When viewed from space, Earth's northern and southern halves appear equally bright. This is particularly surprising since the Southern Hemisphere is mostly covered by dark oceans, while large parts of the Northern Hemisphere are covered with bright areas of dry land. For years, the reason for this symmetry between the halves has remained a mystery. In a new study, published this week in the scientific journal Proceedings of the National Academy of Sciences of the United States of America (PNAS), Weizmann Institute scientists and their colleagues reveal the relationship between storm intensity and cloud rates and the degree of reflectivity of solar energy in each hemisphere. In this they are offering a solution to the mystery, and even estimate how future radiation reflections may change due to climate change.
As early as the 1970’s, when scientists analyzed the data of the first meteorological satellites, they recognized that surprisingly both hemispheres reflect the same amount of solar radiation; The reflectivity is referred to in professional language as "albedo". To understand better what an albedo is, think about driving at night: how easy it is to spot the white dividing line, which reflects the light from the headlights well, but it is difficult to see the dark asphalt. The same is true when looking at the Earth from space – the ratio between the sun's energy that hits each region of the Earth and that reflected from that region depends on various factors, including the ratio between dark oceans and light land, which, just like asphalt and the dividing line, differ in their degree of reflection of light. In the Northern Hemisphere, the land area is about twice as large as in the Southern Hemisphere, and indeed, when measured near the surface in clear skies, there are differences of more than 10% in the albedo. However, at the top of the atmosphere, the hemispheres appear equally bright.
In the new study, led by Prof. Yohai Kaspi and research student Or Hadas from the Institute's Department of Earth and Planetary Sciences, the scientists focused on a high-altitude factor that reflects radiation from the sun – clouds. The scientists analyzed data from some of the most advanced databases in the world, including cloud cover data collected by NASA satellites (CERES) and data from SERA, a global weather database with information collected from a variety of air and ground sources starting from 1950. Information in the SERA database has enabled the completion of cloud cover data and the cross-reference of 50 years of that data with equivalent information on storm intensity.
The scientists classified the storms that occurred during these years into three groups according to their intensity and revealed a direct link between the intensity of the storms and the quantity of clouds formed around them. They observed that land areas, and the Northern Hemisphere as a whole, are characterized by weaker storms – while moderate and severe storms are more common in the Southern Hemisphere and over the oceans. The analysis of the data revealed that the relationship between the quantity of clouds and the intensity of storms provides a good explanation for the differences in cloud cover between the two hemispheres. "The cloud albedo as a result of the severe storms in the southern hemisphere was found to balance, with a high degree of accuracy, the albedo originating from the large land area in the northern hemisphere," says Hadas, adding: "This proves that storms are the link between surface brightness and clouds, thus maintaining the albedo symmetry between the two hemispheres."
Will climate change make one hemisphere darker?
In recent years, planet Earth has been changing rapidly. To test whether and how climate change will affect albedo symmetry, the researchers used CMIP6, a set of models run by climate research centers around the world in an attempt to predict climate change. One of the main drawbacks of these models is their limited ability to provide predictions about the degree of cloud cover. However, the link found in the study between the intensity of storms and cloud cover makes it possible to infer the degree of cloud cover from the storm forecast.
According to these models, climate change will lead to a decrease in the frequency of all storms in the Northern Hemisphere and of weak and moderate storms in the Southern Hemisphere. However, the severe storms in the southern hemisphere will intensify. The reason for these predicted differences is "Arctic amplification" in the Northern Hemisphere, a phenomenon in which the Arctic warms at least twice as fast as the Earth’s average. Ostensibly, one would expect this difference to break the symmetry in albedo, but the findings of the study indicate that in very severe storms the quantity of clouds reaches saturation. Therefore, a further increase in their intensity will not change the degree of cloud cover, and symmetry may be maintained.
"It is not yet possible to determine with certainty whether the albedo symmetry will be broken in the face of global warming," says Prof. Kaspi, "However, the new study answers a fundamental question and expands our understanding of the Earth's radiation balance and the factors that influence it. In dealing with climate change, it will be necessary to develop geoengineering solutions that will enable us to live alongside it. I really hope that understanding fundamental climatic phenomena, such as albedo symmetry, will help develop these solutions."
Dr. George Datseris and Prof. Björn Stevens of the Max Planck Institute of Meteorology in Hamburg, Dr. Joaquín Blanco and Prof. Rodrigo Caballero of Stockholm University and Dr. Sandrine Bonnet of the Sorbonne University in Paris also participated in the study.