Optical measurement of melting ice

The Kiel geographer Natascha Oppelt took part in a Polarstern expedition to the Arctic, to gather size, depth and optical data of melt ponds. The aim is to gather such data, which is important for climate modelling, by means of remote sensing in the future.

Three persons on the ice.
© Niels Fuchs, AWI Bremen

Well protected in bright red survival suits, Natascha Oppelt (left) and two colleagues measure, among other things, the depth of the melting ponds in the artisanal sea ice, the thickness of the ice lying under the ponds as well as optical properties of the pond water and the surrounding sea ice surface.

When ice melts in the Arctic summer, ponds form on the ice floes. And these melt ponds are interest­ing for research, because the ice melts faster if ponds are present. The reason for this is that »Ice and snow reflect incoming solar radiation very strongly, which means that very little radiation energy actually reaches the ice. Water, on the other hand, absorbs the radiation energy and thus makes the ice thaw faster,« explained Professor Natascha Oppelt.

The physical geographer heads the Earth Observation and Modelling working group at Kiel University’s Department of Geography, and was on board the research vessel Polarstern when it drifted with an ice floe in the central Arctic in June 2017. »We were anchored directly to an ice floe for two weeks, and when the weather allowed it, we went onto the floe to take measurements. There were several ponds, and in the course of two weeks some new ponds also formed.«

Exploring the ice floe was not without risk. »You are exploring unknown terrain and could fall through the ice at any time. That is why we were only allowed to leave the ship and explore the ice floe wearing a survival suit, and took along special metal hooks which you can use to pull yourself to the edge.« It becomes particularly adventurous if the largest land carnivore crosses the researchers’ path. Each research group was also only allowed onto the ice with an armed guard in case of polar bears . »On the second day we were there, a polar bear mother and her cub visited the research station. She investigated all the equipment and then moved on again. We could watch everything from the ship - not even 100 meters away,« reported the Dean of the Faculty of Mathematics and Natural Sciences.

Natascha Oppelt and her team member Marcel König investigated the melt ponds using various methods, together with colleagues from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) in Bremerhaven and the German Aerospace Center (DLR). They determined, amongst other things, the extent and depth of the pond, the thickness of the underlying ice, the optical characteristics of the pond water and the surrounding sea ice surface.

In addition to measurements on the ground, a helicopter equipped with special cameras flew over the area to capture hyperspectral data, which provides very accurate images of the reflected light. Whereas the human eye only sees white snow or turquoise-blue melt ponds, the special camera records the reflected light from very many closely-spaced wavelengths. Correctly interpreting the colour signal of the different colours of ice and water of varying depths, or calibrating them, requires on-site measurements. Thus, the ground measurements are used to validate the data recorded with the helicopter.

This data should be used to develop a method that allows important parameters of melt ponds (extent of coverage, depth) to be derived from satellite data. The aim is ultimately to use satellite data to document how the melt ponds develop spatially and chronologically, to better understand their role in the Arctic climate and ecosystem.

»The fact is, the Arctic ice is shrinking. A logical consequence would be that the melt ponds become bigger and deeper. We can’t yet prove this is true everywhere,« said Oppelt, who has taken on this challenge, and will also participate in the expedition to the Arctic again next year.

Author: Kerstin Nees


Arctic ice is melting faster than predicted

The polar sea ice is an important factor for climate models, but one that has not yet been adequately mapped. For years, it has been disappearing faster than all climate models have predicted. Compared with 1979, the start of satellite observation, the sea ice cover (measured in September each time) has almost halved. The Arctic is warming approximately twice as fast as the rest of the planet. This phenomenon, known as Arctic amplification, is still not fully understood.

Apparently the melting ice sets off a self-perpetuating chain of events, which leads to more melting and more heat. One reason for this is that a bright ice surface reflects almost all the incoming solar energy. A dark ocean, on the other hand, absorbs the energy and heats up, which makes the formation of new ice more difficult. (ne)