Satellite data suggest that the world is getting darker.
A new study published in the Proceedings of the National Academy of Sciences (PNAS) by Norman G. Loeb and colleagues reveals a concerning shift in Earth’s energy balance. Satellite observations over 24 years indicate an emerging hemispheric asymmetry in Earth’s radiation, with the Northern Hemisphere darkening at a faster rate than the Southern Hemisphere.
This finding challenges the traditional understanding of Earth’s albedo symmetry and raises important questions about the implications for the planet’s future climate.
The Emerging Asymmetry
Historically, it was believed that the Northern and Southern Hemispheres reflected similar amounts of solar radiation, with the Northern Hemisphere emitting more longwave radiation into space.
However, data from NASA’s Clouds and the Earth’s Radiant Energy System (CERES), collected between 2001 and 2024, show that although both hemispheres are darkening (i.e., absorbing more solar radiation), the Northern Hemisphere is doing so at a significantly higher rate.
The trend difference between the Northern and Southern Hemispheres in absorbed solar radiation (ASR) is 0.34 ± 0.23 Wm⁻² per decade. This means the Northern Hemisphere is absorbing more solar energy compared to the Southern Hemisphere. Although there is also a difference in outgoing longwave radiation (OLR), with the Northern Hemisphere cooling more rapidly, the difference in ASR is more pronounced — resulting in a net energy imbalance.
Causes and Attributions
The study attributes this emerging asymmetry to a combination of factors:
- Aerosol–radiation interactions: The reduction in aerosol pollution in the Northern Hemisphere since 2000 (due to pollution control efforts in China, the United States, and Europe) contributes to darkening, as fewer aerosols mean less sunlight is reflected back into space.
- Surface albedo: Changes in surface albedo, such as the decrease in sea ice concentration and snow cover in the Northern Hemisphere, also contribute to the hemisphere absorbing more solar radiation.
- Water vapor: Increases in atmospheric humidity in the Northern Hemisphere also play a role in the darkening.
- Clouds: Although clouds are a complex factor, the study suggests that changes in cloud cover contribute to a greater hemispheric contrast in ASR, but their magnitude is small due to opposing trends in tropical and extratropical regions. The hypothesis that clouds naturally compensate for forced hemispheric asymmetries in non-cloud properties is challenged by these new data.
Implications for Future Climate
The breakdown in hemispheric albedo symmetry raises crucial questions about how the coupled atmosphere-ocean circulation will adjust to produce a cloud distribution that restores hemispheric albedo symmetry in the future.
The implications include:
- Uneven warming: The fact that the Northern Hemisphere is absorbing more energy means that global warming may not occur uniformly, with more pronounced impacts in regions such as North America and Europe.
- Atmospheric and oceanic circulation: The radiation asymmetry is intrinsically linked to the general atmosphere-ocean circulation. Changes in this balance could affect global climate patterns, such as the Intertropical Convergence Zone (ITCZ).
- Climate modeling: Current climate models may need to be revised to incorporate this emerging asymmetry in order to more accurately predict future climate changes.
The observations by Loeb and his team provide clear evidence of an emerging hemispheric asymmetry in Earth’s radiation, with the Northern Hemisphere darkening at an accelerated pace.
This discovery not only deepens our understanding of Earth’s energy balance but also underscores the need to reassess how climate systems respond to regional imbalances.
How clouds and atmospheric circulation will adapt to this new reality remains an open question — and one of vital importance for future climate projections.
🛰️ Reference:
Loeb, N. G., Thorsen, T. J., Kato, S., Rose, F. G., Hodnebrog, Ø., & Myhre, G. (2025). Emerging hemispheric asymmetry of Earth’s radiation. Proceedings of the National Academy of Sciences, 122(40), e2511595122. Available at: doi/10.1073/pnas.2511595122
