Due largely to energy and agriculture practices, fossil fuel burning, fertilizers, etc. CFCs - significant destroyers of atmospheric ozone, as well as important greenhouse gases. Molecule for molecule, CFCs are 10,000 times stronger than carbon dioxide in greenhouse effect. They do not occur naturally, but are the result of use of aerosol spray can propellants, foam blowing, refrigerator coolants and cleansers. Residence time of these gases long and even curtailing production will not stem the effects they have in atmosphere over the next few decades.
Ozone is good in the stratosphere, but bad in the troposphere.
The global temp. is only 5 degrees higher than it was during the coldest period of the last ice age. Some models predict a rise of 3 degrees C by the end of the next century. Doubling of carbon dioxide levels could occur as early as 2015, or as late as 2050, based upon diff. These may cancel out. 120,000 - 150,000 years ago sea level may have been 5 meters higher than it is today.