Many years ago, scientists noticed something odd: Earth’s northern and southern hemispheres reflect roughly the same amount of sunlight into space. This symmetry is strange because the northern hemisphere has more land, cities, pollution, and industrial aerosols. All of this should lead to a higher albedo, meaning more sunlight is reflected than absorbed. Most of the southern hemisphere is ocean, and the ocean is darker and absorbs more sunlight.
But new satellite data suggests that the symmetry is breaking down.
From balance to imbalance
In a new study published in the Proceedings of the National Academy of Sciences, climate scientist Norman Loeb of NASA Langley Research Center and colleagues analyzed 24 years of observations from NASA’s Cloud and Earth Radiant Energy System (CERES) mission.
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They found that the Northern Hemisphere darkens faster than the Southern Hemisphere. This means they absorb more sunlight. This change could alter weather patterns, rainfall, and the global climate for decades to come.
Since 2000, CERES has recorded how much sunlight is absorbed and reflected, and how much infrared (longwave) radiation escapes into space. Loeb used these measurements to analyze how the Earth’s energy balance changed between 2001 and 2024. Energy balance tells scientists whether a planet is absorbing more energy than it emits, and how that difference varies between hemispheres.
“Every object in the universe has a way of maintaining equilibrium by receiving energy and releasing energy. That’s the basic law that governs everything in the universe,” said Zhanqing Li, a climate scientist at the University of Maryland who was not involved in the study. “The Earth maintains its equilibrium by exchanging energy between the sun and the long-wave radiation emitted by the Earth.”
The researchers found that the northern hemisphere absorbs about 0.34 watts more solar energy per square meter per decade than the southern hemisphere. “This difference may not seem like a big deal, but on a global scale, it’s a huge number,” Lee said.
To find out what is causing this imbalance, the scientists applied a technique called partial radiative perturbation (PRP) analysis. The PRP method separates the effects of factors such as clouds, aerosols, surface brightness, and water vapor from calculating how much sunlight each hemisphere absorbs.
They found that three main causes of the Northern Hemisphere’s darkening were melting snow and ice, reduced air pollution, and rising water vapor.
“It made a lot of sense,” Loeb said. “The surface of the Northern Hemisphere is darker because snow and ice is melting, exposing the land and oceans beneath. And places like China, the United States and Europe are seeing less pollution. This means there are fewer aerosols in the air that reflect sunlight. In the Southern Hemisphere, the opposite is true.”
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“The north is warming faster, so it’s holding more water vapor,” Loeb continued. “Water vapor absorbs sunlight rather than reflecting it. That’s another reason why the northern hemisphere absorbs more heat.”
Curiosity about cloud cover
One of the study’s interesting findings is something that hasn’t changed over the past 20 years: cloud cover.
“Clouds are a puzzle to me because of the symmetry of this hemisphere,” Loeb said. “We were wondering if this is a fundamental property of the climate system. If so, clouds should compensate for it. We should be seeing more cloud reflections in the Northern Hemisphere than in the Southern Hemisphere, but we weren’t seeing that.”
Loeb used models to understand these clouds.
“I’m not sure about the clouds,” Loeb said.
“Understanding the interaction between aerosols and clouds remains a major challenge,” Lee agreed. “Clouds remain a major factor regulating our energy balance,” he says. “That’s very important.”
Still, Lee said, “Dr. Norman Loeb’s research… [the asymmetry] It exists, but it’s important enough to worry about what’s behind it. ”
Loeb is “excited that new climate models are coming soon” and looks forward to how they will advance his research. “It would be interesting to revisit this question using the latest and greatest models.”
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