A commenter sent me off in pursuit of the Thomas Jefferson climate change quote, with good results. The full text is here (search on "Beck", second occurrence), a July 16, 1824 letter from Jefferson to one Lewis Beck, who had apparently sent Jeffeson something he had written on the climate of the west. The quote excerpted in Glantz's book shows Jefferson's interest in understanding anthropogenic climate change (another commenter pointed to the widespread notion afoot at the time that "rain follows the plow"), but the full text also shows Jefferson with a broad and keen interest in gathering good baseline climatological data on the continent:

Years are requisite for this, steady attention to the thermometer, to the plants growing there, the times of their leafing and flowering, its animal inhabitants, beasts, birds, reptiles and insects; its prevalent winds, quantities of rain and snow, temperature of fountains, and other indexes of climate. We want this indeed for all the States, and the work should be repeated once or twice in a century, to show the effect of clearing and culture towards changes of climate.