YubaNet (EUA)

More carbon dioxide will dry world’s rainforests

Publicado em 08 julho 2021

One was: what would happen over the next 100 years if the ratio of carbon dioxide in the atmosphere reached 588 parts per million? For most of human history, this ratio hovered around 288ppm. Worldwide, since the global exploitation of fossil fuels began 200 years ago, this ratio has already soared beyond 400 ppm. And under various climate scenarios, the 588 ppm figure could happen by 2050, or 2080.

The second question was: what would happen over a century if the entire forest - it spreads across nine nations - was cleared for grassland? Much of the forest enjoys notional official protection but is still being cleared, lost or degraded anyway.

“To our surprise, just the physiological effect on the leaves of the forest would generate an annual fall of 12% in the amount of rain, whereas total deforestation would lead to a fall of 9%,” Professor Lapola said. “These numbers are far higher than the natural variation in precipitation between one year and the next, which is 5%.”

“The wind gives rise to the convection responsible for heavy equatorial rainfall”

At the heart of the puzzle of plants and precipitation is the physiology of green growth: the stomata that control the exchange of atmospheric gases on all foliage. These tiny portals open to capture carbon, and emit water vapour. With more CO2 in the atmosphere, they would remain open for shorter spells. The result: less water vapour, reduced cloud formation, lower rainfall.

But there is a second factor: trees are tall and very leafy, with six times the leaf area per square metre of grass, which is low and earthbound. If the entire forest was replaced by pasture, leaf area would be down by two-thirds. And both rising greenhouse gas ratios and deforestation would also influence wind and the movement of the air masses that carry the potential rainfall.

“The forest canopy has a complex surface made up of the tops of tall trees, low trees, leaves and branches. This is called canopy surface roughness. The wind produces turbulence, with eddies and vortices that in turn produce the instability that gives rise to the convection responsible for heavy equatorial rainfall,” Professor Lapola said.

“Pasture has a smooth surface over which the wind always flows forward, and without forest doesn’t produce vortices. The wind intensifies as a result, bearing away most of the precipitation westward, while much of eastern and central Amazonia, the Brazilian part, has less rain.” -

Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.

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