Carlos Peres: Unthinkably, wild fires are breaking out in rainforests

Not so long ago it was thought that tropical rainforests in the Brazilian Amazon region were immune to fire thanks to the high moisture content of the growth beneath the top tree cover. But the severe droughts of 1997-98, 2005 and 2010 have changed that.

These severe shortages were probably driven by interacting, large-scale climatic events, with the warming of the Atlantic – which also drives hurricanes affecting the Caribbean and south-eastern United States – increasingly outweighing the drying effects of El Niño in the Pacific.

The droughts are becoming more frequent and more severe and are generating the conditions conducive for the wildfires affecting vast areas of previously unburnt Amazonian rainforests which are the largest in the world and home to 21 million people. It is a wake-up call about things to come as the climate changes.

This manifested itself again in August and September while I was working in Alta Floresta, a city in the southern Amazonian state of Mato Grosso.

I arrived in Mato Grosso to witness the worst drought in living memory, following 110 consecutive days without rain. Two days later the local mayor escorted me to the largest watershed and freshwater reservoir supplying the the city.

The perennial streams in the reservoir's catchment had dried up and the reservoir was almost unbelievably bone-dry, requiring urgent and expensive water-provision measures for the first time in its existence.

Meanwhile forest fires were ranging across several other Mato Grosso counties. In a neighbouring town a major fire had burned dozens of houses, shops, churches and schools to the ground.

Few people consider the hydrological ecosystem services of tropical forest cover in buffering extreme climatic conditions.

In terms of their very hydrological viability, tropical rainforests sit on a tight-rope. Depending on local soil conditions, extreme droughts can rapidly breach their flammability threshold, setting in motion a process in which local fires unprecedented in the area lead to bigger and more intensive fires. Little in the evolutionary history of these great forest ecosystems prepares them for the massive levels of tree mortality to which they are likely to succumb.

Last year's mega-drought was yet another reminder. The predicament of the Amazon and other major tropical forest regions has never been so uncertain. Now we simply wait for the next conflagration.

Carlos Peres is a Brazilian conservation biologist and a Professor of Tropical Ecology at the University of East Anglia