The world takes on a brilliant, luminous quality on a sunny snow-covered day. Light seems to emanate from every direction and it’s almost painful to be outside without sunglasses. That’s because snow reflects most of the light shining down on it. In fact, fresh snow can reflect up to 95% of the Sun’s light. As the snow surface gets older and dirtier, it reflects less light. When the snow thins and dirt patches emerge, the surface further darkens, and of course bare ground is not particularly reflective at all.
This concept is called albedo, or reflectivity. Albedo is a simple ratio of how much sunlight falls on a surface compared to how much bounces off. While fresh snow reflects 95% of light that hits it, open ocean reflects 10%, and pavement only reflects about 5%. Whatever light is not reflected is absorbed, which is why dark pavement gets so hot on a summer day.
Even though this is a very simple concept, it plays a big role in the climate system. In the case of Greenland, the surface is mostly white, thanks to its resident ice sheet. But in recent decades it’s becoming a little less white. The implications of a slight change in color shed light on how small changes in the climate system can, in fact, have large effects.
As Earth loses snow and ice in Greenland and elsewhere, the bare ground that is left behind is less reflective than snow, which means that the ground absorbs more energy from the Sun. This causes the surface to warm up, which contributes to further melting of nearby snow. The more snow that melts, the more solar energy is absorbed and the more the ground warms up. This is an example of a self-reinforcing cycle, or positive feedback cycle, meaning that once the cycle gets started, it has a tendency to strengthen itself.
So as the Greenland ice sheet melts around the edges, the dark, rocky surface underneath absorbs more solar energy and accelerates further warming and melting. As the ice thins, dark particles within the ice become concentrated, making the ice darker and less reflective, allowing it to melt even faster. All tolled, as the surface of Greenland becomes less white and less reflective, warming speeds up in the region.
Why is this significant? Because it’s one example of how initial warming in the climate system can trigger even larger effects, such that the resulting warming is much greater than the initial input. It shows how global climate change has the potential to create ‘runaway’ effects that, once initiated, pick up steam. The Greenland ice sheet contains enough water to raise sea level by 20 feet, and thus we are wise to keep an eye on it.
This same effect takes place on the oceans as well. As sea ice melts and is replaced by open water, the loss of reflectiveness allows the water to warm up. Scientists think this is the underlying cause of the wobble in the ‘Polar Vortex’ that has created newsworthy winter weather across the US this year. These are but a few examples of why the Arctic climate is of particular importance for Earth’s climate.
Greenland Ice Sheet. From NOAA’s Arctic Report Card: Update for 2013.
Springtime melt in Greenland: Late start, rapid spread. From the National Snow and Ice Data Center
Greenland – an albedo feedback laboratory. From Meltfactor: the ice and climate blog of Jason Box, PhD.
Satellite data reveal the rapid darkening of the Arctic. From Scripps Institution of Oceanography
Written by Karin Kirk
Karin Kirk is a freelance writer and educational curriculum developer. She holds Bachelor’s and Master’s degrees in geology and has taught undergraduate geology, environmental science and climate change in both face-to-face and online courses. Prior to joining the Visionlearning team, she worked for the Science Education Resource Center at Carleton College where she was part of several educational projects focused on improving undergraduate science education. In the winter months, she continues her passion for education in her role as a ski instructor at Montana’s rugged Bridger Bowl ski area.