New satellite observations reveal a subtle but significant change in Earth’s reflectivity, suggesting that the planet is absorbing more solar energy than before. This phenomenon, known as darkening, reflects an imbalance between the northern and southern hemispheres, with the north losing brightness more rapidly. According to a study published in Proceedings of the National Academy of Sciences (PNAS), the hemispheric symmetry once thought to characterise Earth’s albedo, the fraction of sunlight reflected by the planet, is breaking down. The implications extend beyond optical brightness, as this shift could influence global circulation patterns, atmospheric behaviour, and the long-term stability of Earth’s climate system.
What NASA discovered about the darkening of Earth
The study examined twenty-four years of satellite data from NASA’s Clouds and the Earth’s Radiant Energy System (CERES). Researchers measured absorbed solar radiation (ASR) and outgoing longwave radiation (OLR) across both hemispheres, finding that while both the northern hemisphere (NH) and southern hemisphere (SH) are darkening, the NH is doing so more rapidly. The NH–SH trend difference in absorbed solar radiation reached approximately 0.34 watts per square metre per decade, indicating that the northern half of the planet is retaining more solar energy.This growing contrast challenges the long-held assumption that hemispheric albedo symmetry is a fundamental and self-regulating property of Earth. The study found that while both hemispheres emit more longwave radiation as they warm, the NH shows stronger radiative cooling, a pattern offset by its even faster solar absorption. Together, these changes suggest that the natural energy exchange between hemispheres may be weakening, potentially altering the balance that underpins weather and ocean circulation systems.
How aerosols, clouds, and surface changes contribute to the darkening
The observed asymmetry is not due to a single factor but rather a complex interaction of atmospheric and surface processes. Using a partial radiative perturbation (PRP) analysis, the researchers attributed the hemispheric differences in solar absorption primarily to variations in aerosols, surface albedo, water vapour, and clouds. Over the past two decades, a steady decline in pollution levels over industrial regions such as China, the United States, and Europe has reduced reflective aerosols in the NH atmosphere. Meanwhile, large-scale natural events like the 2019–2020 Australian bushfires and the 2021–2022 Hunga Tonga eruption temporarily increased aerosol presence in the SH.These patterns explain the stronger positive contribution from aerosol–radiation interactions to the NH–SH difference. At the same time, the NH has experienced larger decreases in snow cover and sea ice, both of which naturally reflect sunlight. More exposed land and ocean surfaces mean greater heat absorption. Although clouds typically compensate for these imbalances by reflecting incoming radiation, their contribution here was unexpectedly weak. This suggests that cloud systems may no longer be fully offsetting hemispheric disparities caused by human and natural changes.
Where the hemispheric changes are most visible
The northern subtropics, between 20 and 42 degrees latitude, show the most pronounced darkening, with an estimated 0.51 watts per square metre per decade increase in absorbed solar radiation. This region, which includes parts of North Africa, southern Europe, and Asia, has become a focal point for radiative imbalance. The stronger radiative cooling observed in NH mid and high latitudes further highlights how warming and energy redistribution are geographically uneven.The study also detected patterns in precipitation and surface temperature that align with this trend. The NH is warming faster than the SH by about 0.16 degrees Celsius per decade, and rainfall patterns are shifting accordingly. There is an increasing trend in tropical precipitation in the NH relative to the SH, indicating that warmer northern regions are also becoming wetter. These hydrological and thermal imbalances support the idea that large-scale circulation systems, including the Intertropical Convergence Zone (ITCZ), are slowly migrating northward in response to asymmetric heating.
When atmospheric and oceanic circulation respond to energy imbalance
Earth’s radiation budget governs the transfer of heat between the atmosphere and the oceans. On average, the SH gains more radiative energy at the top of the atmosphere, while the NH typically experiences a net loss. This imbalance has historically been corrected by cross-equatorial energy transport through air and ocean currents. However, the current data indicate that the NH’s increasing absorption is narrowing this difference, suggesting that the compensatory circulation may be changing.The connection between radiative asymmetry and climate dynamics has long been explored in modelling studies. Earlier research showed that heating anomalies in one hemisphere can shift tropical rainfall patterns toward that side, reinforcing atmospheric feedback loops. The PNAS findings align with this view, implying that ongoing NH darkening may lead to structural shifts in global weather zones. The results also echo observed trends such as the poleward displacement of storm tracks and a gradual narrowing of the ITCZ, all signs that the planet’s circulation is adjusting to a new energy regime.
How these findings reshape understanding of Earth’s climate resilience
The discovery that the NH is darkening faster than the SH introduces uncertainty into assumptions about the stability of Earth’s climate feedbacks. Previously, scientists believed that cloud dynamics would naturally balance hemispheric differences by redistributing reflection and absorption. Yet, this study suggests limits to that self-correcting mechanism. The NH’s ongoing darkening, driven by reductions in reflective aerosols and ice alongside increased water vapour, appears insufficiently balanced by compensatory cloud behaviour.While climate models can estimate how far this asymmetry might grow, discrepancies between simulations remain large, leaving uncertainty about its long-term trajectory. If the trend continues, hemispheric contrasts in surface warming and albedo could intensify, influencing patterns of rainfall, storm activity, and oceanic heat flow. For now, the data underline the importance of sustained satellite monitoring to understand how Earth’s radiation budget evolves and whether the planet’s natural systems can restore equilibrium as human and environmental pressures accelerate.Also Read | A 480-million-year-old fossil changes what we know about the origin of parasites
