Is global burned area declining due to cropland expansion? How much do we know based on remotely sensed data?

Globally, the amount of fire activity detected by satellite observations seems to have changed substantially in the last two decades. Discussions regarding the main force behind the current trends have dominated research in recent years, with several studies attributing the global decline in wildfires to the expansion of agricultural areas. Nevertheless, most studies have failed to acknowledge that the regions where these changing patterns have been observed are known to be data-poor.

Here we discuss the uncertainties and limitations of remotely sensed data used to determine global trends in burned area and changes in their potential drivers. Specifically, we i) quantify changes in the amount of burned area and cropland area and illustrate discrepancies between commonly used datasets, ii) state the limitations of remote-sensed fire and land cover products, and iii) highlight recent fire-trend studies and hypothesize likely effects of the choice of datasets on their conclusions. We argue that to legitimately conclude that the reported global decline in fire activity is driven by cropland expansion, three conditions must be met. First, negative trends should be global in scope, not localized to a specific continent or region. Second, the trends in both fire activity and agricultural areas must be supported by data that definitively demonstrate such trends over time. And third, the decline in the amount of burned area must be documented within or proximal to the areas of cropland expansion.

We demonstrate that a drastic decline in fire activity in the last 20 years was only observed within regions highly sensitive to coarse resolution fire product biases, while most regions/continents did not experience significant changes in the amount of burned area. Additionally, the analysis of several global land cover products reveals the lack of consistency in the direction and magnitude of the trend in cropland land cover type. And finally, based on the available data, no clear spatial relationship can be detected between areas experiencing cropland expansion and areas where the amount of burned area is declining.

Therefore, our knowledge of anthropogenic effects on fire, while growing, remains incomplete, particularly the effects of cropland expansion on wildfires, due to the low confidence in estimated fire trends within human-managed land and lack of understanding of the spatial distribution of cropland expansion/loss.