  Consider an arithmetic group $\mathbf{G}(O_S)$, where $\mathbf{G}$ is an
affine group scheme with connected, simply connected absolutely almost simple
generic fiber, defined over the ring of $S$-integers $O_S$ of a number field
$K$ with respect to a finite set of places $S$. For each $n \in \mathbb{N}$,
let $R_n(\mathbf{G}(O_S))$ denote the number of irreducible complex
representations of $\mathbf{G}(O_S)$ of dimension at most $n$. The degree of
representation growth $\alpha(\mathbf{G}(O_S)) = \lim_{n \rightarrow \infty}
\log R_n(\mathbf{G}(O_S)) / \log n$ is finite if and only if $\mathbf{G}(O_S)$
has the weak Congruence Subgroup Property.
  We establish that for every $\mathbf{G}(O_S)$ with the weak Congruence
Subgroup Property the invariant $\alpha(\mathbf{G}(O_S))$ is already determined
by the absolute root system of $\mathbf{G}$. To show this we demonstrate that
the abscissae of convergence of the representation zeta functions of such
groups are invariant under base extensions $K \subset L$. We deduce from our
result a variant of a conjecture of Larsen and Lubotzky regarding the
representation growth of irreducible lattices in higher rank semi-simple
groups. In particular, this reduces Larsen and Lubotzky's conjecture to Serre's
conjecture on the weak Congruence Subgroup Property, which it refines.
