Pre-industrial human activities which changed the atmospheric greenhouse gas or aerosol loading, or which modified the properties of the earth's surface, such as albedo, roughness, or vegetation cover, had the potential to modify the regional or even global climate. The primary activities which could have produced these effects were deforestation, burning, and agriculture. These activities were not independent, and often occurred together. Deforestation could have produced warming or cooling at the surface, and different effects on different scales, depending on the fate of the biomass removed and the new use of the land. Burning is much less now than it was in the past in some regions, which would have produced warming as the burning decreased. This may be a partial explanation for the Little Ice Age. While a thorough survey of such pre-industrial human activities is called for, current information indicates that regional climatic effects were large in some regions, such as western North America, and hemispheric or global effects were possible. Once these pre-industrial human climatic forcing factors are better quantified, existing numerical models of the climate can be used to examine the impacts on regional and global scales.