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The widespread use of earth sheltered and underground construction could have an enormous effect on the global environment. The effects of deforestation and the impact of urban growth on the world’s atmosphere are only now beginning to be understood. In most cities, urban air temperatures are generally higher than their corresponding rural values
This phenomenon, the urban heat island, has been recognized since the turn of this century and has been well documented. Population growth in the 21st century will cause cities to grow and merge with each other, reducing even further the mitigating effects of suburban and rural vegetated areas.
One key to the reduction of the negative effects of human communities on global warming, air and water pollution, and the quality of life in general, lies in technologies designed to return the natural environment to the surface level, and human habitation to an intermediate one. These technologies include green roofs and walls, rooftop and vertical urban farming systems, and earth sheltered construction with the Green-Magic-Homes System.
Evaporative cooling is the process by which a local area is cooled by the energy used in the evaporation process, energy that would have otherwise heated the area’s surface. It is well known that the paving over of urban areas and the clearing of forests can contribute to local warming by decreasing local evaporative cooling, but it was not understood whether this decreased evaporation would also contribute to global warming.
The Earth has been getting warmer over at least the past several decades, primarily as a result of the emissions of carbon dioxide from the burning of coal, oil, and gas, as well as the clearing of forests. But because water vapor plays so many roles in the climate system, the global climate effects of changes in evaporation were not well understood. Evapotranspiration (evaporation and transpiration) from soilvegetation systems is an effective moderator of near-surface climates, particularly in the warm and dry mid and low latitudes. Given the right conditions, evapotranspiration can create ‘oases’ that are 2-8°C cooler than their surroundings.
In extreme oasis conditions, the latent heat flux can be so large that the sensible heat flux becomes negative, meaning that the air above vegetation and over the dry surroundings must supply sensible heat to the vegetated area and the Bowen ratio (ratio of sensible to latent heat fluxes) becomes negative. For example, in deserts, oases can develop with Bowen ratios of 0.26. In more average oasis conditions, Bowen ratios in vegetative canopies are within 0.5-2. In comparison, in urban areas ratios are typically around 5, in a desert it is in the neighborhood of 10, and over tropical oceans, it is about 0.1. Urban areas, with extensive impervious surfaces, have generally more runoff than their rural counterparts.
The runoff water drains quickly and, in the long run, less surface water remains available for evapotranspiration, thus affecting the urban surface energy balance. The lower evapotranspiration rate in urban areas is a major factor in increasing daytime temperatures. Simulations indicate that a vegetative cover of 30% could produce a noontime oasis of up to 6°C in favorable conditions, and a nighttime heat island of 2°C. In conclusion, increases in vegetation in urban areas can result in some 2°C decrease in air temperatures. Under some circumstances, e.g., potentially evaporating soil-vegetation systems and favorable meteorological conditions, the localized decrease in air temperature can reach 4°C.
The long-range benefits of evapotranspiration have often been confused with the question of the Albedo effect, which is a relative measurement of the reflectivity of surfaces and of the planet itself. In general, the more reflective surfaces are (the higher the Albedo), the less radiation they absorb and the less heat they reflect back into the atmosphere.
Since forests are generally attributed a low albedo, (as the majority of the ultraviolet and visible spectrum is absorbed through photosynthesis), it has been erroneously assumed that removing forests would lead to cooling on the grounds of increased albedo. Through the evapotranspiration of water, trees discharge excess heat from the forest canopy. This water vapor rises resulting in cloud cover, which also has a high albedo, thereby further increasing the net global cooling effect attributable to forests