Science: "Forests Are Not Just Carbon Sponges" - Good Article On The Complexities Of Rapid Change
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To account for how forests will affect future climate, researchers must not only tally current trends, such as development-driven deforestation, but also forecast how powerful forces such as surges in wildfire and warmer temperatures might affect forests, sometimes helping and sometimes harming their ability to soak up atmospheric carbon. Historically, researchers have focused much of their attention on the losses side of the balance sheet, for example by quantifying the steady erosion of tropical forests, one of the planet’s major carbon sinks. In the Amazon, the world’s most expansive tropical forest, the news has been almost unremittingly bad. Overall, it has shrunk by about 18% since the 1970s because of deforestation.
In 2007, meteorologist Carlos Nobre of Brazil’s National Institute for Space Research (INPE) warned the ongoing losses could flip the Amazon from acting as a global carbon sink to a significant new source. Simulations of the Amazon’s hydrological cycle, he found, showed deforestation would make rainforests drier, reduce tree growth, and promote tree losses, resulting in a net release of carbon to the atmosphere. That prediction now appears to have been realized, says INPE climate researcher Luciana Gatti. Drawing on measurements of atmospheric carbon collected during 590 research flights over the Amazon between 2010 and 2018, she reported in a July 2021 Nature study that the southeastern Amazon—a region often called the “arc of deforestation,” where agriculture has gobbled vast swaths of trees—had flipped from sink to source. So, in consequence, had the Amazon as a whole. “We have hit a tipping point,” she says.
The years since 2018 have been “even worse” for the Amazon’s carbon storage capacity, Gatti says, as warming temperatures have compounded the effects of deforestation. Longer dry seasons are stressing trees and increasing fire risks, accelerating the conversion of forest to more open savannas. Overall, the Amazon’s total carbon storage could drop by one-third in coming decades if regional temperatures rise by 4°C, modeling studies conducted by climate scientist Chris Jones and colleagues at the United Kingdom’s Met Office conclude.
In the meantime, however, some tropical forests are continuing to sequester large amounts of carbon. For example, one longterm field study in the lowland forests on the island of Borneo recently reported that intact 1-hectare plots, where tree deaths remain infrequent, hold an average of 20 tons more carbon today than they did in 1958, primarily because of CO2 fertilization. But ongoing warming is working against tropical forests, even those that are still intact. An international study that has tracked 300,000 trees in more than 500 plots of intact tropical forests over 30 years finds that even without deforestation, their ability to capture CO2 peaked in the 1990s and has since declined by one-third. The decline began in the Amazon, and since 2010 has extended to tropical Africa, says co-author Simon Lewis, a plant ecologist at University College London. Remote-sensing techniques that assess changes in the total leaf area produced by trees and other plants also suggest many tropical forests are slowing their carbon intake.
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https://www.science.org/content/article/trees-help-curb-climate-change-can-also-contribute-warming-reducing-earths-reflectivity