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Abstract

While the concentration of landmasses and atmospheric aerosols on the NorthernHemisphere suggests that the
Northern Hemisphere is brighter than the Southern Hemisphere, satellite measurements of top-of-atmosphere
irradiances found that both hemispheres reflect nearly the same amount of shortwave irradiance.Here, the authors
document that the most precise and accurate observation, the energy balanced and filled dataset of the Clouds
and the Earth’sRadiant Energy System covering the period 2000–10,measures an absolute hemispheric difference
in reflected shortwave irradiance of 0.1 W m22. In contrast, the longwave irradiance of the two hemispheres
differs by more than 1 W m22, indicating that the observed climate system exhibits hemispheric symmetry in
reflected shortwave irradiance but not in longwave irradiance. The authors devise a variety of methods to estimate
the spatial degrees of freedom of the time-mean reflected shortwave irradiance. These are used to show that the
hemispheric symmetry in reflected shortwave irradiance is a nontrivial property of the Earth system in the sense
that most partitionings of Earth into two random halves do not exhibit hemispheric symmetry in reflected
shortwave irradiance. Climate models generally do not reproduce the observed hemispheric symmetry, which the
authors interpret as further evidence that the symmetry is nontrivial. While the authors cannot rule out that the
observed hemispheric symmetry in reflected shortwave irradiance is accidental, their results motivate a search for
mechanisms that minimize hemispheric differences in reflected shortwave irradiance and planetary albedo