Global warming exists, we know this because the Earth is getting warmer. Global warming and Global Cooling occur naturally. The question is - what impact is mankind having on global temperature?
"If I walk down a busy high street and find a fifty pound note, it does not mean that the appearance of the note is a new thing. It could have been there for weeks but everyone else ignored it." How likely is it that many people chose to ignore the hole in the ozone layer/global warming? Has the increase in change of temperature coincided with human industry, or does the rate of change of temperature vary to the extent currently experienced? Given the evidence available to us it appears that man has made a significant impact on the environment.
Since the industrial revolution mankind has pumped obscene amounts of CO2 into the atmosphere AND cut down large amounts of plant life. I think anyone would be hard pressed to proove that this had no effect on the amount of carbon dioxide in the atmosphere.
With regards to Global Warming it is the rate of change of temperature that is important, along with the peak temperature of the atmosphere (or we'll end up like venus*). It may take the earth 500 years for the temperature to rise X degrees, or with mankinds effect on the atmosphere it may take only 50 years. Is it better to have 500 years to prepare for that temperature rise, and the effects associated with it, or 50 years?
The ozone layer, and the "hole" in it are often misunderstood. The ozone layer has only a small effect on global warming and cooling. It is a different problem. We released of large quantities of manmade organohalogen compounds into the atmosphere. This is bad for the ozone layer, as the compounds react first with the UV-rays from the sun, and then with the O3 in the ozone layer. Now the right amount of ozone is imperative to life on this planet. We, like all oother species on this planet, evolved with a certain amount of UV radiation from the sun. We like that amount of radiation, not more, not less. UV radiation damages DNA, the more UV radiation that gets through the higher the likelyhood of damage from it(e.g. skin cancer). With a thinner ozone layer more radiation gets through. This is bad. One of the often missed problems with global warming will increase ozone depletion, and that's likely to be as bad for us as the obvious effects of global warming.
Sorry that was so long. On another topic which may be of interest - *current knowledge of global warming and climate modelling have been applied to the planet venus, which may have suffered a thermal runaway a while ago, causing it to be a bloody awful place.