Mapping sea-level rise

A major reassessment of 18 years of satellite observations has provided a new and a detailed view of sea-level change around the world.

This study reaffirms that ocean waters globally are rising by just over three millimeters per year. But that figure, according to the reassessment, hides some very big regional differences. The Philippine Sea, for instance, has seen increase of 10 millimeters per year.

Part of that signal reflects the great fluctuation in winds and sea-surface temperature across the Pacific Ocean known as the El Nino or La Nina-Southern Oscillation. Steven Nerem of the University of Colorado explains, “The trend map is really a way of looking at average field changes over the 20 years.” The places where you see high trends probably won’t have high trends in another 20 years. Nerem further posits: “A lot of this is decadal variability that will average out over the longer time series, which is why we need more missions to understand where this variability lies.” Paolo Cipollini from UK’s National Oceanography Center suggests, “Many of the features in the trend map indicate changes in heat storage and correspond to long-term variations in the ocean currents.”

This observation is evident if you look for some of the well-established mass movements of water. For instance, the Gulf stream arching across the North Atlantic from the eastern US, or the Kuroshio extension reaching out from Japan into the Pacific.

The research will help scientists to tease out the scale of the various contributions, to long-term sea-level rise and understand better the annual and interannual changes that can occur. It is crucial to identify the extent to which sea-level rise may be accelerating, and pull out any long-term oscillations in ocean behaviour that could confuse the signal.

This study was completed as part of the climate change initiative project by European space agency member states at their ministerial meeting in 2008. 

Key indicator of change

The climate change initiative seeks to deliver long-term observations on a large set of “essential climate variables.” Sea-level change is taken up as one of the most important indicators of global change. Routine observations began with Europe’s ERS-1 spacecraft in 1991, and this has subsequently been followed up by international missions. The benchmark is arguably Jason or Poseidon, a co-operative venture between the U S and Europe (principally France). Now in its third incarnation, the Jason satellite circles the globe making a topographic map of 95 per cent of earth’s ice-free oceans every 10 days.

It uses a radar altimeter, which constantly bounces microwave pulses off the sea surface to accomplish this task. Through timing as to how long the signal takes to make the return trip, this instrument determines sea-surface height. Jason’s data needs to be tied into satellites which view parts of the world it cannot see, and also mapped on to tide gauge information. An important tool that has been introduced recently is the gravity satellite or the two US Grace spacecraft.

This duo can weigh the amount of ice held in Antarctica and Greenland, and the quantity of water stored on the continents. “The key challenge in the coming years is to ensure that we keep acquiring altimetry data, that we are able to calibrate it and ensure its quality,” says Maurice Borgeaud from Applications and future technologies department of ESA’s earth observation science.