Escape the heat; Urban Heat Islands, global warming

Satellite-produced maps of Providence city’s urban heat island. Providence has a significantly strong heat island: Credit: (NASA, 2010)

The problem

The emergence of urban heat islands(UHIs) from the urban-industrial expansion of the great acceleration is notable in assessing urban environmental impacts. The decline in vegetation cover associated with urbanization decreases the transpiration and sensible heat flux capacity in cities, while proliferation of low albedo surface paving, roofing and asphalt causes a greater absorption of solar energy and heat retention. These anthropogenic domes of warm air from human urban industrial and transportation activities, have led to debate around their contribution to global warming.

Figure 1: Surface temperatures vary in typical heat island effect of US city: Credit: (NASA, 2010) & EPA

Figure 2: Calculation of effects of heat dumping from Air Conditioning use during heatwave crisis in Paris, 2003, with actual AC effect(Real AC) and projected tEmperatures without AC use: Credit: (NASA, 2010) & Météo France/Cécile de Munck

UHIs have been shown to cause localized ambient temperature increases of 3-10⁰C in city centers compared to surrounding areas (See figure 1), yet there is contention around how UHIs impact the occurrence of global atmospheric warming. Some claim an impact from 15-21% of warming effect, while a more conservative model holds this value at 2-4%. Chinese studies have suggested 30%.

Regardless of the actual value, consensus holds in acknowledging UHIs as priorities for urban sustainability, as the reduced evapo-transpiration, direct material warming and changes in sensible, latent and radiative energy fluxes add permanent stressors to climate systems,  leading to positive feedback warming of nearby regions.

High Energy

In addition to this, UHIs are associated with a spike in city energy consumption for building cooling, with increases of 67% in Athens, 22% in Osaka, 20-37% in Boston and 27-45% in London compared to their rural outliers. Along with greater net carbon release and subsequent warming through greenhouse effects, increased air conditioning(AC) use has further impacts on the ambient air temperature, exemplified in the heat wave crisis in Paris in 2003, where mass heat dumping from AC units exacerbated the UHI intensity and fatalities(See figure 2) . With the development of new infrastructure in the arctic region, UHIs push questions of the more direct arctic impacts of future and existent urban developments accompanying such infrastructural installations.

Some solutions

Installation of light colored roofing, the implementation of cool pavements and roofing with greater albedo and proliferation of greater city vegetation coverage can go far in tackling this issue, in increasing albedo and evapo-transpiration cooling. Check out this video concerning high albedo paving: