Research aircraft from the Alfred Wegener Institute have been surveying the ice-covered Arctic Ocean for 30 years. The immense effort of the past 52 expeditions has paid off: 40,000 km of measurement data document the significant decrease in pack-ice thickness as a result of climate change. The time series is the only aeroplane- and helicopter-based measurement series in the world that has been carried out in the Arctic over such a long period. Currently, two Basler BT-67 aircraft are in operation: the Polar 5 and Polar 6.
This year’s Polar 6 measurement campaign, called IceBird, took the team led by Dr Thomas Krumpen, sea-ice physicist at the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), from Inuvik, Resolute Bay and Eureka in Canada to Station Nord in Greenland. On April 26, the research aircraft returned from the Arctic to Bremen, where it has been documenting changes in the pack ice from various locations over the past four weeks. According to Krumpen, this year’s expedition leader: “This is the 52nd time we have flown over the sea ice. Taken together, all these measurements would span the globe: we’re talking about 40,000 km of ice data. Much of it was collected in low-level flights at an altitude of about 60 metres. This is very demanding for the pilots and requires a good team and good planning, especially as the weather is constantly changing and difficult to predict.”
The immense effort required for such large-scale flights in the Arctic is well worth it; in some regions, the researchers are seeing a clear reduction in ice thickness as a result of climate change. “Based on the 52 airborne surveys starting in 1993, we can construct time series for different regions, showing how sea-ice thickness and surface roughness vary from year to year and whether there is a trend. For example, in Fram Strait, ice thickness in the summer months has decreased by about 20 to 25 percent over the past two decades,” says Krumpen. “The observed decrease in thickness affects the heat flux between the ocean and the atmosphere, further accelerates the sea-ice retreat in the summer months and has consequences for the entire Arctic ecosystem.”
The researchers are also observing similarly drastic changes in the regions north of Canada, where the older and thicker ice is usually found. In some cases, the effects of melting sea ice can also be seen directly. In the nineties, the Adventfjord in Svalbard was frozen solid. When the AWI research vessel Polarstern was anchored in the main town of Longyearbyen, the researchers could still reach the ship on foot from land. Today, however, the fjord no longer freezes over at all and is navigable all year round.
Prof Christian Haas, head of the Sea Ice Physics Section at the AWI, says: “The AWI’s measurements in the Arctic are the only aeroplane- and helicopter-based observations over such a long period that consistently show the decrease in ice thickness and deformation in the Arctic. This makes them an important basis for socio-political and scientific decisions. In addition, the data facilitates the calibration of satellite-based measurements, which provide year-round and Arctic-wide information, but with less accuracy.”
The first scientific flights over the sea ice took place in March 1993, launching from Svalbard. At that time, two Dornier DO228s, Polar 2 and Polar 4, were in service at the research institute. The latter can now be seen in the Dornier Museum in Friedrichshafen. As AWI atmospheric physicists Dr Jörg Hartmann, participant and leader of many earlier expeditions, recalls: “The survey flights with the polar planes, but also the evaluation of the data, were very time-consuming at the time due to the limited computer and storage capacities. Although the thickness of the sea ice could not be determined directly back then, we were able to create a terrain model of the ice’s surface using laser distance measurements.”
Today, 30 years later, the research institute relies on two Basler BT-67 aircraft: Polar 5 and Polar 6 are equipped for flights in the extreme environmental conditions of the polar regions. The 80-year-old polar aircraft can be used more flexibly than their predecessors and can carry significantly more sensors. A towed system specially designed to measure the total (sea-ice + snow) thickness (the “EM-Bird”) is stowed directly under the fuselage during take-off and landing. Once in the target area, the torpedo-shaped sensor is lowered by a winch and towed at roughly 15 metres above the ice. This measurement method is the only one of its kind. Over the years, the laser systems on board have evolved from single-beam lasers to laser scanners, which have been used to measure the ice’s surface since the 1990s. An ultrawideband microwave radar system complements the measurements by providing information on the thickness of the snow, while a camera system from the German Aerospace Center records the ice’s radiometric signature.
The IceBird aerial surveys with Polar 5 and Polar 6 will continue in late winter and summer in the coming years.
