Urban green spaces: Insights from Valencia

Visiting the city of Valencia in Spain for the first time, we were pleasantly greeted by the subtle aroma of orange blossoms in the air and the sight of beautiful oranges dangling from the orange trees that line the street pavements. Whilst many parts of the city are bordered with trees, there are also other green spaces,  such as beautiful small parks (like the one in the picture  shown below),

complete with Mediterranean flora such as palms, ficus, cycas, and orange trees. 

 The most impressive green space is perhaps the 9 km green belt that runs through the city-  the Jardi delTuria (Garden of the Turia), which is a credit to the city. The Turia is a Spanish river that empties into the Mediterranean near the city of Valencia. In 1957, it flooded and devastated Valencia. Consequently, the course of the river was artifically changed- now running along the city edge before meeting the Mediterranean. The original course of the river continues has been converted into an enormous green space resplendent with beautiful trees with pedestrian paths, cycle paths, and occasionally dotted with football grounds, cafes, athletics tracks, and gardens within the garden. Here city dwellers relax, play, or just get about their business, avoiding the traffic that plies overhead on the many bridges that cross the river bed. Buildings tower over either side of the Turia garden, comprising largely of apartment blocks and businesses. Our walk along the Turia garden in sweltering sunny April was very pleasant which made me realize the importance of green spaces in cities.

Urban heat islands and global warming

Cities have their own micro-climates. Human activities- buildings made of concrete,  asphalt roads , vehicular emissions, heat generated by people, and heat arising from equipment use, all contribute to temperature increases in the urban areas to levels that are significantly higher from the adjoining rural areas where temperatures remains close to air temperatures. Such urban areas are called ‘Urban heat islands’, a concept described over 200 years ago by Luke Howard, the father of meteorology. The term has been coined  as the warmer urban air lies in a ‘sea’ of cooler rural air.  According to the EPA, the annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings, going up to temperature difference as much as 22°F (12°C) on a clear, calm night.

Heat islands could not only affect the health of the urban inhabitants by causing heat -related illnesses (a topic discussed in the earlier post), but also contribute to global warming in general, and must not be ignored particularly as predictions indicate that by 2050 about 64% of the developing world and 86% of the developed world will be urbanised - leading to amplification of the issue. Whilst urbanisation cannot be halted, it must be made sustainable.

Mitigation strategies for urban heat island effects

Among the four major strategies United States EnvironmentalProtection Agency (EPA) has suggested for mitigating the effects of urban heat islands, the first one is increasing tree and vegetation cover. The others are the installation of green roofs (roofs with plants grown over a waterproof membrane), installation of cool and reflective roofs (built from materials with high solar reflectance and high heat emittance), and building cool pavements (made of material with high solar reflectivity and good water permeability).

In this post, we will look at the EPA’s primary recommendation. This, in my opinion, is the one with the maximum impact and the least costliest. The major benefits of increasing trees and vegetation covers are the following:

They reduce surface and air temperatures by providing shade and by the process of evapotranspiration. Estimates indicate that evapotranspiration, alone or in combination with shading, can help reduce peak summer temperatures by 2–9°F (1–5°C). By providing shade to buildings, trees decrease the demand for air conditioning and indirectly decrease the need for energy use which, in turn, lowers greenhouse gas emissions and provides better air quality. Trees also function as ‘sinks’ for air pollutants and carbon dioxide. Trees improve the quality of life of city dwellers by enhancing the aesthetic value of their surroundings and promote biodiversity by providing habitats for diverse species.

Interestingly, it  also appears that for some trees living in an urban environment might not be bad after all. In a study in 2012, seedlings of oak were grown for one season at four sites along an urban–rural transect from Central Park in New York City to the Catskill Mountains in upstate New York  with a difference in average maximum temperatures of 2.4 °C and  difference in minimum temperatures of 4.6 °C.  Additionally, seedlings were cultured in growth cabinets simulating the seasonal differential between the city and rural sites. The researchers found that warmer temperatures associated with the urban environment, especially high night-time temperatures, lead to enhanced growth in these seedlings.

The necessity for a Green Channel Programme for our cities

Coming back to our example - Valencia- it appears that the city is going in the right direction with its tree and vegetation cover initiatives. The green spaces and the green belts are not only aesthetically pleasing, but also reduce the heat (personal experience). Increasing the vegetation cover in cities is something where more is less. Thus, Valencia and other cities who have such initiatives cannot rest on their laurels. We really need to have green channels through our cities to cool the urban heat islands.

References

http://www.valenciavalencia.com/sights-guide/turia.htm

http://www.epa.gov/heatisland/about/index.htm

Searle, S., Turnbull, M., Boelman, N., Schuster, W., Yakir, D., & Griffin, K. (2012). Urban environment of New York City promotes growth in northern red oak seedlings Tree Physiology, 32 (4), 389-400 DOI: 10.1093/treephys/tps027http://dx.doi.org/10.1093/treephys/tps027">10.1093/treephys/tps027

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