Does Urban Heat Island effect add to the global warming trend?

It's well established that urban areas are warmer than surrounding rural areas. However, does Urban Heat Island (UHI) contribute to the global warming trend? Short answer, no. Two thirds of global temperature data comes from ocean records, free of UHI effect. For land records, urban trends are compared to nearby rural data - anomalous urban trends are homogenized to match rural records (Hansen 2001). However, in most cases, the urban temperature trend is observed to be little different to the rural trend. A new paper Urbanization effects in large-scale temperature records, with an emphasis on China (Jones et al 2008) looks at this in more detail.

Comparing rural and urban sites in London and Vienna

The paper begins by looking at 5 sites in and around London. Figure 1 shows absolute temperatures, clearly indicating a UHI influence on the urban sites at London Weather Centre (brown) and St. James Park (dark blue). The coolest record is the rural based Rothamsted (dark green). However, the excess urban warmth has no effect on the temperature trend - all sites show the same overall trend.


Figure 1: Annual temperature trends for five sites in and around London. Brown and dark blue are urban sites, green are rural.

A similar comparison was made between two sites in Vienna. Again, the absolute temperature is greater for the urban site but both sites show near identical trends.


Figure 2: Annual temperature trends for two sites in Vienna – Hohewarte in the center (brown) and the rural location of Grossenzersdorf (green).

Comparing rural and urban networks in China

So established urban areas show the same trends as surrounding rural areas. What about urban areas that are still developing? China, in contrast to Europe, has experienced rapid economic growth over the last 30 years with a dramatic increase in its city areas. If there were to be significant urban-related warming, it ought to be in this region and over recent decades. Figure 3 compares a range of temperature datasets:


Figure 3: Annual average temperature anomalies. Jones et all (dotted green and brown) is a dataset of 42 rural and 42 urban sites. Li et al (solid green and brown) is a homogenized dataset of 42 rural and 40 urban sites. Li (blue) is a non-homogenized set of 728 stations, urban and rural. CRUTEM3v (red) is a land-only data set (Brohan et al., 2006). This plot uses the 1954–83 base period.

That there are hardly any differences between the six series tells us several things. Small datasets of 40 stations show the same result as the 728 station dataset. In other words, for a region of this size, the average can be constructed from a limited number of sites, implying that for the 728 station network there is considerable redundancy.

As the scale increases, the overall impact of homogeneity adjustments diminishes. This might be a bit heartbreaking for those hard working boffins who spend hundreds of hours pouring meticulously over station data, ensuring the data is all homogenised (but of course, they don't do it just to calculate global trends).

And of course, the most significant finding: the trend is the same for both urban and rural groups over any of the periods. Even in the case of developing urban areas, when averaged out over large areas, urban heat island has little impact on the warming trend.

Globle warming

What is global warming?   Global warming is when the earth heats up (the temperature rises). It happens when greenhouse gases (carbon dioxide, water vapor, nitrous oxide, and methane) trap heat and light from the sun in the earth’s atmosphere, which increases the temperature. This hurts many people, animals, and plants. Many cannot take the change, so they die.     What is the greenhouse effect? The greenhouse effect is when the temperature rises because the sun’s heat and light is trapped in the earth’s atmosphere. This is like when heat is trapped in a car. On a very hot day, the car gets hotter when it is out in the parking lot. This is because the heat and light from the sun can get into the car, by going through the windows, but it can’t get back out. This is what the greenhouse effect does to the earth. The heat and light can get through the atmosphere, but it can’t get out. As a result, the temperature rises. The sun’s heat can get into the car through the windows but is then trapped. This makes what ever the place might be, a greenhouse, a car, a building, or the earth’s atmosphere, hotter. This diagram shows the heat coming into a car as visible light (light you can see) and infrared light (heat). Once the light is inside the car, it is trapped and the heat builds up, just like it does in the earth’s atmosphere. Sometimes the temperature can change in a way that helps us. The greenhouse effect makes the earth appropriate for people to live on. Without it, the earth would be freezing, or on the other hand it would be burning hot. It would be freezing at night because the sun would be down. We would not get the sun’s heat and light to make the night somewhat warm. During the day, especially during the summer, it would be burning because the sun would be up with no atmosphere to filter it, so people, plants, and animals would be exposed to all the light and heat. Although the greenhouse effect makes the earth able to have people living on it, if there gets to be too many gases, the earth can get unusually warmer, and many plants, animals, and people will die. They would die because there would be less food (plants like corn, wheat, and other vegetables and fruits). This would happen because the plants would not be able to take the heat. This would cause us to have less food to eat, but it would also limit the food that animals have. With less food, like grass, for the animals that we need to survive (like cows) we would even have less food. Gradually, people, plants, and animals would all die of hunger.    What causes global warming? Many things cause global warming. One thing that causes global warming is electrical pollution. Electricity causes pollution in many ways, some worse than others. In most cases, fossil fuels are burned to create electricity. Fossil fuels are made of dead plants and animals. Some examples of fossil fuels are oil and petroleum. Many pollutants (chemicals that pollute the air, water, and land) are sent into the air when fossil fuels are burned. Some of these chemicals are called greenhouse gasses. We use these sources of energy much more than the sources that give off less pollution. Petroleum, one of the sources of energy, is used a lot. It is used for transportation, making electricity, and making many other things. Although this source of energy gives off a lot of pollution, it is used for 38% of the United States’ energy. Some other examples of using energy and polluting the air are: Turning on a light Watching T.V. Listening to a stereo Washing or drying clothes Using a hair dryer Riding in a car Heating a meal in the microwave Using an air conditioner Playing a video game Using a dish washer