Investigation of Thermal Heat Mapping and Vegetation Cooling Impact using Landsat-5, -7, -8, and MODIS Imagery: A Case Study of Greater Beirut Area in Lebanon

Climate change and urban expansion are together playing a critical role in disrupting urban microclimate. Lebanon is an example country that is suffering from such effects. Particularly, the capital city “Beirut”, which covers only 0.95% of Lebanon’s total area, consumes 12% of the total national energy and has ~36% of the total population. Metropolitan areas continue to expand along adjacent mountains and leading to diminishing green areas. With the absence of plans to monitor this urbanization, it is critical to examine the interplay between urban temperatures and land use patterns to avoid high-intensity urban heat islands (UHI).

In this research study, we use thermal remote sensing technology to analyze the urban heat mapping of the Greater Beirut Area (GBA) at various spatial and temporal scales. Furthermore, we estimate the vegetation cooling impact as a response to rising urban temperatures. The investigation is conducted within a time frame that spans over 3 decades from 1990 to 2020. We use Landsat-5, -7, -8, and MODIS thermal infrared imagery, with a dataset of 524 images of the surface reflectance. For each year, we calculate the normalized difference vegetation index (NDVI) and land surface temperature (LST) statistics. We also create an overlay between each image and GBA topography to measure the vegetation area. Based on LST statistics, we retrieve the urban heat index (UHIndx) in GBA. Then, we examine the correlation between vegetation and temperature. The spatial-temporal analysis is conducted to relate heat mapping in GBA to topography based on altitude and land cover.

Overall results show that the temperature in GBA has increased over 3 decades, with an increase in the vegetation and urban LST by 1.1 °C and 1.26 °C, respectively. According to the land’s altitude, the highest LST values with a mean of 34.36°C are recorded at the lowest altitudes between 0-30 m at the coastal area, even if it is away by a 6 km distance. A substantial drop in LST values is witnessed when the elevation increases to 100s meters, at which rural areas or mountains with vegetation growth exist. Hence, this temperature dependency on the elevation also aligns with the results obtained from the LST statistics according to the land cover. Results show that green areas are cooler than urban areas, and dense forests witness lower temperatures than clear forests. Therefore, local analyses show that vegetation and altitude have a cooling effect, with temperatures dropping in the high and green mountains. Results of NDVI statistics show that urbanization has reduced the vegetation in the GBA with an 11% drop in green areas over the 3 decades. The vegetation cooling impact is demonstrated by negative spatial correlations between LST and NDVI. Finally, the urban heat index is higher in cities than in rural regions.

This study shows that UHI begins to appear in GBA due to the degradation of green spaces that reduce LST peak values. It highlights the need to consider urban heating in the legal urban planning code, which doesn’t strongly consider urban climatology or UHI.