In the northern hemisphere only, what percentage of the surface is land?
I get 40.44%
With a workflow using a tiny bit of Python.

After Polygorial‘s comment I am adding (Feb 17, 2021 update) 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.

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.

Store as Numpy array.

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

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

Calculate percentage of white pixels.



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%