While the Southern Hemisphere is mostly covered with dark oceans, the Northern Hemisphere has a vast, much brighter land area. However, when looking at the Earth from space, its hemispheres – northern and southern – appear equally bright.
For years, the reasons for this remained a mystery until Weizmann Institute of Science researchers and their collaborators revealed a strong correlation between storm intensity, cloudiness, and the solar energy reflection rate in each hemisphere. They offer a solution to the mystery, alongside an assessment of how climate change might alter the reflectivity of solar radiation, known in scientific lingo as “albedo.”
The ratio of the solar energy hitting the Earth to the energy reflected by each region is determined by various factors. One of them is the ratio of dark oceans to bright land. The two elements differ in reflectivity, just like asphalt and intermittent white lines. When the skies are clear, there is more than a 10 percent difference in albedo. Still, both hemispheres appear to be equally bright from space.
In a recent study, researchers led by Prof. Yohai Kaspi and Or Hadas of Weizmann’s Earth and Planetary Sciences Department analyzed data derived from the world’s most advanced databases. This included cloud data collected via NASA satellites (CERES) as well as data from ERA5, a global weather database, which was utilized to complete cloud data and to cross-correlate 50 years of this data with information on the intensity of cyclones and anticyclones.
Next, the scientists classified storms of 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 the Northern Hemisphere and land areas in general are characterized by weaker storms, over the Southern Hemisphere’s oceans, moderate and strong storms are more prevalent. Data analysis showed that the link between storm intensity and cloudiness accounts for the difference in cloudiness between the hemispheres.
“This suggests that storms are the linking factor between the brightness of Earth’s surface and that of clouds, solving the symmetry mystery,” Prof. Hadas says.
The scientists used CMIP6, a set of models run by climate modeling centers around the world to simulate climate change. One of these models’ major shortcomings is their limited ability to predict the degree of cloudiness. Nevertheless, the relation found in this study between storm intensity and cloudiness enables scientists to assess future cloud amounts, based on storm predictions.
As global warming continues, geoengineered solutions will become vital for human life to carry on alongside it. “It is not yet possible to determine with certainty whether the symmetry will break in the face of global warming,” says Prof. Kaspi. “However, the new research solves a basic scientific question and deepens our understanding of Earth’s radiation balance and its effectors.”
