I would say it is cars, trucks , coal power plants, and so on, the ozone layer filters out the suns harmfull UV rays,and the ozone layer is breaking down from all the emitions , this is why there is so much more UV rays and people are getting skin cancer from it.
On the subject cows are the cause, i very much doubt it, there has always been wild cattle/horses even dinosaurs and so on living on this planet.
scientistscan created Ozone and fill a chamer with its gasses, they have also added car emittions to this chamer and mesured the brake down the ozones on a huge scale so yes they have proved it.
greenman:D Reviewed by greenman:D on . Global Warming The reason for Global warming is...... Rating: 5