The New York Times has an article on historical observations of anthropogenic climate change - Ben Franklin on Global Warming .
In the 1780s, Thomas Jefferson opined in his “Notes on Virginia” that “both heats and colds are become much more moderate within the memory even of the middle-aged,” expressing views articulated as early as 1721 by Cotton Mather: “Our cold is much moderated since the opening and clearing of our woods, and the winds do not blow roughly as in the days of our fathers, when water, cast up into the air, would commonly be turned into ice before it came to the ground.”
The weather historian James R. Fleming has noted that the vexing scientific challenge in the climate debate has always been “the response of a large, complex, potentially chaotic system to small changes in forcing factors.” Benjamin Franklin understood climatic forcing factors better than anyone, surmising in a 1763 letter to Ezra Stiles that “cleared land absorbs more heat and melts snow quicker.” Franklin, our meteorologist emeritus for his seminal work on everything from lightning to northeasters, later surmised (correctly) that a prevailing haze over parts of North America and northern Europe was associated with the eruption of the Laki volcano in Iceland in June 1783, and was possibly the source for the exceptional chill experienced in the winter of 1783-84 in the new United States.
On the other side of the developing weather controversy in the late 18th century, Webster quarreled with Jefferson, insisting that he relied too heavily on the memories of “elderly and middle-aged people” for his observation that the climate had moderated. While Webster conceded an anthropogenic influence might still be at work, he argued that it caused something less than climate change: “All the alterations in a country, in consequence of clearing and cultivation, result only in making a different distribution of heat and cold, moisture and dry weather, among the several seasons.”
Hugh Williamson, astute in his understanding of the hydrological cycle, a key component in any climate change debate, wrote, “The vapors that arise from the forests are soon converted into rain, and that rain becomes the subject of future evaporation, by which the earth is further cooled.” A century and a half later, land-use studies would confirm quantifiable relationships between clearing trees for extensive farmland and changes in soil temperature, moisture distribution and local and regional climate responses, as well as the urban heat-island effect. In our time, we have learned that tropical deforestation is linked to as much as 15 percent of the world’s global warming pollution, largely due to the release of carbon dioxide, one of several “greenhouse gases” that trap and re-radiate terrestrial heat.