Years ago, scientists noted something odd: Earth's 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 — more sunlight reflected than absorbed. The Southern Hemisphere is mostly ocean, which is darker and absorbs more sunlight.

New satellite data, however, suggest that symmetry is breaking.

In a new study a climate scientist at NASA’s Langley Research Center, and colleagues analyzed 24 years of observations from NASA’s Clouds and the Earth’s Radiant Energy System mission.

They found that the Northern Hemisphere is darkening faster than the Southern Hemisphere. In other words, it's absorbing more sunlight. That shift may alter weather patterns, rainfall, and the planet's overall climate in the decades ahead.

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's energy balance changed between 2001 and 2024. The energy balance tells scientists whether the planet is absorbing more energy than it releases and how that difference varies between hemispheres.

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.

The results pointed to three main reasons for the Northern Hemisphere darkening: melting snow and ice, declining air pollution, and rising water vapor.

Loeb is “excited about the new climate models coming out soon" and how they will further his work. "It'll be interesting to revisit this question with the latest and greatest models."

Source: PNAS

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