Sep 10, 2008
Monitoring the melting of the Arctic
Rising global temperatures are causing increasingly large amounts of ice in the Arctic to melt, with possibly huge environmental consequences. Liz Kalaugher joins a research ship in the Canadian Arctic to find out how microwave imaging can reveal where and how fast the ice is melting
Standing on the deck of the icebreaker Amundsen in the Arctic Ocean, I am bathed in blazing June sunshine. The weather has been like this all week since I joined the ship – a research vessel that set sail from Quebec in Canada last summer – as a visiting science journalist. It would be tempting to think that such conditions are typical, but most areas of the Arctic are in fact cloudy for 80% of the time in the spring and summer due to moisture in the air from melting ice and from exposed areas of the ocean.
As it is usually cloudy, it is difficult to use optical sensors on board aircraft or satellites to monitor in detail the condition of the Arctic ice. Moreover, the Arctic sees little or no daylight for six months of the year, which means that during the winter visible light cannot be used for imaging at all. But researchers are desperate to find out more about ice conditions in this part of the world, which have been changing rapidly for the last 30 years as a result of global warming.
Thankfully, microwaves, which do penetrate clouds, can help researchers gain a clearer, year-round picture of the Arctic ice. In particular, microwave techniques have been used to analyse the area covered by the ice, which grows and shrinks naturally throughout the course of the year. Such studies reveal that between 1979 and 2000 the average minimum extent of the ice was 6.74 × 106km2. In September 2007, however, the ice had shrunk so much that it covered a record low area of just 4.13 × 106km2.
The jury is still out on whether this year’s figure will be even less, but the consensus among the researchers on board the Amundsen during my stay was that the ice began melting three weeks earlier than usual. However, new experiments using a microwave transmitter and receiver on board the ship could help scientists to interpret satellite microwave images better and thus paint a much more precise picture of the extent of the Arctic ice and of the process by which it melts.
Into the gap
The Amundsen is a 98 m long vessel belonging to the Canadian coastguard that has been used as a scientific research ship for the past five years. It arrived last autumn at a site about 100 km off the coast of northern Canada, near Banks Island, at a latitude of 71°N. There are 40 researchers on the Amundsen at any one time, supported by 40 crew members, although the personnel changes roughly every six weeks as new scientists arrive by plane.
Operated by the University of Manitoba for the Canadian government, the Amundsen’s $40m trip to the Arctic is one of the largest projects being undertaken as part of International Polar Year, which runs from March 2007 to March 2009. There are 10 different projects on board, all of which are designed to monitor various physical and biological changes in the Arctic. These include studying plankton, the levels of carbon dioxide in the water and the extent to which visible light can pass through ice.
But what has made this particular research trip unique is that the Amundsen spent the winter in the Arctic. This meant that researchers were in situ in early spring and so could document changes that occur as the Arctic ice beings to melt. Normally, it is hard to reach this area by boat until after the ice has thinned significantly, which means that vital data can not be obtained while the ice is in a major state of flux.
The researchers on board have been focusing on an area in which a gap opens up from time to time between the "fast ice" that is attached to land and the "sea ice" that moves with the ocean currents. (Sea ice is frozen ocean water, as opposed to icebergs, which are chunks of ice broken off glaciers.) This area is known as the "circumpolar flaw-lead system" because it extends in a circle around the pole and creates a "flaw" in the ice surface. It is an ideal place for studying the Arctic because the reduced ice cover is highly sensitive to physical changes in the atmosphere and ocean.
So what happens as ice in the Arctic melts? We know that frozen ice reflects about 80% of the sunlight that hits it. But as the ice disappears and the darker ocean beneath is revealed, the reflectivity, or "albedo", of the surface decreases. If all the ice has melted and only ocean is left, the albedo drops to just 10%. That means that more of the incoming long-wave visible and infrared radiation from the Sun is absorbed by the surface, which can cause even more ice in the region to melt.
While melting of some Arctic sea ice is perfectly normal in the summer, rising air temperatures in recent years have caused a greater proportion of the Arctic ice to melt than in the past. As more ocean is exposed, this can trigger a potentially dangerous mechanism of positive feedback, whereby less light is reflected, causing further warming and thus more ice to melt. Indeed, Wieslaw Maslowski, an oceanographer from the Naval Postgraduate School in Monterey, California, who was not on the Amundsen, has predicted that the Arctic could be completely ice-free in the summer as early as 2013.
