Abstract
The CERES (Clouds and the Earth’s Radiant Energy System) onboard Aqua and Terra has been providing measurements of outgoing shortwave and longwave fluxes since 2000. The analysis of more than 20 years of CERES TOA (top of the atmosphere) clear-sky outgoing radiation flux data revealed significant negative trends in reflected shortwave radiation (i.e., positive shortwave radiative forcing) over northwest India. Trends in surface reflectance, AOD (Aerosol Optical Depth), NDVI (Normalized Difference Vegetation Index), and land surface temperature observed by the MODIS during the same period over the same region have also been investigated to better understand the origin of these secular negative trends in the TOA reflected shortwave radiation flux. Surface reflectance and AOD are both decreasing, whereas NDVI is increasing over this time period. Surprisingly, despite positive shortwave radiative forcing, the land surface temperature significantly declined during the day and increased only marginally at night. The sensitivity study using the radiative transfer model indicates that TOA shortwave fluxes are more responsive to changes in surface reflectance than AOD changes. In recent years, various irrigation projects in northwest India have expanded agricultural practices, which have resulted in greater vegetation. Increased vegetation reduces surface reflectance and aerosol production, which in turn reduces TOA shortwave radiative flux. Furthermore, declining trends in daytime land surface temperature despite positive shortwave radiative forcing suggest that increasing greenery is one of the most efficient strategies to minimize global warming through evapotranspiration and photosynthesis processes. The study of regional trends in climatic variables is vital for understanding climate change and its quantification; hence, the results of this study will have a significant bearing on climate change impact studies.