The problem we investigate concerns the choice, among conformal cartographic representations, of the one that causes as small areal deformations as possible. To that aim, we define an index of the global areal deformation through a functional T depending on the form of the cartographic representation. Since conformal representations are realized by a couple of harmonic functions, one of which depends on the other, we introduce a suitable topological space E of harmonic functions. The problem now becomes the minimization of the functional T of the space E. In this paper a theorem of existence of the minimum of T on E is demonstrated by means of classic variational techniques.