A steady increase in mean global temperatures and violent weather over the previous several decades has provided circumstantial evidence that significant changes in global climate are underway (Stone, 1999). Numerous efforts are underway to understand the cause and to explore the technological and management strategies to minimize the implications.
In recent years, the role of human activities in the process of global climate change has attracted a growing level of attention within the scientific community. Perhaps more significant in the short term, however, is the impact human settlement patterns are having on climates at the regional level. Sustainability of human kind is often linked with global climate change but the climate change at city or regional scale is paid little attention by the policy makers and academicians in both the developed and the developing countries. Changing climate in the dense mega-cities around the world is a well-documented phenomenon. Cities like Bangkok, Manila, Shanghai,Tokyo, Los Angeles and San Francisco are becoming warmer and warmer everyday. The urban heat environment is worsening in other mega-cities around the world regardless of the development stages and its income level. Heat environment is neglected in most cities in terms of awareness, mitigation policies, researches and this poses a clear threat to urban sustainability. Policy makers and the people are less aware of the implications of worsening urban heat environment to the society and the urban system. Contributing to the potential for detrimental ecological impacts within cities in particular is a more regionalized process of temperature change known as the urban heat island effect.
The Urban Heat Island Phenomenon
“Urban Heat Island” is a climatological phenomenon wherein large urbanized regions have been shown to physically alter their climates in the form of elevated temperatures relative to rural areas at their peripheries. Temperatures of urban areas are usually higher (about 2.5 to 6˚C) than those of its surrounding, and this phenomenon have been reported inside dense and highly urbanized cities around the world.
Rapid urbanization and population growth in the mega-cities has resulted into massive infrastructure built-up and dense settlements. Urbanization has a dynamic relationship with the physical environment. As cities and urban areas expand (called Sprawl), thousands of hectares in naturally vegetated surfaces are being lost each year -- replaced with asphalt, concrete, rooftops and other man-made materials. While urban growth affects the physical environment (usually negatively), urban environmental changes also affects the qualityof life in these areas. The latter lead to biochemical, epidemiological and psychological responses in the urban dwellers.
Urban Sprawl not only results in the loss of native habitats (where animal and plant species are becoming extinct or endangered), but creates Urban Heat Islands -- where man-made materials such as asphalt store much of the sun's energy producing a dome of elevated air temperatures over the urban area. In urban areas, buildings and paved surfaces have gradually replaced preexisting natural landscapes. As a result, solar energy is absorbed into roads and rooftops, causing the surface temperature of urban structures to become 50-70˚F higher than the ambient temperatures. As surfaces throughout an entire community or city become hotter, overall ambient air temperature increases. This phenomenon can raise air temperature in a city by 2-8˚F (World Meteorological Organization, 1984).
Dr. J. Marshall Shepherd and colleagues at NASA's Goddard Space Flight Center, Greenbelt, Md., found that urban areas with high concentrations of buildings, roads and other artificial surfaces retain heat and lead to warmer surrounding temperatures, and create urban heat islands. This occurs because in urban areas, there are fewer trees, and other natural vegetation to shade buildings, block solar radiation and cool the air. In addition, roof and paving materials absorb more of the sun’s rays, causing both surface temperature and over-all ambient air temperature in an urban area to rise. This increased heat may promote rising air and alter the weather around cities.
Man-made changes to the urban environment have been the traditional sources of the worsening urban heat environment. In the process of urbanization, vegetated land surfaces are converted into concrete and asphalt. These changes in the nature of surface have primarily affected solar reflectivity (popularly called albedo), evaporative efficiency and roughness of the land surfaces. Building density and type, amount of road surface, and energy use, as well as local topography and regional wind patterns, all work together to modify a city’s climate. These causes can be classified according to the following - alterations to urban thermal properties, changes in vegetation cover, heat trapping by urban geometry and man-made (anthropogenic) heat input.
