When it comes to doing research in the Amazon, Oliver Phillips says the worst part is the sweat bees. Phillips, an ecologist from the University of Leeds who has been working in the Amazon for 30 years, says the bees don’t bite or sting or carry diseases, like many of the rainforest’s other insects, but “they’re just all over you.” The instant you start sweating—which is the instant you set foot in the Amazon—they swarm your limbs, your nose, even your eyes, ravenously feeding on the salt in your perspiration. “They don’t do any damage except drive you crazy,” Phillips says.

For Phillips, sweat bees are an occupational hazard. He’s hiked—on purpose—into swamp forests in Peru, wading through stagnant water that smells like rotten eggs. He’s returned to areas of forest he thinks he knows well, only to find them flooded. But no matter the obstacle, he gets out his measuring tape and doesn’t leave until he and his colleagues have recorded the height and girth of every single tree in a 100 square meter plot. A few years later, they’ll come back to the same spot and do it all again.

Large trees are measured above the roots with the use of ladders. Roel Brienen/rainfor

Any individual tree doesn’t tell Phillips much about how the Amazon is reacting to climate change, of course. But thousands of them, measured regularly for decades? That’s some of the most valuable climate data to come out of the world’s largest rainforest. Phillips coordinates a project called Rainfor, which aims to census and re-census the trees in hundreds of plots in the Amazon for as many decades as funding will allow. The oldest plots in the network were first censused in the 1970s; over 400 scientists, many of them from Amazonian countries, have been involved in the fieldwork so far.

By tracking “the growth and the history and the death of every single tree” within those plots, Phillips says, Rainfor can calculate how much carbon the Amazon sucks up and stores, thereby keeping it out of the atmosphere where it would contribute to global warming. Tracking changes in the amount of carbon stored and released by the rainforest indicates how the Amazon is responding to—and potentially influencing—climate change.

And if scientists want to have any hope of understanding how climate change will affect the world, they must have data from the Amazon. Every year, the world’s largest rainforest cycles through 18 billion tons of carbon per year as its 6 million square kilometers of trees breathe in carbon dioxide and release it back into the atmosphere when they die. That’s more than twice as much carbon as fossil fuels emit all over the world. Mess with this system and the consequences will reverberate around the world.

Working in flooded forests in Peru. Roel Brienen

But scientists are less sure about what exactly those consequences will be. The fact is, climate models have no idea what to do with the Amazon as global warming hits its stride. They don’t know whether the rainforest will grow voraciously, nourished by the extra carbon dioxide in the atmosphere, or die off completely, becoming a savanna. They don’t know whether annual rainfall will increase or decrease over the next 100 years. They can’t tell scientists whether or not the devastating droughts of 2005 and 2010 are anomalies or the new normal. When fed all the data scientists have, climate models don’t even spit out the current, documented weather patterns in the Amazon. “All the models [predict] rainfall in the morning,” says Maria Assunção Faus da Silva Dias, an atmospheric scientist and climate modeler at the University of São Paulo. “But nature does it after lunch.”

The problem is that scientists don’t have all that much data on the Amazon, especially in proportion to its importance in global climate systems. Which brings us back to the sweat bees. Fieldwork in the Amazon is rough, and huge swaths of the forest remain inaccessible even to the most determined scientist. This presents a paradox to field researchers. They want to collect long term climate data from parts of the Amazon that will likely remain untouched by deforestation for several decades to come. But that pristine forest will avoid the axe precisely because it’s the hardest to get to.

The Best Research Is Also the Toughest

This was the challenge faced by the team constructing the Amazon Tall Tower Observatory, a 325-meter (nearly 1,000 feet) high, Erector-set-looking steel tower that aims to study the rainforest’s unique atmospheric physics and chemistry. But to make the project work, the researchers needed the top of the tower to be able to sample clean air 30 years. They needed somewhere remote, but also within a day of Manaus.

Eventually the researchers found a site that is only accessible by dirt road. And then by boat. And then another dirt road. The journey takes six hours each way and is unpleasant enough that the scientists hope it will deter loggers and settlers from following their footsteps. They finished building the tower this year; now comes the hauling of delicate scientific instruments. Not made to survive in the jungle, they will need to be constantly monitored and frequently replaced.

To understand why all the hassle is worth it, you really have to climb to the top of the tower (a journey that takes two hours in and of itself, up a narrow, zig zagging staircase). “You just see forest for 360 degrees,” says Paulo Artaxo, an atmospheric physicist at USP. “There are very few other places in the world where in any direction for more than 150 kilometers, there is just forest.” Upwind of Manaus, the forest and its atmosphere are as pristine as they were in the 18th century, Artaxo says. That makes climbing ATTO a bit like going back in time.

Meanwhile, 150 kilometers to the southeast, there’s Manaus. A booming city of 2 million and counting, it’s on track to be “a megacity in the middle of nowhere,” Artaxo says. With people and industry comes pollution. Another ongoing project called GoAmazon is flying planes directly into that pollution, as well as downwind of it, to see how it interacts with the forest air.

RAINFOR team monitoring Amazon canopy, Peru. Kuo-Jung Chao/rainfor

The preliminary results are troubling, to say the least. Pollution from the city has filled the rainforest atmosphere with an unprecedented number of aerosol particles, changing the first step in the complex process by which clouds and rain form. Since precipitation that forms over the Amazon eventually ends up in the water supplies of cities like São Paulo and Brazil’s huge agricultural areas, changes to the rainforest’s hydrological could have far reaching and potentially devastating effects.

Rainfor’s work also contributes to a growing sense of dread. Back in 1998, Phillips and his colleagues used their network of tree plots to show that the South American rainforest was sucking up more carbon than it was releasing, thus keeping carbon dioxide out of the atmosphere and mitigating climate change. The Amazon’s role as an important carbon sink has been accepted dogma ever since, bolstering conservation efforts and guiding climate models.

But Phillips and his team didn’t stop measuring trees, braving sweat bees and swamp forests. “In the first week everyone still has a lot of energy,” says Roel Brienen, another University of Leeds ecologist who works on Rainfor. “But after two or three weeks when the food runs low and when the morale runs low, then it’s very difficult to get a whole team going.” Yet slowly they expanded the network to 391 plots. When Brienen crunched the numbers, a startling result emerged. Last month the team reported that the Amazon isn’t pulling in as much carbon as it used to, contradicting the many models that predicted that the rainforest would thrive for another several decades thanks to increasing carbon dioxide levels. In many parts of the forest, the trees do appear to be growing faster and bigger, Phillips says. But they are also dying younger, riding the high of resource abundance until they suddenly flame out. Once a tree dies, its carbon travels back to the atmosphere—exactly where we don’t want it be.

For now, the Amazon is still sucking up more carbon than it’s releasing. But the Rainfor data suggests that a tipping point is coming. As skeptics have long asserted and scientists themselves suspected, models of the future of Earth's climate did indeed get it wrong. But not in they way anyone would hope. As Phillips puts it: They were “too optimistic."