Amazon Watch: What Happens When the Forest Disappears?

Scientists are worried about a crucial tipping point: The time when the Amazon ceases to be a carbon sink.

A car drives along a section under construction of the Trans-Amazonian highway (BR230) near Ruropolis, Para state, Brazil.Nelson Almeida/Getty

This story was originally published by Yale Environment 360. It appears here as part of the Climate Desk collaboration.

The contrast is staggering. On one side of a narrow track is cool, moist rainforest, stretching northwest for hundreds of kilometers through the almost intact Xingu indigenous reserve. On the other side is hot, bare ground being prepared to plant soy on a farm the size of 14 Manhattans. This, says my guide, earth systems scientist Michael Coe, is the front line of deforestation in the Amazon—where the rainforest meets agribusiness, but also where a rainforest ecosystem is being degraded into savanna grassland.

It is also “the perfect laboratory” for exploring how forests interact with climate, and how that changes when the forest disappears, says Coe, of the Woods Hole Research Center in Massachusetts. And it is where Brazilian and American scientists are keeping watch for the long-predicted tipping point—the moment when the Amazon, the world’s largest rainforest, begins a process of runaway degradation, when so much forest has been lost that the transition to savanna is irreversible. That will be the moment when the Amazon ceases to be a carbon sink that helps protect the planet from climate change, and turns into a global source for carbon emissions.

We are on Tanguro Farm in the Brazilian state of Mato Grosso, one of the world’s largest soy farms. A 16-hour bus ride from Brasilia, the farm is in the basin of the Xingu River, one of the Amazon River’s largest tributaries. A century ago, when the area was still remote jungle, eccentric British explorer Percy Fawcett disappeared here while searching for the rumored “Lost City of Z,” and where some 100 people died seeking to rescue him.

There may never have been a “lost city.” But the modern-day local town of Canarana is full of grain silos, bars, and John Deere franchises, servicing the big farms. One of the largest, Tanguro, was partly cleared for pasture in the 1980s, and converted to cultivation starting in 2003 by the Amaggi corporation, the world’s largest soy farming conglomerate. Today, half is comprised of fragments of forest; the rest consists of giant fields growing soy, corn, and, starting this year, cotton.

Thanks to a deal struck at a chance meeting between Woods Hole researcher Dan Nepstad and the company’s CEO and then-state governor Blairo Maggi, American researchers and colleagues from the Amazon Environmental Research Institute (IPAM) have since 2004 been monitoring the forest and researching how the climate is changing in and around it.

And following the widespread fires set this year on the fringes of the Amazon—breaking a run of 15 years during which deforestation had been dramatically reduced — places such as this are on the front line as the Amazon faces its most fundamental crisis, with temperatures rising, dry seasons lengthening, and rainforest trees being replaced by savanna species.

More than a third of the Xingu Basin, a region bigger than New York State, is now deforested. Scraps of tree cover are all that remain outside the still densely forested Xingu indigenous reserve at its heart. I am shown around by Coe and Divino Silvério, the son of a local farmer, whose research work on the station has garnered him a doctorate and a string of highly regarded scientific papers. “We have over a decade of data here. Nowhere else in the tropics has that,” says Coe.

What they are seeing is alarming.

Deforestation is dramatically raising local temperatures. The air over the farm is on average 5 degrees Celsius hotter than in the forested reserve over the fence: 34 degrees C, rather than 29 degrees C. The difference rises to a staggering 10 degrees at the end of the dry season, says Coe.

And the dry season is lengthening. Across the Xingu Basin and through the southern Amazon region known as the “arc of deforestation,” it lasts almost four weeks longer than half a century ago.

Why these huge changes? The answers lie not in global climate change but in the impact of deforestation, says Coe. In the old days, the trees of the rainforest acted as water pumps, recycling most of the rainwater, which they pumped from underground and released into the atmosphere from the pores in their leaves, a process known as transpiration.

Transpiration requires large amounts of energy, taken from solar radiation. “Every square meter of forest removes the heat equivalent of about two 60-watt [light] bulbs burning 14 hours per day,” Coe calculated in one study. So it cools the air of intact forest. But take away the forest, and the air is instantly much hotter.

The transpiration of a typical large Amazon tree also releases around 500 liters of water a day into the atmosphere. The moisture creates clouds and rain that sustain the forest. Three-quarters of the rain falling in the forested parts of the Xingu Basin is recycled back into the air in this way. But that proportion falls to 50 percent or less if the trees are replaced by pasture or croplands.

The Amazon currently still generates about half its own rainfall, with some rain blowing on the trade winds from the Atlantic Ocean falling and then transpiring back into the air five or six times as it crosses the vast basin. But deforestation has reduced annual moisture recycling in the Xingu Basin by 35 cubic kilometers in the past two decades, according to Silvério.

So deforestation creates what Coe calls “a giant change to the water and energy balance. The climate shifts.” Permanently steamy jungle is replaced by a hotter, drier climate, with dust devils replacing transpiring trees.

The change is especially important at the end of the dry season. Tapping water deep underground, trees keep transpiring even after months without rain. In fact, thanks to the energy from the unrelenting sun, they transpire even more in the dry season than in the wet season. Research at Tanguro has confirmed that this is vital to ending the dry season, because it provides the first moisture for the rains to resume, says Coe.

