Earth’s east–west albedo symmetry

Article Earth’s east–west albedo symmetry https://doi.org/10.1038/s41586-026-10624-2 Jianhao Zhang1,2 ✉, Jake J. Gristey1,2,3 & Graham Feingold2 Received: 25 November 2025 Accepted: 5 May 2026 Published online: xx xx xxxx Open access Check for updates 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 symmetry1–6. 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. The Earth reflects about 29% of the incoming solar radiation back to space, an intrinsic property of a planet known as the planetary albedo1. An intriguing observation has emerged since spaceborne monitoring of Earth’s radiative fluxes became available half a century ago: the NH and SH reflect almost identical amounts of sunlight1–4,7–9. Although this symmetry is known to arise from a cloudier SH balancing more reflective clear skies in the NH1,5,8,10, the lack of satisfactory mechanistic explanations for how the north–south (N–S) symmetry is maintained leaves open the possibility of a coincidence, and the N–S symmetry remains a mystery6,8. This possibility is further fuelled by the fact that observations may now suggest a departure from N–S symmetry11–13. Given that the Earth is approximately spherical, it is unsurprising that one can divide it into two non-overlapping hemispheres that reflect equal amounts of sunlight. However, the chance of achieving such a hemispheric symmetry within 0.1 W m−2 is less than 3% (ref. 7), making the hemispheric symmetry unlikely to be coincidental. An unexplored question is what we can learn from these rare hemispheric pairs in which symmetry holds, insights that could be fundamental to understanding the Earth’s climate system. Similarities or differences across symmetry pairs could shed light on the underlying couplings among Earth system components. This work presents the first investigation of the Earth’s hemispheric albedo symmetry as a function of longitude. We divide the planet into EHs and WHs at every longitudinal great circle, with one-degree increments, and identify 27° E (with 153° W completing the great circle) as the unique meridian that produces an E–W albedo symmetry using 25 years of satellite-measured top-of-atmosphere (TOA) shortwave (SW) fluxes. The finding of the E–W symmetry at 27° E is notable for its persistence and the fact that it represents the only longitudinal divide yielding such balance. Moreover, it coincides with the meridian that separates the Earth into two hemispheres containing almost the same amounts of ice-free ocean. Put differently, the ratio of land to ocean correlates strongly with the reflected solar flux in the two hemispheres, a feature in strong contrast to the N–S symmetry. In a hypothetically cloud-free world, this correspondence might not be surprising, given the contrast in surface reflectivity between land and ice-free ocean. However, for the cloudy planet we inhabit, the persistence of this symmetry becomes nontrivial, underscoring the need to understand the role of clouds in maintaining the observed E–W symmetry. Using more than two decades of Clouds and the Earth’s Radiant Energy System (CERES) observations14–17, we show that the E–W symmetry at 27° E arises from a high-cloud-dominated EH balancing a low-cloud-dominated WH, and from an EH with more reflective oceanic and ice-covered regions balancing a WH with more reflective ice-free continental regions. This E–W symmetry, coinciding with an even divide of hemispheric ice-free-ocean fractions and a near symmetry in cloud radiative effect (CRE)—a triple symmetry—is not captured by climate models. We find a strong correlation between the interannual variability of the E–W hemispheric albedo symmetry and the phase of ENSO, suggesting that large-scale atmospheric circulation, particularly the Walker circulation, may play an important role in maintaining this symmetry. 1 Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado Boulder, Boulder, CO, USA. 2Chemical Sciences Laboratory, National Oceanic and Atmospheric Administration (NOAA), Boulder, CO, USA. 3Laboratory for Atmospheric and Space Physics (LASP), University of Colorado Boulder, Boulder, CO, USA. ✉e-mail: Nature | www.nature.com | 1 25 23 20 21 20 19 20 17 15 20 13 20 11 20 09 20 07 20 05 20 03 20 01 20 20 25 20 23 21 20 Longitude (°) −2 19 180 20 90 17 27 20 10 20 1 15 −0.20 −10.0 −1 20 27° E 0 13 −0.15 ΔR 20 −7.5 N–S symmetry (equator) E–W symmetry (27° E) 1 11 ΔCRE Starting year 20 −0.10 ΔRclr 2 09 −5.0 c 20 −0.05 Symmetry longitude (° E) 07 −2.5 20.0 26 27 28 20 0 05 0 22.5 03 0.05 25.0 20 2.5 27.5 01 0.10 32.5 30.0 20 5.0 Median 20 0.15 Consecutive years averaged 7.5 Mean 12 11 10 9 8 7 6 5 4 3 2 1 20 b 0.20 ΔR (W m−2) 10.0 ΔO (EH minus WH) EH minus WH (W m−2) a Symmetry longitude (° E) Article Year Fig. 1 | Demonstration of Earth’s E–W hemispheric albedo symmetry and its persistence. a, 25-year mean differences in reflected SW radiation based on CERES EBAF51 as a function of longitude and coloured by the difference in ice-free-ocean fraction (ΔO ) between the EH and the WH (EH minus WH). All-sky reflection difference (ΔR ) is shown by circles, clear-sky reflection difference (Δ R clr ) is shown by upward triangles and CRE difference (ΔCRE) is shown by downward triangles. Values at 27° E are highlighted with black outlines. An approximate triple symmetry at 27° E, for O , R clr and CRE, is a unique feature of the E–W symmetry (...truncated)