It's a well-known fact that cities are warmer than their surrounding rural areas. This is because of the so-called 'urban heat island' effect created by buildings, roads and human activities, which absorb heat during the day and release it slowly during nights. But did you know that these warmer conditions also affected plants? Now, researchers from Boston University have quantified this phenomenon and say that cities are greener longer than neighboring rural regions. Using information from NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on the Terra satellite, they found that growing seasons are about two weeks longer in urban areas than in rural ones, one in the spring and one in the fall. In "Urban heat islands make cities greener," NASA adds that this effect can be seen up to six miles from cities, extending the growing season for people living outside cities. So is this time to transform city parks into corn fields?
Here is what says Boston University.
Recent NASA-sponsored research from a team of geographers in Boston University’s Center for Remote Sensing shows that the growing season for vegetation in about 70 urban areas in North America is, on average, 15 days longer than that in rural regions surrounding the cities studied. Led by Xiaoyang Zhang, a research assistant professor in BU’s Geography Department, the team found that, like many urban-dwelling humans, urban greenery lives at a more intense pace, getting as much as a seven-day jump-start on spring and up to eight additional days before winter dormancy than vegetation in surrounding rural areas.
Below is a false color image of downtown Atlanta, produced from data collected by the Airborne Terrestrial and Land Acquisition Sensor (ATLAS) in 1997. Trees and other vegetation appear in red in the color image while man-made structures appear in light blue and black. (Credit: NASA).
This image comes from this page at NASA's Goddard Space Flight Center. Unrelated to the subject, you'll also find there a small but very spectacular movie which zooms from Central Park, New York, to the whole city to end by the Earth in space (4.26 MB). Very cool!
NASA, which partially funded the project, gives some precise numbers.
Zhang added that urban heat islands provide a very good model to assess the effects of global warming on plant growing seasons and ecosystems. As temperatures warm due to climate change, growing seasons will likely change as well. Zhang and colleagues found that for every 1 degree Celsius (C) or 1.8 Fahrenheit (F) that temperatures rose on average during the early springtime, vegetation bloomed 3 days earlier.
And here is how much the rural zones around cities are affected by this warming effect.
The researchers found that the effect urban heat islands have on plants' growing seasons is exponentially weaker the further away from the city one travels. Significant effects were seen up to 10 kilometers (6 miles) from city lines. In other words, the impact of urban climates on ecosystems extended out 2.4 times the size of a city itself.
The research work has been published by Geophysical Research Letters in June 2004. Here is a link to the abstract of this paper called "The footprint of urban climates on vegetation phenology."
Human activity, through changing land use and other activities, is the most fundamental source of environmental change on the Earth. Urbanization and the resultant “urban heat islands” provide a means for evaluating the effect of climate warming on vegetation phenology. Using data from the Moderate Resolution Imaging Spectroradiometer, we analyzed urban-rural differences in vegetation phenological transition dates and land surface temperatures for urban areas larger than 10 km2 in eastern North America. The results show that the effect of urban climates on vegetation phenology decays exponentially with distance from urban areas with substantial influence up to 10 km beyond the edge of urban land cover, and that the ecological 'footprint' of urban climates is about 2.4 times that of urban land use in eastern North America. The net effect is an increase in the growing season by about 15 days in urban areas relative to adjacent unaffected rural areas.
A French humorist, Alphonse Allais, once said we should build cities in the countryside. Now, is this time to bring back fields in the center of our cities to get better crops?
Sources: Boston University news release, July 29, 2004; NASA's Goddard Space Flight Center news release, July 29, 2004; Geophysical Research Letters, Vol. 31, L12209, June 25, 2004