Assertion : Presently, the global atmosphere is warming up. Reason...

The warming up of global atmosphere present day is due to the increase in green house effect. Green house effect (GHE) describe the roles of water vapour, $\mathrm{CO}_2$ and other trace gases in keeping the Earth's surface warmer. These gases are relatively transparent to incoming shortwave radiation (visible spectrum), but are relatively opaque to outgoing reradiating (infrared rays) longwave radiation. The latter radiation, which would otherwise escape to space, is trapped by these gases within the lower levels of the atmosphere. The subsequent reradiation of some of the energy back to the surface maintains surface temperature higher than they would be if the gases were absent (without the green house effect the Earth's average global temperature would be -18 degree celsius rather than the present 15 degree celsius). There is concern that increasing concentration of the green house gases including $\mathrm{CO}_2, \mathrm{CH}_4$, and man made chlorofluorocarbons (CFCS), may enhance the green house effect and cause global warming. Ozone layer present in the stratosphere is depleted by the aerosols and chloroflurocarbons. Ozone protects the earth from high energy ultra violet radiation.