Urban Heat Islands and Global Warming

This article may seem outside the scope of roofing; however, it's not when you consider reflective roof coatings constitute a primary part of the solution to the urban heat island problem. Well, it's a solution each home owner can take some action on. Consider that it's substantially more difficult for us to change government policies or the layouts, designs, and materials used in existing communities. In fact, getting a NON-reflective heat absorbing roof will most likely increase your cooling bill during the summer and contribute to this problematic phenomena. There are more extreme options such as "green roofs" which can accommodate vegetation, gardens or even rooftop trees. However, buildings with these types of roofs must be specially designed, and this is not a viable option for existing properties.

Picture: A visual of the Urban Heat Island Effect.

Are Urban Heat Islands Linked to Global Warming?

Short Answer: No--not at all. They are completely different.

Urban Heat Islands vs Global Warming = Local vs Global
Urban Heat Islands = Caused by Dark Heat Absorbent Surfaces
Global Warming = Caused by Global Pollution, both man made or from natural events, such as ash from a volcanic eruption

What is Global Warming?

Global warming occurs due to increased levels of pollutants in our atmosphere which creates a 'greenhouse effect' in that heat cannot escape from the atmosphere. As the sun warms the atmosphere and the heat is unable to escape, theoretically, with enough added 'greenhouse effects' the polar icecaps would melt, and the 'grain belt' in both the northern and southern atmosphere would shrink, making less land usable for food production, and also reducing the amount of available land globally (a catastrophic effect). An urban heat island is just a proliferation of dark surfaces absorbing sunlight and artificially heating small areas of a city to a few degrees warmer than normal. The Environmental Protection Agency (EPA) has released studies which imply Urban heat islands may increase the heat of Urban heat islands are isolated to small local areas, while of course global warming is global...

What are Urban Heat Islands?

Heat islands are areas in a city that are ten to twenty degrees warmer than normal due to black asphalt, dark roof materials and other non reflective surfaces. These dark, man made surfaces absorb heat in contrast to the normal natural sources such as trees and plants which naturally cool through evaporating water and reflection of heat. In fact, studies have shown that black a tar and gravel roof can easily exceed 150 degrees Fahrenheit on just a ninety degree day! Multiply this by all the buildings and black asphalt streets in a city and you'll begin to see how the net effect is an artificial heat island! A simple (but partial) solution to a large portion of this problem is to use reflective cool roof acrylic / elastomeric coatings. Reduced airflow created by tightly constructed sprawl, and waste heat and pollutants from both industrial and vehicle engines all contribute further to the heat island effect.

Can an Urban Heat Island Effect Local Weather? Yes.

Urban heat islands can be large enough to effect local weather. When a cold front of air undercuts a warm front, pushing the warmer air upwards into the atmosphere, the air begins to cool. At lower temperatures, the air cannot hold as much moisture, and when the dew point is reach, condensation, or rain results. So, when cooler air collides with an Urban heat island several things may happen including rain. Although the typical heat island might not be severe enough to create rain, effects such as clouds, fog, or strange winds may occur.

Are there Negative Economic Consequences due to Urban Heat Islands?

Due to the 10 degrees of additional heat produced by the "urban heat island" effect, many people are forced to use air conditioning or fans which they would otherwise leave off. Heavily populated cities in areas such as Southern California waste millions of dollars in extra energy costs to power all these cooling devices in both residential and commercial buildings to counter-act the heat island effect.

Below: A visual explanation of the Urban Heat Island Effect

Government Involvement and Recommended Actions to Counter-Act Urban Heat Islands

Natural parks and vegetation help counter-act the urban heat island effect. Buildings constructed with intensive or extensive green roofs, which accommodate rooftop trees, plants and gardens also help to dampen the heat island effect. The US Government has recognized this phenomena as a growing problem and the Department of Energy instituted "cool communities", a nationwide program to encourage hortatory legislation on the part of lawmakers and incentives for contractors to build greener construction. Energy Star also encourages the use of reflective, cool roof coatings, which also prevent a building from contributing to the urban heat island effect.

in short, the "urban heat island" effect is a negative phenomena, but it's completely unrelated to global warming, except to the extent that both phenomena are mostly man-made.