Impact of Human Activities on Natural Hazards Essay

Natural hazards are naturally occurring phenomena that have disastrous impact on humanity. These phenomena had been in existence even before the advent of humanity. The hazardous dimension of these natural phenomena are in the context of the impact that such a phenomenon would have on human population in the area affected by that phenomenon. In this essay, the effect that human activity has on these natural hazards would be analyzed. Some human activities may be exacerbating the factors that cause the natural hazard, like the impact of excessive and unplanned logging on floods and droughts. In certain other cases the human activities may cause subsequent or supplementary hazards to a primary hazard event, like building dams in earthquake prone zones may lead to flash floods and landslides in the event of a rupture. A hazard can be defined as an event that has the potential to cause harm. This potential may be on account of its unexpected timing of occurrence or the actual intensity of the event itself. Human societies can withstand these events within a normal scale of occurrence. However, human societies become vulnerable when these events occur unexpectedly or are of an intensity or duration that falls beyond that normal scale (O’Hare and Rivas, 2005). Natural hazards can be broadly classified under the heads of geological, hydrological, climatic and diseases. This essay would limit its scope to analyzing causal relationships, if any, of human activities on landslides, floods and drought and the secondary hazards triggered by those activities in the event of an earthquake. Of all human activities that have a direct or indirect impact on natural hazards, deforestation is by far the most significant. Deforestation is the removal or destruction of forest cover of an area. It may occur due to unscientific logging practices without regeneration and may be accompanied by subsequent conversion to non-forest usage like agriculture, pasture, urban, mining or industrial development, fallow or wetland. At a very broad level, it has been argued that deforestation is a major cause of global climatic changes. It has been predicted that removal of forest cover will lead to violent and unpredictable environmental fluctuations. At a smaller landscape, deforestation has a direct bearing upon the climatic, hydrological, edaphic and biological aspects of that area. Deforestation is associated with higher levels of soil erosion and landslides, sedimentation in river beds and changes in fluvial geomorphology (Haigh, 1984). Quite a few of these effects of deforestation have a direct bearing on the natural hazards that will be covered in this essay. One of the major functions of a forest is to maintain the humidity level in the atmosphere. Trees withdraw groundwater through their roots and transpire the excess water through their leaves. Forests return a major part of the rainfall received by them through evapotranspiration. Annual evapotranspiration in tropical moist lowland forests ranges up to 1500 mm per year, with transpiration accounting for a maximum of 1045 mm per year (Bruijnzeel, 1990). This process of evapotranspiration in the leaves of trees takes the latent heat of evaporation from the surrounding atmosphere. Thus evapotranspiration has a cooling effect on the atmosphere that aids precipitation. Deforestation denies the atmosphere of this cooling effect and is thus a contributing factor to lowering of annual rainfall in an area. Further, the effects of deforestation generally compound the severity of drought. Lack of trees translates to the lack of root fibers that hold the topsoil. In the event of a drought, the topsoil flakes and gets blown by the wind, leading to severe dust storms. This phenomenon had devastated the American Great Plains for close to a decade in 1930s. The dust bowl covered farming areas in Colorado, Kansas, north west Oklahoma, north Texas and north east New Mexico. The fertile soil of the plains was exposed due to lack of vegetation cover and actions of the plow. These farming techniques that led to severe soil erosion, coupled with prolonged periods of extremely low rainfall, led to a series of severe dust storms that ranged up to the Atlantic coast. Much of the fertile topsoil was lost in the Atlantic (Cartensen et al. , 1999). Direct causal relationship between human activity and drought is yet to be conclusively established. However, there are studies available that point to a positive correlation between the two. For example, climate-modeling studies have indicated that the 20th century Sahel drought was caused by changing sea surface temperatures. These changes were due to a combination of natural variability and human induced atmospheric changes. The anthropogenic factors in this case were rise in greenhouse gas levels and aerosols (GFDL Climate Modeling Research Highlights, 2007). The effect of human activities like deforestation is rather more direct and pronounced in case of hydrological hazards like fluvial floods. Fluvial floods occur when the discharge of a river exceeds its bankfull capacity. Forests create deep, open textured soils that can hold large quantities of water. When the forest cover is removed through logging, the soil becomes compacted. More rainwater is converted to runoff or near surface flow and less proportion percolates as groundwater. Research has shown significant increase in monthly runoff following logging activities (Rahim and Harding, 1993). The runoff rainwater carries with it considerable amounts of loose soil particles. Removal of vegetation cover through excessive logging activities or overgrazing leaves the soil bare. In such a situation, the upper layer of the soils becomes susceptible to erosion by surface runoff. These suspended soil particles are deposited on the riverbeds. The effect of this type of soil erosion by surface runoff is even more pronounced when the deforestation happens in the riparian zones as well.