|
E. Linacre |
11/'97 |
The atmosphere is affected by surface conditions on land, including the forests, which cover about 30% of the Earth's land surface (1). The replacement of the world's largest primary forest, the Amazon rainforest, by grassland may have a significant effect on the regional and global climate.
First principles suggest that the net radiation at the surface would be smaller over an Amazon grassland, because grass has a higher albedo than a forest canopy. Evaporation too would be less because the surface area of foliage is larger for a forest. One might argue that grassland has a lower surface roughness, which means a higher surface wind, which increases the rate of evaporation, but numerical simulations indicate that this effect is smaller.
A lower rate of evaporation implies a lower humidity in the PBL, therefore the cumulus cloud base would be higher and the rainfall less. In terms of surface energy balance, the reduced net incoming solar radiation is offset by the reduced evaporative cooling, so the surface temperatures are hardly affected by deforestation. Actually daytime maxima at 1.5m above the ground would be higher, because of the lack of shading by the forest canopy. Similarly nighttime minima would be lower.
More important to local and global climate is the effect of deforestation on rainfall. This topic is the focus of LBA (the Large Scale Biosphere-Atmosphere Experiment in Amazonia), an international field experiment conducted in the state of Rondonia in southwestern Amazon Basin. This region experiences a clear dry season from May until October, during which most of the clear-cutting and burning occurs. In October the air over Porto Velho is so hazy that on cloud-free days sunsets or sunrises are not visible.
It is hypothesized that the onset of the wet season is delayed because of the abundant availability of cloud condensation nuclei (i.e. smoke particles) (1). Deep convection forming early in the wet season contains more cloud droplets and therefore produces rain less efficiently. The cloud top height of the first rain echo of the day (for convection triggered by surface heating) is usually above the -10° C level, suggesting that the Bergeron-Findeisen process (Note 9.C) is responsible for the precipitation. Later in the wet season the air is much more clear, and there is more competition for CCN. As a result the clouds assume a more marine drop size distribution, and warm rain processes (growth by condensation, autoconversion and coalescence) become more important. Indeed the cloud top height of the first rain echo is often below the freezing level by February, and the median drop size is larger. In summary, the smoke due to deforestation reduces the precipitation efficiency. The LBA experiment intends to examine this hypothesis.
Slash-and-burn agriculture elsewhere (in particular in Indonesia, Southeast Asia and tropical Africa in dry years) may also have a profound effect on climate (2). It can be argued that because the smoke CCN are short-lived, and because the CO2 emissions involved are relatively small, the direct effect of the burning on climate is mostly transient and therefore deforestation is of little importance to climate. However the long-term change in land surface conditions may have a lasting effect on climate through changes in surface heat fluxes, rainfall, and greenhouse gas production (e.g. methane).
Reference