What would the weather or environment be like on a mostly-water-covered world?

Question

I started a Minecraft world one day and decided I'd challenge myself by setting the sea level to 100, a rise of 36m from typical. Besides the fact that the tops of mountains and plateaus are still above water, a quirk of the Minecraft world means that any water in mountain areas above a certain elevation will freeze, so I found myself with a lot of ice surrounding the few bits of land that poked above the water. Now, I know this would probably be unrealistic if it were applied to Earth or any other planet, so I figured I'd see if Worldbuilding had any similar questions. I have not found one yet.

My question is: what might the weather and/or environment look like on Earth after it underwent a sort of Waterworld-like scenario where the water levels rose considerably? Ideally, this would be looking at a situation after whatever had caused the problem in the first place had gone away or calmed down. If whatever caused the Earth to undergo this would by its very nature need to stick around, then by all means let me know and figure it into an answer. Thousands of years may have passed.

I mention the Minecraft scenario because it made me wonder if the tree level of mountains would change, if the trees would start being able to live "further up" than they had been before. Would there be less snow on previously snowcapped mountains? On one hand, perhaps survivable conditions would now exist where they didn't before, such as on Mt Everest. On the other hand, just because the sea level rose, it won't mean that the nutrients in the soil would have improved any.

Answer 1

First, there are two ways to look at this question

1. An earth-like planet that becomes a water world.

2. A world that naturally evolves as a water-world.

Those are important distinctions. An earth-like planet would either need an "explanation for the Biblical flood" solution (e.g., something squeezes all the water from the aquifers type of thing) or a "water was added to the system" type of solution (e.g., an ice asteroid hit the planet bringing enough water to flood it.) Either way, given that things are allowed to "settle down" into their new water-covered state, what would you have?

What you'd have is ice—and a lot of it. From this perspective Minecraft is right. Now you mentioned a rise of only 36 meters from typical. Let's use the seal level simulator from Floodmap.net.

• If you set it to 36 meters, you don't have a water world. Most of the landmass is intact. You don't have ice in this scenario as the world hasn't really changed. You'd have a bit more polar ice, but that's about it.

• If you set it to 500 meters, you have more of a water world, but there's still a LOT of land. I wouldn't expect ice in this scenario, but people are at measurably higher elevation, so a colder climate would be expected. Your polar ice would be pretty big and global warming would no longer be a problem.

• At 1,000 meters I'd claim we have a true water world, but there's still enough land (go look at Greenland, the world's new superpower...) that you could save everyone on Earth today and probably feed them (that's a lot of ocean for fishing...). Except for the ice, which likely covers the earth.

The problem is that simply changing Earth means that you have the normal heating conditions of the sun and what we know of high-elevation temperatures coupled with a higher rate of cooling (a LOT more water in the atmosphere and much more evaporative cooling) and the fact that water is more thermally conductive than dirt. In other words, no more global warming. It would be a permanent ice age with a thriving underwater ecosystem because all that solar energy is still arriving and underwater vents still exist. Thus, I wouldn't predict a thick ice sheet on the ocean, but I would predict all the land covered with ice and substantial ice shelves.

But, what about world #2?

For a moment, let's consider the goldilocks condition for a true water world. In this case the planet is absolutely tropical. It's a bit closer to the sun to balance the cooling effects of the water. There's no ice, anywhere, not even at the poles. What land is exposed is either lush with life or completely barren. I'm going to ignore the analysis of deep-ocean vs. shallow-ocean. That would have a substantial effect on the plant life, but not the average planetary temperature as the orbit would simply be adjusted to balance it out. For fun, we'll assume deep oceans like ours.

That world would be teaming with happy oceanic life. Lost of elbow room.

OK, but what about the weather?

In both circumstances (#1 and #2), you have substantial storms. A TON of water is evaporating into the atmosphere and there's fewer blocks to wind. Ocean currents are still moving around undersea continents, but there's a lot less drama involving them. Which means you have striated currents, striated winds... You'd have a planet with whooping high winds, really BIG storms, and almost constant cloud cover.

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@FinAndTonic brought up an incredibly good point that throws some of my answer into disarray (but I haven't time tonight to rework it all). As sea levels rise, they push the atmosphere up with them.

Here's the problem: the idea that things are colder at "higher altitudes" is based on thinner atmosphere. We humans ofttimes don't respect just how thin the golden biosphere really is. That two kilometer band above sea level is prime happy land: warm and inviting. Above that things start getting dicey until you reach the "death zone" at 8,000 meters.

But if you add water, the water pushes the atmosphere up. So the death zone is still 8,000 meters above the new sea level.

Now, there is a complication with this: the Earth's mass isn't materially changing with the addition of new water. That means that while we're pushing the atmosphere up, the loss of atmosphere to space increases. At this moment I don't know if that increase is enough to lower the atmospheric density to bring the ice back into play.

But it's uber-cool to think about.

Answer 2

With water, it has a high specific heat capacity, so the temperature changes during the day would be less prounounced, and the range of naturally-occuring temperatures at its surface will be smaller. However, as there's a lot more water, there will be a lot more evaporation, which will produce a more humid atmosphere. This, combined with water's low albedo, will make the planet warmer than an Earthlike one that orbits the same sun(s) at the same orbital radius. Also, there will be more condensation, which means that there will be more cloud cover, which will do interesting things with how the planet traps heat. And, the hurricanes there will be larger, stronger, and longer-lasting than those of an Earthlike planet.

Answer 3

If the OP wants an extraterritorial experience in the story/fiction, try Ganymede where a salty ocean of water is expected. Then space exploration, terraforming, technology and manufacturing can be tied into the settings.