Earth Imaging Journal: Remote Sensing, Satellite Images, Satellite Imagery
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During the last few decades, average global temperatures have been on the rise, and this warming is happening much faster in the Arctic and Antarctic. Scientists are hard at work studying how rising temperatures are affecting Earth’s polar regions.

By Matteo Luccio, founder and president of Pale Blue Dot (www.palebluedotllc.com).

 

Sea Ice (1)

Scientists watch from the deck of the U.S. Coast Guard cutter Healy as it cuts through multiyear sea ice in the Arctic Ocean on July 6, 2011.

The extent of the polar ice caps is shrinking. This simple fact has been common knowledge since the late 1970s thanks to the added awareness of aerial and satellite imagery. Since then, researchers have collected continuous, daily measurements of the ice extent in the Arctic and Antarctic. Although this information is useful, more important is the ice’s thickness, which allows scientists to calculate its total volume.

Measuring changes in ice thickness across the Arctic for long periods of time will allow researchers to more accurately predict when it will become seasonal, according to Bruno Tremblay, an associate professor with the Department of Atmospheric and Oceanic Sciences at McGill University.

“Monitoring the changes in the thickness of the thick, multiyear ice just north of Canada and within the Canadian Arctic Archipelago is also crucial to predict whether and when there will be remnant ice in those two regions when the rest of the Arctic will be virtually seasonally ice free,” explains Tremblay. “A good knowledge of the spatial and temporal variability of ice thickness is also important to see whether our global climate models are correctly simulating the sea ice cover.”

Variations in sea ice volume are due to surface forcing, sunlight and clouds as well as melting on the bottom of the ice due to heat in the ocean. Sunlight is one of the main drivers of summer melting.

Why Ice Matters

As researchers have learned more about sea ice, they’ve become increasingly aware of its many functions. One of these functions is to convey fresh water. Sea ice extracts fresh water where the ice forms, leaving the salt behind, and then releases it where it melts. Sea ice can also transport contaminants and biota, according to Stephanie Pfirman, Hirschorn professor and co-chair of the Environmental Science Department at Barnard College, Columbia University. Sea ice is a moving habitat, with algae growing on its bottom.

According to Pfirman, the key scientific questions regarding sea ice are its thickness, age, quality and variability. Tremblay also has more specific questions:

  • Assuming various scenarios in terms of reducing greenhouse gas emissions, when will the Arctic become seasonally ice free (i.e., free of summer ice)?
  • How will the retreat take place (geographically) and at what rate? Are there strong positive feedbacks that will lead to a rapid decline?
  • Assuming business-as-usual scenarios in terms of fossil fuel burning, when will the Arctic become ice free in winter and summer?
  • How much do greenhouse gas emissions need to be reduced to maintain a summer sea ice cover in the Arctic? By when do such reductions need to be made?
  • How does the Arctic contribute to keeping the planet cool? Alternatively, how much more will the planet warm when it loses its sea ice cover?
  • What’s the seasonality of a summer ice-free Arctic (i.e., how long will the ice-free period last)?

“When I first started working on the ice, back in 1980, the average thickness was something like 3 meters,” says Pfirman.

“Now it has decreased dramatically, in large part because we’ve lost the old ice.”

Ice Dynamics

Sea_Ice_Goddard (1)

Sea ice can take many forms, as seen in this image of Arctic sea ice from a recent Operation IceBridge aerial survey. Varying thicknesses of sea ice are shown here, from thin layers to thicker, older sea ice covered with snow.

According to Tremblay, there’s a positive trend in sea ice drift speeds since 1979. As ice gets thinner, it offers less resistance to surface wind stress, leading to faster-moving pack ice.

“In the past, people hadn’t realized how dynamic the ice is,” explains Pfirman. “People think the Arctic ice cap is stationary and is melting like an ice cube on a plate. In fact, the ice moves, and every winter sea ice forms across the entire Arctic and extends quite far south. Then, in the summer, the ice starts to melt and retreat. However, because the ice is moving, you have a source for ice, which is over on the Siberian margin, and the winds blow the ice across the Arctic in the Transpolar Drift Stream. Some of the ice splits off and comes down along the east coast of Greenland, and some of it splits off and goes into a big gyre, called the ‘Beaufort Gyre,’ that circulates north of Canada.”

So, according to Pfirman, there’s a linear stream that goes all the way from the coast of Siberia across the North Pole and down along the east coast of Greenland, and there’s another gyre that heads toward Canada and Alaska and rotates clockwise. That’s important, because the stream basically brings the new ice formed along the coast of Siberia across the Arctic, and it piles up. Also, every winter, about 30 centimeters of ice is added to the underside, so the ice gets thicker as it gets older.

“It’s that old ice that has been kind of the thermostat of the Arctic,” says Pfirman. “That core of ice was impossible to melt all the way through in the short summer of June, July and August. Now, however, we have hardly any really old ice anymore.”

