Vic, there is no dispute that CO2 in the atmophere causes a greenhouse effect (though there is undoubtedly dispute over the specifics of what this will be manifested).
The levels of carbon isotopes trapped in things like ice and tree trunks and corals is not controvertial and can be cross referenced against each other and tell us a great deal about temperatures and general conditions.
The fact that particular isotopes are related to human actiivity - the burning of fossil fuels - is again not controvertial.
We don't need to know what the precise temperatures were 500 years ago to have a very good understanding of what is happening now, and fortunately, the IPCC climate models that tell us climate change is a reality don't require such data to work.
Happily (or rather unhappily) the data from all those old models also supports the case for antropogenic global warming. There's a good free source on the methodologies here:
http://www.nap.edu/catalog.php?record_id=11676