The breaking of hemispheric symmetry

The Earth became darker between 2001 and 2024 according to new satellite data. It also suggests an emerging hemispheric asymmetry in Earth's radiation budget, involving significant differences in solar radiation absorption and longwave radiation emission between the Northern Hemisphere (NH) and Southern Hemisphere (SH). The research is published in PNAS by a team led by Norman Loeb from NASA's Langley Research Center.

​This break in hemispheric symmetry challenges a long-held hypothesis that albedo symmetry is a fundamental property of Earth’s climate. ​It also raises questions about whether atmospheric circulation can adjust to restore the symmetry in the future.

Satellite observations from the Clouds and the Earth’s Radiant Energy System (CERES) indicate that both hemispheres are darkening, but the NH at a faster rate with a trend of 0.34 ± 0.23 W/m² per decade in absorbed solar radiation and a weaker trend of 0.21 ± 0.21 W/m² per decade in outgoing longwave radiation. ​It is attributed to changes in aerosol-radiation interactions, surface albedo, and water vapour. ​The NH has seen a decrease in pollution since 2000, which has cleared the air, while changes in sea-ice concentration and snow cover have further contributed to NH darkening. ​

It is well-known that the NH is warming faster than the SH, with a recorded increase of 0.16 ± 0.056°C per decade. ​In addition, tropical precipitation trends show that the NH tropics are getting wetter relative to the SH, possibly due to shifts in atmospheric circulation patterns. ​Overall, it suggests a relative decrease in precipitation and cloud cover in the SH tropics compared to the NH. ​

The increasing NH-SH difference over the past two decades raises questions about the mechanisms that minimise hemispheric differences in albedo. ​It suggests that if clouds do not compensate for the effects, we may be seeing limits in their ability to regulate hemispheric albedo symmetry. ​Future climate models may need to account for the potential increase in hemispheric albedo asymmetry.