The role of temperature changes in environmental sustainability of Tehran

The global temperature in 2017 increased to almost 1˚C above pre-industrial levels. The studies on temperature time series in Iran as well as the city of Tehran demonstrate substantial changes within recent decades. The increase in global warming, particularly in cities, influences human health and ecosystems. Meanwhile, different types of green spaces can be a comprehensive tool to maintain urban environmental sustainability by improving air quality, controlling temperature, and reducing the cost of energy consumption in buildings.On the other hand, through city development, the natural and green spaces are under the pressure of rapid urbanization. It is necessary to evaluate the sustainability of green spaces and their impact on urban sustainability to achieve environmental sustainability. This paper studies the changes in air temperature, land surface temperature, and land use/land cover of the city of Tehran within 30 years from 1988 to 2017 to evaluate the urban environmental sustainability.

The city of Tehran has 212.3mm annual precipitation and an average temperature of 17.6˚C, based on Mehrabad station records from 1960 to 2017. It has a population density of almost 13600 persons per square kilometer. Daily temperature records of Mehrabad and Shemiran stations from 1988 to 2017 were obtained from Iran Meteorological Organization to measure the changes in air temperature. Anderson-Darling, Mann-Kendall, Sen’s slope, and Pettitt’s statistical tests as well as monthly plotting and annual temperature time series with trend components were employed to analyze monthly and annual temperatures.Two daytime Landsat images in summer were used to study the changes in land use/land cover and land surface temperature: images of 18 August 1988 (TM) and 25 July 2017 (OLI TIRS). Through supervised classification, land use/land cover was classified into four classes of water, vegetation, built-up and bare soil. Land surface temperatures were calculated by using thermal bands (band 6 of TM and band 10 of TIRS), calculating brightness temperature, NDVI, proportion of vegetation and emissivity. Consequently, environmental sustainability was evaluated by analyzing the changes in air temperature, vegetation cover and land surface temperature.
 

The Anderson-Darling test results for monthly and annual mean, maximum, and minimum air temperature time series showed that the time series are not normally distributed. Therefore, Mann-Kendall, seasonal Mann-Kendall, Sen’s slope, and Pettitt’s tests were employed to detect trends. The trends in monthly and annual air temperature time series (mean, maximum, and minimum were statistically significant with p-values less than 0.05. Pettitt’s test results showed that the changing points were in the late 1990s and early 2000s. The changes in temperature before and after the changing points show 0.8˚C and 1˚C increase in minimum temperature and 1.5˚C and 1˚C increase in maximum temperature in Shemiran and Mehrabad stations, respectively.Comparing the land use/land cover and the extent of green spaces within the 30 years period, it demonstrates urban sprawl and expansion of built-up areas in western parts of Tehran. Also green patches, particularly farmlands and gardens, significantly shrunk in southwest, west and north Tehran. During this period, the extent of vegetation cover decreased by 6.8%. The minimum land surface temperature threshold within the study period increased 5.3˚C in summer 2017 compared to 1988. The increase in surface temperature was accompanied by high temperatures in southern and western districts of the city, the districts with noticeable green patch decline.
Uncontrolled construction, horizontal expansion of the concrete structures in the city, decrease in the extent and fragmentation of green spaces along with the increase in the mean land surface temperature by 2.6˚C within the study period indicate the unsustainability of green spaces as an important element of urban land use and environmental infrastructure.

According to the results of the study, air temperature in Tehran has increased with a statistically significant trend. The changes in land use/land cover were accompanied by built-up development and vegetation cover decline while the land surface temperatures increased. The course of actions during the last three decades demonstrate the environmental unsustainability of the city of Tehran. Implementing strategies following nature-based approaches, suitable for the urban climate and existing limitations, may help adapt to the impacts of temperature rise and reduce greenhouse gases emission.