As the climate changes, so does the vegetation. Rising temperatures and a longer dry season, both caused by the loss of trees, create water stress that flips ecosystems from rainforest to savanna.

A long dry season also makes the forests more susceptible to fires. And fires in turn accelerate the change in vegetation. As Coe puts it: “Fire is nature’s way of starting over.” And now when it starts over here, it shifts to savannah species.

The combination of rising temperatures, longer dry seasons, and more fires is driving the “savannization” of the forests—a process first predicted in 1991 by Brazil’s pre-eminent climate scientist, Carlos Nobre. “When the dry season becomes longer than four months, tropical forest turns to savanna,” he told me when we met in his hometown outside São Paulo after my visit to Tanguro.

For many years this was just a prediction from climate models. But, says Paulo Moutinho, a senior scientist at IPAM and a fellow of Woods Hole, “Our fire studies at Tanguro were the first to test Nobre’s savannization model in the field. We are demonstrating what Nobre predicted—that fire transforms rainforest into savanna through speeding the invasion of cerrado trees.” Silvério has overseen a detailed inventory of thousands of trees in the forested areas of Tanguro Farm. The second census, currently under way, has found a decline in the number of species in just the past four years. Big rainforest trees in particular are being replaced by fast-growing pioneer species, many more widely known in savanna regions.

Experimental plots at Tanguro, in which patches of forest are subjected to burning, show how following the fires, savanna trees and grasses move in to replace the lost rainforest. The grasses in particular are more flammable, so the next fire burns more fiercely than the first.

The managers on Tanguro Farm have been attempting to follow the government’s Forest Code, which requires them to plant native rainforest tree species onto land near rivers that was illegally cleared by the cattle ranchers that preceded them. But the new savanna climate means rainforest saplings won’t grow, says Coe. “Probably savannah species would grow, but the code says you have to restore what was there before.” So instead, the company leaves these riparian areas fenced off in the hope the native species will find a way to return. Nobody is holding their breath.

Nobre argued in 2007 that there could come a point where savannization is unstoppable across large swaths of the Amazon. He said the tipping point could occur if 40 percent of the forest was lost. More recently he has warned that, with the background global rise in temperatures, that threshold could be much closer—at between 20 and 25 percent loss. With Brazilian government scientists putting the current loss at 19.7 percent, the doomsday could be close.

Some leading Brazilian researchers interviewed for this article questioned whether there is a single tipping point that applies to the entire Amazon. It could be a more gradual process. The more pristine north and west could survive. But other regions in the south and east, including Mato Grosso, are well past 25 percent loss. And in Tanguro, accelerating savannization seems to be under way right now.

This matters for the planet as a whole. For, says Jose Marengo, research director of Brazil’s National Center for Monitoring and Early Warning of Natural Disasters, switching from rainforest to savanna will change the Amazon from its current position as a sink for about a billion tons of atmospheric carbon dioxide a year, into a CO2 source. “It would mean bye-bye Paris,” he says, referring to the 2015 Paris accord aimed at keeping global warming below 2 degrees.

One researcher told me she believes that the switch has already happened. Her study is not yet completed, but it may produce some headline-grabbing findings next year.

For the Brazilian scientists fighting to save the Amazon, the tragedy is that they are seeing decades of work that established the rainforest’s importance for Brazil and the globe apparently undone in the months since Jair Bolsonaro took office as Brazil’s president in January. He has effectively given a green light for forest clearance. This has put into reverse a decline in deforestation of more than 75 percent since 2004, just after environmental activist Marina Silva became the country’s environment minister. She introduced a moratorium on the sale of beef or soy from recently deforested land and policing the Amazon with real-time satellite monitoring of forest destruction.

Bolsonaro says Brazil cannot allow its economic development to be hampered by foreign-imposed restrictions on clearing forests for agriculture. Baloney, say most of his country’s scientists. Most deforestation is carried out illegally by speculators grabbing state-owned forest land. “They cut and burn the trees. Then they put in cattle, and wait for an amnesty to legalize their annexation, after which they can sell,” says Mountiho.

This is not about economic development, Mountiho says. It is about condoning criminality. Genuine economic development would involve more intensive use of already deforested land. In fact, says Nobre, the country could increase beef and soy production while still giving land back for natural forest regeneration. “If you double up livestock intensity, which is entirely feasible, you could free up more than half a million square kilometers for forest restoration,” he told me.

Such progressive policies are now supported by many large agribusinesses, says Nobre. They fear that continued deforestation could bring international consumer boycotts of their beef, soy, and other products. But it remains unclear if the government will continue to back land speculators or listen to an emerging alliance of environmentalists and agribusiness.

With a tipping point looming, the stakes are immensely high for the Amazon and the planet. The forests and much of their biodiversity could, with the right assistance, still recover. Even the huge soy fields at Tanguro are not as lifeless as might be expected. Tall flightless rhea birds wander around looking for seeds. Tapir tracks and droppings are everywhere. Armadillos burrow in the verges. And I even saw a jaguar saunter down a track just 20 meters from a field waiting to be planted with soy.

But on the road back to Canarana, just a few kilometers from the farm gate, Silvério and I encountered the blackened remains of a native forest. It had been engulfed in flame after a fire set to clear scrub on pasture just a week before spread across the road. Was it deliberate or accidental? Out here on Brazil’s wild frontier, nobody knew—or was saying.

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