The National Snow and Ice Data Center reports ice extent, a two-dimensional measure of the Arctic Ocean's ice cover. But sea ice extent tells only part of the story: sea ice is not all flat like a sheet of paper. While freshly formed ice might not be much thicker than a few sheets of paper, the oldest, thickest ice in the Arctic can be more than 15 feet thick -- as thick as a one-story house. Scientists want to know not just how far the ice extends, but also how deep and thick it is, because thinner ice is more vulnerable to summer melt.
But ice thickness is hard to measure, especially on a large scale. Scientists cannot measure the thickness of the entire Arctic sea ice cover by hiking around and drilling a hole every ten feet -- the Arctic Ocean spans millions of square miles and is constantly on the move, swept around by winds and ocean currents. And while some newer satellites can provide estimates of ice thickness, there is no long-term satellite record of ice thickness as there is for ice extent.
With all these limitations, how do researchers study how sea ice is changing underneath its shiny surface?
From the surface
Scientists do sometimes measure ice thickness by drilling holes in the ice. The method seems simple, but it does not work on a large scale. Field researchers also sometimes drag sensors on sleds that can send out pulses of electromagnetic energy to estimate the thickness of the ice below them—these include radar sensors or electromagnetic induction instruments. Researchers can also collect data using submarines that cruise beneath the ice, using an instrument called upward-looking sonar.
It is not practical to try to measure ice thickness over the entire Arctic Ocean on foot, or even by submarine, but field measurements can give scientists a detailed picture of how ice moves, melts, or grows in a particular area. Scientists can also compare field data to other types of data to figure out if other measurements accurately represent the Arctic ice cover.
From the air
Satellite measurements of ice thickness only go back about ten years. Earlier this summer, European scientists reported the first set of ice thickness data from the European Space Agency's new Cryosat 2 satellite. The NASA Ice, Cloud, and Land Elevation satellite (ICESat) collected ice thickness measurements from 2003 to 2009. NASA plans to launch a second ICESat in 2016 to continue those measurements. The tricky part with satellite data is to get a long time series of comparable data. Because Cryosat uses a radar altimeter, while ICESat uses a laser altimeter, the two sets of measurements can be difficult to match up. So researchers plan to look at the trends within each type of data, and then compare the trends they see. They will also look at field data to determine how the satellite estimates match with measurements on the ground.
Some of the same sensors sitting on satellites can also hitch a ride on an airplane or helicopter. From 2009 to 2016, NASA is using airplanes outfitted with ice-sensitive sensors to help span the gap between the two ICESat missions, through the Operation IceBridge Project.
From the computer
To get longer time series, researchers mash up different types of data and use sophisticated computer models.
Researchers Jim Maslanik and Chuck Fowler at the University of Colorado, along with Julienne Stroeve at NSIDC, have tied ice age to ice thickness using a combination of satellite data and location data from buoys that sit atop the ice cover. Sea ice thickens over winter. In its first winter, ice may grow to be only a few feet thick, although thicker ice may form as winds and ocean currents push ice floes atop each other. While some of this ice may simply melt away in summer, some (in particular the thicker ridged ice) survives summer melt, and will thicken again the next winter. In general, the more summer melt seasons that the ice survives, the thicker it gets.
Researchers Jinlun Zhang and colleagues at the University of Washington, meanwhile, have developed a computer model that combines atmospheric observations with ocean conditions and sea ice data. The model estimates sea ice volume, or the total amount of ice in the Arctic. Zhang's colleague Axel Schweiger, who has been working on comparing the model data with observations said, "Because neither data or models are perfect, combining them takes advantage of both. Models can provide information on things we cannot directly observe, and data can help keep the models on track." Another group of researchers at the University of Washington has developed a unified sea ice thickness data set using submarine and other data.
Together, all these methods provide convincing evidence that as Arctic sea ice has declined in extent, the ice has also become thinner and younger. Stroeve said, "Thickness data collected using various methods indicate the average thickness of sea ice in the Arctic Basin has declined from more than 3 meters (10 feet) in winter to less than 2.5 meters (8 feet) in winter."
As researchers come up with better ways to evaluate ice thickness, they will be better able to say how the actual volume of Arctic sea ice is changing with the decreasing ice extent. Schweiger said. "Changes in ice volume tell us how much ice has grown or melted and this is directly connected to the amount of energy involved in the growth or melt of ice."