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Sunday, April 14, 2024

A Looming El Niño Could Dry the Amazon

On paper, the Amazon rainforest is a static expanse: perpetually wet, impenetrable, consistently humming with biology. But in reality, the region endures periodic droughts when the rains dwindle, trees stress out, and wetlands parch. Boom and bust. As with forests around the world, that’s part of the natural order.

One of the drivers of Amazonian droughts may soon kick off, potentially piling yet more stress on an ecosystem already ravaged by the deforestation and fires caused by human meddling. The El Niño-Southern Oscillation is a Pacific Ocean phenomenon in which a band of water develops off the coast of South America that transitions from neutral to exceptionally cold or warm. The past few years of cold “La Niña” conditions are weakening, potentially giving way to warm “El Niño” conditions later this year, according to modeling by the National Oceanic and Atmospheric Administration. And for the Amazon, that can cause drought.

It’s still too early to tell when El Niño will arrive, and how severe it may end up being. But scientists recall how bad things got during the El Niño eight years ago. “In 2015-2016, we observed that air temperature over Amazonia was the highest in maybe the last century,” says Juan Carlos Jiménez-Muñoz, a physicist and remote-sensing specialist at the University of Valencia. “In particular, over Amazonia [El Niño] suppresses the rain, and in general you can expect a widespread drought.” But, Jiménez-Muñoz cautions, “every El Niño is different—you can have different regional or local impacts.”

That’s because El Niño widely transforms atmospheric circulation. When that warm blob of water forms in the Pacific, it creates more evaporation, sending moist air into the sky. That water eventually falls as rain over the ocean. This messes with the Walker circulation, sending sinking, relatively dry air over the South American landmass, leading to less rain over the Amazon. “In general, the rain falls more on the ocean,” says Earth systems scientist James Randerson, of the University of California, Irvine. “It just doesn’t rain as much on global land. The continents lose water, especially South America.”

When El Niño isn’t active and conditions are normal, moisture evaporates off the Amazon and ascends to the sky before falling on the forest as rain. The Amazon may recycle up to half of its precipitation this way. “The Amazon is a factory of atmospheric moisture,” says Paola A. Arias, a climate scientist at the University of Antioquia in Colombia. “When you have these drought events, you also typically have reductions in this precipitation recycling.”

Because El Niños vary in their magnitude, they vary in how much they suppress rain over the Amazon. They also vary in where exactly they spawn droughts, and for how long. If the development of an El Niño is more focused in the central Pacific Ocean, it tends to create drought focused in the northeastern part of the Amazon. If it’s more focused in the eastern Pacific, the drought can be more widespread and last a bit longer. But for 2023, it’s too early to say how any of this will play out—Randerson says that scientists should have a better idea this spring. “The fact that we’re in this sustained La Niña for so long,” says Randerson, “I think it’s more likely that you’re going to shift to a stronger El Niño state.”

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What’s abundantly clear right now is that climate change is driving temperatures ever higher and generally making droughts more frequent and intense. That creates unprecedented peril for the Amazon. “Deforestation is increasing, fires are increasing, and in general degradation of the forest is increasing,” says Jiménez-Muñoz. “Every future El Niño will occur in a global warming scenario, but also in a scenario where the Amazon forest is more degraded. So this will lead to potentially more and more damages in the future.”

The atmospheric issue is twofold: Drier air sucks more water out of soils and plants, plus there just isn’t as much rainfall to hydrate the enormous amount of vegetation in the rainforest. Plant species in wetter parts of the Amazon are particularly sensitive to drought, while in drier southern parts plants are adapted to ride out seasonal periods of dryness. But only to a point, especially considering that dry seasons are already getting longer. 

“They may be able to deal with a three-month, four-month, even five-month dry season. But then increase it by three weeks or a month or two months, and you may be in trouble,” says Paulo Brando, a tropical ecologist at Yale University and Brazil’s Amazon Environmental Research Institute. “If plants are water-stressed, there is a greater chance that they’re going to drop their leaves and twigs to survive the drought. And that increases forest vulnerability.”

This vulnerability is particularly precarious along the forest’s edges. Deforestation is driven largely by ranchers and farmers clear-cutting to make way for their crops and animals. This leaves fragmented forest edges exposed to the dry open air, which desiccates plants that would normally enjoy the humidity of a thick canopy. Then people set fire to what they’ve clear-cut, leading to out-of-control blazes. 

If plants stressed by drought are dropping leaves, they’re piling up yet more fuel for these fires. At the same time, the air is drier and hotter—the perfect conditions for blazes to spread. “You’re undermining the ecological firebreak,” says Brando, “because you’re increasing the likelihood that what is on the ground is going to burn.”

Scientists are concerned that deforestation and wildfires are making the Amazon less resilient to dry spells. Last year, researchers published a paper showing that since the early 2000s, three quarters of the Amazon has been slower to grow back its biomass after being disturbed by changes like drought. Areas that get less rainfall, or that are closer to human activity, have been suffering more than wetter ones. This dynamic kicks off a nasty feedback loop: Less vegetation means fewer plants are sending moisture into the atmosphere, which would normally fall back on the Amazon as rain. With less moisture, degraded parts of the rainforest may be approaching a tipping point at which they will transform into grassy, savannah-like landscapes—a transition of no return.

Today in the journal Nature Climate Change, Brando and his colleagues calculate that as the world warms and dry periods intensify, the shrinking of South America’s humid regions could account for 40 percent of the biomass loss across all the world’s tropics. “When you look at the broader picture using our empirical models, dry-season intensity tends to increase in the southeast Amazon, and in the eastern Amazon especially,” says Brando. “And so there are very large losses of biomass in that region projected by our model.” 

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Yet it isn’t fully clear how the frequency and severity of both El Niños and La Niñas will change as the world warms. The Intergovernmental Panel on Climate Change says that both phenomena are likely to increase in frequency, with extreme El Niño events becoming twice as common in the 21st century compared to the 20th. But if La Niñas become more frequent, they would have the opposite effect on the Amazon. “If we do have more La Niñas, then the trend is to have more precipitation than usual,” says climate change ecologist David M. Lapola, who studies the Amazon at Brazil’s State University of Campinas. “It’s still an open field of active research. But if we base our knowledge on what the IPCC says, it points to a trend of drier climates.” 

Plus, the El Niño-Southern Oscillation isn’t the only driver of drought in the Amazon. The Amazon’s severe drought of 2005, for instance, was due to intense storms in the North Atlantic that—similar to El Niño—created atmospheric circulation that suppressed precipitation over the rainforest. 

Still, scientists are already finding that all these stressors—longer dry seasons, drought, deforestation, wildfires—are conspiring to flip a critical carbon switch. The Amazon’s vegetation absorbs CO2 and exhales oxygen as it grows. But if people chop down the forest and light it on fire, that carbon heads right back into the atmosphere. And less growth due to water stress means less CO2 gets sequestered and global temperatures will rise higher.

So instead of being a reliable tool for pulling our carbon emissions out of the atmosphere, the Amazon could be turning into a climate problem. Drought will only make matters worse. “There are some regions in the Amazon that are not able to capture carbon anymore—they are actually becoming a source of carbon, which is even more dangerous,” says Arias. “That is a physiological change that is already happening.”

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