The Year Without Summer by William K. Klingaman, Nicholas P. Klingaman Read Free Book Online Page B

Pennsylvania in 1771. Dunbar, who frequently exchanged meteorological observations
     with Jefferson, claimed that deforestation actually made summers and winters more
     extreme. “I would enquire,” he wrote in an article published in the Transactions of the American Philosophical Society , “whether a partial clearing extending 30 or 40 miles square, may not be expected
     to produce a contrary effect by admitting with full liberty, the sunbeams upon the
     discovered surface of the earth in summer, and promoting during winter a free circulation
     of cold northern air.”
    Timothy Dwight, a Massachusetts cleric and educator who, like his contemporary Jefferson,
     loved to collect weather data, also rejected the argument that American winters were
     growing milder. Dwight pointed to numerous very cold and snowy winters in the thirty
     years since independence that rivaled any of the formidable seasons of the seventeenth
     or early eighteenth centuries. Besides, discussions of changing climates seemed pointless
     to Dwight without adequate statistical data. “Few, if any, registers were kept in
     former times,” Dwight noted, and fewer still had been published. “Hence the comparisons
     of our present climate with that of former periods must be extremely defective.”
    Climate scientists now know that deforestation of large areas can cause prolonged
     droughts and exaggerate seasonal variations in temperature, such that summers become
     much warmer and winters much colder. Dunbar was partially correct in his conclusions,
     although he failed to understand how forest canopies maintain the climate beneath
     them. Forests insulate their environment not only by reflecting sunlight but also
     by trapping moisture; plant roots help to retain water in the ground, while the canopy
     prevents water vapor from escaping into the air above. Remove the forest, and the
     moisture in the soil quickly escapes; winds then transport the water vapor hundreds
     or thousands of miles away. This starts a vicious cycle: Less water in the soil leads
     to less evaporation into the air, which can lead—when applied to an area of hundreds
     of square miles or more—to less rainfall, which in turn leads to less water in the
     soil. What rain does fall will often be unable to penetrate into the dry, hard soil,
     further increasing the risk of devastating droughts.
    Summers become hotter in deforested areas not only because more sunlight reaches the
     surface, as Dunbar argued, but also because there is less moisture in the soil to
     cool the ground through evaporation. Water in the soil performs the same function
     as sweat does in humans; with little moisture to evaporate, bare ground quickly warms
     in the sunlight. Without the insulating effects of the forest canopy, winter temperatures
     can drop rapidly as the heat stored in the soil is lost to the atmosphere. There is
     no evidence to support Dunbar’s link between deforestation and stronger northerly
     winds, although generally forests do act as a brake on the local wind speeds, regardless
     of the direction. The effects of deforestation on local temperatures and rainfall
     can be mitigated where the forests are replaced with other ground cover, such as shrubs
     or crops, instead of simply left as bare soil.
    If deforestation had, in fact, transformed their climate, Americans were ambivalent
     about the desirability of that change. On the one hand, the early colonists viewed
     the virgin North American forests as dangerous and evil places, the preserve of the
     devil (and, not coincidentally, Native Americans). They and their descendants believed
     they had a duty to level what Nathaniel Hawthorne termed the “heathen wilderness.”
     Turning a dense and dark forest filled with “stagnant air” and “rank vegetation” into
     productive farmland to support a Christian community seemed to fulfill God’s plan
     for the New World. Yet by the early nineteenth century, Americans in the