Cryosat mission delivers first sea-ice map
This is the best view we have yet had of the thickness of sea-ice across the entire Arctic Ocean basin.
It is the first fully processed map from Europe’s new Cryosat spacecraft.
It only covers the months of January and February, but the UK team behind the data says it can now roll out the information on a continuous basis.
The extent of Arctic sea-ice has become a major issue in recent years, with summer melting appearing to outstrip what many climate models had predicted.
But a proper assessment of the status of the sea-ice requires knowledge also about its thickness – something scientists have only recently had the tools to measure from space.
“Some years the wind will push the ice out of the way or pile it up, and it may look from the area coverage like it’s all melted,” explained the Cryosat mission’s principal investigator, Professor Duncan Wingham.
“But it’s only when you combine the area coverage information with the thickness information that you get the product – volume. And that’s what you really need to know to answer the question about melting,” he told BBC News.
Professor Wingham presented Cryosat’s first ice map here at the Paris Air Show in Le Bourget, a major event in the space calendar.
The European Space Agency (Esa) launched its “ice explorer” last year. It carries one of the highest resolution synthetic aperture radars ever put in orbit.
The instrument sends down pulses of microwave energy that bounce off both the top of the Arctic sea-ice and the water in the cracks, or leads, which separate the floes.
By measuring the difference in height between these two surfaces, the Cryosat team is able, using a relatively simple calculation, to work out the overall volume of the marine ice cover in the far north.
Wingham’s group at the Centre for Polar Observation and Modelling, University College London, has spent the past year learning how to interpret the radar data and turn it into a form that the research community can use.
How to measure sea-ice thickness from space
- Cryosat’s radar has the resolution to see the Arctic’s floes and leads
- Some 7/8ths of the ice tends to sit below the waterline – the draft
- The aim is to measure the freeboard – the ice part above the waterline
- Knowing this 1/8th figure allows Cryosat to work out sea ice thickness
This has involved calibrating the instrument and then validating its output by comparing it with independent assessments.
One such assessment employed a German Alfred Wegener Institute (AWI) aeroplane.
It obtained thickness information by flying a laser altimeter to record the distance to the top of the ice and a conductivity sensor to identify the location of seawater on the underside of the floes. Being an aeroplane, it could only record limited lines of data, but these strongly correlated with the Cryosat observations.
Another independent assessment called on a different type of radar satellite instrument known as a scatterometer, which, as its name suggests, looks at how much of the energy beamed down from space is reflected back or scattered away. This type of instrument can discern the thin flat seasonal ice from the rough terrains associated with floes that have been around for many years.
Again, what the scatterometer saw with its approach was an excellent match for the Cryosat map.
“We’re now processing the rest of the Cryosat data to get it to the same standard as we’re showing you here,” said CPOM researcher Dr Katharine Giles. “Then we will use that data to look at how the ice cover is changing. This is only two months of data – and we’re very excited to have our first map – but we need to compare year-on-year changes.”
Scientists already have a number of insights on sea-ice thickness in the Arctic – from buoys, from submarine sonar data, from field expeditions, from aircraft sorties such those by the AWI, and from previous generations of satellite radar and laser altimeters. But Cryosat should be a big boost to that data haul, not least because it sees the entire Arctic basin, right up to two degrees from the pole.
In addition to its sea-ice mission, Cryosat is also tracking changes in land-ice.
For this, the radar instrument carries a second antenna. By listening to the radar echoes with an additional device offset from the first by about a metre, the satellite can sense much better the shape of the ice below, returning more reliable information on slopes and ridges.
This is especially important in Greenland and Antarctica where past missions have struggled to discern events at the edges of the ice sheets – the very locations where some of the biggest, fastest changes have been taking place.
Here at Le Bourget, an elevation model of Antarctica, built from Cryosat data, was displayed. Again, this covered just a couple of months earlier this year.
• As with the Arctic sea-ice map, this height model of Antarctica incorporates just two months of data at the start of the year
• The outer ring shows the closest older satellites could get to the pole. The inner hole is the only portion unseen by Cryosat
• The exaggerated model has been sliced open like a cake to show the position of the Antarctic bedrock under the ice
• By subtracting ice-surface height from bedrock height, Cryosat can derive ice thickness across the entire continent
Cryosat was given an initial mission plan to 2013, but engineers fully expect it to keep working until perhaps 2017. To pay for that extension, Esa will need to get new funding from Europe’s space ministers when they meet for their big conference in Italy next year.
Dr Volker Liebig, the director of Earth observation at the agency, said Cryosat had already made a compelling case for the additional support – and perhaps for something even bigger down the line.
“Cryosat is a [one-off] science mission that is part of Esa’s Envelope Earth observation programme. I think it has the makings of a [on-going] operational mission,” he told the air show presentation.
“The need for this data will not stop when we retire this mission. The first step is to run Cryosat for as long as possible, but then we have a couple of missions that have the potential to become operational – and Cryosat is one of them.”
Esa itself does not launch repeat missions. It leaves this to Europe’s meteorological satellite organisation, Eumetsat. One option also for a recurring Cryosat series would be to incorporate it into the European Union’s forthcoming Sentinel Earth-observation project. These satellites are being launched to acquire high-priority environmental data-sets long into the future.