  We study the thermal properties of the lowest multiplet of the QCD
light-flavor scalar resonances, including the $f_0(500)/\sigma$,
$K_{0}^{*}(700)/\kappa$, $f_0(980)$ and $a_0(980)$, in the framework of
unitarized $U(3)$ chiral perturbation theory. After the successful fits to the
meson-meson scattering inputs, such as the phase shifts and inelasticities, we
obtain the unknown parameters and further calculate the resonance poles and
their residues at zero temperature. By including the finite-temperature effects
in the unitarized meson-meson scattering amplitudes, the thermal behaviors of
the scalar resonance poles in the complex energy plane are studied. The masses
of $\sigma$ and $\kappa$ are found to considerably decrease when increasing the
temperatures, while their widths turn out to be still large when the
temperatures reach around $200$ MeV. In contrast, both the masses and widths of
the $f_0(980)$ and $a_0(980)$ are only slightly changed.
