2 - Tempering the Climate, c. 1600–1840

Summary

A two-thousand-year-old tradition, dating to Greek and Roman scholars, holds that deforestation changes the climate. This belief arose again during European settlement of the Americas with the belief that deforestation was improving the cold winter climate. Opening the land to sunlight was thought to warm the surface and melt the snow. Evaporation was known to cool the surface, and forests, by transpiring considerable quantities of water, were thought to contribute to the coldness. Other scholars theorized that the differential heating of forested and open lands arranged in a patchwork mosaic across the landscape created mesoscale circulations that altered temperature. By the early 1800s, the belief that deforestation had altered the climate of the United States and Europe was common. Alexander von Humboldt and others began to develop a theoretical framework in which forests influence climate by shading the ground, through transpiration, and through radiative exchanges with the atmosphere. Other scholars looked to long-term meteorological measurements and could find no signal of deforestation in temperature records.