It's pretty simple:
1 - we must be having an effect on global climate - our idustry levels, etc. are simply to high to not be having an effect
2 - the exact impact is unknown, because
a - we have no previous data to go on, we are the only data set for an industrial society to use
b- we cannot be exact on external impacts. The global climate changes over time all by itself
3 - it is of course prudent to move steadily to cleaner approachs because:
a - it would be better to known we are reducing any possible impact than not
b - we have, I believe, a moral obligation to reduce extinctions and other human directed impacts
c - it is in our own species long term interest anyway
Put simply, even without evidence of climate and other global impacts, we should as a matter of course move ever forward with cleaner, purer technology, anything else is a short term cop out.
Try to be reasonable... its easier than you think...
