Although Earth’s two hemispheres lie on opposite sides of the planet and differ in many ways, they share a peculiar commonality—or at least they used to. The Northern and Southern Hemispheres reflected nearly the same amount of sunlight back into space. This balance, though long considered odd, is now coming undone, as new data reveals that one side of Earth is darkening faster than the other.

Using 24 years’ worth of data from NASA’s Clouds and the Earth’s Radiant Energy System (CERES) mission, a team of scientists discovered that the Northern Hemisphere is absorbing more sunlight than the Southern Hemisphere. This shift in Earth’s energy balance could have lasting impacts on weather patterns, rainfall, and the overall climate in the coming decades, a new study published in the Proceedings of the National Academy of Sciences suggests.

A delicate balance

Earth receives energy from the Sun and reflects part of it back to space. This reflectivity is known as albedo, a measure of the percentage of sunlight reflected by a surface. Throughout the year, the Southern and Northern hemispheres receive the same amount of energy from the Sun, with each one receiving more sunlight at different times of the year.

The Southern Hemisphere is dominated by oceans, which absorb more sunlight, while the Northern Hemisphere has more land and less sea ice, which absorbs heat faster and reflects less sunlight. In the early 2000s, satellite data revealed that the two hemispheres reflect the same amount of solar energy back into space. Scientists had expected the hemispheres to differ, but data showed that clouds in the Southern Hemisphere were slightly thicker and more reflective, therefore balancing out the darker land surfaces in the Northern Hemisphere.

The new study, however, suggests this once-balanced system is coming to an end. NASA’s CERES, launched in 1997, measures the amount of sunlight absorbed by Earth and the amount of infrared energy emitted to space. The team behind the study analyzed CERES data from 2001 to 2024 and found that the Northern Hemisphere is absorbing around 0.34 watt more solar energy per square meter per decade than the Southern Hemisphere.

The team’s analysis pointed to three main driving factors behind the emerging imbalance: melting snow and ice, declining air pollution, and rising water vapor.

“It made a lot of sense,” Norman Loeb, a climate scientist at NASA’s Langley Research Center and lead author of the new study, told Eos. “The Northern Hemisphere’s 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’s the opposite.”

Additionally, the Northern Hemisphere is warming faster and therefore holds more water vapor, which tends to absorb sunlight rather than reflect it. “That’s another reason the Northern Hemisphere is taking in more heat,” Loeb added.

The study notes that clouds naturally compensate for hemispheric asymmetry. As a result, there should be more cloud reflection in the Northern Hemisphere than the Southern Hemisphere. The data, however, shows no change in cloud cover thus far. “How clouds respond to this hemispheric imbalance has important implications for future climate,” the study reads.