Urban Warming – Where Does It Come From?

By Ed Caryl

My last article, Anthropogenic Global Warming is Real (But It’s Not From CO2, contained the statement: “No one (to my knowledge) has researched what part of the global temperature rise is due to energy use. All energy use ultimately goes to heat. This is what causes the heat islands.” This raised eyebrows in at least one reader, who commented, to paraphrase, that in his opinion, heat islands were caused by solar energy acting on and with the structures in the city, pavements, buildings, etc. The answer is: both solar and anthropogenic heat contribute to heat islands.

There have been studies done on some towns and cities. I cited the study done on Barrow, Alaska in A Light In Siberia. The heat island there is 2.2°C in winter when there is no sunlight at Barrow’s latitude north of the Arctic Circle. At Barrow, it is clear that the heat island is due to the fuel used to heat the village. To quote from that study, “There was a strong positive relation between monthly UHI magnitude and natural gas production/use. Integrated over the period September–May.”

Fairbanks, Alaska, has also been studied for heat island. There, the heat island was shown to grow with increasing population over the 50 year time period from 1949 to 1999. The temperature difference at the end of this period was 1°C in mid-winter at Fairbanks airport, 7 kilometers from downtown, compared to Eielson Air Force Base, about 40 kilometers away, and nil in the summer. Again, as Fairbanks is nearly at the Arctic Circle, solar heating cannot be the cause.

What about heat islands further south? This author has lived in the Pacific Northwest of the US for most of his life, the last 50 years in Seattle and Spokane, Washington, and Portland, Oregon. In winter, there have been several hundred occasions when it was raining in the downtown areas, and snowing in the suburbs or surrounding rural areas. These cities are well known for cloudy and rainy climate, especially in the winter, when the sun may not shine for months at a time. These winter heat islands cannot be solar caused.

In Anthropogenic Global Warming is Real, the article stated that the U. S. used 29 PetaWattHours of energy in 2005. This is from all sources: fossil fuels, hydroelectric, wood, solar, wind-turbines, and pedal-power included. The last three contributed insignificantly, so can be ignored. The total land area of the contiguous 48 states is just over 8 million square kilometers. In 2002, urban area was less that 3% of the land area, or 240,000 km² and 79% of the population lived in these urban areas. If each person consumes roughly the same amount of energy, then roughly 23 PetaWattHours was used in less than 240,000 km², or almost 10¹⁰ WattHours per square kilometer, or 100 kilowatthours per square meter annually in urban areas. This is 6.1% of the sun’s energy on that same square meter. This will raise the temperature in that square meter by 17.4°C.

This figure seems a little high, as it is more than the NASA heat images reveal (10 to 15°C), and more than most studies have stated. This is probably because more energy is consumed between urban areas, for transportation (jet fuel, long-haul trucks, auto travel), farm energy use, electricity transport losses, and losses from the wind simply blowing it away to the rest of the northern hemisphere. But it does indicate that energy use heating may be the large factor in urban warming.

But what about the city structure itself absorbing solar energy and providing it back to the environment? This undoubtedly happens. The ability of a land surface to reflect solar energy is called albedo. The scale is zero for no reflectance, or complete absorbance, to 1 for perfect reflection. Lamp-black has an albedo of nearly zero, and fresh fallen powder snow has an albedo of nearly 1. Cities are fairly dark because asphalt city streets and parking lots, along with most roofing materials, are a large percentage of the surface. Most cities have an albedo of between 0.1 and 0.2. The average albedo of the earth is between 0.29 and 0.35, but it varies widely from about 0.9 or more for Antarctica and Greenland, to 0.2 for forests and vegetated areas. Deserts and dry grasslands have an albedo of 0.4 to 0.5. The oceans have an albedo of less than 0.1.

Figure 1. Terrestrial earth albedo. Source: NASA Earth Observatory, http://www.eoearth.org/article/Albedo?topic=54300

A city with an urban core that has an albedo of less than 0.2, surrounded by desert or grasslands with an albedo of 0.4 to 0.5, will be warmer than the surrounding area because it absorbs more solar radiation. Any heat energy produced by the population will simply add slightly to that heat. Examples would be Phoenix, Arizona or Cairo, Egypt.

There is a study that compared energy use and albedo for several cities. The link is below. If a city is mostly light colored concrete, with an albedo of 0.2, and is surrounded by suburbs with green lawns and asphalt streets, with an albedo of 0.11 to 0.16, the city core may be cooler than the surrounding area because the city core reflects more heat than the darker surroundings. An example would be Los Angeles, California.

Cities north of about 45° latitude, such as New York City, Montreal, Moscow, and of course Fairbanks and Barrow Alaska, will tend to have more anthropogenic heat than solar heat. Cities south of that line will tend to solar heating. A compact city will have more anthropogenic heat than one that is spread out because the anthropogenic heat will warm a smaller area. Again, New York City, Providence, Rhode Island, USA, and Moscow, Russia, are good examples of compact cities with extreme heat islands driven by energy consumption. St. Louis, Missouri, USA and Berlin, Germany are the opposite examples with heat islands that are predominately driven by the sun. A city’s location, days of sunshine, density, and the character of the structures, will define whether the heat island is predominately caused by the sun or by the people.

One of the commenters on Anthropogenic Global Warming is Real…wanted an explanation on why some urban surface stations show a cooling trend. The answer lies in the local trends in albedo. If cities or regions get lighter, (white concrete streets replacing asphalt, increasing numbers of white sheet metal, or white painted roofs replacing tarred or composition roofs, increasing parks and open spaces, etc.) conditions would favor declining temperatures. A recent paper about land use changes in Spain cites an example where greenhouse horticulture in the southeastern part of the country changed the albedo locally by +0.09 changing the local temperature trend to –0.3°C/decade. Changes in land use, population density, vegetation, or even changes in local building codes may change the local albedo and change the trend in temperature. Very small changes in local albedo will lead to noticeable changes in local temperature trends.

Temperature change driven by urban warming or cooling, and albedo changes are what makes the total global surface temperature trend so maddingly difficult to ascertain. In the absence of man’s influence on albedo or burning of fossil fuel (ignoring any possible CO2 influence), the earth may be warming, or it may be cooling. We simply do not know. Albedo changes and energy use are masking the truth.