BBC_Earth_UK_-_January_2017

combined with data on the surface area of sea ice, are

allowing researchers to get a better handle on how much ice

there is and just how much is being lost.

‘The thickness of the ice is crucial,’ Shepherd says.

Concentrating on surface area alone is deceiving because

you can have a metre-square of ice five centimetres high or a

metre-square 10m high and they tell very different stories.

‘If you don’t measure the thickness, it could appear there

is a lack of ice but it will just be piled up in a smaller area,’ he

says. Indeed, this September, the US National Snow and Ice

Data Center reported that the area covered by sea ice in the

Arctic was at one of the lowest levels on record. But CryoSat

showed that that ice was often thicker than in previous years.

‘A thinner ice pack is less resilient to changes in climate,’

says Shepherd. ‘So although we have had 30- or 40-year

records of sea ice extent, with no information about how thick

it is, you don’t know how much ice is really there. If the sea ice

in the Arctic is 10cm thick, you’d say it’s quite vulnerable to

changes in climate because that can change pretty quickly.

But if it’s five metres thick, you’d say it’s in good health.’

Early warning

It means CryoSat’s data gives us a better idea of volume. This

is important since diminishing ice cover is seen as an early

indicator of climate change – even the slightest alteration in

temperature can affect states of water in our planet’s coldest

areas. By measuring volume using CryoSat, scientists could

work out that the growth of ice was slower in November

than usual. ‘The evidence is that it’s warmer in the Arctic this

month than it usually has been and that is going to slow down

the rate at which the sea ice grows,’ says Shepherd.

CryoSat is powered by fixed solar panels and reaches

latitudes of 88 degrees north and south, giving it an overview

of large swathes of the poles. It uses an instrument called the

Synthetic Aperture Interferometric Radar Altimeter, or SIDAL,

which is especially designed for measuring the height

of ice – previous radar altimeters were built to survey

water and land.

The SIDAL sends out a radar wave from space to Earth every

50 microseconds, then listens out for the echo as it hits the

ice. By keeping a tab on where it is at any point by tracking

the position of the stars, the satellite can measure the sea

level and detect the smallest variations in the height of the ice

sitting above the water, allowing for highly accurate readings.

The data is then sent to two CryoSat ground stations based in

northern Sweden. The mission team can issue commands to

the satellite and process the information for scientific use.

Although the technology on CryoSat has also been used on

aeroplanes, the satellite has a distinct advantage. Rather than

give definitive figures for a specific location, CryoSat offers

wider coverage to provide a more comprehensive overview.

‘From time to time, the sea ice gets thicker and thinner in

To ensure CryoSat is not

delivering duff data, dozens of

scientists have been sent to

the Arctic and Antarctic by the

European Space Agency to take

snow-depth measurements on

the ground and use aeroplanes

fitted with laser scanners to

measure the thickness of the ice.

The researchers gain a better

understanding of how overlying

snow affects ice elevation

measurements and can correct

any natural seasonal changes.

The studies are supplemented

by other expeditions, including

those carried out by NASA’s

Operation IceBridge. By

comparing their manually

collected data with CryoSat, the

scientists can test it is doing its

job effectively.

How do we know CryoSat is accurate?

CryoSat sends a signal

to Earth every 50

microseconds

November Arctic sea ice volume

Ye a r

Volume (km

3 )

15,000

10,000

5,000

0,

(^201120122013201420152016)
092 / / JANUARY 2017