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Human-caused fires growing faster than lightning fires in the Western US
image:
The spatial distribution of the ecoregions in the western United States.
view moreCredit: Li et al.
A study shows that there are almost twice as many risky days for large human-caused fires in the American West as there are for lightning-caused fires, due to differences in the level of heat and aridity under which each type of fire is likely to occur. The discrepancy is not accounted for in most fire early warning systems. In addition, risky days for human-caused fires are growing faster than risky days for lighting-caused fires as the climate warms.
Fa Li and colleagues focused on Vapor Pressure Deficit (VPD), which captures both dryness and heat, reflecting the difference between the actual air water vapor content and saturation. The authors used a Bayesian inference algorithm to model the relationship between VPD and the probability of large fires for the largest 10% of fires in each ecoregion in the western United States. The estimated VPD threshold for large fires ranged from 1.1 to 2.1 kPa for human-ignited large fires and 1.8 to 3.1 kPa for lightning-ignited large fires. One reason for this difference is likely related to the location of the first ignition. Lightning strikes from above, hitting the forest canopy, which is often living and therefore moist. Human-caused fires often ignite at ground level in dried grass or fine dead branches, material which tends to be very dry. Thus human-caused fires can catch and spread when the atmosphere is wetter. Across the west, from 1979-2020, about 30 days a year were sufficiently hot and dry for lightning-ignited large fires, while about 58 days a year were sufficiently hot and dry for human-ignited large fires. The number of flammable days for human-caused fires increased 21% more rapidly than the number of flammable days for fires caused by lightning over the same period. Anthropogenic greenhouse gas emissions were responsible for 81% of the increases in human-related flammable days. According to the authors, the results can help build more accurate models of fire risk.
flammable days
Mean annual number of flammable days from 1979 to 2020. Each point signifies the mean annual number of flammable days in a grid cell (0.25 degree). The whiskers represent the 5th and 95th quantiles, with boundaries indicating the 25th, 50th, and 75th quartiles.
Credit
Li et al.
Journal
PNAS Nexus
Article Title
Exacerbating risk in human-ignited large fires over western United States due to lower flammability thresholds and greenhouse gas emissions
Article Publication Date
11-Feb-2025
Models show intensifying wildfires in a warming world due to changes in vegetation and humidity; only a minor role for lightning
Institute for Basic Science
image:
Global changes in lightning, fires and smoke, expressed per oC global warming. Figure (A) shows the modeled change in lightning flash rate density, (B) the percentage fraction of land burned per year, and (C) the aerosol optical depth changes due to fires, which is an indicator for the amount of smoke in the atmosphere. In the model simulations the amount of CO2 quadruples over a 140-year period with respect to the pre-industrial period; this corresponds to a global mean surface warming of about 4oC.
view moreCredit: Institute for Basic Science
Extreme fire seasons in recent years highlight the urgent need to better understand wildfires within the broader context of climate change. Under climate change, many drivers of wildfires are expected to change, such as the amount of carbon stored in vegetation, rainfall, and lightning strikes. Quantifying the relative importance of these processes in recent and future wildfire trends has remained challenging, because previous climate computer model simulations did not capture the full coupling between climate change, lightning, wildfires, smoke and corresponding shifts in solar radiation and heat.
A new study published in the journal Science Advances by an international team of climate scientists presents the first realistic supercomputer simulation that resolves the complex interactions between fire, vegetation, smoke and the atmosphere. The authors find that increasing greenhouse gas emissions will likely increase the global lightning frequency by about 1.6% per degree Celsius global warming, with regional hotspots in the eastern United States, Kenya, Uganda and Argentina [Figure 1A]. Locally this could intensify wildfire occurrences. However, the dominant drivers for the growing area burned by fires each year [Figure 1B] remain shifts in global humidity and a more rapid growth of vegetation, which can serve as wildfire fuel.
The study further identifies regions, where the intensification of fires caused by global warming will be most pronounced [Figure 1B]. Among the regions exhibiting the strongest anthropogenic trends in biomass burning are southern and central equatorial Africa, Madagascar, Australia, parts of the Mediterranean and western North-America. “Our results show that with every degree global warming the global mean area burned by fires each year will increase by 14%. This can have substantial effects on ecosystems, infrastructure and human health and livelihoods.” says Dr. Vincent VERJANS, former postdoctoral research fellow at the IBS Center for Climate Physics (now at Barcelona Supercomputing Center) and lead author of the study.
Moreover, the researchers also highlight that with more fires on a global scale, also the levels of fire smoke will increase [Figure 1C]. Smoke plumes emerging from wildfires will have an effect on air pollution and also lead to reduced penetration of sunlight. The latter changes the heat and infrared radiation in the atmosphere. “Our new computer model simulations show for the first time that accounting for these effects in a comprehensive earth system model, can influence regional temperatures. Fire regions and their downwind smoke plume extensions will experience on average somewhat reduced warming due to the solar dimming effect.” says co-author Prof. Christian FRANZKE from the IBS Center for Climate Physics at Pusan National University, South Korea. However, in addition to reducing sunlight (direct aerosol effect) which is accounted for in the new computer simulations, aerosols from biomass burning can also change the formation of clouds (indirect effect). “This part is still somewhat uncertain, and more research needs to be conducted to understand how fires will impact clouds and subsequently surface temperatures,” adds Prof. Franzke.
While this study makes important strides in representing climate-lightning-wildfire interactions in the current generation of Earth System models, it also identifies key aspects that require further consideration. A critical example is the extent to which Arctic wildfires will increase in a warmer world. In their model simulations, the increase in Arctic wildfire activity is weaker than the observed trends in recent years. “This may indicate that current climate models underestimate future Arctic wildfire risks. Among other things, this would have important consequences for predictions of aerosols released from wildfires, which in turn will affect the climate and influence air quality,” says Dr. Vincent VERJANS.
Journal
Science Advances
Method of Research
Computational simulation/modeling
Subject of Research
Not applicable
Article Title
Quantifying CO2 forcing effects on lightning, wildfires, and climate interactions
Article Publication Date
12-Feb-2025
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