The mystery of Earth’s Uniform Shine may finally have a solution: ScienceAlert

Take a look at Earth far away in space, you will notice that it looks uniformly bright. Given that the Southern Hemisphere’s disproportionate expanse of dark ocean water should reflect less sunlight back, it’s not what we’d expect down here on the surface.

It’s a mystery that has intrigued scientists since the early 1970s, when we were first able to get satellites into orbit to measure reflected solar radiation, which is technically known as albedo. Now that mystery may have been solved.

The explanation is that while the oceans in the Southern Hemisphere absorb more sunlight, they also produce a higher number of storms – and the resulting storm clouds act as reflectors that offset the solar radiation sent back to space.

“The new research addresses a fundamental scientific question and deepens our understanding of the Earth’s radiation balance and its effectors,” said Yohai Kaspi, a geophysicist at the Weizmann Institute of Science in Israel.

While all sorts of factors affect our planet’s albedo – whether or not there is snow on the ground, for example – measurements closer to the Earth’s surface show a difference in albedo between the hemispheres.

However, this is not the case when looking at the planet from further outside.

The team collected data from various reports, including those from the NASA Terra satellite and the global weather dataset ERA5, which also incorporates satellite readings. They then cross-referenced 50 years of cloud cover data against information on storm location and intensity.

Based on the patterns of cloud-generating cyclones and cloud-suppressing anticyclones formed by the interaction of temperature and pressure in the atmosphere, the researchers were able to show a balancing out of the Earth’s albedo.

Cloud cover was analyzed as part of the research. (NASA)

“Sky albedo originating from strong storms over the Southern Hemisphere was found to be a high-precision compensator for the large land area in the Northern Hemisphere, thus preserving the symmetry,” says climate scientist Or Hadas of the Weizmann Institute of Science.

“This suggests that storms are the connecting factor between the brightness of the Earth’s surface and the brightness of the clouds, solving the symmetry mystery.”

The next question is how global warming might affect this balance. Based on climate models, it is likely that as the planet warms, the Northern Hemisphere will have fewer storms overall, while the Southern Hemisphere will have fewer weaker and moderate storms.

However, the number of severe storms in the Southern Hemisphere is expected to increase due to ‘Arctic strengthening’; spillover effects of faster warming at the North Pole. Theoretically, it should unbalance the albedo symmetry.

According to the researchers’ data, the more severe weather events may not contribute to more cloud albedo over the Southern Hemisphere, as cloud levels reach saturation in strong storms. A Southern Hemisphere bias toward stronger storms with fewer storms overall could mean a similar decrease in albedo over both hemispheres, maintaining symmetry.

At this stage, however, it is difficult to predict exactly what will happen to the Earth’s overall brightness.

“It is not yet possible to determine with certainty whether the symmetry will break in light of global warming,” says Kaspi.

“As global warming continues, geotechnical solutions will become essential for human life to continue alongside it. I hope that a better understanding of fundamental climate phenomena, such as hemispheric albedosymmetry, will help develop these solutions.”

The research is published in PNAS.

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