The really obvious problem with your argument is that we will never get anything done.
Think of this scenario - I currently have a $1500 tax liability to the ATO this financial year. I refuse to pay on grounds that me paying my $1500 taxes will have extreme negligible effect on the Australian economy. The government is not going to go bankrupt if I don't pay my taxes, in fact, nothing at all is likely to happen as a result of me not paying my taxes. So is this a good reason for me to not pay any tax??? Surely you can see how absurd this is.
Likewise, if we allow one nation to not contribute to the reduction of carbon emission on the grounds that it will have no effect on the environment when they do it alone, then everyone can use that line of argument and we will never get anywhere. It's completely fallacious.
http://www.theaustralian.com.au/bus...conomists-survey/story-e6frg9if-1226094172989
According to this article the majority of surveyed economists supports the carbon tax.
I am not an economist, I am unable to make economic assertions. All I can do here is 'rely on the experts'. You say damage>benefit, I say benefit>damage - so this won't get us anywhere. All that we can expect our politicians to do is follow the experts' opinion.