MANAUS - Deep in the heart of the world's greatest rainforest, a nine-day journey by boat from the sea, Otavio Luz Castello is anxiously watching the soft waters of the Amazon drain away.

Every day they recede further, like water running slowly out of an immense bathtub, threatening a worldwide catastrophe.

Standing on an island in a quiet channel of the giant river, he points out what is happening. A month ago, the island was under water. Now, it juts 5m above it.

It is a sign that severe drought is returning to the Amazon for a second successive year. And that would be ominous. New research suggests that one further dry year beyond that could tip the whole vast forest into a cycle of destruction.

The day before, top scientists delivered much the same message at a remarkable floating symposium on the Rio Negro, on the strange black waters beside which Manaus, the capital city of the Amazon, stands.

They told the meeting - convened on a flotilla of boats by Ecumenical Patriarch Bartholomew of the Greek Orthodox Church, dubbed the "green Pope" for his environmental activism - that global warming and deforestation were pushing the entire enormous area towards a "tipping point", where it would start to die.

The consequences would be awesome. The wet Amazon Basin would turn to dry savannah at best, desert at worst. This would cause much of the world to become hotter and drier.

In the long term, it could send global warming out of control, eventually making the world uninhabitable.

Nowhere could seem further from the world's problems than the idyllic spot where Otavio Luz Castello lives. The young naturalist's home is a chain of floating thatched cottages making up a research station in the Mamiraua Reserve, halfway between Manaus and Brazil's border with Colombia.

Rare pink river dolphin play in the tranquil waters around the cottages, kingfishers dive into them, giant, bright butterflies zig-zag across them and squirrel monkeys romp in the trees on their banks.

There is little to suggest that it may be witnessing the first scenes of an apocalypse. The rivers of the Amazon Basin usually routinely fall 9m to 12m - greater than most of the tides of the world's seas - between the wet and dry seasons. But last year they just went on falling in the worst drought in recorded history.

At one point in the western Brazilian state of Acre, the world's biggest river shrank so far that it was possible to walk across it.

Millions of fish died, and thousands of communities whose only transport was by water were stranded.

And the drying forest caught fire; in September, satellite camera images showed 73,000 blazes in the basin.