Scientists from the National Oceanography Centre and Newcastle University have proposed a method that will estimate the mass of the world ocean by weighing it at a single point.
Global sea levels are currently rising at a rate of around 3mm per year, but it has been difficult to predict the rise over the last century – with estimates ranging from 30cm to over a metre.
Sea levels can rise in two different ways: the water in the oceans can warm and expand resulting in the same weight of water taking up more space; or alternatively, the melting of land ice results in more water added to the ocean.
The researchers are trying to ascertain how much of this rise is due to an increase in the amount of water. Weighing the ocean is an ideal method because an increase in water will result in an increase of weight but an expansion of existing water will not.
Professor Christopher Hughes of the NOC and his team have shown that making accurate measurements of changing gravitational pressure at a single point in the ocean will indicate its mass.
Hughes said: “we know where to place such an instrument – the central tropical Pacific where the deep ocean is at its quietest. The pressure gauge needs to be located away from land and oceanic variability.”
By a chance of luck, pressure measurements have been made in the Pacific Ocean since 2001, as part of the US National Tsunami Hazard Mitigation Program which detects small pressure fluctuations produced by the deep ocean waves the become tsunamis at the coast.
“The necessary instruments just happened to be in the right place at the right time,” Hughes told Laboratory News.
The team were able to use the measurements to show that a net 6 trillion tonnes of water entered the ocean between late March and late September each year; enough to raise the sea level by 1.7cm.
Unfortunately, the instruments used to make these measurements don’t work well on a large timescale.
“Oceanography instrumentation has basically remained the same for 40 years. A totally different approach is now needed. Hopefully now, new measurement systems resistant to erosion and high pressures can be developed to provide us with a very simple way of working out how much the amount of water in the world ocean is increasing.” Hughes explained.