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This is a water world with depths ranging from 2-3 kilometers deep up to vast shallows of only a few meters deep usually around volcanic islands, where there are coral islands not bigger than Hawaii's Big Island.

The planet has a 23 hour day, gravity is just slightly lower than the Earth, the large moon causes tides to be twice as strong as on Earth, it has an axial tilt of 14.6 degrees and has permanent ice at both poles, while the temperature at the equator is on average 30 degrees Celsius.

The main feature on this world are the multitude of floating islands of vegetation that are several hundred kilometers in diameter with a handful that are over a thousand kilometers in diameter.

These plant islands come in several varieties from massive Sargassum like seaweed creating soggy mats of weeds that break up and come together regularly, to colonies of plants that twine together on the surface hardening into a wood like island between 2-10 meters thick and several hundred kilometers in diameter while its roots form an entire ecosystem under the water.

The islands would be too low to affect the wind, but with tens of thousands of these islands floating around the ocean, would it help keep waves from becoming too large, and possibly help to dampen the numerous storms and hurricanes?

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Seemingly unrelated, but very related at the core:
How do you spot oil slicks on the open ocean?
Answer: You look at the waves!

This is what happens when SAR imagery from planes or satellites is used to spot illegal oil dumps. The trick is, that the radar reflectivity of the sea surface depends on the presence of surface waves of a specific wavelength. And the oil slicks look dark in SAR images, because they dampen the short waves that are abundant everywhere else on the ocean. This works even with the thin contaminations that result from ships cleaning their tanks.

So, if a thin film of oil can sufficiently dampen waves with a wavelength of several centimeters, it stands to reason that a thick floating island can easily dampen waves of many meters in length.

Of course, the shore line of your floating island will go up and down with the waves, but if you go some hundred meters away from the shore, all but the largest waves should be entirely dampened out. The total dampening of a floating island should actually be more than the dampening of a rock island, because the floating island won't reflect the waves. It will actually absorb them and turn them into heat.


Tsunamis are no threat to floating islands, by the way: Tsunamis are waves with extremely long wavelengths. They only become dangerous when their wavelength is reduced by the low water depths near shores. As long as there is enough water beneath a floating island, it will just move a bit up and down when a tsunami passes underneath.


Storms are a big problem for your floating islands, though. Hurricanes have immense power, and they will tear at the trees on the floating islands. And with the high equatorial temperatures, you have to expect many strong hurricanes. The hurricanes will threaten to tear your floating islands apart.

You might work around this problem by suggesting that the floating islands consist of plants with extra long horizontal roots, which actively entangle themselves with each other. They actually evolved to stabilize the floating islands. If you have a several meters thick layer of entangled roots, you should get some robustness against even hurricanes. This should work out, since the forces exerted on land by storms are similar for nearby points: The whole island will feel an enormous amount of force acting in a certain direction, but the forces trying to tear the island apart will be orders of magnitude smaller.


The greatest danger to your floating islands, however, is that they will drift between different climate zones. The ecosystem of the floating islands needs to be able to survive drastic climatic changes. They need to be able to strive with hot, wet, equatorial weather, as well as with dry sunny weather, or frosty conditions, changing on a timescale of years.

How you adapt your flora and fauna to this, is up to you. I wouldn't think, its impossible to do, but it requires some thought to come up with a sketch of an ecosystem that can tolerate such drastic climate changes. Maybe some species will drastically change their appearance with the changing climate, while some others will become extinct outside of their home climate and only survive in the form of spores. This could lead to quite some interesting dynamics, imho.

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  • $\begingroup$ I was thinking about the climate zones, that at least with some of the islands, they would have broad leaves on the topside, creating a forest like appearance of man size and bigger sails. As one side gets too cold or too hot the leaves change position and slowly shift the island with the wind. It wouldn't always work, but it would help keep most of them where they should be. Others would just bear it until they got really far out of their comfort zone, then breaking apart explosively, throwing off spores and seeds so some survive. As for the roots, they'd be thick and long to stabilize it. $\endgroup$ – Dan Clarke Feb 28 '18 at 17:08
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Waves

Depending on how close or far the islands are from each other, the surfaces waves would break on the islands further from the center, keeping the ones near the center from seeing large waves. Also, with the fact that the islands are coral, that means that the energy from the wave can break on the coral before they actually reach the shores of the island, reducing the size of the waves that the islands are seeing.

Hurricanes and Storms

As for hurricanes, the sheer number of them prevent any storm from really getting too bad, because in order for hurricanes to gain any power, they need to have vast, open, areas for them to gain energy due to cold air interaction with the warm water. As for storms though, they can form anywhere so they won't really be influenced. Given the fact that the equator temperature is 30 degrees Celsius you would almost certainly be seeing regular tropical storms around it, that could travel with the wind to any of the islands, although tropical storms would be the worst your planet would see.

Tsunamis

The biggest issue that the islands would have is flooding due to fluxes in weather or tsunamis caused by island rock falling into the body of water that it's floating on. The coral around the islands will help dampen these but they won't stop them completely. Now the free floating bodies of dried seaweed like plants would help mitigate the effects of them if they were stable enough to survive a heavy impact, because if it were to hit one of those large bodies then the tsunami would loose it's power because of the amount of energy transfer that happen at once.

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  • $\begingroup$ Tsunamis will definitely be a problem. I'm thinking that they, plus fire and disease will be the main reason for the wooden islands to break apart. I like the idea of several tropical storms a year, rather than hurricanes, adds danger without being overpowering. Thanks for the great answer. $\endgroup$ – Dan Clarke Feb 28 '18 at 15:46
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Tidal waves: these are formed involving the entire volume of water. Something affecting only the surface like your floating islands will do pretty much nothing on them. I expect their influence to be noticeable where the height of the highland is no less than half the depth of water.

Surface waves: here your islands, provided they are sufficiently clustered, will scatter the waves and mitigate their influence on the coasts.

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They might affect the wind more than the waves.

If the islands are flexible, they will move up and down with the water, and won't sap much energy from the wave.

With the sun hitting the island, making it warmer and changing the air pressure, it might cause the wind to change.

A lot of weather is created by the differences of land & water. Without land your planet's weather might be a lot more homogeneous, and might not get many hurricanes.

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  • $\begingroup$ Thanks, about the weather, I've read that it could be near constant hurricanes, or very boring and homogeneous. The floating islands, while also being a cool image and setting, is a way to deal with both of those problems, by adding in constantly changing variables. Good food for thought. $\endgroup$ – Dan Clarke Feb 28 '18 at 15:44
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I know things like floating ice or lots of jellyfish do dampen the waves (at least near the shore)

Dampening waves should have no effect on wind speed or rain during storms and hurricanes.

Changing surface area of the ocean, and its ability to absorb or reduce heat will have effect on weather, but I do not know enough to see which way it will go

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