BioEd Online

How salty are the oceans? Back in the 1970s, scientists dreamed that a satellite could provide the answer by measuring microwave emissions from the seas.

That dream is rapidly becoming a reality. NASA plans to launch a new instrument, Aquarius, on 9 June, which should allow researchers to monitor global salinity measurements to help answer some pressing climate questions. Because salinity is linked to both evaporation and water density, the new data could help scientists explore questions about precipitation trends as well as about ocean circulation and the uptake of carbon dioxide by seawater.

"It's going to be a leap forward for the science of oceanography," says NASA project scientist Eric Lindstrom. In particular, he says, Aquarius could help scientists confirm theories about how the global water cycle — experienced in everyday life as surface evaporation, rainfall and snowfall — is changing in response to global warming.

Aquarius will fly as part of a joint mission with Argentina's National Commission for Space Activities, which built the main satellite as well as other instruments on board and which will take the lead in managing the mission from the ground.

The mission comes on the heels of two disasters for NASA's Earth observations projects. In 2009, the Orbiting Carbon Observatory, which would have monitored carbon dioxide levels, failed to launch; earlier this year, the Glory probe, which would have advanced solar monitoring and global aerosol measurements, suffered the same fate (see Mourning Glory).

Salty signals

The heart of Aquarius is a set of three ultrasensitive radio receivers that will pick up the weak microwave radiation emitted naturally by the ocean. Those emissions vary according to the electrical conductivity of the water, which is directly tied to its salinity. In combination, the three instruments will be able to gather data across a swathe of ocean nearly 390 kilometres wide, allowing Aquarius to cover the whole globe once every seven days, measuring changes in salinity down to 2 parts per 10,000 in seawater.

Given that most of the salinity data going back 50 years comes from measurements from ships, this represents a huge advance, says Tim Boyer, an oceanographer with the National Oceanic and Atmospheric Administration in Silver Spring, Maryland. "It's definitely good enough to see large-scale seasonal cycles" such as the Amazon outflow, Boyer says, but scientists won't know exactly how much they can do until they see what the data looks like. "Until you put the satellite up, you don't know what you are going to get."

Salinity levels vary widely across the ocean. River systems dilute seawater around deltas, and evaporation can increase the salinity in one area of the ocean only to produce precipitation that reduces salinity in another. Or a strong sun, combined with hot dry air blowing from the Sahara, might increase salinity off the west coast of Africa while fuelling storms that can grow into hurricanes across the Atlantic Ocean.

In recent years, scientists have begun collecting salinity data using the Argo ocean observing network. These probes collect data in the deep ocean and periodically surface to transmit the measurements to scientists on shore. But that system only collects salinity data below a depth of about 4 metres. Scientists are now working to deploy around 100 Argo floats that produce a salinity profile all the way up to the surface.

Those sensors will help scientists to bridge the data gap between Argo and Aquarius, says Steve Riser, an oceanographer at the University of Washington in Seattle. And if it turns out that surface readings correspond well with the readings at 4 meters, Riser adds, scientists might be able to extrapolate surface salinity from the entire network of some 3,200 floats. "Based on what we know so far," Riser says, "I would suspect that most of the Argo floats will have some validity."

For Raymond Schmitt, a senior scientist at the Woods Hole Oceanographic Institution in Massachusetts, Aquarius could help resolve an apparent discrepancy between global climate models and historical observations of ocean salinity.

Global warming is expected to speed up the water cycle. Because warmer air holds more water, one would expect to see more evaporation, more precipitation and, consequently, more extreme weather. Observations over the past 50 years seem to confirm that these changes are leaving their imprint on ocean salinity: salty regions have become saltier, and less salty regions have grown even fresher.

While the salinity data would seem to suggest a massive acceleration of the water cycle, Schmitt says, climate models tend to suggest that weaker winds will offset the effects of higher temperatures, leading to a more moderate increase in the water cycle. It is also possible that global warming is driving shifts in ocean circulation that could contribute to the changes in salinity, but that is unlikely to explain the whole effect.

"We have reason to be concerned that the water cycle is changing a lot faster than predicted, and that could be serious" because this would translate into more extreme weather in the years and decades to come, says Schmitt. Answering these questions won't be easy, even with new data. "Aquarius is trying to do a hard thing," he says, "but to me, ocean salinity is the best gauge we have on what these water cycle changes are going to be."