Impact of vegetation greening on TOA clear-sky shortwave radiation in Northwest India.

Since 2000, Clouds and the Earth's Radiant Energy System (CERES) instruments on Aqua and Terra satellites have recorded top-of-atmosphere (TOA) clear-sky outgoing radiation fluxes. Over northwest India, analysis of these two decades of data revealed significant negative trends in reflected shortwave radiation, indicating positive shortwave radiative forcing. To understand the drivers of this trend, we investigate changes in the surface reflectance, aerosol optical depth (AOD), Normalized Difference Vegetation Index (NDVI), and land surface temperature from MODIS observations during the same period. We find decreasing trends in surface reflectance and AOD, while NDVI shows a pronounced increase. Notably, despite positive shortwave radiative forcing, daytime land surface temperature exhibits a significant decrease, with only a slight nighttime increase. Sensitivity analysis using a radiative transfer model identifies decrease in the surface reflectance as the primary contributor to the decrease in TOA shortwave fluxes. This suggests that the observed greening, potentially linked to expanded irrigation projects, has played a crucial role by reducing both surface reflectance and aerosol production. Our findings highlight the potential for increasing vegetation as a locally effective strategy to mitigate the effects of global warming through enhanced evapotranspiration process, even in the presence of positive shortwave radiative forcing. Investigating regional climate trends is vital for understanding and quantifying climate change; these results highlight the need for future climate change impact studies to consider interplay between vegetation dynamics and regional climate. [ABSTRACT FROM AUTHOR]