Years ago, scientists noted something odd: Earth\u2019s Northern and Southern Hemispheres reflect nearly the same amount of sunlight back into space. The reason why this symmetry is odd is because the Northern Hemisphere has more land, cities, pollution, and industrial aerosols. All those things should lead to a higher albedo\u2014more sunlight reflected than absorbed. The Southern Hemisphere is mostly ocean, which is darker and absorbs more sunlight.<\/p>\n
New satellite data, however, suggest that symmetry is breaking.<\/p>\n
From Balance to Imbalance<\/p>\n
In a new study<\/a> published in the Proceedings of the National Academy of Sciences of the United States of America, Norman Loeb<\/a>, a climate scientist at NASA\u2019s Langley Research Center, and colleagues analyzed 24 years of observations from NASA\u2019s Clouds and the Earth\u2019s Radiant Energy System (CERES)<\/a> mission.<\/p>\n They found that the Northern Hemisphere is darkening faster than the Southern Hemisphere. In other words, it\u2019s absorbing more sunlight. That shift may alter weather patterns, rainfall, and the planet\u2019s overall climate in the decades ahead.<\/p>\n Since 2000, CERES has recorded how much sunlight is absorbed and reflected, as well as how much infrared (longwave) radiation escapes back to space. Loeb used these measurements to analyze how Earth\u2019s energy balance changed between 2001 and 2024. The energy balance<\/a> tells scientists whether the planet is absorbing more energy than it releases and how that difference varies between hemispheres.<\/p>\n \u201cAny object in the universe has a way to maintain equilibrium by receiving energy and giving off energy. That\u2019s the fundamental law governing everything in the universe,\u201d said Zhanqing Li<\/a>, a climate scientist at the University of Maryland who was not part of the study. \u201cThe Earth maintains equilibrium by exchanging energy between the Sun and the Earth\u2019s emitted longwave radiation.\u201d<\/p>\n The team found that the Northern Hemisphere is absorbing about 0.34 watt more solar energy per square meter per decade than the Southern Hemisphere. \u201cThis difference doesn\u2019t sound like much, but over the whole planet, that\u2019s a huge number,\u201d said Li.<\/p>\n Results pointed to three main reasons for the Northern Hemisphere darkening: melting snow and ice, declining air pollution, and rising water vapor.<\/p>\n To figure out what was driving this imbalance, the scientists applied a technique called partial radiative perturbation (PRP) analysis. The PRP method separates the influence of factors such as clouds, aerosols, surface brightness, and water vapor from calculations of how much sunlight each hemisphere absorbs.<\/p>\n The results pointed to three main reasons for the Northern Hemisphere darkening: melting snow and ice, declining air pollution, and rising water vapor.<\/p>\n \u201cIt made a lot of sense,\u201d Loeb said. \u201cThe Northern Hemisphere\u2019s surface is getting darker because snow and ice are melting. That exposes the land and ocean underneath. And pollution has gone down in places like China, the U.S., and Europe. It means there are fewer aerosols in the air to reflect sunlight. In the Southern Hemisphere, it\u2019s the opposite.\u201d<\/p>\n \u201cBecause the north is warming faster, it also holds more water vapor,\u201d Loeb continued. \u201cWater vapor doesn\u2019t reflect sunlight, it absorbs it. That\u2019s another reason the Northern Hemisphere is taking in more heat.\u201d<\/p>\n Curiosity About Cloud Cover<\/p>\n One of the study\u2019s interesting findings is what didn\u2019t change over the past 20 years: cloud cover.<\/p>\n \u201cThe clouds are a puzzle to me because of this hemispheric symmetry,\u201d Loeb said. \u201cWe kind of questioned whether this was a fundamental property of the climate system. If it were, the clouds should compensate. You should see more cloud reflection in the Northern Hemisphere relative to the Southern Hemisphere, but we weren\u2019t seeing that.\u201d<\/p>\n Loeb worked with models to understand these clouds.<\/p>\n \u201cWe are unsure about the clouds,\u201d said Loeb.<\/p>\n \u201cUnderstanding aerosol and cloud interactions is still a major challenge,\u201d agreed Li. \u201cClouds remain the dominant factor adjusting our energy balance,\u201d he said. \u201cIt\u2019s very important.\u201d<\/p>\n Still, Li said that \u201cDr. Norman Loeb\u2019s study shows that not only does [the asymmetry] exist, but it\u2019s important enough to worry about what\u2019s behind it.\u201d<\/p>\n Loeb is \u201cexcited about the new climate models coming out soon\u201d and how they will further his work. \u201cIt\u2019ll be interesting to revisit this question with the latest and greatest models.\u201d<\/p>\n \u2014Larissa G. Capella (@CapellaLarissa)<\/a>, Science Writer<\/p>\n Citation:\u00a0Capella, L. G. (2025), New satellite data reveal a shift in Earth\u2019s once-balanced energy system,\u00a0Eos, 106, https:\/\/doi.org\/10.1029\/2025EO250399<\/a>. Published on 23 October 2025.<\/p>\n