CAAPIRANGA, Brazil (Reuters) - The river loops low past its bleached-white banks, where caimans bask in the fierce morning sun and stranded houseboats tilt precariously. Nearby sits a beached barge with its load of eight trucks and a crane. Its owners were caught out long ago by the speed of the river's decline.
This is what it looks like when the world's greatest rainforest is thirsty. If climate scientists are right, parched Amazon scenes like this will become more common in the coming decades, possibly threatening the survival of the forest and accelerating global warming.
The environmental and economic consequences could be huge -- for Brazil, for South America, for the planet.
An intense months-long drought through November drained the mighty Negro river -- a tributary of the Amazon -- to its lowest since records began in 1902, drying up the network of water that is the lifeblood of Brazil's huge Amazonas state. More than 60,000 people went short of food and many lacked clean drinking water as millions of dead fish contaminated rivers.
It was a "once in a century" kind of weather event. The weird thing is, it came just five years after another severe Amazon drought that meteorologists had described in the same way. Last year, massive floods in the region killed dozens and made hundreds of thousands homeless, fitting a pattern of more extreme weather that climate models forecast for this century.
Years like this add credence to predictions that by the middle of this century, the forest will suffer "mega-droughts" lasting years, killing trees en masse.
That in turn would reduce rainfall over the remaining forest, creating a vicious cycle that would turn much of the Amazon into a savannah-like state by 2100. Ecologists and climatologists say there may come "a tipping point" after which the death of the forest becomes self-sustained by higher temperatures, dwindling rain levels and destructive fires.
The latest drought came as little surprise to Erli Perreira, a skinny 19-year-old who was fishing for his family's dinner in the shadow of the barge, which lay on a tributary of the Solimoes river about 60 miles from the central Amazon city of Manaus. The sun has been getting hotter for years, he said, making it impossible to work in the fields after mid-morning and causing his fish catch to plunge during the annual "burning season" when farmers take advantage of the dry conditions to clear the forest with fire.
"Many things in the Bible are coming to pass," said Perreira, wearing a soaked Guns N' Roses T-shirt and holding a gasping fish in one hand. "At the End of Times many things change, like the sun getting hotter."
Their predictions may be less biblical, but climate scientists and ecologists are worried too. As leaders gather this month for a new round of global climate talks in Cancun, Mexico, the recent weather extremes have sent climate scientists around the world scrambling to study whether they represent a freak or a more sinister sign of climate change.
Rosie Fisher, a project scientist at the National Center for Atmospheric Research (NCAR) in Colorado, has always viewed apocalyptic Amazon scenarios with a dose of skepticism. Many of the complex models that seek to map future climate, including NCAR's own, show that Amazon rainfall may in fact change little over this century.
But she got a shock when she saw maps showing the paltry rainfall over the Amazon this year, less than half average annual levels. The drought of 2005 was severe, but maps showing water deficits over the region this year painted an even drier picture.
"The map that I'm looking at now looks like the extreme bit of my scenario, and it's happening right now. I'm genuinely quite alarmed by this," said Fisher, who specializes in the interactions between climate and forests.
"In some ways it kind of reminds me of when they figured out that the Greenland ice sheet was melting much faster than the climate models predicted it would."
Accounting for more than half of the world's remaining rainforest, the Amazon's trees are a vital global air conditioner, helping to keep the world cool by soaking up atmospheric carbon totaling about 2 billion tons each year. When they die or wither, as they did in large numbers during the 2005 drought, they become part of the global-warming problem by releasing carbon.
The 2005 drought released more greenhouse gases than the annual emissions of Europe and Japan, an international study found last year, showing how the forest can shift rapidly from carbon sink to source. If that study was right, this year's drought is likely to have released at least as much carbon.
"We don't need a big catastrophe in the Amazon to change the earth's system, we just need that sink to disappear," said Oliver Phillips, an ecologist at Leeds University who co-authored the study.
The Amazon -- spanning nine countries and viewed as the world's greatest caldron of biodiversity -- is expected to be hotter by the end of the century than it has been since before the last Ice Age. Depending on greenhouse gas emissions, climatologists say a rise of 3 to 5 degrees Celsius (5.4-9 degrees Fahrenheit) is likely.
For the region's people, who still largely live off its land and water, that is likely to mean an ever tougher struggle to survive. Brazil's agriculture boom, which has seen it become one of the world's breadbaskets, would also be at risk from a breakdown of the region's great rain-making machine.
The consequences for the forest's mind-boggling universe of fauna and flora and for the fight against global warming could also be grave. A large-scale Amazon "dieback" is among a handful of potential events that could drastically intensify climate change, along with the melting of ice sheets in Greenland and Antarctica and the breakdown of the Gulf Stream ocean current. In some ways, it is the most worrying of all because of the speed it could occur and the huge amount of carbon it could pump into the atmosphere as trees die -- estimated at about 15 years worth of human-caused emissions.
