Bio-co-albedo?

At EGU I had the pleasure of talking about BioSNICAR and biological albedo reduction with two of the big-names in albedo research. A very interesting point they raised was that the term ‘bioalbedo’ does not precisely describe the concept that it is attached to. This is true. The term bioalbedo was not coined by spectroscopy or remote sensing experts, but by microbiologists and glaciologists, and is now well-baked into the literature. I will outline here the reasons why we should be cautious of this terminology.

Albedo is the survival probability of a photon entering a medium. Light incident upon a material partly reflects from the upper surface, the remainder enters the medium and can scatter anywhere there is a change in the refractive index (e.g. a boundary between air and ice, or ice and water, etc). Where there are opportunities for scattering, light bounces around in the medium, sometimes preferentially in a certain direction depending upon the optical properties of the medium (ice is forward-scattering) but always changing direction to some extent each time it scatters, until it is either absorbed or it escapes back out of the medium travelling in a skywards direction. The albedo of the material is the likelihood that the down-welling light entering the medium exits again later as up-welling light. The more strongly absorbing the material, the more likely the light is to be absorbed before exiting. Ice is very weakly absorbing in blue wavelengths (~400 nm), becoming generally more strongly absorbing at longer wavelengths into the near infra-red (hence ice often appearing blue). Solar energy is mostly concentrated within the wavelength range 300 – 5000 nm and the term albedo concerns the survival probability of all photons with wavelengths within this range either at a particular wavelength (spectral albedo) or integrated over the entire solar spectrum (broadband albedo).

This means that a photon entering a material with a broadband albedo of 0.8 has an 80% chance of exiting again. Therefore, when a material is bombarded with billions of photons, 80% of them are returned skywards and 20% are absorbed, and the surface appears bright. A lower albedo therefore means less likelihood of photon survival.

For a single material, its absorbing and scattering efficiencies are described using the scattering and absorption coefficients. The ratio of these two coefficients is known as the single scattering albedo (SSA), which is a crucial term for radiative transfer. A higher SSA is associated with a greater likelihood of a particle scattering a photon rather than absorbing it. a particle with SSA = 1 is non-absorbing.

Therefore, with these definitions we can see why the term bio-albedo is not semantically perfect. The term bio-albedo implies that the relevant measurement is the light reflected from biological cells, which is really the inverse of the measurement of interest. Algal cells are strongly absorbing and their effect on snow and ice albedo is to increase the likelihood of a photon being absorbed rather than scattered back out of the medium. For this reason, the better term to use would be bio-co-albedo, where co-albedo describes the fraction of incident energy absorbed by the particles (i.e. 1-SSA).

Bio-co-albedo is more technically correct terminology, but it is also quite a subtle distinction, and arguably if we have calculated the single scattering albedo, we have by default calculated the co-albedo (co-albedo = 1 – single scattering albedo), and the outcome is the same. The meaning of the term ‘bio-co-albedo’ is not obvious to those outside of spectroscopy and remote sensing communities, which i think is a major issue since the topic is so broadly interdisciplinary. The more aesthetic and simpler ‘bio-albedo’ is justified in most cases, especially because it is already well-used in the literature and more widely accessible. From a utilitarian perspective, bio-albedo wins out.

As an aside, it reminds me that I have often wondered whether ‘evolution’ is really an acceptable word for cryosphere scientists to use to describe the temporal development of – for example – a snowpack or ice surface. Evolution implies changes resulting from inherited characteristics passed through successive generations plus random mutations that are selected for or against based on goodness of fit for the specific environment. A melting snowpack cannot ‘evolve’ as there are no ancestors, no selection, no inheritance, no generations. People also age over time, influenced by external factors, but we do not describe individuals as evolving – same applies to a snowpack or glacier. Overall, I suspect splitting hairs over terms like bio-co-albedo does more to dissuade non-specialists from joining the conversation than it does to improving understanding of the processes involved.

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