BioEd Online

The first satellite to accurately measure how fast the Earth's polar ice caps are shrinking will launch on 8 October.

Unlike previous radar satellites, CryoSat carries twin radar antennae that give it three-dimensional vision, so it can see not only how much of the planet's surface is covered with ice, but also how thick the ice is. The satellite should be able to detect changes in thickness of just a few centimetres, and can even see through thick cloud.

The result is the most precise radar system ever sent into space, according to the chief scientist on the project, Duncan Wingham of University College London.

The craft's main goal is to watch floating sea ice around the Arctic. This ice plays an important part in moderating our climate by reflecting the Sun's heat from the Earth. When it melts, this will not only raise sea levels but also expose the darker oceans that greedily soak up the Sun's rays, accelerating ice loss.

Many climatologists believe that as fresh water floods into the Atlantic Ocean from the melting ice, it could disrupt the ocean currents that play a vital role in climate (see ' Disaster Movie Makes Waves' ). But no one really knows how significant this effect will be, says Wingham. "The only way to get the answer is to measure the ice thickness."

NASA's orbiting ICESat uses lasers to do a similar job. But although lasers have a better resolution than radar, they have a hard time distinguishing between ice and water, says Wingham. CryoSat's radar is much better suited to tracking the floating ice of the Arctic.

Fading fast

On 28 September, scientists using NASA's NIMBUS-7 satellite reported that Arctic sea ice was at its lowest extent since satellite monitoring began in 1978. But Wingham points out that these measurements reveal nothing about the thickness of the ice, so the amount of ice loss could in fact be even greater than current estimates.

NIMBUS-7 also suffered from only being able to take snapshots of certain areas of the Arctic at certain times, rather than getting the full picture. "The great difficulty at present is in figuring out whether the changes in ice cover are due to melting or to changes in the winds that shift the ice around," says Wingham. "The only way to do this is to examine the entire Arctic at the same time."

CryoSat will make a complete survey of the Arctic and Antarctic every month. It orbits the Earth around the poles, completing a circuit every 100 minutes.

The satellite will also measure ice thickness on the landmasses of Greenland and Antarctica. Previous ice-monitoring satellites ERS-1 and ERS-2 have made more than ten years of measurements, but their radar equipment works best on the central, flat parts of these continents. (see ' East Antarctica puts on weight'). CryoSat should be able to pick up ice loss at the steep edges of the ice sheets as well, filling in vital details about the where the polar caps experience most melting.

Only the beginning

According to computer models produced at the Hadley Centre for Climate Prediction and Research in Exeter, UK, average temperatures in the Arctic will rise by at least 5 °C in the next hundred years, which would completely melt all the sea ice there.

That loss is not inevitable, however, says Alan Thorpe, chief executive of the UK Natural Environment Research Council, which has contributed part of the project's €135-million (US$160-million) budget. "If we could stop emitting greenhouse gasses, this trend could be reversed," he says.

The mission should start to return useful data after about two years in orbit, and will stay there for at least five years. This will not be long enough to observe long-term trends, admits Wingham. "But if you don't fly the first mission, you'll never fly a second or a third," he says. Researchers hope that if CryoSat proves a success then similar probes will be commissioned.