A Reduction in Human Activities can Enhance the Urban Heat Island Intensity: Insights from the COVID-19 Lockdowns

Abstract:

The COVID-19 lockdown period (April-May 2020) led to a drastic reduction in human activity, emulating a controlled experiment on human-land-atmosphere coupling. Here, we examine the impact of the lockdown restrictions on this coupling in the Indo-Gangetic Basin (IGB), one of the most populated and polluted regions in the world. TROPOMI observations show large (<-14%) decreases in column number densities of NO₂ and SO₂ and a slight increase in O₃ over this region. While this reduction in air pollution reduced the MODIS-derived Aerosol Optical Depth (AOD) by around 5%, the MERRA-2 reanalysis shows a slight decrease in incident solar radiation at the surface due to a large increase (~15-20%) in cloud fraction during this period compared to a 5-year baseline, which is also confirmed by satellite observations. During this period, there was also an increase in surface vegetation based on satellite-derived normalized difference vegetation index (NDVI), probably caused by the delay in rabi crop harvest due to lockdown regulations. Using a theoretical framework implemented using the MERRA-2 data, we demonstrate that the increase in evaporative cooling due to this surface greening dominated the effect of lockdown-induced radiative forcing on the change (decrease) in surface temperature in the study area. Since this greening was higher in the rural references of urban clusters in this region, it caused a striking (~87%) increase in the daytime surface urban heat island (SUHI) intensity. This is confirmed by considering other relevant atmospheric and surface factors, including surface albedo (α), cloud fraction, AOD, and emissivity (ε) using linear and multiple linear models, as well as ensembles of decision trees. Our study is an illustration of the role of rural areas in modulating the SUHI intensity. More broadly, we demonstrate the human-induced control on atmosphere-biosphere interactions in this region, supporting ongoing efforts to represent human activity in earth system models.