The Amazon Basin, traditionally considered a bulwark against global warming, may be becoming a net contributor of carbon dioxide (CO2) as a result of deforestation, researchers said on Wednesday.

In an overview published in the journal Nature, scientists led by Eric Davidson of the Woods Hole Research Center in Massachusetts say the Amazon is "in transition" as a result of human activity.

Over 50 years, the population has risen from six million to 25 million, triggering massive land clearance for logging and agriculture, they said.

The Amazon's carbon budget -- the amount of CO2 that it releases into the atmosphere or takes from it -- is changing although it is hard to estimate accurately, they said.

"Deforestation has moved the net basin-wide budget away from a possible late 20th-century net carbon sink and towards a net source," according to their paper.

Mature forests such as the Amazon are big factors in the global-warming equation.

Their trees suck up CO2 from the atmosphere through the natural process of photosynthesis.

But when they rot or are burned, or the forest land is ploughed up, the carbon is returned to the air, adding to the greenhouse effect.

The paper estimates that the biomass of the Amazon contains a whopping 100 billion tonnes of carbon -- the equivalent of more than 10 years of global fossil-fuel emissions.

Global warming, unleashing weather shifts, could release some of this store, it warned.

"Much of the Amazon forest is resilient to seasonal and moderate drought, but this resilience can and has been exceeded with experimental and natural severe droughts, indicating a risk of carbon loss if drought increases with climate change."

The paper also noted that there had been extreme droughts and floods on the Tocantins and Araguaia basins, whose rivers drain the heavily deforested Cerado region.

"Where deforestation is widespread at local and regional scales, the dry season duration is lengthening and wet season discharge is increasing," it warned.