Years ago, scientists noticed something strange: Earth’s northern and southern hemispheres reflect the same amount of sunlight into space. The reason this balance is odd is that the Northern Hemisphere has more land, cities, pollution and industrial aerosols. All of these things should lead to a higher albedo – more sunlight is reflected than absorbed. The southern hemisphere is mostly ocean, which is deeper and absorbs more sunlight.
New satellite data, however, suggests the balance is tipping.
From equilibrium to disequilibrium
In a new study published in Proceedings of the National Academy of Sciences of the United States of Americaclimate scientist Norman Loeb of NASA’s Langley Research Center, and colleagues analyzed 24 years of observations from NASA’s Clouds and Earth’s Radiant Energy System (CERES) mission.
They found that the Northern Hemisphere is darkening faster than the Southern Hemisphere. In other words, it is absorbing more sunlight. This change can lead to changes in weather patterns, precipitation and the overall climate of the planet over decades.
Since 2000, Ceres has recorded how much sunlight is absorbed and reflected, as well as how much infrared (longwave) radiation is returned to space. LOEB used these measurements to analyze how the Earth’s energy balance changed between 2001 and 2024. The energy balance tells scientists whether a planet is absorbing more energy than it is emitting and how this difference varies between orbits.
“Anything in the universe has a way of gaining energy and maintaining a balance by giving up energy. This is the fundamental law governing everything in the universe,” said Xangqing Li, a climate scientist at the University of Maryland. “The Earth maintains a balance by exchanging energy between the Sun and the longwave radiation emitted by the Earth.”
The team found that the Northern Hemisphere is absorbing 0.34 watts more solar energy per square meter per decade than the Southern Hemisphere. “It’s not like much of a difference, but across the planet, it’s a huge number,” Lee said.
The findings pointed to three main causes of Northern Hemisphere darkening: melting ice and snow, reduced air pollution, and increased water evaporation.
To find out what’s driving this imbalance, the scientists applied a technique called partial radiative perturbation (PRP) analysis. The PRP method separates the effect of factors such as clouds, aerosols, surface brightness, and water vapor to account for each hemisphere’s absorption.
The findings pointed to three main causes of Northern Hemisphere darkening: melting ice and snow, reduced air pollution, and increased water evaporation.
“It makes a lot of sense,” Loeb said. which exposes the land and sea below. And pollution has decreased in places like China, America and Europe. This means there are fewer aerosols in the air to reflect sunlight. In the Southern Hemisphere, it’s the opposite.”
“Because the north is warming faster, it also has more water vapor.” “Water vapor doesn’t reflect sunlight, it absorbs it. That’s another reason why the northern hemisphere is getting more heat.”
Curious about cloud cover
One of the interesting findings of the study is did not Change in the last 20 years: Cloud cover.
“Clouds are a puzzle to me because of this hemispheric balance,” Loeb said. “We kind of questioned whether this is a fundamental property of the climate system.” If it were, the clouds should compensate. You should see more cloud reflection in the Northern Hemisphere than in the Southern Hemisphere, but we weren’t seeing that.”
Loeb worked with models to understand these clouds.
“We don’t believe in clouds,” Loeb said.
“Understanding aerosol-cloud interactions is still a huge challenge,” Lee agreed. “Clouds continue to be the dominant factor in adjusting our energy balance,” he said. “That’s very important.”
Still, Lee said, “Dr. Norman Loeb’s study shows that not only does this happen [the asymmetry] There is, but it’s important enough to worry about what’s behind it.
Loeb is “excited about new climate models coming out soon” and how they will advance his work. “It would be interesting to revisit this question with the latest and greatest models.”
alar Larissa G. Capela (@capellalarissa), science writer