"You can have a lot of the carbon released within a few decades whereas the ice sheet is going to take many hundreds of years," said Peter Cox, a professor of climate system dynamics at the University of Exeter in Britain.
"The Amazon, if it happens, will be more catastrophic because there's this feedback between drying and fire and fire and carbon dioxide release that is quite fast."
About 870 miles southeast of Manaus, the east of Brazil's Mato Grosso state may be an early indicator of what the worst case scenario could look like. Here the country's expanding cattle and soy farming frontier collides with the forest, often with fiery consequences.
This year's drought turned the region around the huge protected Xingu indigenous Indian park into a tinder box. Fires, often set by small-time farmers to clear their land, raged indiscriminately through farmland and forest.
The number of fires in Mato Grosso -- which means thick forest -- surged to more than 36,700 so far this year from 8,135 last year, razing cattle pasture, killing livestock and often jumping into the region's remaining pristine forest. A NASA satellite image from the period shows a huge pall of smoke blanketing the center of South America.
For Edimar dos Santos Abreu, it was an exhausting few months. As chief of a new six-member fire brigade trained by the U.S. Forest Service's elite "smokejumper" firefighters, he was in charge of putting out the blazes across the sprawling region.
"We would hardly arrive back home before getting a call about another fire," said the soft-spoken 36-year-old.
One of the 30 or so fires they tackled this year lasted nine days, he said. And unlike in the past, fires that spread to the forest continued to burn at night -- a stark sign of the drier conditions.
"I think what we're seeing in Mato Grosso is a dieback process. It's a process that's going to take 15 or 20 years to come to a new equilibrium," said Daniel Nepstad, a U.S. ecologist with 26 years of experience in the Amazon.
Fires continuing to burn and even intensify at night, when normally in the Amazon they would be extinguished by dew and falling temperatures, are a particularly worrying sight.
"You could get into a situation where there are mega-fire conditions -- we see that in California," said Nepstad, who is a senior scientist for Brazil's Amazon Environmental Research Institute (IPAM).
John Carter, a Texan who has been a rancher in the region for 15 years and who set up the fire brigade, is no bleeding-heart environmentalist. "I don't really care about the biodiversity -- it's just a consequence," says the 44-year-old, a veteran of the first Gulf War who has had his fair share of violent showdowns in the often lawless Amazon region.
But he says the warming of the region since he arrived is striking and increasingly threatens farm production as well as the forest. Without keeping more forest intact, "we're probably shooting ourselves in the foot for short-term gains in the next 20 years," he said.
"I don't have any doubt that if we don't get our act together in the next five years it might be a little bit too late because of the drying effect of logging and further deforestation and wildfires. By 2030-2040, we're just going to have a big brush pile."
Indigenous Indian farmers who used to plant their fields in August now do so in October or November because of the later rains. Among fields whose blackened fence posts betray the fires that raged here weeks earlier, chief Damiao Paridzane leaned on his hoe and spoke wistfully of a youth spent under trees and fruit before the "white man" made contact with his Xavante (Warrior) tribe.
Now, the 1,000 people or so in his community live on a reserve east of the Xingu park that has been almost totally deforested and widely invaded by land-grabbers.
"If I was born today, I would be born weak because there isn't nature, there isn't forest or good air for us to breath," said the 58-year-old. "The climate has worsened and will get hotter. If this cassava I'm planting doesn't get rain we will get thin because of these changes."
Farmers, loggers and land speculators have destroyed nearly a fifth of the original Amazon forest, but the rate of destruction has fallen dramatically in the past few years. About 2,700 square miles (7,000 square km) was lost in 2008-09, a more than 70 percent fall over five years, a dramatic change that the government says is largely a result of better monitoring and enforcement of laws.
But that is only part of the story in a region the size of western Europe where Brazil's environmental agency has just six helicopters. The fall in deforestation also coincided with a slump in global commodity prices and a worldwide recession, suggesting that it could be a temporary lull.
"I think we're in the eye of a hurricane," said Carter, whose Alianca da Terra (Alliance of the Land) group is working with farmers to improve their environmental and fire-prevention standards. "When people don't have cash and they're leveraged to the hilt they're not going to spend money on deforestation."
Even if deforestation directly caused by human intervention were to fall to zero, the balance of evidence suggests that the forest is intensely vulnerable in a warming world.
A World Bank report this year drawing on 24 global climate models and Japan's Earth Simulator super-computer predicted a slighter wetter Amazon in the relatively unspoiled northwest this century with increasing droughts in the south. But once effects on vegetation from warmer temperatures, deforestation, and greater fire risk were taken into account, it concluded that there was a "substantial probability" of Amazon dieback with a particularly severe and near-term risk in the east.
