Do Forests Make the Wind?

Meteorology is not my field, but I’ve been paying much closer attention to trees (they’re everywhere in Portland!) and forests (harder to get to in these stay-at-home days) ever since I read The Overstory this past winter-spring. So I was naturally intrigued when my eye fell on Weather Makers in Science last month (19 June 2020, 1303). The subtitle says it all, “Forests supply the world with rain. A controversial Russian theory claims they also make the wind.” I was surprised twice over.

In my mind, rain comes from the ocean. Evaporation of water from the Pacific creates the clouds that keep Pacific Northwest skies gray most of the year. When the conditions are right, the clouds drop their load and turn my garden green. As for wind, I remember an instructive encounter during junior high. A kid said to me, “I’ll teach you where the wind comes from. The hot air rises (he raised his left hand dramatically high over his head and I tilted my head back to watch), and the cold air comes in to takes it place (and with that I got a quick smack on my cheek from his right-hook).”

But little did I know. In the 1990s scientists documented atmospheric rivers that lift moisture exhaled by forests and then carry it downwind. One so-called “flying river” over the Amazon provides 70% of the rain for southeastern South America. Another flying river brings China 80% of its rain from the west, starting in Europe and getting recycled through the forests of Scandinavia and Siberia. Cut down a forest and the land downwind turns into a desert.

The newest wrinkle is a proposal from two Russian scientists, Anastassia Makarieva and Victor Gorshkov, that forests also function as “biotic pumps” that create air flow, that is, wind. Doubters say this is likely a minor effect, while believers say that it is a significant factor in making the weather. The article gives ample space to both sides in the debate, and it also includes these important words:

Even those who doubt the theory agree that forest loss can have far-reaching climatic consequences. Many scientists have argued that deforestation thousands of years ago was to blame for desertification in the Australian Outback and West Africa. The fear is that future deforestation could dry up other regions, for example, tipping parts of the Amazon rainforest to savanna. Agricultural regions of  China, the African Sahel, and the Argentine Pampas are also at risk, says Patrick Keys, an atmospheric chemist at Colorado State University, Fort Collins.

In 2018, Keys … track[ed] the sources of rainfall for 29 global megacities. He found that 19 were highly dependent on distant forests for much of their water supply, including Karachi, Pakistan; Wuhan and Shanghai, China; and New Delhi and Kolkata, India. “Even small changes in precipitation could have big impacts on the fragility of urban water supplies,” he says.

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