A total absence of ice in the Arctic would not just affect local wildlife and indigenous communities but could also have huge implications for the global climate. In particular, the ice helps to drive worldwide ocean circulation by creating dense salty water that sinks to the ocean floor. Given these concerns, it is clear why it is vital to monitor exactly what is going on in the Arctic, which is where microwave remote sensing comes to the rescue.
"Using microwave remote-sensing imagery really is a key to a larger view of Arctic change that field observations don’t give us," says Randy Scharien, a geographer from the University of Calgary and one of the researchers on the Amundsen. Indeed, the Canadian Ice Monitoring Service has been using the country’s RADARSAT-1 Earth-observation satellite, which was launched in 1995, to look at ice levels using horizontally polarized microwaves with a frequency of 5.3GHz.
Monitoring with microwaves
When it comes to examining ice with microwaves, however, there is one snag: pools of water, known as "melt ponds", that form on the ice surface as it melts can look very like the open ocean. With this in mind, researchers are turning to "fully polarimetric" microwave remote sensing to provide more information. Although such systems have been used on aeroplanes for roughly 15 years, the first satellite to carry them was RADARSAT-2, which was launched by the Canadian Space Agency in collaboration with MacDonald, Dettwiler and Associates in December 2007.
RADARSAT-2 can transmit and receive waves that are either horizontally (H) or vertically (V) polarized. This means that images can be built up from four different combinations of transmitted and received radiation (HH, HV, VH and VV) as well as from the phase difference between the H and V signals. But to validate and interpret RADARSAT-2’s images, which have a resolution of 3m, Scharien and colleagues are turning to a mini, groundbased version of the satellite, just a few metres in size, which can be left at appropriate positions on the ice.
This fully polarimetric “scatterometer” collects data over an area of roughly 3m × 5 m. It transmits microwaves at the same frequency as RADARSAT-2 before measuring what is reflected back from the surrounding area. Given its size, the device can collect information from just one type of surface at a time, whereas RADARSAT-2 obtains data over a much larger area. "With the scatterometer you can look at specific targets and see how they react in terms of microwaves," explains Scharien.
The team, led by Calgary’s John Yackel, has not only kept the device on board the ship to study conditions in the ocean but also lugged it out onto the ice to examine melt ponds. Doing experiments on the ice is no easy matter: it generally takes about six hours to drag the kit onto the ice and back off again. But it is worthwhile because fully polarimetric microwaves are so useful.
For example, horizontally polarized microwaves can lose their polarization when they penetrate and scatter off air bubbles in "multiyear ice" – ice that never melts completely from one summer to the next and thickens again each winter. But when horizontally polarized waves reflect off wind-roughened ocean, they do not depolarize. In other words, a high HV backscatter signal indicates multiyear ice, and a low HV signal reveals areas of ocean. Fully polarimetric studies can also provide extra information on the internal structure of the ice that is not accessible if waves with only one form of polarization are used.
In addition to the scatterometer data, Scharien and colleagues have also been measuring the physical properties of the ice using various probes as well as data on wind and wave roughness. This information can then be compared with data from the RADARSAT-2 satellite, which in springtime cannot distinguish between pools of water on the surface of the ice and exposed sea water. The team expects to spend about a year analysing all the data but it has already noticed that microwave scattering properties are closely related to the optical properties of the surface.
"We can use our microwave signatures to decompose the network of melt ponds versus snow and ice patches to get a relative percentage coverage of melt ponds," says Scharien. "So we can use microwaves to give us an idea indirectly of the albedo or the optical properties of the sea ice and how it’s processing sunlight." This is vital because it will enable scientists to monitor the melt process as it occurs. Whether anyone can prevent the Arctic ice from disappearing, though, is another matter altogether.
About the author
Liz Kalaugher is editor of the website environmentalresearchweb. Her trip was funded by the World Federation of Science Journalists. This article originally appeared in the September edition of PhysicsWorld.