The plunge — which followed a summer of international attention on the fires, and the Brazilian government’s response to them — caught Amazon watchers by surprise.
Since measuring began two decades ago, September, the peak of dry season, has almost always been more flammable than August. That history fueled expectations that the problem would only worsen as the dry season deepened.
Analysts said rain and the government response — hundreds of soldiers were dispatched to fight the fires — might have been factors.
“It will take some time to determine the impact of government interventions,” said Daniel Nepstad, the executive director of the Earth Innovation Institute in San Francisco.
The overall number of fires in the Amazon biome from January through September was nonetheless 43 percent higher than the same period last year, according to the institute.
Fire is a key tool in the process of deforestation. Analysts blame most of it on land grabbers burning the forest to clear land for cattle and soy farms.
Fire is used when the forest is too thick or too remote to knock down with bulldozers. It can also sow nutrients into the ground that can help boost agricultural production.
The September drop remained a mystery.
“It is not simple to find out the reasons for the decline in fires while deforestation continues to increase,” said Carlos Nobre, a Brazilian scientist. “The army was sent to combat fires, and not necessarily to combat the environmental criminals doing the deforestation. It is obviously simple to detect a fire due to the smoke plumes, and the Army and police can act readily against it.”
Many of the fires are small — but in a forest ill-equipped to withstand any flame, they can wreak devastating damage.
Scientists worry that increasing rates of deforestation and fire could push the Amazon to a tipping point, at which the ecosystem is knocked off balance and large swaths convert into an arid savanna.