figures as 19 years). For a tidal value, particularly a range, to be considered a true mean, it must be either based upon observations extended over this period of time, or adjusted to take account of variations known to occur during the nodal period.

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