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This is the first of possibly several posts about the following scenario, in which Earth's sea-level rises 2 km overnight, generating a world map looking like this:

enter image description here

Question: What would be the climate after 30 years?

Would temperatures (and ice-sheets) rise, fall, or be redistributed somehow? What would trade winds and ocean currents be like: are there areas where the surviving landmasses are significant enough to generate gyres? I imagine storms will become more powerful, but when and where will they be concentrated?

  • I'm specifically interested in the area around the west of the main Tibetan landmass and the islands which used to be Iran.

  • There is some potentially helpful info here, though it addresses fully ocean planets and the results are inconclusive.

  • I'm a humanities student, please try to avoid jargon if possible.

  • I do not have set ideas about the average temperature or salinity of the new water, so please feel free to either assume that this is the same as that already present on Earth (average 5 degrees centigrade).

  • Don't worry about how the water arrives on Earth; this takes place through a non-scientific process. Simply assume that it happened, and 30 years later, what are the consequences?

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    $\begingroup$ Funny thing about objects of extra-terrestrial origin: they come with a lot of kinetic energy. One kilogram of water of extra-terrestrial origin comes with at least 62.5 MJ of kinetic energy, which is enough heat to vaporize about 25 kg of water. The oceans would boil. (Explanation: the escape velocity from Earth is about 11.2 km/s; the latent heat of vaporization of water is about 2.2 MJ/kg.) $\endgroup$ – AlexP Sep 9 '18 at 18:01
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    $\begingroup$ Where did you get that map? Is there a map generator that can accommodate 2000m? Link please. Or if you did it yourself, well done! $\endgroup$ – Willk Sep 9 '18 at 18:18
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    $\begingroup$ Sorry, I was unclear. The water is not arriving by some sort of comet strike, it comes via a series of portals near to the Earth's surface, so there is no massive impact. I just said 'extraterrestrial' to avoid people pointing out that there isn't enough water on Earth to generate that kind of rise. The map is a simple screenshot from: calculatedearth.com/index.php. $\endgroup$ – Montefeltro Sep 9 '18 at 18:24
  • $\begingroup$ It is pretty important to describe how this happened. If it was global warming/climate related, the climate will be warmer, which will affect winds, storms, and everything else. Climate affects ocean temperature and vice versa. If it was not climate related, what stops the ice sheets from re-forming? $\endgroup$ – John Locke Sep 9 '18 at 18:25
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    $\begingroup$ What temperature is the water? Climate effects will be very different if the water added is 30 C vs 3 C. Also, specifying may help quiet some of the astrophysical pedants $\endgroup$ – Dubukay Sep 9 '18 at 20:52
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Your proposed planet would essentially be a water planet, despite the small amounts of land still found on the surface. As such, we can make a few simplifying assumptions and work with some best-guess scenarios.

Air currents

Air currents are greatly simplified by this change. Rather than navigating complex topography, the surface of the Earth has been dramatically smoothed as it follows the ocean surface, which in turn follows the geoid. As such, the air currents will follow the idealized atmospheric cell circulation shown below (if this is confusing, check out my answer here for a more detailed explanation of why these patterns form):

Diagram of atmospheric circulation

The equator is still going to be the warmest area, as it's receiving the most solar radiation per square meter. Most of the evaporation will happen here, and the ITCZ will remain intact but will actually align nearly-perfectly with the equator, rather than being deflected to the north as it is in today's Earth.

The atmospheric cells will radiate from there, following idealized Hadley and Ferrel models. As pointed out by the Reddit link you found, the jet streams will grow a bit stronger because they no longer need to meander quite as much. You can check out what the jet stream is doing today at this cool website. They'll still run into the Himalayan Plateau and the Andes, but those will each be easier to avoid.

The winds across the planet will probably get stronger, leading to more powerful storms and more destruction when they do run into the remaining land. There's an article here that argues it won't be that large of a change, but this has been disputed by other papers.

