Why are Earth's Hemispheres the Same Brightness? New Research Solves a 50-year-old Mystery.

NASA’s Apollo program most notably explored the Moon. But it also helped us study the Earth as well, as it provided some of the first high-resolution images of our whole planet, like the famous “Blue Marble” photo taken by the Apollo 17 astronauts.

However, these full-Earth photos revealed a mystery.  Scientists expected that Earth’s two hemispheres, the north and south, would have different albedos, a difference in the amount of light they reflect. This is because Earth’s northern and southern hemispheres of Earth are quite different from each other. The southern hemisphere is mostly covered with dark oceans, while the northern hemisphere contains vast land areas that are much brighter than the oceans

Yet, when observing Earth from space, the two hemispheres appear equally bright.

This symmetry in brightness has been a puzzle for over 50 years. But now, a new study shows that the albedos are roughly the same because of the increased clouds and storms in the southern hemisphere.

“Cloud albedo arising from strong storms above the Southern Hemisphere was found to be a high-precision offsetting agent to the large land area in the Northern Hemisphere, and thus symmetry is preserved,” said Or Hadas of the Weizmann Institute’s Earth and Planetary Sciences Department in Canada.  “This suggests that storms are the linking factor between the brightness of Earth’s surface and that of clouds, solving the symmetry mystery.”

Global cloudiness map, based on data collected by the Aqua research satellite over more than a decade (2002-2015). Clouds are not distributed uniformly but rather concentrated in hot spots. Photo: NASA

For their study, Hadas and co-author Yohai Kaspi analyzed data that included cloud data collected from NASA’s CERES program, The Clouds and the Earth’s Radiant Energy System which gathers data from several satellites that provide observations of clouds and the amount of radiant energy Earth reflects. They also used data from ERA5, which is a global weather database containing information collected using a variety of sources from both the air and on the ground, dating back to 1950.

From this data they used the cloud data to cross-correlate with information on the intensity of cyclones and anticyclones. They discovered a direct link between storm intensity and the number of clouds forming around the storm. The northern hemisphere generally has weaker storms above oceans while the southern hemisphere had strong to moderate storms.

Their data analysis showed that the link between storm intensity accounts for the difference in cloudiness between the two hemispheres, even though the land area of the northern hemisphere is about twice as large as that of the southern hemisphere. Hence, the increased cloudiness in the southern hemisphere accounts for the similarity in albedo.

In addition, the team’s research provided an assessment of how climate change might alter the reflection rate in the future. Models predict that global warming will change the frequency of storms in both the north and south, and some have suggested that we could one day find a gap in reflectivity between the two hemispheres.

But this research shows a lot of uncertainly in this line of thinking.

“It is not yet possible to determine with certainty whether the symmetry will break in the face of global warming,” said Kaspi, in a press release. “However, the new research solves a basic scientific question and deepens our understanding of Earth’s radiation balance and its effectors. As global warming continues, geoengineered solutions will become vital for human life to carry on alongside it. I hope that a better understanding of basic climate phenomena, such as the hemispheric albedo symmetry, will help in developing these solutions.”

This study was published in the Proceedings of the National Academy of Sciences.