The Stacks project

Proof. We first reduce (2) to (1). By Sheaves on Stacks, Lemma 96.21.2 we see that $R^ if_*\mathcal{F}$ is the sheaf associated to the presheaf

Here $y$ is a typical object of $\mathcal{Y}$ lying over the scheme $V$. By Lemma 103.9.2 it suffices to show that these cohomology groups are zero when $y : V \to \mathcal{Y}$ is flat. Note that $\text{pr} : V \times _{y, \mathcal{Y}} \mathcal{X} \to \mathcal{X}$ is flat as a base change of $y$. Hence by Lemma 103.9.2 we see that $\text{pr}^{-1}\mathcal{F}$ is parasitic. Thus it suffices to prove (1).

To see (1) we can use the spectral sequence of Sheaves on Stacks, Proposition 96.20.1 to reduce this to the case where $\mathcal{X}$ is an algebraic stack representable by an algebraic space. Note that in the spectral sequence each $f_ p^{-1}\mathcal{F} = f_ p^*\mathcal{F}$ is a parasitic module by Lemma 103.9.2 because the morphisms $f_ p : \mathcal{U}_ p = \mathcal{U} \times _\mathcal {X} \ldots \times _\mathcal {X} \mathcal{U} \to \mathcal{X}$ are flat. Reusing this spectral sequence one more time (as in the proof of Lemma 103.5.1) we reduce to the case where the algebraic stack $\mathcal{X}$ is representable by a scheme $X$. Then $H^ i_\tau (\mathcal{X}, \mathcal{F}) = H^ i((\mathit{Sch}/X)_\tau , \mathcal{F})$. In this case the vanishing follows easily from an argument with Čech coverings, see Descent, Lemma 35.12.2. $\square$