The monsoon season would be largely minimized. Monsoons are caused by the movement of the ITCZ, which would be significantly weakened for the same reasons described above. Without a strong ITCZ, the monsoons will likely be insignificant.

Ocean currents

The oceanic currents are a bit more complex and we don't have as clear of an idea what will happen with them. Some papers find that the Earth becomes latitudinally stratified, with ice caps reaching down much further and the equator becoming warmer. Other papers find that the ocean takes over where the atmosphere fails and succeeds at transporting huge amounts of heat between the equator and the poles, leading to less a less-stratified planet that's more homogeneous temperature-wise.

The gyres themselves would largely dissipate - there's no continental mass to deflect them, so the currents would continue in their normal direction and would wrap around the planet.

Atmosphere composition

I'm not certain on this one, but it also seems likely that the increased ocean surface area will dramatically increase the amount of water in the atmosphere. Because water is a significant greenhouse gas, this will cause an increase in longwave radiation absorption and an increase in the global temperature. This could lead to a runaway greenhouse effect like L. Dutch points out in his answer.

Energy balance

Finally, the albedo of the ocean is much less than the albedo of land. This means that less incoming radiation would be reflected and more would be absorbed. Extrapolating from this site, the Earth's energy budget would increase by about 25 watts per square meter, or maybe 3% of the current amount. This would also contribute to a warmer planet and more greenhouse-like effects.

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  • $\begingroup$ Fantastic, thanks for this. Given that the scientific jury is still out on what would happen to the currents, let's say I adopt the model in which the ocean efficiently redistributes heat. Returning to my specific focus on the Iranian-Himalayan area, it sounds like from what you and other answers are saying that this will have a steamy, tropical climate in which there are steady north-east trade winds all year round and no storm season as such (the ICTZ's northward summer movement probably being unable to reach this far north, since it only just manages it at present). That sound reasonable? $\endgroup$ – Montefeltro Sep 10 '18 at 6:22
  • $\begingroup$ Edit: I may have got that wrong, particularly as these areas are more north than I thought and sit almost exactly on the horse latitudes. I think that means weaker winds, more fog and haze, and (relative) dryness. Might the seasonal northward shift of the subtropical ridge be enough to change prevailing wind direction and lead to increased cyclones in the summer? $\endgroup$ – Montefeltro Sep 10 '18 at 6:40
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Water cycle

I don't think the other answers so far have discussed the effect on the water cycle in great enough length, so here goes.

There will be no problem with lack of rain. Even areas that were previously snowy mountaintops will now have a tropical climate. As other answers have pointed out, the water vapor will significantly increase temperature, which means a ton of water going into the atmosphere as water vapor, which will cause a ton of rain. I suspect that humans will be able to deal with the rain and humidity, but the temperature poses a threat to humans if it becomes a positive feedback cycle with the water vapor.

In this scenario, I am imagining that the water entering the ocean is salt or brackish water, or that its salt concentration is not low enough to make the oceans drinkable.

The major problem in this scenario is getting fresh water. The areas on the map that are above water have small land areas and are stretched out. That means there won't be many freshwater lakes or streams. Most of the aquifers are under water too, and there aren't many (if any) aquifers on mountains. This seriously affects the feasibility of humans being able to live with such high water levels. If humans have the resources, though, they will build rain-catching devices to provide their water. Without fresh water, humans will quickly perish of dehydration.

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As hinted by AlexP in their comment, any object of extraterrestrial origin brings along a lot of kinetic energy.

This would result in the added water to boil off, resulting in the atmosphere being saturated with water vapor.

Water vapor is a greenhouse gas way more effective than CO2 at blocking IR radiation, therefore the immediate result would be turning the planet into a sauna. At that point it's very likely there would be a second hell-like planet a la Venus orbiting the Sun.

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  • $\begingroup$ Howdy L.Dutch. It appears that about the same time you answered the OP commented with clarification and it obsoleted your answer. See his comment to his question about portals delivering the water. $\endgroup$ – JBH Sep 9 '18 at 22:51

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