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Assume the entirety of Earth (all the land) is covered in a city. What would the climate be like in different parts of it? I.e., what would it be like on the coast, or inland?

I imagine it would be unbearably hot inland because you have a planet-wide heat island effect plus hardly any wind. Interior parts of the city would also almost certainly suffer from near-perpetual drought in my mind. However, these are just my thoughts, and I am by no means an expert on climate nor the effects of urban development on local and global climate.

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    $\begingroup$ Are you thinking of a world that's gone completely urbanized, like Trantor in Asimov's Foundation series? If so, why would it matter? The entire enclosed city would be climate-controlled. $\endgroup$ – Jeff Zeitlin Apr 2 at 19:15
  • $\begingroup$ If you're covering all of the land with city then that raises a lot of questions as to the specifics which depending on their answer will lead to dramatically different climates. For one there's the question of what the cities are actually made of since if you are covering all the landmass then you're likely to have moved beyond concrete (plus you may be deliberately using paint to alter albedo). There's also the question of how dense the city is. Plus of course co2 levels may be different in this sort of scenario. $\endgroup$ – Vakus Drake Apr 2 at 19:16
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    $\begingroup$ We need a lot more detail to be able to provide an answer. What have your city builders done about mountains, oceans, and rivers? What is the city made out of? How are they converting all that CO2 back into O2 without any plants? What kind of pollution is being created? There's just... thousands of different ways this could go depending on the details. $\endgroup$ – Morris The Cat Apr 2 at 19:56
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What you are describing is a giant urban heat island (UHI).

I'm oversimplifying it here, but basically concrete and asphalt absorb more heat and release it slower than rural areas and this effect can be exaggerated where there are tall buildings and narrow streets as this traps the air and therefore the heat.

The difference in temperature between urban and rural areas is usually greatest in the evening as the cooling process is slowed in urban areas.

This warming effect isn't known to be exaggerated over time, i.e. cities don't continuously get warmer.

City sizes can affect the evening cooling process, so the center of an area with tall buildings and narrow streets will cool after the outskirts of that area. But they won't necessarily reach a higher temperature in the first place just because they are the center of this urbanised area.

So in general, your planet sized city is likely to be a few degrees warmer than it would be if left rural, especially at night.

UHIs in hotter climates are known to effect rainfall, as clouds will rise over these warm air pockets leading to precipitation. So more rain around the coast of you giant UHI, this will especially be true the closer you get to the equator and areas where the prevalent wind comes from the ocean. Of course, with no rural land, that is a lot of water to drain away. Precipitation will still be affected by natural geography to a point though (e.g. how far above sea level an area is).

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The thing which makes heat islanding such a big deal is partially that it's in pockets, contrasting with the cooler land forms around the hardscaped area, and that differential then drives local convection loops, trapping the heated air mass above the urban scape.

The whole prospect changes absent the contrast; we need to know SO much more about the planet overall to posit reasonable responses:

  • land/sea ratios
  • topography (mountains, plains, valleys)
  • exposed geography / plantscapes in whatever un-urbanized areas there are (mountains - rocky or vegetated?)
  • whether there are extensive parks (Paris, London, Tokyo, Bejing, New York, Philadelphia, Denver, Boston) or disastrously few parks (Fresno, Jacksonville, Mesa)

We also need to know how carefully this has been thought out and planned in process - there are some strategies which could make large scale urbanscaping far less climate-unfriendly:

  • greenrooves
  • pervious / impervious paving
  • grass pavers vs asphault
  • intentionally designed mico-climate generating ratios of heat islanding between differing sections and heights to intentionally generate currents, winds and pressure differentials
  • alignment of taller structures with prevailing winds to cool large masses / generate wind-power
  • power generating base: coal or oil-fired, nuclear fission or fusion, wind and tide turbines, massive solar on all rooves, antimatter/matter reactors, Schwartschild Discontiuity generators
  • widely used high efficacy mass transit - so no cars, nor significant commuting pollution

Hopefully you get the idea.

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As @KMo mantioned, the Heat Island Effect (HIE) won't really effect windfall or overall wind patterns. You will still get relatively normal winds.

What will have a big effect on rainfall will be the height of the city. As wind is diverted upward, it's density/pressure decreases and it can't hold as much water. That water then precipitates as rain.

If the mega city is similar to our own cities where most buildings are 1 to 10 levels tall, then there won't be much affect on anything. The leading edge of the city on the coast will act as low rolling hills and might slightly increase rainfall there.

If the buildings are taller 100-200 levels then the edge effects become more pronounced and more rainfall will happen at the edge.

If the buildings are 500+ levels, they start to have the effect of mountains and you will tend to get very little rain away from the coasts.

I see 3 issues. The one relating to the above is water management. The water collected at the coast has to get transported toward the center. The second is the lack of O2 from the loss of plants. The third, relating to the second issue is massive starvation due to lack of food.

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There is talk about how it would create a hot island effect but I find that whenever people envision a world city, they think in terms of our cities today - concrete, glass, and steel (mainly) and so it is understandable to end up at such a point. My thought has always been that a city in general, but especially if the city were to sprawl across the entirety of the world I think that the world would need to adjust accordingly.

One thing is what about new building materials, which would likely be developed by the time a world could advance to the point of covering the planet. I don't think that the materials that would be used would be the sort of materials of current time.

The next thing is that the rooftops of buildings are a great place for greenery. You can easily put grass, trees, and flowers on top of the buildings. From a satelite, it would look as if the planet was completely green. It wouldn't be until you got closer that you would see the city underneath. From there, you can still cover the sides of structures with greenery as well. These are things already in place around the world and so I imagine a world-city would likely go this route.

I am sure that this would still have a major impact on the climate, but I think that this would minimize any hot island effect and would make the place more pleasant to view and live in.

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Well it all depends on how your city is constructed. If most of your buildings are one or two storeys then lack of wind is hardly going to be an issue (as opposed to if every building was 20 storeys tall.

Additionally, it depends on how many green spaces you have in the city. For example, New York City is not just one big slab of concrete with buildings on it, you have central park, a large green space in the middle of the city. In this global city, you could have a lot of these green spaces and still claim the world is covered by a city.

Something to consider, you would still need crop farms, tree farms and orchards in your planet-wide city, otherwise you will not have any any food or wood. Even if these are built in the city, they still need to exist, reducing the heating effect.

Finally, consider the original temperature of the planet, if most of it originally was a frozen wasteland, having this mega city could instead make it a comfortable temperature.

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