Presently, the global atmosphere is warming up.

The warming up of the global atmosphere in the present day is primarily due to the increase in the green house effect. Green house gases (GHG) like $C{O}_2$, $C{H}_4$, and $CFCs$ are relatively transparent to incoming shortwave solar radiation but opaque to outgoing longwave infrared radiation, trapping heat in the lower atmosphere. While the depletion of the ozone layer by aerosols and $CFCs$ increases ultraviolet radiation reaching the earth, it is not the direct cause of global warming. Thus, both assertion and reason are true, but the reason is not the correct explanation of the assertion.