220.127.116.11 Feedbacks Demonstrated Through Simple Models
In semi-arid systems, the occurrence and amounts of precipitation can be highly variable from year to year. Are there mechanisms whereby the growth of vegetation in times of adequate precipitation can act to maintain the precipitation? Various analyses with simple models have demonstrated how this might happen (Zeng et al., 2002; Foley et al., 2003; G. Wang, et al., 2004; X. Zeng et al., 2004). Such models demonstrate how assumed feedbacks between precipitation and surface fluxes generated by dynamic vegetation may lead to the possibility of transitions between multiple equilibria for two soil moisture and precipitation regimes. That is, the extraction of water by roots and shading of soil by plants can increase precipitation and maintain the vegetation, but if the vegetation is removed, it may not be able to be restored for a long period. The Sahel region between the deserts of North Africa and the African equatorial forests appears to most readily generate such an alternating precipitation regime.
18.104.22.168 Consequences of Changing Moisture Availability and Land Cover
Soil moisture control of the partitioning of energy between sensible and latent heat flux is very important for local and
regional temperatures, and possibly their coupling to precipitation. Oglesby et al. (2002) carried out a study starting with dry soil where the dryness of the soil over the US Great Plains for at least the first several summer months of their integration produced a warming of about 10°C to 20°C. Williamson et al. (2005), have shown that flaws in model formulation of thunderstorms can cause excessive evapotranspiration that lowers temperatures by more than 1°C. Many modelling studies have demonstrated that changing land cover can have local and regional climate impacts that are comparable in magnitude to temperature and precipitation changes observed over the last several decades as reported in Chapter 3. However, since such regional changes can be of both signs, the global average impact is expected to be small. Current literature has large disparities in conclusions. For example, Snyder et al. (2004) found that removal of northern temperate forests gave a summer warming of 1.3°C and a reduction in precipitation of 1.5 mm day-1. Conversely, Oleson et al. (2004) found that removal of temperate forests in the USA would cool summer temperatures by 0.4°C to 1.5°C and probably increase precipitation, depending on the details of the model and prescription of vegetation. The discrepancy between these two studies may be largely an artefact of different assumptions. The first study assumes conversion of forest to desert and the second to crops. Such studies collectively demonstrate a potentially important impact of human activities on climate through land use modification.
Other recent such studies illustrate various aspects of this issue. Maynard and Royer (2004) address the sensitivity to different parameter changes in African deforestation
experiments and find that changes in roughness, soil depth, vegetation cover, stomatal resistance, albedo and leaf area index all could make significant contributions. Voldoire and Royer (2004) find that such changes may affect temperature and precipitation extremes more than means, in particular the daytime maximum temperature and the drying and temperature responses associated with El Niño events. Guillevic et al. (2002) address the importance of interannual leaf area variability as inferred from Advanced Very High Resolution Radiometer (AVHRR) satellite data, and infer a sensitivity of climate to this variation. In contrast, Lawrence and Slingo (2004) find little difference in climate simulations that use annual mean vegetation characteristics compared with those that use a prescribed seasonal cycle. However, they do suggest model modifications that would give a much larger sensitivity. Osborne et al. (2004) examine effects of changing tropical soils and vegetation: variations in vegetation produce variability in surface fluxes and their coupling to precipitation. Thus, interactive vegetation can promote additional variability of surface temperature and precipitation as analysed by Crucifix et al. (2005). Marengo and Nobre (2001) found that removal of vegetation led to a decrease in precipitation and evapotranspiration and a decrease in moisture convergence in central and northern Amazonia. Oyama and Nobre (2004) show that removal of vegetation in northeast Brazil would substantially decrease precipitation.