Origionally posted at Climate Etc
The albedo of Earth
Graeme L. Stephens, Denis O’Brien, Peter J. Webster, Peter Pilewski, Seiji Kato, and Jui-lin Li
Abstract. The fraction of the incoming solar energy scattered by Earth back to space is referred to as the planetary albedo. This reflected energy is a fundamental component of the Earth’s energy balance, and the processes that govern its magnitude, distribution, and variability shape Earth’s climate and climate change. We review our understanding of Earth’s albedo as it has progressed to the current time and provide a global perspective of our understanding of the processes that define it. Joint analyses of surface solar flux data that are a complicated mix of measurements and model calculations with top-of-atmosphere (TOA) flux measurements from current orbiting satellites yield a number of surprising results including (i) the Northern and Southern Hemispheres (NH, SH) reflect the same amount of sunlight within ~ 0.2Wm2. This symmetry is achieved by increased reflection from SH clouds offsetting precisely the greater reflection from the NH land masses. (ii) The albedo of Earth appears to be highly buffered on hemispheric and global scales as highlighted by both the hemispheric symmetry and a remarkably small interannual variability of reflected solar flux (~0.2% of the annual mean flux). We show how clouds provide the necessary degrees of freedom to modulate the Earth’s albedo setting the hemispheric symmetry. We also show that current climate models lack this same degree of hemispheric symmetry and regulation by clouds. The relevance of this hemispheric symmetry to the heat transport across the equator is discussed.
Published in Reviews of Geophysics; [link] to full manuscript.
Excerpts from the Introduction:
There are many reasons why it is important to understand the variability of the Earth’s albedo and the factors that define it:
1. Simple energy balance models of the climate system are unstable to small changes in the amount of energy reflected to space. In these simple models with an albedo overly sensitive to surface temperature, relatively small changes in the absorbed solar energy can swing these models from a near ice-free Earth to a fully ice covered state.
2. It is also speculated that albedo changes potentially regulate the climate system. Lovelock’s Gaia hypothesis, exemplified in the study of “Daisyworld”, suggests that regulation of the system albedo by the adaptation of biota of differing albedos to climate change might in fact buffer the system from the instabilities inherent to earlier energy balance models.
3. The reflection of sunlight by clouds provides an important climate change feedback mechanism. Our inability to quantify these feedbacks with any certainty is recognized as one of the major obstacles in climate change predictions .
4. More locally, the Earths albedo appears to be resilient to other internal changes that might otherwise alter the system albedo. Perturbations to the albedo through effects of aerosol on clouds appears to be buffered by compensating processes that restrict local albedo changes to changing aerosol influences. The implications of these more local compensations to concepts proposed to mitigate climate change through geoengineering cloud albedo are thus profound.
5. Regulation of the Earth’s albedo is also central to other important climate feedbacks, including the snow/ice surface albedo feedback as well as cloud feedbacks.
6. It has also been conjectured that the characteristics of the total energy transport from low to high latitudes are insensitive to the structure and dynamics of the atmosphere-ocean system and are determined primarily by external controls such as the solar constant, the size of the Earth, the tilt of the Earth’s axis, and the hemispheric mean albedo.
We show, as in other studies, that the Northern and Southern Hemispheres (NH and SH) reflect the same amount of sunlight within 0.2Wm2. We show clearly how this is achieved as a consequence of reflection from increased amounts of SH clouds offsetting precisely the increased reflection from the larger NH land masses . The spectral distribution of this reflected energy exhibits clear differences between the hemispheres that reinforce our understanding of how the hemispheric symmetry is established.