  In this paper, we prove that any non-positively curved 2-dimensional surface
(alias, Busemann surface) is isometrically embeddable into $L_1$. As a
corollary, we obtain that all planar graphs which are 1-skeletons of planar
non-positively curved complexes with regular Euclidean polygons as cells are
$L_1$-embeddable with distortion at most $2+\pi/2<4$. Our results significantly
improve and simplify the results of the recent paper {\it A. Sidiropoulos,
Non-positive curvature, and the planar embedding conjecture, FOCS 2013.}}
