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Technical note. The sudden increase in Amazon fires early in the 2019 fire season made global headlines. While it has been heavily speculated that the fires were caused by deliberate human ignitions or human-induced landscape changes, there have also been suggestions that meteorological conditions could have played a role. Here, we ask two questions: were the 2019 fires in the Amazon unprecedented in the historical record, and did the meteorological conditions contribute to the increased burning? To answer this, we take advantage of a recently developed modelling framework which optimises a simple fire model against observations of burnt area and whose outputs are described as probability densities. This allowed us to test the probability of the 2019 fire season occurring due to meteorological conditions alone. The observations show that the burnt area was higher than in previous years in regions where there is already substantial deforestation activity in the Amazon. Overall, 11 % of the area recorded the highest early season (June–August) burnt area since the start of our observational record, with areas in Brazil's central arc of deforestation recording the highest ever monthly burnt area in August. However, areas outside of the regions of widespread deforestation show less burnt area than the historical average, and the optimised model shows that this low burnt area would have extended over much of the eastern Amazon region, including in Brazil's central arc of deforestation with high fire occurrence in 2019. We show that there is a 9 % likelihood of the observed August fires being caused by meteorological conditions alone, decreasing to 6 %–7 % along the agricultural–humid forest interface in Brazil's central states and 8 % in Paraguay and Bolivia dry forests. Our results suggest that changes in land use, cover or management are the likely drivers of the substantial increase in the 2019 early fire season burnt area, especially in Brazil. Burnt area for September in the arc of deforestation had a 14 %–26 % probability of being caused by meteorological conditions, potentially coinciding with a shift in fire-related policy from South American governments.

The sudden increase in Amazon fires early in the 2019 fire season made global headlines. While it has been heavily speculated that the fires were caused by deliberate human ignitions or human-induced landscape changes, there have also been suggestions that meteorological conditions could have played a role. Here, we ask two questions: were the 2019 fires in the Amazon unprecedented in the historical record?; and did the meteorological conditions contribute to the increased burning? To answer this, we take advantage of a recently developed modelling framework which optimizes a simple burnt area model, and whose outputs are described as probability densities. This allowed us to test the probability of the 2019 fire season occurring due to meteorological conditions alone. We show that the burnt area was indeed higher than previous years in regions where there is already substantial deforestation activity in the Amazon, with 11 % of the area recording the highest early season (June–August) burnt area since the start of our observational record. However, areas outside of the regions of widespread deforestation show less burnt area than the historical average, and the optimized model shows that there is a 71 % probability that this low burned area would have been expected over the entire Amazon region, including regions already witnessing deforestation and of high fire occurrence in 2019. We show that there is a < 7 % chance of the observed June–August fires being caused by meteorological conditions alone, decreasing to < 1 % in Paraguay and Bolivia dry-forests and at the eastern end of the Amazons arc of deforestation. This suggests that changes in land use and land cover or land management are the likely drivers of the large increase in the 2019 early fire season burnt area. Burnt area for the peak of the fire season in September returned to levels expected from meteorology conditions in the arc of deforestation, potentially coinciding with a shift in policy from South American governments, but remained high in Bolivia and Paraguay.