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A space mission that will be critical to our understanding of climate change has launched from California.

The Jason-2 satellite will become the primary means of measuring the shape of the world's oceans, taking readings with an accuracy of better than 4cm.

Its data will track not only sea level rise but reveal how the great mass of waters are moving around the globe.

This information will be fundamental in helping weather and climate agencies make better forecasts.

The satellite left Earth at 0746 GMT atop a Delta-2 rocket from the Vandenberg Air Force Base.

The spacecraft, built by Thales Alenia Space, represents the joint efforts of the US and French space agencies (Nasa and CNES), and the US and European organisations dedicated to studying weather and climate from orbit (Noaa and Eumetsat).

Down below

Jason-2 will provide a topographic map of 95% of the Earth's ice-free oceans every 10 days. Although we think of our seas as being flat, they are actually marked by "hills" and "valleys", where the highs and lows may be as much as two metres apart.

Elevation is a key parameter for oceanographers. Just as surface air pressure reveals what the atmosphere is doing above, so ocean height will betray details about the behaviour of water down below.

The data gives clues to temperature and salinity. When combined with gravity information, it will also indicate current direction and speed.

The oceans store vast amounts of heat from the Sun; and how they move that energy around the globe and interact with the atmosphere are what drive our climate system.

"The ocean constitutes the long-term memory of the climate system; the time-scales over which the ocean is changing are the climatic timescales," explained Mikael Rattenborg, the director of operations at Eumetsat.

"In order to understand climate, in order to be able to predict the evolution of the atmosphere over months, years, and decades even, you need to understand the ocean."

Number one

Jason-2 is a continuation of a programme that started in 1992 with the Topex/Poseidon mission and is currently maintained by the Jason-1 satellite launched in 2001.

JASON-2 SPACECRAFT 1. Advance Microwave Radiometer - measures signal delay caused by water vapour 2. GPS antennas - ensures knowledge of precise orbit path 3. Poseidon-3 altimeter- measures sea level 4. Doris antenna - tracking and positioning control 5. Laser Retroreflector Array (LRA) - tracks and calibrates measurements Satellite mass: 525kg (1,155lb) Power generation: 511 watts Satellite height: 3m (9ft 8in) Orbit: 1,338km (831 miles) (Source: Eumetsat, Cnes, Nasa)

The project provides the global reference data for satellite-measured ocean height.

Although other spacecraft in service today can acquire similar data sets, none can match the precision achieved by Jason-1; and Jason-2, when in service, will be the benchmark against which all other spacecraft will be judged and calibrated.

At the heart of the latest mission is the Poseidon 3 solid-state altimeter. The instrument constantly bounces microwave pulses off the sea surface. By timing how long the signal takes to make the return trip, it can determine sea surface height.

Additionally, the signal can indicate the height of waves and wind speed.

"It is not a revolution between Jason-1 and Jason-2; it is an evolution, because the main objective is to ensure continuity," explains Francois Parisot, the Jason-2 project chief at Eumetsat.

"Nevertheless, there are some improvements in the instruments. We hope to make better measurements closer to the coast [and over inland waters and rivers]; and also, we will deliver near-realtime products - products that will be available within three hours of the measurements."

Whale watching

The latter will be particularly useful in storm prediction. Jason will see the surface waters rise as warm eddies fuel hurricanes. The data will tell meteorologists how a storm is likely to intensify and allow them to issue better, more timely warnings.

Jason-2 data will have many other uses that may not be immediately obvious. Industry will take the information to make decisions about when conditions are most suitable for undersea drilling or cable laying.

Jason can help identify where wreckage or pollution will drift; and the satellite will assist marine biologists as they track whales by pinpointing waters with the potential to be prime feeding and breeding grounds.

One very important use will be in maritime navigation.

"Now that the fuel price is going up, saving fuel for the companies that run ships has become very sensitive; and knowing the currents, you can select your route so that you go faster and save fuel," said Philippe Escudier, a space oceanography at CLS (Collecte Localisation Satellites), Toulouse, France.

"You can save up to 5% on fuel consumption by making best use of the currents."

Formation flying

Jason-2 will spend its first few months flying a "tandem mission" with Jason-1.

The two spacecraft will be positioned so that they sweep around the Earth, one following the other, with a separation of just 60 seconds.

This will enable, essentially, the two satellites to measure the same patch of ocean surface at very nearly the same time.

Changes in ocean height can be a key indicator of climate cycles More details

Scientists will use this opportunity to cross-calibrate the instruments so that when Jason-1 is retired (or fails), the future data collected by its successor will be directly comparable with past records.

This continuity of information will be critical in recognising long-term trends in ocean behaviour. It is the data which underpins the observation that global sea level is rising by about three millimetres per year.

Once the tandem phase is completed, Jason-1 will be moved to the side, doubling the return of data. The importance of the Jason programme means both spacecraft will almost certainly be run for as long as they are serviceable.

Discussions are already in progress on a Jason-3 satellite. Given Europe's role in the project, there is a compelling case for the next mission to be included in the GMES (Global Monitoring for Environment and Security) programme. This would attract significant EU money.