Planetary albedo and reflected shortwave flux: Basic characteristics, mechanisms of change and future projections

Abstract

The Earth's planetary albedo (PA) and reflected solar radiation (RSR) are critical for energy distribution and variability, significantly influencing the climate system and its response to climate change. This review presents an updated assessment of the characteristics, mechanisms, model simulations, and future projections of PA and RSR, providing valuable insights into their implications for the Earth's climate system. We summarize the trends and long-term variations in PA/RSR and their key drivers. Over the past two decades, global mean PA/RSR has significantly decreased, exacerbating Earth's energy imbalance, which is attributed to reduced low/mid-level cloud cover in tropical and subtropical oceans, retreating snow/ice cover at high latitudes, and reduced aerosol scattering at Northern Hemisphere mid-latitudes. Note that cloud fraction dominates RSR variations in most areas, but snow/ice coverage plays a larger role in polar coastal regions. Furthermore, we review the potential mechanisms that maintain hemispheric PA symmetry, highlighting the combined effects of tropical cloud movement and asymmetries in extratropical baroclinic activities. By applying the emergent constraint method and observations, we reduce the uncertainty of future projected RSR by 76 %. We find a significant decline in both global and hemispheric RSR this century, with trends slowing under low and medium emission scenarios, but accelerating under high emission scenarios. Finally, we emphasize future challenges in paleoclimatic radiation studies and the need for accurate long-term radiation data, and we suggest that strategies like emission reductions and reforestation may be vital for stabilizing Earth's PA on a long-term scale.