Testing Cloud Adjustment Hypotheses for the Maintenance of Earth's Hemispheric Albedo Symmetry With Natural Experiments.

Earth's Northern and Southern Hemispheres reflect essentially equal amounts of sunlight. How—and whether—this hemispheric albedo symmetry is maintained remains a mystery. We decompose Earth's hemispheric albedo symmetry into components associated with the surface, clear‐sky atmosphere, and different cloud types as defined by cloud effective pressure and optical thickness. Climatologically, greater reflection by the surface, aerosols, and high clouds in the Northern Hemisphere is balanced by greater low and mid‐level cloudiness in the Southern Hemisphere. Both hemispheres have darkened at similar rates over the past two decades; whether the darkening from more rapidly declining aerosol in the Northern Hemisphere is causing a departure from all‐sky symmetry remains uncertain. Natural experiments including long‐term trends, sea ice loss, and volcanic eruptions provide strong evidence against the hypothesis that extratropical low clouds compensate changing clear‐sky asymmetries on annual‐to‐decadal timescales but some evidence that tropical high cloud shifts may do so. Plain Language Summary: Mysteriously, the Northern and Southern Hemispheres reflect the same amount of sunlight as each other, but scientists are not yet sure why, how, or even whether this phenomenon is sustained by the Earth system. The Northern Hemisphere is brighter in clear skies because it contains more pollution particles in the atmosphere and has more land area, whereas the Southern Hemisphere is cloudier. We break down this cloudiness contrast into components related to different cloud types defined by their height and thickness. Tropical high‐altitude clouds increase reflection preferentially in the Northern Hemisphere but are overcompensated by low‐ and mid‐level clouds in the Southern Hemisphere, especially in the subtropics and midlatitudes. Both hemispheres have darkened over the past two decades, but whether the Northern Hemisphere is darkening faster than the Southern Hemisphere due to decreasing particulate pollution or if they are darkening at the exact same rate remains uncertain. Based on long‐term trends and "natural experiments" like sea ice loss and volcanic eruptions, we can rule out the hypothesis that low‐level clouds in the Southern Ocean act to balance out clear‐sky asymmetries at yearly‐to‐decadal timescales, but we cannot rule out the hypothesis that high‐altitude tropical clouds do so. Key Points: Greater reflection from Northern Hemisphere clear‐skies and high clouds is balanced by Southern Hemisphere low and mid‐level cloudsEvidence points against extratropical low cloud adjustments compensating for clear‐sky albedo changes on decadal timescalesLimited and conflicting evidence for whether shifts in tropical high clouds compensate clear‐sky changes at annual to decadal timescales [ABSTRACT FROM AUTHOR]