May 2, 2007

Global View of Wildfires

Filed under: Droughts

Wildfires have been associated with the global warming scare for decades, and nearly two million websites found for “Forest Fires and Global Warming” continue to stoke the flames. The argument seems rather simple – the world warms and in the absence of a compensating increase in precipitation, many areas of the world become drier. The drier vegetation is more vulnerable to fires, and more wildfires result. A few pictures of Yellowstone Park and/or San Diego hillsides on fire will help make the point. Add comments about how vegetation is stressed trying to cope with rapid environmental change, and fires seem even more inevitable. Furthermore, a not-so-complicated set of feedbacks can make it seem even worse – note that fires contribute enormously to the atmospheric carbon dioxide concentration and then describe how a darker surface resulting from fires can decrease the reflectivity of the surface (making it as black as a parking lot) thereby resulting in even more warming. Given that wildfires are impacting some part of the Earth every day, there is no end to up-to-date photo opportunities, there are plenty of folks are willing to speculate that “we may be seeing the impacts of climate change,” and a pillar of the greenhouse earth is reinforced. Through in some heart-wrenching pictures of animals coping with a burned forest or grassland, and to say the least, you are part of the global warming fraternity.

A recent article in Global Change Biology caught our eye with its title “Global spatial patterns and temporal trends of burned area between 1981 and 2000 using NOAA-NASA Pathfinder.” Given that we had not seen any press coverage for this piece, we suspected it would be a perfect fit for World Climate Report. Sure enough, a few sentences into the abstract we found “The total annual burned area has not increased in the last 20 years” and we suddenly knew full well why we have not heard of this research. Announcing that burn area has not increased is not going to sit well with the fraternity members!

A research team from the University of California–Davis and several institutions in Spain noted from the outset that “About 32% of the carbon monoxide and 40% of the carbon dioxide released to the atmosphere globally come from burned biomass. This atmospheric forcing is a driver for climate warming and can have positive feedback with increased fire frequency.” Riaño et al. made the case that determining the burn area and/or biomass consumed would be critical in understanding various dimensions of the global ecosystem, but that to date, the data have been collected annually using highly irregular criteria from country to country. The United Nations’ Food and Agriculture Organziation has tried to collect the statistics, but their data are notoriously suspicious in terms of accuracy. Riaño et al. convincingly argue that “A single remote sensing data source can provide globally coherent multitemporal spatial information, not only from the visible part of the spectrum but also reflected solar infrared, which can be used to obtain consistent environmental monitoring at the global scale.” They state “The main purpose of this study was to identify global patterns of burned area using the 20 years of NOAA-AVHRR data which will provide a more consistent spatial and temporal basis for interpretation than statistics compiled from different sources or short periods (e.g. annual studies).”

They collected the 8 km resolution global satellite-based AVHRR (Advanced Very High-Resolution Radiometer) data from July 1981 to December 2000. The developed a computer algorithm that could spot pixels that had recently burned, and the algorithm could basically count the number of burned pixels, calculate total burn area, and determine percentage of burned area for any defined region of the world. The global and regional data were ultimately assembled on a monthly basis.

In analyzing trends in the burn area data, the team found that “The global trend statistics in the total number of pixels burned in any month or annually were not significantly different from 0 (at alpha = 0.10 significance level). Therefore, no significant upward or downward global trend was found in the burned area data.”

The research team conducted a cluster analysis to identify areas with similar fire histories, and they report that cluster group #1 did show a significant increase in burned area. As seen in the map below (Figure 1), cluster group #1 includes the western United States, southern Europe, and interior Eurasia. Before the greenhouse crusade could jump for joy, the team reported a significant decrease in burn area for cluster group #9 that includes Central America and much of Southeast Asia. When viewed globally (and we like to view the world globally), Riaño et al. once again reported “There was also no significant upward or downward global trend in the burned area for any individual month.”


Figure 1. Global land cover map showing the cluster number for each land cover tile (from Riaño et al., 2007)

How inconvenient! The study period, 1981 to 2000, was a two decade time period when the greenhouse gases increased substantially in the global atmosphere, it was a time when the Earth warmed, and it was a time when the media breathlessly covered every fire from Indonesia to Australia to the western United States. Many scientists are quoted in the nearly two million websites on wildfires claiming that the increase in fire activity is well underway. However, the Riaño et al. team developed a truly global dataset on burn area, they conducted an analysis that is uniform across the globe, and they found no trend in the global burn area data? Of course, had they found an increase, they would need press agents to handle to global media coverage. No global trend in burn area means coverage in World Climate Report, not the front pages of the world’s newspapers.

Reference:

Riaño, D., J.A. Moreno Ruiz, D. Isidoro, and S.L. Ustin. 2007. Global spatial patterns and temporal trends of burned area between 1981 and 2000 using NOAA-NASA Pathfinder. Global Change Biology, 13, 40–50.




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