  This article is devoted to the study of the critical dissipative surface
quasi-geostrophic $(SQG)$ equation in $\mathbb{R}^2$. For any initial data
$\theta_{0}$ belonging to the space $\Lambda^{s} ( H^{s}_{uloc}(\mathbb{R}^2))
\cap L^\infty(\mathbb{R}^2)$, we show that the critical (SQG) equation has at
least one global weak solution in time for all $1/4\leq s \leq 1/2$ and at
least one local weak solution in time for all $0<s<1/4$. The proof for the
global existence is based on a new energy inequality which improves the one
obtain in \cite{Laz} whereas the local existence uses more refined energy
estimates based on Besov space techniques.
