The burning question

That many low-latitude regions are becoming increasingly desiccated and hotter is now well beyond any reasonable doubt. Yet nothing prepares the 21 million people occupying the Brazilian Amazon alone to see vast tracts of the largest tropical rainforest on Earth burning.  Not so long ago, it was thought that tropical moist forests were essentially immune to fire disturbance thanks to the high moisture of the shaded understorey underneath heavy canopy cover, but the severe droughts of 1997-98, and then 2005 and 2010 rapidly changed that perception. However, these ‘mega-droughts’ were driven by different, and likely interacting, large-scale climatic events, with the importance of high Atlantic sea surface warming — which also fuels the hurricanes affecting the Caribbean and southeastern US — increasingly outweighing the drying effects of El Niño events in the Pacific. These droughts are also becoming more frequent and more severe, generating the right conditions for recurrent wildfires affecting vast areas of previously unburnt Amazonian forests.

Once again this became obvious last August-September while I was working in Alta Floresta, a 60,000 strong cattle ranching town in the southern Amazonian state of Mato Grosso. I arrived in northern Mato Grosso to witness the worst drought and water shortage in living memory, following some 110 consecutive days without rain. Two days later the local mayor escorted me to the largest watershed and freshwater reservoir supplying the 8,000m³ water demand of the urban center of Alta Floresta alone. The perennial streams in the Mariana catchment had dried up and the reservoir was inconceivably bone-dry, requiring urgent and expensive water-provision measures for the first time ever. Meanwhile forest wildfires were ranging right across several other Mato Grosso counties; in a neighbouring town a major fire had burned countless houses, shops, churches and public schools to the ground, providing yet another wake-up call of things to come. Yet few people ever consider the hydrological ecosystem services of tropical forest cover in buffering extreme climatic conditions.

In terms of their very hydrological viability, tropical moist forests sit on a tight-rope. Depending on local soil conditions, extreme droughts can rapidly breach their flammability threshold, setting in motion a positive feed-back process in which unprecedented fires beget subsequent more intensive fires. Little in the evolutionary history of these great forest ecosystems prepares it for the massive levels of tree mortality, which they will likely succumb to. This dramatically reduces the value of the great forests as treasure troves of primary forest biodiversity and storages of biomass and carbon that would otherwise further fuel greenhouse gases and even more severe droughts. Logging operations in old growth forests further aggravate understorey desiccation because they puncture the otherwise closed forest canopy and add to the dry fuel load. Last year’s mega-drought is yet another reminder. The predicament of the Amazon and other major tropical forest regions has never been so uncertain. We live in wonder about the fire next time.


 

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

This article was also published on The Independent