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This here is a plain, simple, good old-fashioned map of our planet:

enter image description here

As everyone here would know, land makes up only 29% of the planet's surface. But suppose for a moment that someone has pulled the plug off the ocean to drain enough of the ocean's waters to raise the percentage of land to exactly 50%, not a fraction more. Using our current knowledge of ocean bathymetry, what would a map of such an Earth look like?

Quick disclaimer--I'm not asking for how this change would affect the atmosphere, the ocean currents, the geography, the climate--that's all for later. Simply what the plain, basic, good old-fashioned map would look like. That's it.

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  • $\begingroup$ Look at an elevation map. Colour in the highest elevations in the ocean and when they are all coloured in do so with the next highest elevation until you get to 50% land. $\endgroup$
    – DKNguyen
    Dec 28, 2021 at 2:47
  • $\begingroup$ watch out, though. With the weight of the water lifted off, the land underneath will rise in elevation, so the ratio of water and land will change over 10-15,000 years. The great lakes are gradually vanishing as they rebound from the compression of the glaciers. $\endgroup$
    – DWKraus
    Dec 28, 2021 at 6:31
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    $\begingroup$ This question was also adressed in a "what-if" by xkcd-author Randall Munroe what-if.xkcd.com/53 $\endgroup$
    – Wolter
    Dec 28, 2021 at 13:31
  • $\begingroup$ @Wolter No, it wasn't. It didn't specifically ask for 50-50. $\endgroup$
    – JohnWDailey
    Dec 28, 2021 at 14:31
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    $\begingroup$ @JohnWDailey Maybe not exactly (re:xkcd), but some clever photoshop to tell what percent of those images is each color could give you a very good starting point, maybe around 3km to 5km depth removed? $\endgroup$
    – TCooper
    Dec 28, 2021 at 14:43

2 Answers 2

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The continental shelves appear quite quickly. I can't give you an exact match to your figure, but my rheumy, ancient eyes suggest that this is about right:

A drop of (approx.) 3,500 metres gives us this:

Map supplied by NASA

Or perhaps a touch more land, a 3,700 metre drop:

Second map, slightly more land.

NASA's nifty online tool lets you view the entire process by sea-level drop against the changing map (the runtime-bar slightly obscures the key). Unfortunately there's no percentage, but if anyone cares to write a short bit of code to count the relative proportion of dark pixels, then please chime-in with an edit.

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    $\begingroup$ I was about to go to town using townsean.github.io/canvas-pixel-color-counter but then I remembered: Mercator! The apparent size of polar land masses is bloated up! How to use something like the Nasa tool with a virtual globe? $\endgroup$
    – Willk
    Dec 28, 2021 at 3:46
  • $\begingroup$ @Willk If only NASA had provided that tool so we could just input the topography. Is there a programmer in the house? One willing to put in the time to contribute to a custom toolkit for the site. $\endgroup$
    – Escaped dental patient.
    Dec 28, 2021 at 3:54
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    $\begingroup$ It should be noted that while this is strictly correct from an elevation standpoint, it is not entirely correct wrt "dropping sea levels". This is because once a section is closed off from the rest of the oceans (such as the Caribbean) it becomes a lake and is no longer part of the sea. Thus dropping sea levels would not be expected to drain the Great Lakes or the Caspian sea, etc. $\endgroup$
    – RBarryYoung
    Dec 28, 2021 at 13:27
  • $\begingroup$ Good point, didn't think of that. @RBarryYoung $\endgroup$
    – Escaped dental patient.
    Dec 28, 2021 at 13:30
  • $\begingroup$ Give pixels nearer the poles less "weight" toward the sum of pixels. If the weight function references in some way the inverse-Mercator function, then that should do it. Maybe a linear/quadratic decrease in weight could be accurate enough, though. $\endgroup$
    – BMF
    Dec 28, 2021 at 13:40
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As comments to the other answer point, closed basins will remain over the overall ocean level, although in the long term it's hard to predict its level - some of them will keep full and spilling out to the ocean becoming lakes, and others will lose more water to evaporation than gain from runoff and will become inland seas with endorheic basins.

However, Randal Munroe tried to predict what would happen if the oceans were drained with a drain on the Marianas Trench. The maps for 3 km and 5 km water level drop may give an insight of what the OP is looking for.

enter image description here

enter image description here

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  • $\begingroup$ But are either of them 50-50? THAT is what I am looking for. $\endgroup$
    – JohnWDailey
    Dec 28, 2021 at 14:31
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    $\begingroup$ 50% likely lays somewhere in between, probably more close to the 5 km map. Btw, I don't expect my answer to be the definitive answer, just a somehow useful answer until somebody codes and computes an exact answer. $\endgroup$
    – Pere
    Dec 28, 2021 at 14:36
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    $\begingroup$ I like that Australia starts to look like a kangaroo $\endgroup$
    – SeriousBri
    Dec 28, 2021 at 19:17

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