Post by Jason Schatz
Densely built urban areas tend to be warmer than their rural surroundings, a phenomenon known as the urban heat island effect. Urban heat islands are nearly always described in terms of temperature—i.e., how much warmer cities are compared to their rural surroundings.
However, there are other ways to describe a city’s effects on its climate. For instance, how does urban warming affect demands for heating and air conditioning in buildings? What does it mean for the vegetation and natural areas within a city?
We were curious about these questions for the Yahara Watershed, since understanding the effects of the urban heat island can indicate ways we might make our cities more resilient as the climate warms. Our findings describe these effects in detail for Madison, the watershed’s major city, and were recently published in the International Journal of Climatology.
Urban plants may be protected from freezing for longer
Madison’s heat island extends the “freeze-free season” by several weeks per year, compared to rural areas. This means that when rural Dane County experiences mild freezes in late spring and early fall, downtown Madison often stays above freezing.
For example, in rural places around Dane County, the last spring freeze in 2012 occurred on April 25th, on average. In downtown Madison, however, no freezes occurred after April 7th.
Similarly, in the fall of 2012, the first freeze occurred on September 29th in rural areas, but downtown Madison did not experience its first freeze until October 23rd.
So in both spring and fall, Madison’s heat island added a few freeze-free weeks to the growing season. This could provide a potential buffer against freeze risk for urban plants and gardens. There are a couple of important caveats to keep in mind, however.
First, we recorded our data at a height of 11 feet, using temperature sensors installed on utility poles. But many sensitive plants grow nearer to the ground, where they may encounter different temperatures and freeze risks.
Second, small-scale differences in where plants grow—such as out in the open or under the shelter of a tree—can determine whether freezes and damaging frosts occur. Plants in open areas are more vulnerable to freezing at night, while plants under shelter tend to be more insulated.
Nonetheless, Madison’s urban heat island prevented many late spring and early fall freezes at 11 feet, and we suspect this buffering effect occurs near the ground as well.
Lower heating, but higher cooling costs
Because urban heat islands raise outdoor air temperatures, they affect the amount of energy needed to heat and cool buildings. By raising temperatures, heat islands tend to increase air conditioning during the summer and lower heating requirements during the winter.
In Madison, we measured the heat island’s effects on how many days in a year buildings need to be heated or cooled. Between 2012 and 2015, downtown Madison recorded an average of 25 percent more air-conditioned days and 6 percent fewer heated days than rural areas.
This suggests that a building located downtown would require 25 percent more air conditioning and 6 percent less heating than the same building in a rural area, all else being equal.
At first glance, the increased cooling demand seems to dwarf the energy savings in heating. However, Madison uses more energy for heating than for cooling, so the two effects roughly cancel each other out in an average year—although not necessarily in an unusually hot or cold year.
Nevertheless, there is a circular relationship between heat islands and building energy demands. By simply changing outdoor air temperatures by a few degrees, heat islands can affect indoor temperature needs. In other words, buildings create heat islands, which in turn modify the heating and cooling needs of the buildings.
It’s hard to talk about the implications of this interesting factoid in a meaningful way, however. Essentially, the best (and virtually only) way to lower energy costs is to make buildings more efficient, change thermostat settings, and/or install solar panels. But understanding the heat island’s influence helps us better understand how the dynamic system that is a city would work under the effects of a changing climate.
Schatz is a postdoctoral researcher on the UW-Madison Water Sustainability and Climate project.