Biomass burning in South America: Impacts on the regional climate

By Gillian Thornhill

Deforestation in south America has many environmental impacts, including loss of habitat, soil erosion, changes to the water cycle and the reduced capacity of the CO2 sink the vegetation provides. Where the vegetation is burned, an additional climate impact comes from the release of smoke aerosols into the atmosphere, which affects the regional climate due to changes in the radiation reaching the surface and changes in cloud cover resulting from atmospheric heating by the aerosol.  The wind circulation, surface temperatures and precipitation in the region are also affected by increases in aerosol from biomass burning.

In order to investigate the impact of biomass burning aerosols (BBA) on the regional climate, we compared two simulations of the global atmosphere using the Met Office Unified Model HadGEM3. This work was undertaken as part of the South American Biomass Burning Analysis (SAMBBA) project, which included aircraft observations of biomass burning aerosols to provide constraints on the aerosol properties in the model. We used two realistic levels of biomass burning emissions, one case taken from a high emissions year and one from a low emissions year, and ran the model for 30 years to average out inter-annual variability. The model output from the two cases was compared for September (the month with highest smoke emissions), by taking the September means over the 30 year run.

Figure 1 shows the September mean difference in the biomass burning aerosol optical depth between the high emissions case and the low emissions case, the largest differences being over the areas with largest smoke emissions, as we might expect. The aerosol can affect cloud cover by increasing cloud burn-off as the aerosol absorbs radiation and heats up the atmosphere around it, reducing the cloud fraction at the altitude of the aerosol layer (referred to as the semi-direct effect). In Figure 2 we see the decrease in cloud cover over the area of the main biomass burning aerosol, extending up to the north-east and slightly beyond the main area of biomass burning. There are also changes in the boundary layer height and an increase in the boundary layer stability due to the increased amount of aerosol, which can affect the formation of higher convective clouds; we think this mechanism is responsible for reducing higher level clouds in this area. The absorption of downwelling shortwave radiation by the BBA (Figure 3) results in a reduction at the surface, which lowers the surface temperature slightly (Figure 4); this effect competes with the reduction in cloud cover, which tends to increase shortwave radiation at the surface. In areas with the highest biomass burning, the reduction in the downwelling shortwave from absorption by the aerosol is the stronger process. Finally there is a drying effect in the region, with a reduction in the precipitation occurring in the high emissions case (Figure 5).

Figure 1 Difference in the Aerosol Optical Depth (AOD) at 0.44 microns for September between the high emissions case and the low emissions case (H-L). Stippling represents the 95% confidence level.

As the amount of biomass burning varies from year to year, investigating the impact of high emissions versus low emissions gives us an insight into how the level of biomass burning may affect the regional climate.

Further details and discussion

Figure 2 Difference in cloud fraction for September between the high and low emissions case (H-L). Stippling represents the 95% confidence level.

Figure 3 Difference in downwelling shortwave radiation at the surface for high-low emissions cases. Stippling represents significance at the 95% confidence level.

Figure 4 Difference in surface temperature in Sep. for high-low emissions cases. Stippling represents significance at the 95% confidence interval.

Figure 5 Difference in precipitation in September for high-low emissions cases. Stippling represents significance at the 95% confidence interval. (Note different contour colour scale)

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