The amount of area and material that is burned each year in tropical Africa is staggering. The series of maps shown here is from NASA, showing burning across Africa during 2005 (note they aren't monthly, just 10 day composites ranging from January to August). Globally, biomass burning is estimated to consume somewhere around 8700 Tg of dry matter and release nearly 4 Tg of carbon to the atmosphere. Much of that carbon is returned to the biosphere, though, because the majority of the burning isn't to clear forest, but to clear cropland in savannah regions. People seasonally set the fires to keep harmful plants and pests out of their farmland. The chemical consequences of this huge efflux of gas and aerosol each season across the tropical belt is still only crudely understood. The global impact is poorly understood, as there are processes that are indirectly related to the burning, such as the planetary albedo, cloud and precipitation effects, and chemical effects in both the troposphere and stratosphere. See here for a little more description, especially on the chemical side.
While not exactly analogous to other forms of anthropogenic changes to the climate system, this is an obvious and large perturbation that is mostly human induced. Understanding the consequences, both locally and remotely, may help us understand impacts of climate change globally.
Another interesting aspect of the African seasonal biomass burning is that it could represent an interaction between the climate system and human culture. The burning is seasonal, as I mentioned, but the extent and severity of the burning and resulting smoke depends on when and where the fires are started. That in turn depends on the previous rainy season. Where the smoke goes depends on the atmospheric circulation, and where the material ends up may determine the remote impact. For example, in some circulation patterns, the central African smoke is transported toward the Indian Ocean and is mixed into the westerlies, which will disperse the plume rather quickly. In other situations, the plume is transported over the southeast Atlantic, which is an are of large-scale sinking motion, so the smoke is contained within a layer and slowly moved over the ocean. This can affect radiation budget and the clouds in the area, which then have potential effects on other aspects of climate variability (through changes in the ocean surface temperature, for example, which may feed back on Atlantic Nino activity). These links are only tenuously understood, and are worth a good think if you have the time.
 |
| MODIS burning product, see LINK |
