Climate: The Little Ice Age
In the second half of the 18th century and early decades of the 19th century, which is to say during the time covered by Jefferson’s weather records, the Northern Hemisphere was continuing to experience a period of cooling that began in the 1300s. In the 20th century scientists and historians began to call this period the Little Ice Age, as it was marked by some extension of glaciation. The cooling trend was the product of a convergence of multiple causes, including a reduction in solar radiation, changes in oceanic and atmospheric oscillations, and volcanic eruptions. It was over by about 1850.
Although this was a general climatic trend throughout the northern half of the globe, the effects varied over time and in different locations. A significant value of the Jefferson observations and others from Early America is the detailed information they provide about day-to-day weather and longer trends during the period.
Jefferson and his contemporaries also show something of the limitations of individual perception and anecdotal evidence. He and some other thoughtful observers were convinced that European colonization of North America and subsequent changes in land use were moderating the cold climate and making it warmer by the second half of the 18th century. As Jefferson wrote in Notes on Virginia in the early 1780s:
A change in our climate however is taking place very sensibly. Both heats and colds are become much more moderate within the memory even of the middle-aged. Snows are less frequent and less deep. They do not often lie, below the mountains, more than one, two, or three days, and very rarely a week. They are remembered to have been formerly frequent, deep, and of long continuance. The elderly inform me the earth used to be covered with snow about three months in every year. The rivers, which then seldom failed to freeze over in the course of the winter, scarcely ever do so now.
Jefferson’s assessment was not correct: Virginians and other Americans would experience some terrible winters in the decades to come after he wrote that passage. Yet he was not alone, for others, including Benjamin Franklin, also asserted that winters were becoming milder. While Jefferson, Franklin, and their contemporaries were right to think that human activity can affect climate, they were incorrect in believing that forest-clearing and agriculture by colonists had moderated the climate of North America since the 17th century.
Stefan Brönnimann, Sam White, and Victoria Slonosky, “Climate from 1800 to 1970 in North America and Europe,” in Sam White, Christian Pfister, and Franz Mauelshagen, eds., The Palgrave Handbook of Climate History (London, 2018), 309-20.
Brian Fagan, The Little Ice Age: How Climate Made History, 1300–1850 (New York, 2000).
Thomas Jefferson, Notes on the State of Virginia, ed. William Peden (Chapel Hill, N.C., 1955), 80.
Karen Ordahl Kupperman, “The Puzzle of the American Climate in the Early Colonial Period,” American Historical Review, 87 (1982), 1262-89.
Sam White, A Cold Welcome: The Little Ice Age and Europe’s Encounter with North America (Cambridge, Mass., 2017), 19-22.
Sam White, “North American Climate History (1500–1800),” in Sam White, Christian Pfister, and Franz Mauelshagen, eds., The Palgrave Handbook of Climate History (London, 2018), 297-308.
Sam White, “The Real Little Ice Age,” Journal of Interdisciplinary History, 44 (2014), 327-52.
Anya Zilberstein, A Temperate Empire: Making Climate Change in Early America (New York, 2016), 3-5, 148, 167.