The Development of Cities

[JDR Dragoon]

can I suggest however that there is a diferance between Western europe & the americas, one obviously being size,

Depends on the timeframe and location. The Pre-columbian Americas were a mix of essentially quasi Paleolithic and Neolithic tribes (especially in what is today the USA and Canada) and highly evolved civilizations (Inca, Aztec etc.) to the South. The more evolved civilizations had cities with high populaton densities and overall bigger populations. So I suspect this doesn´t really have a lot to do with any phenomenon unique to Europe. Least of all the climate.

but more pertinant to europe that of it's phisyical geography and climate, and I would suggest that it's through climate that there is empirical evidence;

I sincerely doubt it.

think the climatic optimum c800-1300 AD and 'the Little Ice Age' of the early Modern period + earlier ones that drove the tribes West into the Roman empire etc, etc.

There is no evidence that anything "drove" any tribes anywhere. Least of all into the Roman empire. The barbarians entered the Roman empire for various specific reasons (some of which are today wholly unclear to us), not because they were conditioned to do so by the climate or any racially conditioned "wanderlust"