Using Cartopy by respecting 1 to 1 km aspect ratio
The aspect ratio of a Mercator projection isn't actually 1°:1°, but is instead 1:1 in cartesian "Mercator units". The Mercator projection doesn't preserve distance along the meridians, and therefore the higher (or lower) you go in latitude in the Mercator projection, the longer (in physical distance) 1 Mercator unit is in the y direction.
As 1° has approximately the same length anywhere on the Earth (assuming a perfect sphere), you are actually looking for a projection that does preserve the 1° x 1° aspect ratio. At high-resolution, many map projections are good representations of 1km:1km, but at global scale there are only a few good choices.
Essentially, you are looking for a projection that is equidistant along both parallels and meridians, and the most obvious projection with that property is Plate Carree / Equirectangular (all cylindrical projections are equidistant along parallels, but Plate Carree is equidistant along meridians too).
I'm using cartopy to plot images on geographic axes. I used the Mercator projection to plot my data. However I realized that the equal aspect ratio is based on degrees and not in km. How can I do to force the map to respect a 1 km x 1 km aspect ratio instead of a 1° x 1° aspect ratio.
Thanks