How Much Does the Ocean Weigh?

Water does weigh something; about 8.3 pounds per gallon. In research published this week, scientists from the National Oceanography Center and Newcastle University have proposed an idea that will assess the mass of the world ocean by weighing it at a single point. But there is a catch. Global sea level is currently rising at about 3 mm per year, but predictions of rise over the century vary from 30 cm to over a meter. There are two ways global sea level can increase. The water in the oceans can warm and expand, leading to the same weight of water taking up more space. In other words water density can vary which must be taken into account. Alternatively, more water added to the ocean from melting of land ice will increase the ocean’s weight.

The National Oceanography Centre’s Prof Christopher Hughes said: “We have shown that making accurate measurements of the changing pressure at a single point in the Pacific Ocean will indicate the mass of the world ocean. And we know where to place such an instrument — the central tropical Pacific where the deep ocean is at its quietest. This pressure gauge needs to be located away from land and oceanic variability. The principle is rather like watching your bath fill: you don’t look near the taps, where all you can see is splashing and swirling, you look at the other end where the rise is slow and steady.”

By a lucky chance, pressure measurements have been made in the Pacific Ocean since 2001, as part of the U.S. National Tsunami Hazard Mitigation Program, which focuses on detecting the small pressure fluctuations produced by the deep ocean waves that become tsunamis at the coast.

From these measurements, the team including Dr Rory Bingham, based in the School of Civil Engineering and Geosciences at Newcastle University, have been able to show that a net 6 trillion tonnes of water enters the ocean between late March and late September each year, enough to raise sea level by 1.7 cm, and leaves the ocean in the following six months.

Prof Hughes: “Of course, what we are most interested in is how much water accumulates in the ocean each year, and this is where we currently have a problem. While present instruments are able to measure pressure variations very accurately, they have a problem with long term trends, producing false outcomes.”

By knowing the weight an estimate of how much the ocean in increasing would be known which would be related to how much global warming is occurring.

“This is a challenging goal. The pressure changes are smaller than the background pressure by a factor of about 10 million, and the deep ocean is a hostile environment for mechanical components with erosion and high pressures. However, there are many other measurement systems with this kind of accuracy and there is no reason, in principle, why someone with a new idea and a fresh approach could not achieve this.

Article appearing courtesy Environmental News Network.

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