About greenhouse effect.

Q: How is greenhouse effect ?

"Greenhouse effect" is the trapping of long wavelength infrared radiation, which is usually emitted by objects on earth with a temperature much lower than that of the sun, in the atmosphere. Such infrared radiation would warm the envirnoment, thus causing some adverse effects like climatic changes.
Q: Why density of CO2 increase, greenhouse effect worsen?
CO2 acts like a blanket covering the atmosphere. CO2 in the atmosphere doesn't allow long wavelength infrared to pass through but reflects it back to earth. Thus, an increase of CO2 density would enhance greenhouse effect.

Q: How the thinning of ozone layer affects the environment?
Ozone could absorb ultraviolet radiation from the sun. Thus we are protected from the hazard of utlraviolet radiation by the ozone layer in the atmosphere. Thinning of ozone layer results in more ultraviolet radiation reaching the earth surface. This not only affects human health, but also would have adverse effects on other animal species on earth.
The following web-page gives more details:
http://www.umich.edu/~gs265/society/ozone.htm