High above the Earth, satellites track melting ice

That’s crucial to the operation of yet another type of orbiting instrument known as an altimeter. As the name suggests, an altimeter measures the satellite’s altitude above the surface - if that altitude changes over time due to rising sea level, say, or dwindling ice, a satellite altimeter can track that change down to the millimetre. It works similarly to an active microwave instrument, except that rather than looking at where the ice is and where it isn’t, an altimeter measures the surface’s height, by gauging how long it takes the beam to travel down and back.

NASA’s ICESat, which broke down last fall after six years in orbit, did its altimetry by laser. Before it malfunctioned, its instruments followed up on GRACE’s detection of mass loss in Greenland by measuring a physical thinning of the huge island’s seaward-flowing glaciers, and making similar measurements of retreating glaciers in West Antarctica. As recently as 2007, the Intergovernmental Panel on Climate Change’s Fourth Assessment Report declared that there wasn’t sufficient information available about changes in glacier and ice sheet dynamics to gauge their likely effect on sea level. Now, thanks in part to ICESat, that information is emerging.

The European Space Agency’s CryoSat-2 satellite uses radar rather than lasers, so its beam is less sharp and thus takes in a bigger, fuzzier patch of ice at a time. Like ICESat, CryoSat-2 can measure not only the changing thickness of land ice but also that of sea ice. Thinner first-year ice is easier to melt, so the amount of it relative to the thicker stuff makes a big difference when summer comes. By measuring “freeboard” - that is, the height of the ice above sea - CryoSat-2’s altimeter can differentiate between first-year ice and multi-year ice, because a thicker slab of ice floats higher in the water. That kind of information is important to Arctic ice experts, since a decline in thicker, multi-year sea ice sets the stage for more rapid melting in the future.

CryoSat-2’s projected lifetime is five years; by the time it runs out of steam, NASA is hoping to have ICESat-2 in orbit to replace its own defunct satellite. In the meantime, both agencies will be launching ice-detecting instruments aboard new satellites. And just as next year’s new crop of laptops and cell phones will outshine last year’s, these instruments will be more compact, powerful and feature-laden than their existing counterparts.

“This is a very exciting time for us,” says Drinkwater. “We’ve got six more already approved, so when we want to put the big picture together, we’ll have a lot of tools up there.”