Every spring, the scientists and flight crew of Operation Ice Bridge fly low over the Arctic, studying the Greenland ice sheet and nearby sea ice with high-resolution digital and thermal imaging cameras, a sophisticated radar suite, a laser altimeter and other instruments.
The team just returned from its seventh Arctic mission: 10 weeks in Greenland and a total of 33 eight-hour flights over the ice in a modified C-130 cargo plane. The sea ice around the North Pole is critical to regulating global temperature, ocean currents, and atmospheric circulation, and it's melting away faster every year. That's because Arctic sea ice is also more sensitive to rising temperatures than any other part of the planet, so it's no wonder that NASA sent an airborne science mission to monitor the situation.
Back in 2003, NASA launched its Ice, Cloud, and Land Elevation Satellite, aptly named ICESat, which kept an eye on the ice, along with other environmental concerns, from near-polar orbit until its instruments stopped working in late 2009. Its successor, the cleverly named ICESat-2, won't launch until 2017. To fill that gap, NASA created Operation Ice Bridge.
Welcome to the Poles
As the National Snow and Ice Data Center -- which hosts the publicly available data from Operation Ice Bridge -- puts it, "Antarctica is a continent surrounded by water, while the Arctic is an ocean surrounded by land," and they're reacting differently to climate change. Antarctica is actually gaining sea ice, while sea ice is vanishing rapidly from around the Arctic.
Sea ice forms in the ocean, unlike glaciers and ice shelves that form on land and may end up floating on the oceans. Although Arctic land ice, like the Greenland ice sheet, is thinning too, sea ice in the Arctic is vanishing much more quickly and may have a bigger impact on global climate.
Because it's so light in colour, ice reflects about 80 per cent of the sunlight that hits it back into space, so its high albedo, or reflectivity, actually helps the ice cool. Open ocean, on the other hand, is dark enough to absorb about 90 per cent of the sunlight that reaches it, so it warms much more easily. Warmer ocean water melts sea ice from below. A warmer ocean also warms the air above it, which also speeds the melting of the ice. Melting ice means more open water to absorb heat from the Sun, creating what scientists call a positive feedback loop.
If the poles get too warm, it will alter global patterns of ocean and atmosphere circulation, creating a bigger positive feedback loop that accelerates the whole process of global warming.
Vanishing Sea Ice
Of course, there's always some seasonal variation in the ice; every summer, the ice melts until it reaches what scientists call the "sea ice minimum" at the end of the season. As winter sets in, the ice grows again until it reaches the "sea ice maximum." The problem is that over the last few decades, the summer melting season has lasted longer, and the ice has recovered less during the winter.
Even in the middle of the polar summers, there's always some sea ice left. Some of it, known as multi-year ice, sticks around for season after season. But that may not always be true, unless global warming slows dramatically -- which isn't looking too likely at the moment. "A lot of this depends on what we, as a society, do in the future, but all the models really point to what's called an ice-free summer in terms of the sea ice loss," said Operation Ice Bridge's chief scientist, Nathan Kurtz. Climate models vary, but most agree that within the next century, the Arctic sea ice cover will largely be completely gone during the summer. The average prediction, Kurtz said, is about 30 years.
That's bad news for polar wildlife. Near Alaska, where there's already much less sea ice around in the summer than there used to be, the polar bears who live and hunt on the multi-year sea ice have had to swim longer and longer distances to reach the ice. Seals may also suffer in the years to come; Kurtz explained, "Because the ice isn't around all year, it doesn't collect as much snow as it used to, and seals, in particular, need a certain amount of snow cover because they use the snow to build their dens."
So how quickly is the Arctic sea ice thinning? The ice around Greenland tends to be the thickest in the region, and it's thinned from about seven meters to between three and four meters since the first submarine measurements in the 1950s.
An Arsenal of Instruments
But it's not a process that's visible to the naked eye from one year to the next. "When you actually look at the data, as an example, there are places in Greenland that are thinning at meters per year. This is a huge signal, when you look at the actual data itself, but maybe not necessarily something you can see with your eyes," said Kurtz.
Operation Ice Bridge relies on a suite of instruments to see how the ice changes from year to year. A laser altimeter is the main instrument in Ice Bridge's arsenal. The team fires a laser pulse at the ground and measures how long it takes the pulse to reflect off the surface and return to the plane. With a very precise knowledge of the plane's position and altitude, scientists can use the laser to measure how high the ice is above mean sea level and therefore how thick it is.
A shallow radar reveals how deep the snow on top of the sea is. A new layer of snow accumulates each year, and scientists can use the shallow radar to see those layers and understand how much snow accumulated on the ice sheet every year. It's a bit like reading the rings on a tree.
With another type of radar, Ice Bridge's scientists can look much deeper, all the way down to the bedrock beneath the Greenland ice sheet. Understanding how thick the ice is over the bedrock, and how the bedrock itself is structured, is important in building models that predict how the ice might change in the future.
And on past missions, Ice Bridge has carried an instrument called a gravimeter, which measures the gravity field in an area. "This is important for essentially determining when you're flying over floating ice," said Kurtz. "The gravimeter taking these gravity measurements allows us to determine, essentially, how deep the ocean is underneath these ice shelves, which helps determine how the ocean is circulating, how much heat is being put into the ice, and things like that."
