The UK Natural Environment Research Council (NERC) has funded a consortium of UK scientist to work together with their international collaborators on issues related to why the melting of the Greenland Ice Sheet is accelerating. Global warming alone is not enough to account for the increasingly rapid melting of the ice sheet. Other factors are darkening the ice sheet surface, which results in greater rates of melting. The main focus of this large research project is based on our hypothesis that microbes thrive and bloom on melting snow and ice surfaces, and darken the ice sheet surface as a consequence.
Life exists wherever there is liquid water on the Earth’s surface, and ice sheet surfaces are no exception (see Figures 1). Just one drop of ice melt contains up to 10,000 microbes. Many of these are tiny organisms have green chlorophyll, similar to plants, to capture sunlight and grow va photosynthesis. They also develop their own dark-coloured sun block, often coloured red, purple or brown, which protects them from damage by the fierce sunlight which shines for 24 hr per day in the height of the Arctic summer. The microbes can turn the surface of the ice sheet purple to black when they multiply rapidly and bloom, which means that the ice sheet surface warms and melts much faster than if the surface were white and lifeless. Microbiologists have long known that snow is discoloured by the growth of snow algae. Indeed, water melon snow looks and smells like water melons. It is only recently that microbiologists have shown that dark-coloured microorganisms grow in melting ice.
Figure 1. Examples of coloured snow algae from the Greenland Ice Sheet . Clean, fresh snow has an albedo of ~90%. This means that ~90% of the incoming solar radiation is reflected back into the atmosphere, and that only 10% is used for melting the surface snow. The snow and ice algae give rise to albedos of 35-49%, which means that 51-65% of the incoming solar radiation is used to melt the surface snow.
Scientists at the Universities of Bristol (Martyn Tranter, Alex Anesio, Jon Bamber, Jo Laybourn-Parry, Nicholas Roberts, Jemma Wadham and Marian Yallop), Leeds (Liane Benning and Jim McQuaid), Sheffield (Edward Hanna and Andy Hodson) and Aberystwyth (Tris Irvine-Fynn) have been awarded over £3 million during the next five years to study these microbes and to evaluate how their distribution and growth combine with the other factors to darken the ice surface and enhance the melting of the Greenland Ice Sheet . They are joined by a talented team of international partner scientists, including Carl BØggild (Copenhagen, Denmark), Jason Box (GEUS, Denmark), Michiel van den Broeke (Utrecht, Netherlands), Brent Christner (Louisianna State, USA), Gianfelice Cinque and Burkhard Kaulich (Diamond Light Source, UK), Xavier Fettweis (Liege, Belgium), Paolo Formenti (Paris, France), Alex Gardner (NASA JPL, USA), Stef Lhermitte (Leuven, Belgium), Martin Sharp (Alberta, Canada), Marek Stibal (Prague, Czech Republic) and Nozomu Takeuchi (Chiba, Japan).
Our goal will be to understand what controls the growth and blooming of the microorganisms. We want to know how they stick to the small amounts of dark particles present in the snow and ice, including dust and black soot, and if they retain those particles at the surface for long periods.
We will be making some of the first detailed measurements of how the surface or the ice sheet darkens over the whole spring, summer and autumn, starting with cold snow, going through slush, then ice as the snow melt drains away, and finally to rotten ice, a mix of ice and water. At the moment, we lack detailed field measurements that take account of all the different factors that darken the ice sheet surface. Our detailed field measurements will do just this. Importantly, few measurements of coloured microbes have been made alongside the other factors, which include how wet the snow and ice is and the size of the snow and ice crystals. Wet snow and ice and bigger crystals are usually darker. Details of fieldwork already undertaken on the Greenland Ice Sheet by our partners at GEUS as part of the Dark Snow Project can be found at http://darksnow.org/ .
Finally, we will put all our information on surface darkening into a melt model of the whole ice sheet that will be used to predict how much sea level rise will occur in the future. Currently, sea level is rising by about 1cm per decade, but there are real concerns that this could accelerate in the future. In addition, there are concerns that the input of more fresh water into the seas south of Greenland may decrease the flow of the Gulf Stream, which is responsible for the temperate climate of the UK. Warming leads to greater melting of the ice sheet interior, which is quite flat, and so holds melt water at the surface for longer. This will lead to more growth of microbes, which will darken the surface and increase the amount of melt. We need to be able to predict with more certainty than we can do at present just how much more melting will arise as a consequence, so that we can be confident that our predictions of sea level rise are more accurate.
 Lutz, S., Anesio, A. M., Jorge Villar, S. E. & Benning, L. G. Variations of algal communities cause darkening of a Greenland glacier. FEMS Microbiology Ecology 89, 402-414, doi:10.1111/1574-6941.12351 (2014).