Earth's east-west albedo symmetry

Earth's albedo is fundamental to the planetary energy budget¹. The Northern Hemisphere (NH) and Southern Hemisphere (SH) contribute essentially equally to the planetary albedo-a remarkable yet puzzling phenomenon known as hemispheric albedo symmetry^1-6. Although such symmetry is rare, it is not unique⁷. Nevertheless, other symmetry pairs have remained unexplored, despite their potential to illuminate possible causes of albedo symmetries and implications for the planetary energy budget. Using a 25-year satellite record, here we show that Earth also exhibits a unique and persistent east-west (E-W) albedo symmetry: the 27° E meridian divides the planet into an Eastern Hemisphere (EH) and a Western Hemisphere (WH) that reflect nearly identical amounts of sunlight. In contrast to the NH-SH symmetry, the EH-WH symmetry encapsulates a distinctive 'triple symmetry' in which clear-sky albedo, cloud radiative effect and open-ocean fraction all exhibit hemispheric symmetry around this meridian. This EH-WH symmetry arises from greater high-cloud reflection in the EH balancing greater low-cloud reflection in the WH. Furthermore, interannual variability in the EH-WH symmetry tracks the phase of the El Niño-Southern Oscillation (ENSO), indicating a potential connection to general circulation. This discovery of the EH-WH albedo symmetry and its emergence as a triple symmetry provides a reduced degree-of-freedom constraint for Earth system models (ESMs) and stresses the critical nature of continued Earth radiation budget observations under a rapidly changing climate.