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Urban Heat Island Effect
Context:
In a recent study published in the journal One Earth, researchers sought to answer a longstanding question, what is the extent of urban sprawl’s contribution to global warming?
Urbanisation Impact on Global Warming:
- Global Influence: Urbanisation affects global warming over land, contributing just over 1% of increased land surface warming globally.
- Regional Variations:
- Asia: Rapid urbanisation in areas like the Yangtze River Basin significantly contributed to local warming (up to 40% of observed warming).
- Japan: Urbanisation contributed a quarter of added warming, reflecting significant urban development.
- Europe and North America: Urbanization’s impact was smaller (2-3% of warming) due to earlier development and larger proportions of undeveloped land.
- Urban Climate Signal: Cities influence climate locally and regionally, affecting temperature, air movement, and potentially intensifying extreme weather.
- Global Growth: Urban land cover increased by 226% from 1992 to 2019, primarily in Asia (e.g., China expanded urbanised area by 413%).
- Future Projections: With urban populations rising, accurate urban representation in climate models becomes crucial for regional climate change simulations.
Urban Heat Island Effect and Global Climate Models:
- Representation in Models: Historically, global climate models often neglect cities or simplify their representation.
- Impact Assessment: Recent studies confirm that urbanisation has a statistically significant but minor impact on global land warming.
- Scepticism Addressed: Debunking the notion that urban heat islands dominate large-scale climate warming, attributing it instead to greenhouse gas emissions.
Research Methodology:
- Satellite observations measure infrared radiation from every surface pixel, providing a more comprehensive and unbiased view of land surface temperature changes.
- Satellite data offer a solution by eliminating spatial sampling bias inherent in surface station data.
- Integration of Urbanization in Climate Models:
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- Researchers at PNNL are enhancing climate models to better incorporate urban influences.
- DOE’s investment in projects like E3SM aims to develop comprehensive Earth system models that address critical climate research imperatives.
- Collaboration among seven national laboratories is focused on improving urban representation within these models.
Urban Heat Islands (UHI):
- Urban heat islands form when cities replace natural land cover with dense collections of pavement, buildings, and other surfaces that absorb and retain heat.
- This effect increases energy costs (e.g., for air conditioning), air pollution levels, and heat-related illness and mortality.
- Causes of Urban Heat Islands:
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- Low Albedo Materials:A lower albedo on urban surfaces causes them to absorb more solar energy, resulting in higher urban temperatures and influencing microclimate development accordingly.
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- Dense Construction: Buildings, houses, and shops are constructed close together, insulating and retaining heat.
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- Waste Heat: Energy expended by people, vehicles, and factories is released as heat, contributing to the overall warmth.
- Population Density: High numbers of people in small spaces generate more heat through daily activities.
- Building Materials: Materials used in construction insulate heat effectively, preventing its dissipation.
- Vertical Expansion: Skyscrapers and tall buildings further trap heat within urban areas, as heat cannot easily escape upward.
Effects of Urban Heat Islands:
- Higher nighttime temperatures as heat from buildings and pavement is trapped close to the ground.
- Poor air quality due to higher concentrations of pollutants from vehicles, industry, and dense human activity.
- Warm runoff from urban areas into local streams stresses aquatic species adapted to cooler water.
- UHIs may play a role in the broader pattern of global warming by increasing localised temperatures.
- Increased Energy Demand: Higher temperatures lead to more use of air conditioning and fans, straining energy resources and causing rolling blackouts.
Mitigation Strategies:
- Green Roofs: Installing roofs covered with plants to absorb carbon dioxide and reduce surrounding heat.
- Lighter-Colored Building Materials: Using light-coloured materials to reflect more sunlight and reduce heat absorption.
Effectiveness of Miyawaki Forests in Urban Heat Island Mitigation:
