Biomass burning refers to the burning of vegetation either by natural causes, such as forest fires ignited by lightning strikes, or man-made fires, such as the burning of the rain forests due to deforestation or the burning of savannah grassland for agricultural purposes. Anthropogenic burning presently makes up more than 90% of the fires on the planet. These fires not only damage forests and destroy the habitats for many plant and animal species, but the fires release large amounts of gases and particles into the atmosphere. Due to the global scale of biomass burning these emissions from fires are starting to have an impact on the global climate.
On the one hand, biomass burning increases the amount of greenhouse gases in the atmosphere which tend to warm the climate by trapping infrared radiation emitted from the earth's surface. On the other hand, small particles emitted in the smoke of the fires can both reflect some of the incoming solar radiation back to space, and make clouds brighter, both of which tend to cool the surface of the earth. Therefore, there are two opposing effects on the climate. In addition, fires change the surface characteristics of the continents (vegetation, albedo, etc.) which also influence the local and perhaps global climate.
Unlike most researchers who are interested in how biomass burning may impact the climate, I am interested in how future climate change may impact the frequency and intensity of natural forest fires. Although natural forest fires, generally caused by lightning, are not of major global concern at the moment, these fires are of great importance in mid- to high latitude forests where lightning is a more important factor than man. These lightning-fires normally start in remote regions that are often difficult to reach by fire crews, and sometimes go undetected for many days. Therefore, lightning-caused forest fires generally burn much larger areas than fires caused by man.
Natural forest fires need the right combination of vegetation, climate, and lightning activity to occur. For example, in the tropical rain forests there is plenty of vegetation (fuel) and lightning, however, the climate is so moist that the fires cannot ignite. On the other hand, during the summer months in the boreal forests of Canada, Russia, Alaska and other locations, there is the right combination of vegetation, climate conditions and lightning storms to cause tens of thousands of lightning fires each year. In fact, the area burned by these fires in some regions appears to have increased over the past 50 years. Man or climate change?
We have developed a simple empirical model that relates the number, and area burned, of lightning fires per month to the lightning activity and climate conditions of that area. The model is shown schematically below. The dryness of the vegetation in a regions (on a monthly mean basis) is defined by the difference between the amount of precipitation and the amount of evaporation (or potential evaporation) represented by (P-Ep). The more negative the value of (P-Ep) the drier the conditions. The number of days per month with lightning activity (L) is shown on the y-axis. The model gives the number of fires caused by lightning and the area burned per fire for any values of the two parameters (P-Ep) and L. The drier the conditions the more fires and larger the area burned per fire. If there are too many days with thunderstorms then the wetting effect from the rain reduces the area burned per fire.
Recently we have also looks at the impact
of fires themselves on the production of lightning in the
tropics. We looked at the lightning activity in the burning
season in the Amazon (mainly man-made fires) and compared the amount of
lightning on a regional basis, with the amount of aerosols (particles)
in the atmosphere in that region detected by satellite measurements
from MODIS. The results showed two opposing effects. As the
aerosol optiocal depth (AOD) increased as fires start, the additional
particles in the normally clean atmosphere actually help invigorate the
thunderstorms in the regions. The additional particles act as
cloud condensation nuclei (CCN) and in the cloud these additional CCN
compete for water vapor with other CCN, resulting in the cloud
producing smaller cloud drops overall. Due to their smaller size,
they can be carried higher into the atmosphere, below the 0C isotherm,
where many drops eventually freeze, producing ice crystals, soft hail
(graupel), and supercooled droplets that interact to produce
lightning. So a small addition of smoke particles actually
increases the lightning activity. However, if we pump too many
aerosols into the atmosphere, the layer of smoke starts to block out
the sun from the surface (cooling the surface), while the smoke itself
absorbs some of the solar radiation (warming the smoike layer).
This process results in the stabalization of the atmosphere, producing
unfavourable conditions for thunderstorm development. This
results in a decrease in lightning activity. These two opposing
effects can be seen below in the "boomerang" diagram.
D. Rind, 1994: The impact of a 2xCO2 climate on lightning-caused fires,
J. Climate, 7, 1484-1494.
Goldammer, J. G., and C. Price, 1998: Potential impacts of climate change on fire regimes in the tropics based on MAGICC and a GISS GCM-derived lightning model, Climate Change, 39, 273-296.
Altaratz, O., I. Koren, Y. Yair and C. Price, 2010: Lightning response to smoke from Amazonian fires, Geophys. Res. Lett., 37, L07801, doi:10.1029/2010GL042679.
Global Impact of Biomass Burning
NASA Biomass Burning