LatinContent/Getty Images Deforestation in the Brazilian rain forest is slowing, but it's still happening—with major consequences for the global environment.

I’d say you have to see the Amazon for yourself to understand how vast it is, but I’ve been there—and even I can’t imagine it. The rain forest is more than 2 million sq. miles—two-thirds the size of the continental United States—and the river system of the gigantic basin produces 20% of the world’s freshwater discharge. The forest holds 100 billion metric tons of carbon—equivalent to more than 10 years’ worth of global fossil-fuel emissions. And the Amazon is the global capital of wildlife biodiversity, with more species calling the forest and rivers home than scientists could ever hope to name. It’s safe to say that as the Amazon goes, so goes the planet’s environment.

The problem is that the Amazon is anything but secure. As Amazon basin nations like Brazil have grown economically, they’ve moved to cut down the forest, making room for agriculture. (Which, it should be noted, is exactly what Americans did to their own once vast Eastern forests.) The human population in the Brazilian Amazon has grown from 6 million in 1960 to 25 million in 2010, while forest cover has declined to about 80% of its original area. Deforestation rates have slowed in recent years, but as a new review in this week’s Nature shows, the Amazon basin is changing, under pressure from natural variability in the weather, drought, global warming and deforestation. The question remains: just how resilient is the Amazon?

MORE: Rain Forest for Ransom

From the Nature article, written by Eric Davidson of the Woods Hole Research Center and his colleagues:

Although the basin-wide carbon balance remains uncertain, evidence is emerging for a directional change from a possible sink towards a possible source. Where deforestation is widespread at local and regional scales, the dry season duration is lengthening and wet season discharge is increasing. We show that the forest is resilient to considerable natural climatic variation, but global and regional climate change forcings interact with land-use change, logging and fire in complex ways, generally leading to forest ecosystems that are increasingly vulnerable to degradation.

Specifically, researchers worry about

Climate and weather change: Drought is a fact of life, even in the ultra-wet Amazon. The El Niño effect can produce lengthy droughts, while the corresponding La Niña effect can lead to increased flow and even flooding. The Nature paper notes that the intact Amazon forest—with its deep roots that can access soil water—is resistant to normal seasonal droughts, but that the transitional forests and Cerrado (the tropical savannah) are much more vulnerable. But even the existing forest may find it difficult to withstand lengthy droughts of the sort that may become more common with climate change—the severe 2005 drought in the southwestern Amazon resulted in the loss of several tons of living tree biomass carbon per hectare.

Deforestation and land-use change: It’s not quite true that the Amazon is being clear cut—as the Nature paper describes, more small land holders in the Amazon, even farmers, keep mature or secondary forests on more than half of their land. Much of the forest that’s being lost is being converted to cropland for soybeans—Brazil is a major producer—as well as pastureland for cattle. Still, attempts to curb deforestation in the Amazon appear to be working—forest clearing has fallen from about 11,000 sq. miles a year in 2004 to less than 3,000 sq. miles a year in 2011. But if deforestation continues, it could change the very climate of the Amazon, resulting in less precipitation over the region. There are even models that suggest that deforestation exceeds 40% of the Amazon basin, a tipping point could be reached that would vastly reduce precipitation and result in a forest “dieback.” That would be bad.

Forest fire: It’s no surprise that drought increases forest fire in the Amazon—about 15,000 sq. miles of forest burned during the El Nino-influenced drought of 1998. But as fires become more common, they can reduce rainfall (because of the action of the smoke in the atmosphere) and retard forest regrowth. Fires keep the Amazon from bouncing back.

Greenhouse gases: Right now the Amazon is a major carbon sink, sucking up and storing some of the greenhouse gases we emit—gases that would otherwise accelerate the warming of the atmosphere. Disturb the Amazon—as we’re doing—and the system may become less efficient at storing that carbon, thus speeding climate change. The good news here is that studies indicate that the mature, intact Amazon forest is still accumulating carbon. But as the forest is disturbed, it causes a net loss for carbon, and the Amazon goes from being an ally to an enemy in the fight against warming.

The conclusion here is the Amazon is an unimaginably complex system, once that needs more systematic study before we can know what’s really happening within the forest. But it would be better to understand that now—instead of waiting to see the consequences of change.

MORE: Brazil Takes a Step Backward on Deforestation