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Could a planet's axial tilt change a meaningful amount (what is needed to make inhospitable biomes even more inhospitable: hotter dry hot desert, colder north) over an extremely short period (~100 years), without the interference of technology?

To give a little context, I'm building a fantasy world and would like the seasons to gradually "stabilize" in each area, and am particularly interested in making the desert more dire and have it expand a little with desertification of nearby areas; but without making the high latitudes (> 75-80°) more hospitable.

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  • $\begingroup$ My answer to this other question about forcing the tilt to change may be of interest to you. $\endgroup$
    – JBH
    Feb 16, 2018 at 15:52
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    $\begingroup$ Down-voted for complete and utter lack of research. $\endgroup$
    – RonJohn
    Feb 16, 2018 at 16:04
  • $\begingroup$ @RonJohn You're right about the title question, I don't know how I could not find the answer (as my mother would say: "if it was a dog it would have bitten you") but I really can't find anything related to the short period change. If you think it would be better, I'll edit the question so that it targets that part ("can it happen over just 100 years" instead of "can it happen over time") $\endgroup$ Feb 16, 2018 at 16:23
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    $\begingroup$ By itself, "Can a planet's axial tilt naturally change a meaningful amount in only 100 years from it's current cycle?" is an excellent question. (The answer is going to be "no", though. Waaay too much momentum...) $\endgroup$
    – RonJohn
    Feb 16, 2018 at 23:56
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    $\begingroup$ Note also... for would like the seasons to gradually "stabilize" in each area to happen, the tilt would have to be reduced to 0 degrees. That's a huge change for a planet with Earth-like seasons. $\endgroup$
    – RonJohn
    Feb 16, 2018 at 23:59

2 Answers 2

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Could a planet's axial tilt change over time naturally? ("naturally" meaning without the intervention of technology)

Yes, it can. It happens all the time, actually. The best example we have is our own. From a page in NASA's site:

As the axial tilt increases, the seasonal contrast increases so that winters are colder and summers are warmer in both hemispheres. Today, the Earth's axis is tilted 23.5 degrees from the plane of its orbit around the sun. But this tilt changes. During a cycle that averages about 40,000 years, the tilt of the axis varies between 22.1 and 24.5 degrees.

If so, what would be the fastest - even abrupt - way it could happen?

There are only two ways to do it abruptly (even by geological standards), and those would likely end life on the planet. You see, bending a planet's momentum in any way involves a humongous amount of energy. You could use impactors, but adding just enough energy to Earth so that we have an extra leap second takes enough energy to end life on Earth many times per second. Just imagine the mental exercise on that XKCD page, but with the impacts aimed at tilting the Earth rather than slowing its rotation. The other way would be a flyby of a gas giant/dwarf star/black hole, but that would not only destroy the crust, it could also screw the rest of the solar system along with Earth.

Can it happen gradually and not abruptly? If so, is there a chance it could happen over an extremely short period (~100 years)?

As seen above, it does happen gradually here - over dozens of millennia.

If you could change the tilt of the Earth by one degree in just one century, you would probably cause climate change faster than we are doing today with pollution. In the last 40,000 years, we went through glacial periods and back to "normal" once or twice.

Still from NASA's page:

More tilt means more severe seasons—warmer summers and colder winters; less tilt means less severe seasons—cooler summers and milder winters. It's the cool summers that are thought to allow snow and ice to last from year-to-year in high latitudes, eventually building up into massive ice sheets.

And from the wiki on Milankovitch cycles:

Increased tilt increases the amplitude of the seasonal cycle in insolation, providing more solar radiation in each hemisphere's summer and less in winter. However, these effects are not uniform everywhere on the Earth's surface. Increased tilt increases the total annual solar radiation at higher latitudes, and decreases the total closer to the equator.

(...)

Because most of the planet's snow and ice lies at high latitude, decreasing tilt may encourage the onset of an ice age for two reasons: There is less overall summer insolation, and also less insolation at higher latitudes, which melts less of the previous winter's snow and ice.

Consider that we are already screwed as it is today. Making climate change on your planet faster than that of 21st century Earth does not bode well for life.

Edit due to this comment:

can you add whether or not it could actually change over 100-200 years? Maybe not a full degree, but 0.5°? 0.25°?

We are currently changing tilt at a rate of 0.006 degrees per century. At that rate, species came and went in their own time... Having a change of 0.25 degrees in 200 years would be almost 42x as fast. It would probably extinguish a lot of ecosystems, but I think life could handle it. The planet has gone through five mass extinctions before and here it is, still full of life.

At the end of the day I am no specialist, and even if I were I believe there is not enough data nor context to rule out life resieting and thriving in some way, so it is up to you how your world handles that.

That being said, the general guideline is that less tilt gives you a colder world (and potentially ice ages), whereas more tilt gives you hotter summers and colder winters on temperate zones (should grant you larger, more abundant deserts).

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  • $\begingroup$ Excellent answer, thank you! :) can you add whether or not it could actually change over 100-200 years? Maybe not a full degree, but 0.5°? 0.25°? As I mentioned in the question, I'd like hotter deserts and a colder north, but without devastating life too much $\endgroup$ Feb 16, 2018 at 14:43
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    $\begingroup$ @Hankrecords the last time we had an ice age, the rainforrest in Brazil was a desert. Ice ages reduce sea levels and the amount of water (fresh and sea) available globally. Climate is an infuriatingly complex thing, so you may have more deserts even during an ice age. $\endgroup$ Feb 16, 2018 at 15:10
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    $\begingroup$ @Renan Don't forget the definition of a desert, which is to a very large degree simply about arid climate (lack of precipitation). IIRC, Antarctica qualifies as a desert. $\endgroup$
    – user
    Feb 16, 2018 at 15:18
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    $\begingroup$ @MichaelKjörling You're right, I meant specifically a hot and arid desert. I make the mistake of just generically calling it a desert far too often :) $\endgroup$ Feb 16, 2018 at 16:26
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    $\begingroup$ note the earths axis is fairly stable, as planets go, thanks to our large moon, without it we would see much more wobble. $\endgroup$
    – John
    Feb 16, 2018 at 18:50
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Could a planet's axial tilt change over time gradually and naturally (i.e. without the intervention of technology)?

If so, is there also a chance it could happen over an extremely short period (~100 years)?

Yes to both (almost), but the second part is tricky - and even so, it's not harmless.

You need a very unlikely impactor: a very large, very fast impactor, that must not hit the Earth (because it would destroy most life on it).

It must be large, but comparatively fragile; water ice and loose gravel. Basically a super-comet, several times larger than even the Sarabat comet of 1729.

It must hit the Moon, best if at almost a 90° angle, over the Mare Orientale. The greatest part of the ejecta will fall back on the Moon (some will hit Earth, but that should be survivable). The resulting impact will measurably alter the Moon's orbit, pushing it nearer the Earth. The Moon as seen from the Earth will start to apparently rotate (i.e. its rotation will remain the same, but will no longer match its revolution period).

The "new Moon" should cause a graceful - except for some earthquakes and tsunamis - wobble in the Earth's orbit, and some of the momentum exchanged will go into altering the axis' direction over the years.

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