Across Europe in the early Bronze age man virtually clear felled the native forest and caused a significant change in climate as a consequence. Mans population has ebbed and flowed ever since and the climate has responded in sympathy with his population and its effects on forest cover and land use. man has been a significant influence on global climate since he invented civilization and the agricultural land use changes it brought with it. Climate has also been effected by the resurgence of forestry as mans population has crashed at various times of plague.
AGW is not new, and the signs are there for all to read - the only difference about the current era is the shear scale of land use change and the massive release of geologically stored carbon which has never happened before in mans history. This fact alone means that we are entering uncharted territory.