SCIENTIFIC AMERICAN
20 de febrero, 2023
Emma Bryce
Here’s why the Amazon rain forest is key to protecting Earth from the detrimental effects of climate change
Viewed from high above, the Amazon in South America is a lush emerald quilt, home to millions of animals and the planet’s largest river by volume. It is also key to protecting our planet from the detrimental effects of climate change.
But why? And what could happen to the global climate if we lost it?
The Amazon’s sheer size—it covers nearly seven million square kilometers, or about the area of Australia—makes it shine on the climate stage. With so many trees covering such an enormous swath of land, everything the forest “does” is big and impactful. Some of its actions are downright unique: The Amazon makes its own weather, generating some of its rainfall and keeping itself cool, while also stabilizing regional temperatures. Add that to the gobs of greenhouse gas that its biomass stores and you’ve got a natural climate protector. Yet we are steadily dismantling this valuable landscape, aggravating climate change in several ways scientists are just beginning to understand.
Giant Carbon Sponge
All Earth’s forests impact the atmosphere. Trees draw down carbon dioxide from the atmosphere through photosynthesis, locking away that greenhouse gas in leaves, trunks, roots and nearby soil. The Amazon’s sheer volume of trees makes it one of the world’s largest carbon sinks. “The Amazon today, even with all the deforestation, stores more than 150 billion [metric] tons of carbon,” says Carlos Nobre, an Earth systems scientist at the University of São Paulo in Brazil and a longtime researcher of the climate and ecological effects of Amazon deforestation. Roughly half the Amazon’s carbon store is in the soil. The other half is in its trees, which contain about 20 percent of all the carbon captured by vegetation across the planet.
But when humans cut down those trees, that biomass releases its stored carbon back into the atmosphere as CO2, where it has a warming effect. Like other greenhouse gases, carbon dioxide molecules prevent heat on Earth from escaping back into space. The agriculture and mining industries are steadily cutting down the Amazon’s dense network of 16,000 tree species. Altogether, deforestation has stripped 17 percent of the rain forest’s tree cover since 1970.
Deforesting industries often clear an area by setting fires, which rapidly release stored carbon into the atmosphere. Trees left unburned but cut decompose, also releasing their carbon. The fires help to explain why parts of the Amazon forest now emit more CO2 than they absorb. Fires also send up plumes of soot that screen sunlight and add to warming.
Making It Rain
The warming atmosphere feeds back into the forest. As the globe heats up, the frequency and severity of weather phenomena associated with drought are increasing over South America, says Nobre, who is also co-chair of the Science Panel for the Amazon, which supports research and initiatives to save the rain forest. The effects are creating longer regional dry periods and less rainfall, driving tree die-offs. Dryness and declining forest cover also increase the risk of natural fires.
Tree loss brings other consequences, Nobre says. During the dry season, more intense sunshine causes vast amounts of water—stored in trees and soils from wet periods—to transpire. That water vapor “reaches the lower atmosphere and becomes clouds and rain again,” Nobre explains. “One molecule of water vapor that enters the Amazon recycles between five to eight times,” he adds, illustrating the importance of this hydrological engine in recharging regional rainfall. Ample rains also have a cooling effect, just as sweating can cool a person off after a workout.
If tree cover declines, so will the water store, and with it, more of the forest. Past research led by Nobre showed that a cycle of warming, drying and forest shrinkage could push the Amazon to a minimum threshold of tree cover, below which the forest would irreversibly degrade into a simpler grassland habitat.
What if the Forest Vanishes?
Estimates vary on the exact threshold at which the Amazon would disappear. Nobre and others suggest it could be a loss of just 20 to 25 percent of the Amazon’s predeforestation tree cover (bearing in mind the forest has already lost 17 percent). There are also variable figures on how swiftly the forest could degrade once it passes the threshold. But already, parts of the southern Amazon are transitioning into what Nobre describes as “open canopy degraded ecosystems.” This is a landscape with sparse tree cover, containing “tremendously reduced biodiversity,” that stores a fraction of the carbon of an intact rain forest, Nobre says.
If the forest fades away, and its trees release their massive carbon store, what does that mean for global climate?
The Amazon’s estimated 150-billion-ton-plus carbon bank is the equivalent of more than 10 years’ worth of global fossil fuel emissions, Nobre says. If the entire Amazon degraded into an open, savannalike landscape, local rainfall would decline by up to 30 percent, with consequences felt as far as Colombia and Argentina, where rainfall cycles are partially fed by moisture from the Amazon, Nobre explains. Without the forest’s surface-cooling effect, regional temperatures would rise by several degrees.
The forest loss would resonate across the globe. “If you put [carbon dioxide] into the atmosphere, it quickly diffuses all over the world,” says Elena Shevliakova, a physical scientist at Geophysical Fluid Dynamics Laboratory, part of the U.S. National Oceanic and Atmospheric Administration, who has modeled the climate effects of Amazon loss. Releasing, say, 120 billion metric tons of CO2 (a more conservative estimate of Amazon carbon stores) by removing forest would warm the planet by an estimated 0.25 degree Celsius, she explains.
Even if the world reduced human-caused emissions enough to get on track to fulfill the Paris climate agreement, keeping global warming below a 1.5-degree-C increase from preindustrial levels, the sudden loss of the Amazon and its stored carbon would put that target out of reach. Global emissions reductions are not currently on target, which should raise the concern about the impact of the Amazon’s degradation, Shevliakova says.
To protect this planetary gem, Nobre says the international community urgently needs to curb deforestation and greenhouse gas emissions, and, where possible, reforest the degraded swathes. Saving the Amazon is also entwined with the rights of Indigenous peoples; a growing body of research suggests that lands stewarded by Indigenous peoples are deforested less.
Even if the emerald biome seems like a distant world, everyone on Earth is connected to its fate, Shevliakova says. “Losing the Amazon is going to affect everybody,” she adds.
Tomado de: https://www.scientificamerican.com/article/why-is-the-amazon-so-important-for-climate-change1/