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Across the whole Earth, about 70% of the surface is water and 30% is land.

What I want, though, is to restrict the scope to the northern hemisphere (NH) only. What percentage of NH surface is land?

It would also be good to clarify exactly how we define "land". If I'm not mistaken, the 70/30 split of water/land actually places inland rivers and lakes as land area.

Please also cite a source for this. The only google result I found with an answer was yahoo questions.

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    $\begingroup$ Good question! The best answer will give you a quantitative estimate, and show you how this was done in a GIS or programming language. I hope someone can give you this. $\endgroup$
    – Matt Hall
    Commented Sep 10, 2015 at 11:57

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I get 40.4%

With a workflow using a tiny bit of Python.

  1. After Polygorial‘s comment I am adding (Updated on Feb 17, 2021) point zero to state more explicitly that this method requires a map like the one I used (Gall-Peters projection) with equal area projection, as regions of same area on the Earth’s surface will have the same area on the map, and therefore same number of pixels.

  2. Invert and import B/W image of equal area projection for the Northern hemisphere (land = white pixels). Source of original image (full globe): Wikimedia Commons.

  3. Store as Numpy array.

  4. Calculate total number of pixels in image array (black + white).

  5. Calculate total number of white pixels (1s) by summing the entire array (black pixels (0s) will amount to zero).

  6. Calculate percentage of white pixels.

enter image description here

UPDATE September 20, 2018

As suggested by @JeopardyTempest in a comment, as a validation test I run the script for the full image with both hemispheres, and got the expected 30%

enter image description here

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    $\begingroup$ Love your answer, shows a simpler/ingenious way to solve it, and explains your work process, so very useful. It's of course a bit of a rough estimate depending upon the pixel resolution of the image... but for many purposes, that's plenty fine. Stack Exchange can always use more thoughtful answers like these. Thanks! $\endgroup$
    – JeopardyTempest
    Commented Sep 20, 2018 at 17:13
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    $\begingroup$ (For good measure, happen to be easy to get the Southern Hemisphere number, since it sort of naturally goes with the question? [Plus serves as a further check in how close it is in methodology given the full-Earth values are pretty well publicized]) $\endgroup$
    – JeopardyTempest
    Commented Sep 20, 2018 at 20:20
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    $\begingroup$ Thanks for your comment. Yes, the resolution of the image may limit the accuracy. I was hoping to get high resolution data like the Global Land One-km Base Elevation from the US National Geophysical Data Center (ngdc.noaa.gov/mgg/topo/gltiles.html), but it would have meant merging 8 huge arrays, threshold work, and then reproject. As you imply probably beyond the scope of a Stack Exchange answer. Also we might want to subtract the estimated rivers and streams coverage in @farrenthorpe 's answer. $\endgroup$
    – MyCarta
    Commented Sep 20, 2018 at 22:34
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    $\begingroup$ Always a fan of keep it simple, unless it's something really asking for intense precision :-) $\endgroup$
    – JeopardyTempest
    Commented Sep 20, 2018 at 23:09
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    $\begingroup$ I run the code using the full globe and got the expected 30% land coverage. Updated answer with the details. Thanks for suggesting it. $\endgroup$
    – MyCarta
    Commented Sep 21, 2018 at 3:30
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From http://www.physicalgeography.net/fundamentals/8o.html and http://www.eoearth.org/view/article/154984/

The spatial distribution of ocean regions and continents is unevenly arranged across the Earth's surface. In the Northern Hemisphere, the ratio of land to ocean is about 1 to 1.5. The ratio of land to ocean in the Southern Hemisphere is 1 to 4. This greater abundance of ocean surface has some fascinating effects on the environment of the southern half of our planet. For example, climate of Southern Hemisphere locations is often more moderate when compared to similar places in the Northern Hemisphere. This fact is primarily due to the presence of large amounts of heat energy stored in the oceans.

This equates to a measurement of ~40% land in the northern hemisphere. This agrees with the northern hemisphere azimuthal equal-area map image below.

enter image description here

The surface area of the Earth covered by lakes and rivers is a fraction of a percent, so their classification does not really change the answer. However, you would get a slightly different answer if you consider land to be any continental crust (including seas not on ocean crust). If you define a year and a definition of "land" that you are satisfied with, an exact calculation could be made with GIS software.

From https://www.fba.org.uk/journals/index.php/IW/article/viewFile/502/327

Rivers and streams are likely to cover 0.30–0.56% of the land surface [of Earth]

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    $\begingroup$ So close to a quantitative answer! I hope someone can follow up on your last sentence. $\endgroup$
    – Matt Hall
    Commented Sep 10, 2015 at 11:58
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    $\begingroup$ Are you sure rivers and lakes make up only a tiny fraction? Rivers in particular can be very long and can branch all over the place. All those gullies might add up. $\endgroup$
    – DrZ214
    Commented Sep 10, 2015 at 19:53
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    $\begingroup$ I've added some more info to address this. $\endgroup$
    – f.thorpe ♦
    Commented Sep 11, 2015 at 20:39
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The Northern Hemisphere is 40% land and 60% water. The Southern Hemisphere is 20% land and 80% water.

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    $\begingroup$ Your answer would benefit from citations. Also, it does not make sense to say that the Southern Hemisphere is 80% land and 20% water and call it the water hemisphere. $\endgroup$
    – Isopycnal Oscillation
    Commented Mar 23, 2016 at 23:22
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    $\begingroup$ Fixed the typos so the info on southern hemisphere is correct now. $\endgroup$
    – f.thorpe ♦
    Commented Jan 10, 2017 at 15:42

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