Large wildfires blazing in Australia and the United States have decimated vast swaths of land and captured worldwide attention. Smaller fires are not as accounted for. Now, researchers report that not considering small fires leads to significant misrepresentation of the area burned.
Fire patterns and burned areas are generally estimated using satellite images. The most commonly used images are obtained using the Moderate Resolution Imaging Spectroradiometer (MODIS), an imaging sensor launched aboard NASA’s Terra and Aqua satellites. It provides images of the entire Earth every day or two, with its smallest resolution at about 250 meters. Small fires (less than 100 hectares) are usually not visible at this resolution.
Although MODIS is very useful for capturing landscape changes over time, “the contribution of small fires at a large or global scale was always a challenge,” said said Rubén Ramo of the University of Alcalá, Spain, lead author of a new study on estimating the area burned by small fires.
Higher Resolution, Better Estimates
Ramo and colleagues analyzed a new set of images obtained using the Multispectral Instrument sensor aboard the European Space Agency’s Sentinel-2 satellite. From these images, obtained at a resolution of 20 meters, they created a data set for burned area in Africa in 2016. Africa accounts for about 70% of the total area burned by fire around the world.
Analyzing the data, the team found 4.89 million square kilometers (Mkm2) of land burned in Africa in 2016, about 16% of the continent’s total area. Over the same period of time, the MODIS database showed only 2.72 Mkm2 burned area. Another set of data, obtained from the Global Fire Emissions Database, showed 2.98 Mkm2 of burned area.
The new data suggest fires in Africa were undercounted by almost 80%. The new study was published in the Proceedings of the National Academy of Sciences of the United States of America.
“This difference was predominately related to small fires [less than 100 hectares], which represent 87% of the total,” said Ramo. Only about 2% of fires smaller than 25 hectares were detected by MODIS. These smaller fires are not disasters; they are sometimes wildfires but are more often controlled burns associated with agriculture.
It is not surprising that images obtained at higher resolution provide better estimates of burned area, said Marta Yebra of Australian National University, who studies landscapes by remote sensing and was not involved in the new research. “However, it was shocking to find out that that difference in spatial resolution would lead to missing 80% of the area burned in sub-Saharan Africa in 2016, an area equivalent to the EU,” she said.
Longer Fire Season
The team also found a difference in the fire season in Africa, generally thought to be about 4 months long in the dry season (November to February in the Northern Hemisphere and June to September in the Southern Hemisphere). The new data set showed only about 65%–70% of the total burned area occurred in these months; the rest of the fires occurred during the transition between the wet and dry months.
The longer fire season also means longer periods of poor air quality. The authors estimated that carbon emissions based on the new data set were about 31% higher than the emissions estimated using the Global Fire Emissions Database and about 100% higher than estimates using MODIS data. Apart from affecting health, the increased emission of aerosols and other gases could likely impact regional climate.
The team is now looking to extend the study to other years and then to other regions in the world to understand whether the role and impact of small fires are similar in other regions.
—Lakshmi Supriya ([email protected]), Science Writer