The bright side of Earth

Observed from space, the Earth’s northern and southern hemispheres appear equally bright. This symmetrical brightness is scientifically unexpected because the Southern Hemisphere is mostly covered with dark oceans, whereas the Northern Hemisphere has vast and much brighter land. In a new study, published in the Proceedings of the National Academy of Sciences, Weizmann researchers revealed a possible answer to this mystery: a strong correlation between storm intensity, cloudiness, and the solar energy reflection rate in each hemisphere. This correlation may also indicate how climate change could alter the reflection rate in the future.

Reflectivity of solar radiation is known in scientific lingo as “albedo.” To appreciate albedo, think about driving at night: It’s easy to spot the intermittent white lines, which reflect light from the car’s headlights well, but difficult to discern the dark asphalt. The same is true when observing Earth from space: The ratio of the solar energy hitting the Earth to the energy reflected by each region is determined by various factors, one of which is the ratio of dark oceans to bright land. The land area of the Northern Hemisphere is about twice as large as that of the Southern, and when measuring near the surface of the Earth, when the skies are clear, there is more than a 10% difference in albedo. Still, both hemispheres appear equally bright from space.

In this study, Prof. Yohai Kaspi, in the Department of Earth and Planetary Sciences, and Or Hadas, a grad student in his lab, focused on another factor influencing albedo, located in high altitudes and reflecting solar radiation—clouds. The Kaspi team, with colleagues in Germany, Sweden, and France, analyzed cloud data collected by NASA satellites, and global weather data from sources in the air and on the ground, dating back to 1950. The scientists classified storms from the last 50 years into three categories, according to intensity. They discovered a direct link between storm intensity and the number of clouds forming around the storm. While comparatively weak storms afflict the Northern Hemisphere and land areas in general, above the oceans of the Southern Hemisphere, moderate and strong storms prevail. Data analysis showed that the link between storm intensity and cloudiness accounts for the difference in albedo between the hemispheres.

Earth has been undergoing rapid changes in climate in recent years. To examine whether and how such changes could affect hemispheric albedo symmetry, the scientists used a set of models run by climate modeling centers around the world to simulate climate change.

The models predict that global warming will result in a decreased frequency of all storms above the Northern Hemisphere and of weak and moderate storms above the Southern. However, the strongest storms of the Southern Hemisphere will intensify. One might speculate that this difference should break hemispheric albedo symmetry; but studies show that a further increase in storm intensity might not change the degree of cloudiness in the Southern Hemisphere because cloud amounts reach saturation in very strong storms. Thus, symmetry might be preserved.

“This research solves a basic scientific question and deepens our understanding of Earth’s radiation balance and its effectors,” says Prof. Kaspi. “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.” 

Yohai Kaspi is supported by:
- Susanne and René Braginsky
- Helen Kimmel Center for Planetary Science