FIRES IN THE RAINFOREST

By Rhett A. Butler  Last updated Aug 14, 2020

Note: this page presents mostly historical context on fires in tropical rainforests. If you're looking for the latest updates on fires burning in the topics, please look at the following news feeds: Amazon fires, Indonesia fires and rainforest fires. Mongabay also does occasional reporting on forest fires outside the tropics: here's the full fire feed

Historical context for rainforest fires

Rainforests are increasingly susceptible to forest fires today due to degradation from selective logging, fragmentation, and agricultural activities. Scientists are concerned that much of the Amazon is at risk of burning, and that in the future we could see fires similar to those that so damaged Indonesia in recent el Niño years. The Congo Basin as well is experiencing drying trends and degradation that is resulting in increasingly severe fires in its rainforest.

Today most rainforest fires originate in nearby pasturelands and agricultural fields where fires are used for land clearing and crop and pasture maintenance. Every year, during the burning season, tens of thousands of fires are set by land speculators, ranchers, plantation owners, and small farmers to clear bush and forest. Under dry conditions these agricultural forests can easily spread into neighboring rainforest.

As a result, small fires are not unusual today in the rainforest. Even in "virgin" forests, fires may burn across hundreds of thousands during dry years. While these fires may seem innocuous, with flames reaching only a few inches in height and having virtually no discernable impact on trees or the canopy itself, they cause insidious damage: in passing, the fire sets the path for recurrent fires and subsequent forest loss.Once-burned forests are twice as likely to be deforested as unburned forests, largely because the initial fires—however small—thin out the canopy, allowing more sunlight to reach the forest floor, drying out lead litter and setting the stage for future fires. Previously burned forests, in addition to having more combustible material, are also often adjacent to fire-maintained pastures and therefore are frequently exposed to sources of ignition. Subsequent fires burn with increased velocity and intensity and cause higher tree mortality. Fires intervals of less than 20 years may eliminate all trees in the forest stand.

Under "normal" rainfall and humidity conditions most of these fires are extinguished by the arrival of the rainy season or monsoon. Usually old-growth rainforests function as a sort of humid barrier which prevents the spread of agricultural fires. However, under dry conditions—such as those of an el Niño year—fires can spread from pastures and fields into primary forest. In the 1990s, 90 percent of burning in the Brazilian Amazon occurred in El Niño years.

The unusually strong El Niño of 1997-98 contributed to massive forest fires across the tropics. Ahead of the burning season in the Amazon, humidity in the Basin was 45-55 percent lower than usual and the Woods Hole Research Center warned that 400,000 square kilometers of forest could go up in smoke. In early 1998, some of these fears materialized as 13,200 square miles (34,000 square km) of Roraima state in northern Brazil burned. The fires spread rapidly across the dry savanna and advanced into rainforest usually too humid to burn. As many as 3,800 square miles (10,000 sq km) of intact rainforest were damaged or destroyed by these fires. The government firefighting efforts had virtually no effect and it was only freak heavy showers that extinguished the flames.

Dry conditions again returned in 2005, when the Amazon experienced the worst drought in recorded history. As rivers went dry and communities were left stranded, tens of thousands of fires burned. That calamity was topped in 2010 by an even worse drought that affected a million square kilometers of Amazon forest. In both of these droughts, dry conditions were linked not to El Niño but to warm temperatures in the tropical Atlantic Ocean, raising fears that climate change could worsen droughts in the Amazon.

Besides destroying the rainforest ecosystem and killing wildlife, these fires create other environmental problems, including producing smoke that can cause respiratory distress and disrupt transportation. These fires are significant sources of greenhouse gases. For example, in a four-month period (July-October) in 1987, about 19,300 square miles (50,000 sq. km) of the Brazilian Amazon burned in the states of Para, Rondonia, Mato Grosso, and Acre. The burning released more than 500 million tons of carbon, 44 million tons of carbon monoxide, and millions of tons of other particles and nitrogen oxides.

The tropical forest fires that have made headlines of late will only worsen as more forest is degraded and the area of previously burned forest expands. A study by IMAZON (the Institute for Man and Nature in the Amazon) found that for every acre burned or cleared which shows up on satellite, at least one-acre burns undetected under the forest canopy. These leaf-litter fires can burn for months with warm temperatures and little rain, and subsequent fires in these previously burned areas are more intense and destructive.

Other studies have warned that climate change could significantly dry forests in the Amazon Basin and Africa, increasing their risk of burning. In light of this potential scenario and to better understand the impact of extended drought in the Amazon and the resilience of the forest to fire, in the 2000s the Woods Hole Research Center and NASA are have conducted an extensive series of large-scale experiments in the Brazilian rainforest. Separate findings from NASA suggest that heavy smoke from Amazon forest fires inhibits cloud formation and reduces rainfall. This conclusion, combined with other NASA studies suggesting that deforestation can affect regional climate, means that the Amazon rainforest may be on the verge of a significant environmental transformation—one that will increasingly leave the ecosystem vulnerable to fire.

Current fire tracking efforts

Most fire tracking in the tropics is done using satellites, which can detect "hotspots", representing "thermal anomalies", and aerosol emissions in smoke in the atmosphere. In parts of the Amazon and Indonesia where large tracts of land have been transformed from rainforests to agricultural landscapes due to past deforestation, there is often confusion between the number of hotspots registered via satellite and the actual number of fires burning in forests. In reality, only a small fraction of hotspots represent forest fires. However fires burning on deforested peatlands — which are increasingly common in places like Sumatra and Borneo — can nonetheless be huge sources of carbon emissions and polluting smoke.

The latest news on forest fires

Review questions:

• Why are forest fires getting worse in the Amazon?

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