On Earth all life depends fundamentally on energy, and the energy that sustains life here is from the Sun. The Sun constantly sends out a tremendous amount of energy which comes to us in different forms. Some of it is light energy, which you see reflected during the day. Other energy is thermal energy, which you can feel. Still other energy is ultra-violet (UV), which you probably know is so strong that it can burn your skin! When the energy from the sun reaches the Earth several things can happen, two of which we'll learn about here. The first is that energy can be reflected. That means just what you think it means...the energy bounces off an object and is sent back the way it came. When energy is reflected by an object, it is not used by the object. The second things that can happen is that the energy can be absorbed; it's basically soaked up by the object. When this happens the object has to warm up and re-release it's energy.
As a general rule, the darker an object is the more energy it absorbs, and the lighter it is the more it reflects. This is why you feel cooler when you wear white clothes and hotter when you wear black clothes. It's not magic....the darker clothes are actually absorbing more energy, which heats you up! Geographers use the term albedo to describe what percent of the Sun's energy is reflected and absorbed by an object. An albedo of 100 means that 100% of energy is reflected, whereas an albedo of 0 indicates that all is absorbed (1).
Now, back to the city. When we build cities, we are actually altering the albedo of the area. Cities have lots of roads, many of which are covered with asphalt. What color is asphalt? That's right, it's black. In addition, many roofs are dark, and thus they absorb more energy. The cumulative effect of this increases the temperature of the city. In addition, removing trees alters the energy balance in other ways: trees use a small amount of energy to grow, while at the same time water evaporating (transpiring) from leaves cools the overall environment (2). The HIE effect can increase energy consumption to power air conditioning units as well as increase mortality during heat waves.
On to Jakarta. This city has grown at an incredible rate since the 1970s which has led to a great deal of land conversion. The image to the right is a remotely-sensed picture taken from a satellite (3). This picture is modified to show vegetation (forests, fields) in red and built-up areas in greenish-white. As you can see, there are very few vegetated areas within the core of the city; that's all covered by low-albedo surfaces which cause the temperature to increase! In fact, recent studies indicate that not only is the mercury in Jakarta rising, but the elevated temperatures are also altering the regional climate as well. Two examples of this are increased rainfall downwind (4) of the city and earlier onset of seasonal sea breezes.
Like I mentioned in the introduction, this phenomenon isn't unique to Jakarta. As you can see from the graph I've included, there is a noticeable upward trend in urban temperatures in several cities across Asia (5). I didn't include another graph from the study which shows that rural temperatures are not increasing as fast, but trust me, there is a difference. So what can we do about the HIE? Well, the first step is knowing about it and how it works. And while we're probably never going to completely counter the HIE, we can take steps to mitigate it. These steps range from policy and planning (making sure there are green spaces like parks and that land use is zoned effectively) to technological fixes to changes in our individual behaviors. What kinds of solutions can you think of to address the HIE? Start by doing a search for "green roofs". What can you find, and what can you imagine?
1) What types of surfaces have high albedos? What about low albedos? At night you can try an experiment. Go outside and touch a dark surface, like the street. Then touch a lighter surface. Which one feels cooler? The warmer one is most likely releasing thermal energy it absorbed during the day!
2) I am referring here to the "latent heat of evaporation". It takes a significant amount of energy to change water into water vapor. This is something I'll cover in a future post.
3) The image here was taken by the SPOT satellite. Satellites are neat because they can detect all sorts of different types of energy. Remember that visible light is only one type of energy...there are many others that you can't see, including radio waves, UV waves, IR waves, and microwaves! This satellite can detect infra-red (thermal) energy that your eyes can't detect. Geographers use the term "remote sensing" because the data, in this case the picture, is taken from a remote distance, or far away. You're probably familiar with another remote sensing device...a camera! Remote sensing allows geographers to learn a great deal about the environment. Our department at the University of Hawaii is particularly strong in remote sensing, and our faculty and students are designing new ways to analyze remotely sensed data.
4) Rain in virtually all instances is caused by some mechanism of uplift. In this case the mechanism is convection. This is something I'll cover in a future post!
5) Look at the graph. You can see that in each of the cities, the general trend is increasing. However, you can also see that from year to year there is variation. That means that even though this year is cooler than last year, in the long run the average temperature is increasing! We can apply this to global warming as well. Some people think that just because this winter was really cold and that there was a lot of snow that global warming isn't really happening. But if we look at temperatures over 5, 10, 50, and 100 years we can clearly see an upward trend in temperatures. Can you think of examples of other things that exhibit short term variation but a noticeable long term trend?
REFERENCES AND FURTHER READING
Kataoka, Kumi, Futoshi Matsumoto, Toshiaki Ichinose & Makoto Taniguchi. 2009. Urban Warming Trends in Several Large Asian Cities Over the Last 100 Years. Science of the Total Environment 407:3112-3119.
Based on this article, you kids could probably get published in this journal!
Tokairin, Takayuki, Asep Sofyan, & Toshihiro Kitada. 2009. Numerical Study on Temperature Variation in the Jakarta Area Due to Urbanization. Paper presented to the Seventh International Conference on Urban Climate, Yokohama, Japan, 29 June- 3 July 2009.
This study makes use of data from the “U.S. Geolorogical Survey (USGS)” (2). This is one superfluous syllable away from being the funniest thing I’ve ever seen in my life.Tokairin, Takayuki, Asep Sofyan, and Toshihiro Kita. 2010. Effects of land use Changes on Local Meteorological Conditions in Jakarta, Indonesia: Towards the evaluation of the thermal Environment of Megacities in Asia. International Journal of Climatology 30:1931-1941