Jakarta –
Many years ago, scientists noticed something odd: the Northern and Southern Hemispheres reflect almost the same amount of sunlight back into space. The reason this symmetry is odd is because the Northern Hemisphere has more land, cities, pollution, and industrial aerosols.
All of this should lead to a higher albedo, that is, more sunlight is reflected than absorbed. The Southern Hemisphere is mostly ocean, which is darker and absorbs more sunlight. However, new satellite data shows the symmetry is being broken.
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In a new study published in the Proceedings of the National Academy of Sciences of the United States of America, Norman Loeb, 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 (CERES) mission.
They found that the Northern Hemisphere is darkening faster than the Southern Hemisphere. In other words, the Northern Hemisphere absorbs more sunlight. This shift could change weather patterns, rainfall and the planet’s overall climate in the coming decades.
Since 2000, CERES has recorded the amount of sunlight absorbed and reflected, as well as the amount of infrared (long wave) radiation that returns to space. Loeb used these measurements to analyze changes in Earth’s energy balance between 2001 and 2024. This energy balance tells scientists whether the planet absorbs more energy than it releases and how the difference varies between Earth’s hemispheres.
“Every object in the universe has a way of maintaining balance by receiving and releasing energy. That is 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 research.
“The Earth maintains balance by exchanging energy between the Sun and the long wave radiation emitted by the Earth,” he added.
The team 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 sound small, but across the planet, it’s a huge number,” Li said.
To find out the cause of this imbalance, 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 calculating how much sunlight each hemisphere absorbs.
The results show three main reasons why the Northern Hemisphere is darkening, melting snow and ice, decreasing air pollution and increasing water vapor.
“That makes perfect sense. The surface of the Northern Hemisphere is getting darker as snow and ice melt. That reveals the land and ocean beneath. And pollution has decreased in places like China, the US and Europe. This means that the amount of aerosols in the air that reflect sunlight is decreasing. In the Southern Hemisphere, it’s the opposite,” detailed Loeb.
“Because the Northern Hemisphere is warming faster, it also stores more water vapor. Water vapor does not reflect sunlight, but rather absorbs it. That is another reason why the Northern Hemisphere absorbs more heat,” continued Loeb.
Interesting Findings
One of the interesting findings of this research is what hasn’t changed over the past 20 years: cloud cover.
“Clouds were a puzzle to me because of their hemispherical symmetry. We wondered whether this was a fundamental property of the climate system. If so, clouds should compensate for this. We should see more cloud reflection in the Northern Hemisphere than in the Southern Hemisphere, but we didn’t,” said Loeb, who researched using models to understand these clouds.
“We are not sure about clouds. Understanding the interaction of aerosols and clouds is still a big challenge. Clouds remain the dominant factor governing our energy balance. This is very important,” Li added.
However, Li said that Dr. Norman Loeb shows that the asymmetry not only exists, but is important enough to worry about what is behind it.
(rns/rns)