JGR Paper: Predictive model for bioalbedo of snow

Online Early version here.

The ‘bioalbedo effect’ is the accelerated melting of snow and ice due to living organisms growing on it. Life can change the colour of the snow or ice, increasing its efficiency as an absorber of solar energy and causing it to heat up. Our new paper, just accepted by Journal of Geophysical Research, uses a predictive numerical modelling approach to examine this process for algal cells growing on snow.

A red snow algal bloom in Antarctica next to a white Spectralon reference panel ( 25 cm across) (ph. A Hodson)

The model can be broken down into distinct parts. The first part is a mixing model for pigments that can be present in snow algal cells. The user can define the relative amounts of each pigment in the cells. The cell wall is assumed to be transparent. This then allows the absorption coefficient for each individual algal cell to be calculated.

The second part of the model scatters these cells in a layer of a hypothetical snow pack. The layer can be any thickness and does not necessarily have to be the surface layer. The amount of algae is expressed by the user in g(algae)/g(snow). The radiative transfer model TARTES has been adapted to account for our algal cells and is used to predict the bihemispheric spectral reflectance of the surface.

The albedo of the snow is then calculated using incoming solar radiation (measured or modelled, e.g. using NASA’s COART model). This can be plotted and analysed, or used to drive an energy balance model to determine how much the algae influence snow melt.

Example plot from the model showing spectral albedo (G) and broadband albedo (H) with increasing concentration of primary carotenoid pigments in 1mg(algae)/g (snow). Extracted from Figure 3 in the paper.

The paper shows that algae have the potential to reduce snow albedo, with the magnitude of their effect dependent upon how much algae is in the snow and how pigmented it is. It also shows that specific wavelengths of light are affected more than others by algal cells and that this could be used diagnostically, perhaps allowing us to detect algae from planes, drones or satellites. The energy balance part of the model shows that this affects how quickly the snow melts. A physical mechanism for algal acceleration of snow melt is thereby developed.

Next, we intend to develop the model so that it is applicable to melting glacier ice as well as snow. This will help us to explain the mysterious ‘dark zone’ on the Greenland ice sheet which is probably discoloured by a combination of dust, black carbon soots and algal blooms.

The dark zone in the SW region of the Greenland Ice Sheet (Kangerlussuaq area)> This image is a composite of red, blue and green reflectance from the ESA satellite Sentinel-2, measured on 25th July 2016. The dark stripe on the ice sheet remains unexplained, although dust, black carbon and algae are important light absorbing impurities. Area in image is approx 120 km across.

The model provides a framework for integrating bio-optical and radiative transfer models, demonstrates the potential for algae to melt snow and outlines some of the challenges for empirical bioalbedo measurements. There are improvements still to make to the model, but this work proves the concept that radiative transfer and energy balance modelling can be coupled, and shows how algae can change the colour of snow.

BBC Radio 4: Midweek

Yesterday I visited the BBC to talk about life on ice with Libby Purves on Midweek on Radio 4. It was the first time I’d been to Broadcasting House and that in itself was a real experience. I was whisked upstairs to the recording studio with the other guests – author Ken Follett and musicians Fr David Delargy and Hannah Peel.

The conversation was led by Libby Purves, who began proceedings by asking the group: “Do you like the cold?”

I was able to tell the group about some the life that exists on ice surfaces and the logistics of working on ice sheets, before finally admitting to having questionable choices on my field camp playlist!

Excited to be at BBC Broadcasting House – there’s a Dalek and Tardis just behind the slide doors (true).

Overall a great opportunity to communicate glacier ecology to a new demographic, and great fun.

Thanks BBC, thanks Libby, thanks Ken, David and Hannah.

Available on iplayer here!


November at NASA

I was lucky enough to spend November working at NASA’s Jet Propulsion Laboratory. The purpose was to develop ongoing collaborations with expert glaciologists Johan Nilsson (who joined me in Greenland in the summer) and Alex Gardner, meet new potential collaborators and present my recent work on “bioalbedo”.


It was an extremely productive trip – being face-to-face with these top experts helped to confirm, refine and advance my current work and plans for 2017. I was lucky enough to meet researchers from the Airborne Snow Observatory and Europa Mission and got a tour around some of the labs and talk about potential collaborations.

Speaking at JPL was a huge honour and total boyhood dream-come-true as this is one of the world’s leading centres for cutting edge earth science. I spoke to the Earth Sciences division on the topic of “Bio-albedo” (the effect of microbial life on the colour of ice). The feedback and follow-up conversations were really valuable and will help stimulate some exciting science in 2017!

Title page for my JPL talk on “Bio-albedo”

It also happened to be my 30th birthday while I was out there so to celebrate, Johan, fellow researcher Aumery and I hiked up Mount Baldy before going to the cinema for some appropriately space-themed sci-fi in the evening!

Near the top of Mt Baldy – quite a contrast from shorts and t-shirt weather in Pasadena!
Obligatory summit-selfie with JPL postdocs Johan and Amaury.

So #thanksNASA for hosting me and showing me the incredible Earth and space science being done at JPL.

Rolex Awards for Enterpise


I am very pleased to report that I was recently made a 2016 Rolex Young Laureate. This award recognizes potential in young scientists and entrepreneurs (under 30’s), so it is extremely humbling to have been selected. I’ll be delivering a project (“Ice Alive”) that will advance our understanding of ice as a living landscape.

Part of the living landscape on the Greenland Ice Sheet: biological material is a major component of the dark impurities on the ice.

I have already been able to meet some incredibly inspiring people, including my fellow laureates, the jury and the Rolex team, and this award will stimulate some big adventures in 2017 and beyond!

For more information, see my Rolex profile page here.

Interestingly, I’m the second  researcher from the University of Sheffield’s Geography Department to receive a Rolex Award, after volcanologist Andrew McGonicle (2008).