Earth’s Albedo and Its Symmetry
Creators: name: Datseris, George
name: Stevens, Bjorn
Publisher: name: FID GEO
Year: 2021

description: The properties of Earth's albedo and its symmetries are analyzed using twenty years of space‐based Energy Balanced And Filled product of Clouds and the Earth's Radiant Energy System measurements. Despite surface asymmetries, top of the atmosphere temporally &amp; hemispherically averaged reflected solar irradiance R appears symmetric over Northern/Southern hemispheres. This is confirmed with the use of surrogate time‐series, which provides margins of 0.1±0.28Wm−2 for possible hemispheric differences supported by Clouds and Earth's Radiant System data. R time‐series are further analyzed by decomposition into a seasonal (yearly and half yearly) cycle and residuals. Variability in the reflected solar irradiance is almost entirely (99%) due to the seasonal variations, mostly due to seasonal variations in insolation. The residuals of hemispherically averaged R are not only small, but also indistinguishable from noise, and thus not correlated across hemispheres. This makes yearly and sub‐yearly timescales unlikely as the basis for a symmetry‐establishing mechanism. The residuals however contain a global trend that is large, as compared to expected albedo feedbacks. It is also hemispherically symmetric, and thus indicates the possibility of a symmetry enforcing mechanism at longer timescales. To pinpoint precisely which parts of the Earth system establish the hemispheric symmetry, we create an energetically consistent cloud‐albedo field from the data. We show that the surface albedo asymmetry is compensated by asymmetries between clouds over extra‐tropical oceans, with southern hemispheric storm‐tracks being 11% cloudier than their northern hemisphere counterparts. This again indicates that, assuming the albedo symmetry is not a result of chance, its mechanism likely operates on large temporal and spatial scales.
description: Plain Language Summary: The planetary albedo is the portion of solar radiation reflected by the planet back to space, and is a prime factor deciding whether the planet will warm or cool over time. An intriguing property of the albedo is that, on average, Northern or Southern Hemisphere (NH or SH) have the same albedo, called hemispheric symmetry. The symmetry is surprising, because SH has much more ocean than land, and ocean is less reflective than land, so NH should have higher albedo. Nevertheless, clouds, which also tune the albedo, compensate the surface albedo imbalances of the two hemispheres, leading to an overall symmetric albedo. It is so far unclear how, or why, clouds perform this compensation. Here we show that this cloud compensation comes from the extra‐tropical storm tracks of the SH, which are cloudier than those of the NH. We further analyze satellite radiation measurements in search of indicators of a process between NH and SH which establishes the albedo symmetry. While we find reflected radiation timeseries to be mostly a seasonal cycle superimposed with small noise, we also see that reflection decreases overtime with a significant trend that is identical for both hemispheres and thus hints at some interaction mechanism.
description: Key Points: Surrogate time‐series analysis establishes the hemispheric albedo symmetry and excludes potential mechanisms enforcing it. Decadal trends in reflected insolation, while substantial, do not break Earth's hemispheric albedo symmetry. Hemispheric clear‐sky albedo asymmetries are balanced by hemispheric asymmetries in extra‐tropical storm‐track cloudiness.