Carlos Nobre, one of Brazil's leading climate scientists at the National Institute for Space Research (INPE), said there was a danger that global falls in deforestation would lull the world into a false sense of security. Even if deforestation globally falls to zero, the trajectory of greenhouse gas emissions implies "massive change" in forests like the Amazon, he said.
"Everybody will go home and say 'OK, the forests are safe, biodiversity will be preserved.' No, that's not the case," he said. "It's a very serious situation."
As in 2005, climate scientists say this year's drought was probably caused by a warming of the north Atlantic ocean that causes air over the Amazon to descend, hampering the formation of rain clouds. Some models show that trend intensifying as the planet warms.
By far the scariest reading on the Amazon's future comes from the British Met Office's Hadley Center, whose models show a disastrous rise in regional temperature of 8 degrees Celsius (14.4 Fahrenheit) or more by the end of the century. Under that scenario, the forest retreats to a tiny fraction of its current size.
Other studies have shown that a transition to seasonal forest with longer dry seasons, like those in parts of Asia, is a more likely outcome than scrubland this century. But that could still raise the Amazon's vulnerability to fire and have a severe impact on biodiversity, said Oxford University ecosystem science professor Yadvinder Malhi.
"There could be quite a decline in many tropical species. Some insects and lizards may struggle to cope with warming of 4-5 degrees (Celsius)," he said.
Calling when the forest could pass an irreversible tipping point is an inexact science, depending on complex interactions among the temperature, atmosphere, rainfall and deforestation.
The changes are not all bad news. More carbon dioxide in the atmosphere can fuel tree growth and drought resistance by stimulating the photosynthesis process in some species, for example. Nobody knows for sure how the Amazon's thousands of different tree species will adapt to warmer temperatures and increased droughts.
Many of them are hardy breeds, thanks to deep roots that probe 50 feet or more in search of moisture that keeps them alive during droughts. Studies have found that trees in the Borneo rainforest die much more easily than their Amazon counterparts given the same drought levels.
Brazil's Nobre said that the forest even in areas like Mato Grosso had not yet hit the tipping point -- at least not in terms of changing climate.
"We don't observe any long-term change in rainfall," he said. "Climatically, it's very far from a tipping point."
INPE's research has found that deforestation of the Amazon would have to reach 40 percent, double its current level, to trigger a widespread dieback. But in areas like Mato Grosso, where the remaining forest is fragmented and subject to dry winds and fire, the process is visibly speeding up.
"In those degraded areas, if they continue to use fire, you might reach a point of no return," Nobre said.
In a forest patch the size of a city block in Mato Grosso, Paulo Brando's boots crunch through brittle leaves and twigs among scorched tree trunks.
Every three years, the patch is burned as part of an experiment to compare its resilience to an untouched plot of forest next to it. The result is a sad, wounded landscape -- what Brando, an ecologist with the Amazon Environmental Research Institute, calls an "impoverished" ecosystem.
Up to half the species have been lost and the carbon stored in the vegetation is down by a third over three years. Grasses have invaded the sun-exposed forest floor, providing kindling for future fires, and temperatures are a full 5 degrees Celsius (9 Fahrenheit) higher than in the patch that still has its cooling green canopy.
"If wetter forest becomes drier, those fires are likely to be very intense because you have lots of fuel. If you start having a source of ignition in dry years you're likely to get to this point very quickly," said Brando.
Nearly 30 percent of the Amazon is within 6 miles of a potential fire source, such as a farm or a road.
While the scientific jury may still be out on how more extreme weather will affect the forest, the region's inhabitants are already suffering the consequences. For the second time in five years, drought in Amazonas state, which is the size of Alaska, brought the surreal site of cars driving where people swam just weeks earlier. Some residents desperate for food scooped up endangered manatees from shallow rivers.
Officials in Manacapuru, a small city on the Solimoes river near Manaus, say the extremes of recent years have prompted an influx of environmental migrants.
"It's a consistency of extremes," said vice mayor Joao Messias. "Our city here is literally full. It has filled up a lot after these big floods and droughts."
In the smaller town of Caapiranga, which was mostly cut off from boat transport by the drought, residents complained that many foods had doubled in price and that their crop land had yet to recover from the devastation caused by 2009's floods.
From his shack by the side of a dried-up lake, Manuel Ferreira de Matos squinted through a pair of battered spectacles at the distant water that glistened like a mirage more than a kilometer away.
"By the time I get back home from the fields, I'm dying of thirst," said the 57-year-old father of seven.
"Before I could walk all day, no problem, but now I can't stand it -- it's like the sun got closer."
Editing by Claudia Parsons and Jim Impoco