Bridging the Gap
Ground crews set up a survey field on an ice floe for Ice Bridge. Picture: Norwegian Polar Institute/ Gunnar Spreen
In a way, Ice Bridge is also bridging the gap between large-scale satellite data and local ground observations. The European Space Agency has its own satellite, called CryoSat, in low Earth orbit, where it studies changes in ice thickness at the poles using a radar altimeter. Meanwhile, on the ground and at sea, researchers from several countries brave the Artic's cold and rugged terrain to survey changes in ice. And 1500 feet above the ice, the airborne scientists of Ice Bridge are in the middle of it all, helping to bring together a bigger, multi-layered picture of what's happening to the ice.
"We underflew an ESA satellite, overflew a Norweigan ship, overflew multiple ground experiments, and even did some things in the field such as assisting other scientists selecting targets. We had overflown their targets and gave them data so that they could then better prep for their own field campaigns," said Kurtz.
Ground measurements are much more detailed but much more limited in scope than an aerial survey like Ice Bridge, so combining Ice Bridge's aerial data with the "ground truth" from a few locations also makes it possible to extrapolate those details to what's happening in other areas. Of course, satellites like the ICESats and CryoSat can cover much more territory than aerial surveys. On the other hand, aerial surveys can still cover a lot of ground -- or ice -- and they can capture more detail than satellite views of the same area.
Aerial missions like Ice Bridge can also usually carry a greater variety of instruments than satellites. You can load an awful lot of gear onto a C-130 (they call it Hercules for a reason), or onto the P-3 Orion and DC-8 that have flown the Ice Bridge mission in previous seasons. When it comes to carrying lots of instruments and equipment, satellites are more limited, because it's usually more expensive to fuel a rocket for launch -- about $US10,000 per pound to get something into orbit, unless you can split the cost with other payloads -- than it is to fuel a plane for a season's worth of flights. "But on Ice Bridge, we can really tailor the instruments that we want, to get this comprehensive look at the ice that we can't get, necessarily, from a satellite," said Kurtz.
Ice Bridge was originally meant to be a six year mission, but it's currently funded through 2019, two years after ICESat-2 is scheduled to launch. During those two years, Ice Bridge will help confirm measurements from the new satellite.
Challenging Work Environment
Operation Ice Bridge's modified C-130. Picture: NASA
The Arctic doesn't give up its secrets easily; scientists still have to contend with the formidable polar weather. During part of each year's campaign, Operation Ice Bridge flies out of a base at Kangerlussuaq, Greenland, where there's sometimes not even a heated hangar for the aircraft. Flight crews have to warm up the engines before every flight. "It definitely can be a challenge," said Kurtz.
At Thule Air Force Base in northern Greenland, where the Ice Bridge team spends the rest of each campaign, there are at least heated hangars to park the plane in overnight, but Arctic weather can still be rough. "Sometimes there's these major wind events that happen in Thule, and they just kick up a lot of blowing snow," said Kurtz. "Depending on the storm conditions forecast, we might not even be able to take off. If the storm conditions are bad enough, we're actually not even allowed to leave our living quarters. This has happened on several occasions, where we're stuck in our living quarters while the storm has to blow over."
Even on days without storms, flights have to be planned to avoid cloud cover that the instruments on the plane can't penetrate. "We have to know where we can fly based on are there clouds, are our instruments going to see through it, and is it dangerous to fly here," explained Kurtz, "but once we look at the weather, we decide where it is that we want to fly, based on science priorities that have been identified beforehand."
The scientists start gathering data as soon as the plane is in the air, and most are busy with their instruments. But as the project scientist, Kurtz said, he's responsible for keeping an eye on factors that might impact the mission and its data collection -- a role which allows him plenty of time to look out the window and appreciate the barren beauty of the ice.
"There's some very, very beautiful scenery when we fly over the Arctic. It's just such a very barren place, but it's just not a typical scene that I would ever really see -- I live in Maryland, so there's nothing like it that I've ever seen," said Kurtz. "When we're flying over Greenland, just mountains, snow, and ice mixed with ocean. Sometimes we fly over very chaotic areas where the ice is moving quickly and it gets very crevassed and just very damaged and things like that, and we fly over at very low altitudes, less than 1500 feet, so we get a very close look at the surface. It's sort of this mixture between a close look at the surface but we're high enough up that we can see that this stuff just extends so far."
Free Environmental Data
"This is all publicly available, so when people are interested in the science or if they want to question it, everything the scientists are working with -- and a lot of very big results have come from the Ice Bridge project, you know -- anyone can look at this data where these results came from," said Kurtz.
The first thing to be published every season is called, appropriately enough, the Quick Look data. Kurtz processes the Quick Look data himself as soon as he returns from Greenland, while most of the instrument teams are still in the field. "This is done to help seasonal forecasts," he said. "This is sort of the immediate results and immediate use of the data."
And later, once the project's scientists have had time to process the data, a more detailed data product is also available to the public on the National Snow and Ice Data Center website.