Earth’s albedo—the fraction of incoming sunlight reflected back to space—is a fundamental regulator of the planet’s energy budget. It is well known that the Northern and Southern Hemispheres reflect nearly the same amount of sunlight despite their very different distributions of land and ocean, with clouds compensating for the surface contrast. In this work, we asked whether a similar balance exists in the East-West direction. Using 25 years (2001–2025) of CERES satellite observations, we discovered a unique and persistent East-West albedo symmetry along the 27°E (and 153°W) meridian, published in Nature.

The Science When Earth is split along 27°E, the Eastern and Western Hemispheres reflect nearly identical amounts of sunlight, and this balance holds throughout the satellite record. At this longitude, three components align simultaneously—a triple symmetry: (i) the two hemispheres contain nearly identical fractions of ice-free ocean, (ii) they reflect nearly identical amounts of sunlight under clear-sky conditions, and (iii) clouds contribute equally to the reflected sunlight in each hemisphere. Remarkably, the cloud balance emerges from very different cloud regimes: bright, low-level stratocumulus decks over the subtropical oceans dominate the Western Hemisphere reflection, while more extensive high clouds over Southeast Asia and the Indian Ocean dominate the Eastern Hemisphere.

Year-to-year departures from symmetry track the El Niño–Southern Oscillation (ENSO) through shifts in the Walker circulation: La Niña years favor greater Eastern Hemisphere reflection, while El Niño years favor the Western Hemisphere. Recent changes, such as the thinning of stratocumulus clouds and darkening of clouds over the Amazon, exert a slow pull toward asymmetry, although this trend remains statistically insignificant over the 25-year record. None of the eight state-of-the-art climate models examined reproduce the observed triple symmetry: while they capture the ocean fraction, they fail to simultaneously capture the symmetry in both clear-sky reflection and cloud radiative effects.

Why It Matters The East-West symmetry offers a powerful, reduced degree-of-freedom constraint for evaluating Earth system models and their representation of couplings among clouds, circulation, and the surface. It also advances our fundamental understanding of cloud feedbacks, ENSO, and Earth’s energy imbalance. Finally, it highlights that the impacts of solar radiation modification (SRM) could cascade through Earth system couplings—modifying clouds in one region may trigger responses elsewhere that offset or amplify the intended change.

Reference: J. Zhang, J. J. Gristey, and G. Feingold (2026): Earth’s East-West albedo symmetry. Nature, 654, 676–682. doi:10.1038/s41586-026-10624-2