Alterations to urban thermal properties
Today’s urbanized cities comprise asphalt roads, concrete pavements, parking lots, buildings and these absorb, store and radiate more heat than the vegetated surfaces. This disrupts the natural radiation balance of the surface resulting into the warmer city. The urban heat island effect is often noticed at night when buildings and other constructed surfaces radiate the heat they have accumulated during the day.
The most influential property in the formation of urban heat island is that of albedo. Albedo is defined as the ration between the light reflected from a surface and the total light falling upon a surface. As the picture shows, albedo can range greatly. Clearly, the albedo of vegetation is much greater than that of civil structures, resulting in structures absorbing much more solar radiation than trees and plants.
Changes in vegetation cover
Heat islands are created when city growth alters the urban fabric by substituting man-made asphalt roads and tar roofs and other features for forest growth. Apart from radiation balance, vegetation loss is responsible for decreasing evapotranspiration process in which plant uses heat from the air to evaporate water in the leaf transpiration process. The process releases moisture into the atmosphere. This process is similar to sweating in humans, effectively releasing heat into the atmosphere.
As the water evaporates from vegetation, heat is taken out of the environment. In that way vegetation act as heat sink. They are also responsible for retaining water into the soil and their absence decrease the ability of the soil to retain water thereby decreasing the evaporation rate. Therefore worsening heat environment is partly responsible for the decreased humidity in mega-cities too. Studies in Tokyo have revealed that the temperature has gone up by 2˚C on average and its humidity has fallen by fifteen percent in the last one hundred year.
Heat trapping by urban geometry
Another important reason for worsening of heat environment is the change in the wind pattern. Urban infrastructures increase surface roughness and they lower wind speeds which could have carried away surface heat gain. The formation of urban canopy changes the wind pattern and does not allow wind to enter or to swipe away from the near ground surface effectively, trapping heat inside the canopy.
The canyon structure that tall buildings create enhances warming. During the day, solar energy is trapped by multiple reflections off the buildings while the infrared heat losses are reduced by absorption. The city also changes the overall cooling action of the wind by channeling it into narrow streets. The geometry of high vertical walls and narrow streets also increases the summer heat cities as the high sun is reflected downward and is absorbed, and then reradiated, by the often rocklike street and building surfaces.
Man-made (anthropogenic) heat input
In order for cities to thrive, energy production is a necessity. Great amount of heat is released into the environment by powerplants. Transportation is also contributes large amounts of heat, which is evident to those stuck in rush hour traffic. Clearly, everyday human activity required for a functional society only aggravates the heat island problem. The biggest contributions are from areas of high industrialization, airports and seaports. These areas have enormous energy expenditures, and are highly unlikely to contain vegetation.
Mega-cities are characterized by high population density, high per capita energy consumption and their demand for energy is fulfilled in the physical forms such as electricity, oil, gases and coals which are ultimately discharged as heat into the urban atmosphere. Direct heat discharges are usually categorized as stationary and mobile. Heat discharge from buildings by air conditioning units is the single major source of stationary heat discharge. Although many industrial plants and industries are located far from the cities, still some of them are located in the cities which release heat directly into the urban environment. However they usually discharge heat from tall chimney stacks that are usually easy to swipe away by the wind breeze. Automobiles discharge large amount of heat that is mobile in nature. In the city centers and high traffic zones, the concentration of this discharged heat further increases by congestion and presence of fuel inefficient vehicles. A closer look into a mid-size vehicle for urban driving cycle suggests that nearly thirteen percent of total input energy is converted into the useful work while the rest dissipates as heat.
The cumulative effects of all these factors cause urban environment to be hotter than the surrounding areas. Similar to the effects of global warming, such “urban warming” can have substantial implications for air quality and human health within affected regions Increasing at a rate of 0.25 to 2˚F (0.1 to 1.1˚C) per decade, the heat island effect within the urban cores of rapidly growing metropolitan regions may double within 50 years (McPherson, 1994). In light of the roughly 2.9 billion new residents to arrive in urban regions between 1990 and 2025, there is a pressing need to ascertain the implications of urban warming for metropolitan regions and to identify potential strategies to counteract regional climate change.
Continuation... (you can view it also here)
Friday, February 27, 2009