If severe enough, researchers said, the loss of rainforest could cause the release of large volumes of the greenhouse gas carbon dioxide into the atmosphere and also disrupt plant and animal communities in one of the regions of highest biodiversity in the world.
Researchers at the University of Texas, studying historic rainfall measurement, said since 1979 the dry season in southern Amazonia has been lasting about a week longer per decade.
The most likely explanation for the lengthening dry season is global warming, they said.
"The dry season over the southern Amazon is already marginal for maintaining rainforest," geosciences Professor Rong Fu said. "At some point, if it becomes too long, the rainforest will reach a tipping point."
Water stored in the soil at the end of the wet season is all that the rainforest trees have to last them through the dry season, and longer the dry season lasts -- regardless of how wet the wet season was -- the more stressed trees become and the more susceptible they are to fire, he said.
"The length of the dry season in the southern Amazon is the most important climate condition controlling the rainforest," Fu said. "If the dry season is too long, the rainforest will not survive."
"Because of the potential impact on the global carbon cycle, we need to better understand the changes of the dry season over southern Amazonia."