Pfirman concludes that most of the ice is first- or second-year ice, which is a lot thinner, so it’s a lot easier to melt through it. Therefore, it’s important to know where the old ice is and what its thickness is.

“That’s important from a climate perspective and a habitat perspective,” explains Pfirman. “Many species depend on the ice or prefer to be associated with the ice for various reasons.”

Ice Thinning

sea_ice_lead (1)

Leads of open water in the polar oceans are much darker than the surrounding sea ice and absorb sunlight that otherwise would be reflected away. As global warming causes sea ice to decline, a vicious cycle may lead to more rapid melting.

The ice is thinning because of thermodynamics, which is melting the ice from above and below because the atmosphere and ocean are warming, and dynamics, which is the loss of the thick old ice. The ice’s reflectivity, or albedo, can theoretically range from 0, when all the sunlight is absorbed, to 1, when it’s all reflected. New, flat, sea ice with snow on it—conditions prevalent for much of the year—is almost the brightest white on Earth’s surface, with an albedo of .85. In contrast, the patches of ocean visible through melted ice have an albedo of .1 and look black from space.

“As the snow melts, the albedo of the ice also decreases,” says Don Perovich, a research geophysicist at Dartmouth. “This means the surface absorbs more sunlight, which means there’s more melting. This lowers the albedo even further, and that is a positive feedback. These positive feedbacks are really of interest from a climate perspective, because they are a way you can give the system a general nudge and have it amplified to a big shove.”

Additionally, in the summer, there are puddles of melt water on the ice’s surface called “melt ponds,” and they’re dark, too. Through the open water and these ponds, heat goes into the ocean and melts the bottom of the ice.

“The main thing I do,” explains Perovich, “is look at how much sunlight is reflected by the ice, how much is absorbed in the ice and how much is transmitted into the ocean.”

“With a difference of just a couple of centimeters in ice cover, you can shift from being incredibly reflective to being incredibly absorptive,” notes Pfirman. “That’s the question associated with the extent. The thickness gives you a sense as to how vulnerable the ice is as well as habitat concerns.”

Tools and Techniques

Scientists use a variety of tools and techniques to measure sea ice thickness.

Drilling a Hole through the Ice. A hole can be drilled in sea ice with an auger or a hot water drill, which has a boiler that heats water and a hose with a nozzle.

“That’s a really good way, because you have a precise measurement and don’t have to worry about making some conversion,” says Perovich. “The average sea ice thickness is probably somewhere between 2 meters and 3 meters, so you can drill holes. Those measurements are accurate, but they’re labor-intensive and time-consuming, and you don’t get that many measurements.”

Internal Sea Ice Temperatures. In the winter, the temperature profile has kinks at the interface between the ocean and the ice, the ice and the snow, and the snow and the air.

“In the summer, the water is at minus 1.8 degrees Celsius and the air is at 0 degrees Celsius,” says Tremblay. “So everything is at almost the same temperature. We can no longer identify those kinks in the temperature profile and infer ice thickness from it.”

Autonomous Ice Mass Balance (IMB) Buoys. The mass balance of the ice describes how it grows in the winter and melts in the summer, both on the surface and on the bottom.

“An IMB uses acoustic sensors to keep track of the surface and the bottom,” explains Perovich. “It has temperature sensors in the ice to see how it cools in the winter or warms in the summer; it keeps track of how much snow you get in the winter and how that snow melts in the summer and transmits that back to us via satellite. In a given year, we typically deploy six to eight of these buoys, and we’ve been doing it for 14 years now.”

“In the Beaufort Sea region, we’ve seen large ice losses in the past 10 or 15 years,” he continues. “From our IMB buoys we’ve been able to determine that is due to an increase in bottom melting due to the increased heat in the ocean. We’ve also determined the source of that heat is sunlight.”

“In a different set of measurements, we’ve deployed one of these IMB buoys at the North Pole every spring since 2000,” adds Perovich. “They move and eventually come out by the coast of Greenland. But there we’ve shown that, while there is year-to-year variability, there is no large trend of either increasing or decreasing ice melting.”

Electromagnetic Induction Sensors. These devices consist of an antenna, about six feet long, that’s carried on foot on the ice surface or dragged in a sled. Such sensors emit an electromagnetic wave that reflects at the ice-ocean interface because of the change in conductivity.

“You measure the time it takes for the signal to come back and calculate the distance, knowing the speed of the electromagnetic wave,” explains Tremblay. “One also has to measure the distance from the sensor and the snow or ice using a laser.”

These measurements cover a scale of a few kilometers at a rate of a couple of kilometers per hour. They examine small-scale variability that measurements from aircraft or satellites aren’t able to acquire, according to Perovich. However, devices similar to the electromagnetic induction sensor used directly on the ice can also be carried from low-flying aircraft.

