Forests, drought, and cities

One of the things I’ve learned as a basic concept in studying ecology on the big scale is the way different systems feed each other: the energy isn’t always going simply in one direction.

For example, the climate you have in a given place will affect the forests that grow there – the temperature and precipitation determines what kinds of plants grow and thrive. But forests also affect their local weather (including rainfall) and climate:

Forests exert an influence on their local climates in two ways: by cycling water through evapotranspiration, a phenomenon that includes not only water evaporating from ground surfaces but the water that moves through plants from roots to leaves, where it escapes as vapor into the atmosphere; and by albedo, which involves the degree to which solar energy is reflected or absorbed by an area of the Earth’s surface.

For example, transpiring rainforests in the Amazon contribute to a significant amount of cloud formation and subsequent rainfall. (source)

So cutting down too much forest makes it harder to regrow it, because if decreasing the forest leads to a decrease in rainfall, that will make it more difficult for the trees and other plants to survive in what have become more like drought conditions.

While it is true that droughts occur naturally in many places, there are some people who suspect that the drought being experienced in São Paolo right now is in part due to the vast amounts of deforestation that have happened in that region in recent decades. Historically, it looks like the ancient Mayan city of Tikal disappeared because of drought, which was made worse by local deforestation that reduced local precipitation and urban water management that prevented rainfall from recharging groundwater.

Tikal had a population explosion in the seventh century and continued to adapt. “But they ran all of the resilience out of the landscape. Then, when the rains didn’t come, they had no way to respond and the whole thing collapsed.”

Lentz draws a comparison with a neighbouring city called El Zotz, which had a smaller population, which didn’t modify its landscape as drastically, and was thus able to survive the drought that felled Tikal.

“They ran all the resilience out of the landscape. . . and the whole thing collapsed”

How many times and places has that been the basic story? (For a lovely current example: California’s overuse/inappropriate use of water and worsening drought problems, though it has little to do with deforestation.)


About Fjothr Lokakvan

More or less Northern Tradition polytheist.
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