On a global scale, deforestation is known to be harmful to the environment, as it disrupts the natural exchange of carbon dioxide, radiation, and water between forests and the atmosphere. However, at local and regional scales, things are not so black and white.
For example, global climate models show that deforestation tends to cause cooling at high latitudes, where flatlands replace the former forests and accumulate snow, reflecting more sunlight away from Earth’s surface. Meanwhile, at tropical low latitudes, deforestation reduces evapotranspiration (the process of water being evaporated from the land surface and transpired from plants into the atmosphere) and causes warming.
In another surprising twist, studies have shown that changes in land surface temperature after deforestation can differ during the day and night, a type of phenomenon known as a diurnal asymmetry. This asymmetry can actually produce localized warm conditions at night in the absence of daylight.
However, it’s unclear why, and to what extent, this is true. In a recent study, Schultz et al. explored the biological and physical processes that might work together to produce such a diurnal asymmetry. The team supposed that nighttime forest warming could come either from the trees generating turbulence—small, irregular movements of air—in the layer of the atmosphere where our weather occurs, bringing heat down to the surface, or from the trees storing heat during the day and releasing it at night.
The researchers combined global satellite data from NASA with data collected at flux tower sites (tall towers, higher than the treeline, equipped with sensors to detect atmospheric and other environmental conditions) in Australia, Brazil, Canada, Germany, and the United States. Using these data, they calculated that there is, indeed, a diurnal asymmetry in the changes in land surface temperature in response to deforestation.
Changes in land surface temperature during the day, they found, can be explained by differences in the amount of radiation absorbed from the Sun, as well as the exchange of heat between Earth’s surface and atmosphere, or latent heat flux. At night, temperature changes are driven by a combination of turbulence and stored heat released from trees.
The researchers also confirmed that over most regions of the world, deforestation leads to daytime warming and nighttime cooling. Although the strongest daytime warming is in the tropics, they found, the strongest nighttime cooling is in the boreal zone, or taiga, which includes forests in Alaska, Canada, Russia, and most Scandinavian countries. In boreal forests, which compose about one third of the world’s forest cover, trees have typically grown over ice-covered or glaciated soils. Once deforested, the open lands that remain are cooler than forests.
Regional differences in nighttime cooling, the researchers found, are related to something called a temperature inversion. Instead of warm air expanding, thinning, and cooling as it travels higher up in the atmosphere (which anyone who has hiked up a tall mountain has experienced), a temperature inversion occurs when air close to the ground cools so much and so quickly that it is colder than the air above it. So temperature inversions that occur at high latitudes, where the ground is often cold, will be much stronger than those in the tropics.
This study helps give nuance to our understanding of the environmental effects of deforestation at the local and regional scales. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1002/2016JG003653, 2017)
—Sarah Witman, Freelance Writer