“The problem with this method,” claims Tremblay, “is that you can’t separate the ice thickness from the snow depth. You only measure the depth of the snow plus the ice.”

Sonar. Sonar can be used to measure the distance to the base of the ice, and another sensor is used to measure the distance between the top of the ice and the snow’s surface.

“Both methods send a sound signal and measure the time it takes for the reflected signal to come back,” explains Tremblay. “Knowing the speed of sound as a function of temperature (salinity), one can calculate the distance. From both measurements and the initial ice thickness at deployment we can measure ice/snow thickness change with time. This is how we can derive ice thickness from an IMB.”

Radar. Radar can be deployed from an aircraft or a satellite. The technology was mainly used in a European satellite called CryoSat-2, which “measures how high the top of the ice is floating,” explains Perovich. “Every so often there’s open ocean, so it gets that as a reference site. From the height at which the ice is floating you can calculate how thick the ice is overall just by looking at the density. That’s really good, because you can get maps of the entire Arctic basin many times per year.

However, because the technology is on a satellite, its footprint is tens if not hundreds of meters, so you’re averaging over an area. Also, it requires some interpretation to go from how high the ice is floating to what its thickness is.”

Light Detection and Ranging (LiDAR). LiDAR can be used to measure the distance from a satellite to the ice surface or, when there’s a lead in the ice, to the ocean surface.

“The difference in thickness between the ocean surface and the ice (or snow) surface is called the freeboard, and one can calculate the total ice thickness knowing that about 90 percent of the ice is below the water and only 10 percent sticks out (the freeboard),” explains Tremblay. “From that, you measure the thickness of the ice plus that of the snow, and you need to make an assumption about the snow depth to derive the ice thickness.”

A LiDAR sensor was deployed on NASA’s Ice, Cloud and land Elevation Satellite (ICESat), which operated from 2003 to 2009. ICESat-2 is scheduled for launch in 2017. In the interim, NASA’s IceBridge mission uses an aircraft.

“Instead of going from the surface of the ice, it measures the distance from the top of the snow,” explains Perovich. “Then you need to have an estimate of the snow depth, and from that you can calculate the thickness of the ice.”

Optical Satellites. Optical satellites allow researchers to use pattern recognition, according to Pfirman.

“You have a series of satellite images, and you can actually track where a particularly shaped flow is, and then you track where it is in the next image and then in the next image,” she explains. “Over a month it’s going to change dramatically, but between any couple of images it will look similar.”

Satellite tracking revealed that the ice’s velocity has increased dramatically.

Submarines. Submarines use sonar to map the bottom of the ice by sending a sound pulse to the ice base then calculating the distance knowing the speed of sound in water as a function of temperature/salinity.

“If you know how much is underwater you can add the part that is floating above and get an ice thickness estimate that way,” explains Perovich. “Submarine data show that the ice in the 1990s was appreciably thinner than the ice from the 1950s through the 1970s.”

The same type of measurements can be collected from moorings with upward-looking sensors.

Arctic_sea-ice_thickness_0 (1)

CryoSat is a European Space Agency mission dedicated to precisely monitoring changes in the thickness of marine ice floating in the polar oceans, along with variations in the thickness of the vast ice sheets covering Greenland and Antarctica. Here a 2013 CryoSat-2 map depicts Arctic sea-ice thickness. The map is the result of three years of ice measurements in the northern hemisphere.

Managing Ice

According to Perovich, satellite-based radar and LiDAR provide valuable data because they can cover large areas.

“Their drawback is that they require some interpretation, because they’re measuring how high the ice is floating, and then you have to calculate how thick it really is,” he says.

Additionally, because a satellite’s footprint is wide, extracting the ocean surface height can be challenging. “You need to use a lot of averaging to reduce the error,” says Tremblay.

Such techniques will be invaluable in the coming decades, as a lot of ice is still going to be forming.

“There’s a region in Canada and Alaska that will be the last ice refuge,” says Pfirman. “There’s a strong scientific consensus that if you keep looking for ice all through the summer, this is the location that’s likely to have a continuous ice cover the longest. Once you recognize that, it buys some time to get a handle on greenhouse gas emissions. If we did, this could be an area that potentially could be the seed for this refuge. There would be certain areas that could bring back the polar bears and other ice-associated species from that region if we were to cool down the Arctic.”

Managing this precious resource requires protecting the source of the ice from spills and contamination.

“We’re thinking about something along the lines of co-development with stakeholders,” says Pfirman, “where it would be multiuse, but you would want to be aware of the special status of the ice in this one area.”

“For many years, the only way to get estimates of ice volume was through models,” recalls Perovich. “It’s just in recent years that we have enough complete satellite data that we’re starting to come up with estimates of ice volume and changes in ice volume. It’s really an exciting time to be looking at this!”

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