We analyze the s-dependence of solutions u s to the family of fractional Poisson problemsin an open bounded set Ω ⊂ R N , s ∈ (0, 1). In the case where Ω is of class C 2 and f ∈ C α (Ω) for some α > 0, we show that the map (0, 1) → L ∞ (Ω), s → u s is of class C 1 , and we characterize the derivative ∂ s u s in terms of the logarithmic Laplacian of f . As a corollary, we derive pointwise monotonicity properties of the solution map s → u s under suitable assumptions on f and Ω. Moreover, we derive explicit bounds for the corresponding Green operator on arbitrary bounded domains which are new even for the case s = 1, i.e., for the local Dirichlet problem −∆u = f in Ω, u ≡ 0 on ∂ Ω.