A Magic Definition of Sea Water

It seems like a silly question…what is sea water? But, the answer has all kinds of implications for our scientific understanding of fresh water supply and climate change. Marine scientists, meteorologists, and oceanographers have been searching for the magic formula for measuring salinity – which varies from ocean to ocean and between tropical, temperate and polar regions – for more than 150 years.

Now, the world’s peak ocean science body has adopted a new definition of seawater developed by an international team to help make climate predictions more accurate.

The General Assembly of UNESCO’s Intergovernmental Oceanographic Commission (IOC) accepted the case for the introduction of a new international description of seawater based on its physical, or thermodynamic, properties. The new definition uses a new property called Absolute Salinity to say whether a particular body, lake, estuary, or marginal land is seawater or not.

Variations in salinity and heat influence ocean currents and measuring those variations are central to quantifying the oceans’ role in climate change. These variations drive deep ocean currents and the major vertical overturning circulations of the worlds oceans, which transfer ocean heat towards the Arctic and Antarctic regions.

The last oceanic definition is now three decades old, but a re-assessment was instigated in 2005 when the Scientific Committee on Oceanic Research (SCOR) established a working group, chaired by Trevor McDougall, of Australia’s CSIRO Marine & Atmospheric Research. Supporting him were Rainer Feistel from the Leibniz-Institut für Ostseeforschung in Warnemünde (Germany), Dr Frank Millero, from the Rosenstiel School of Marine and Atmospheric Science at the University of Miami in Florida, Dan Wright of the Bedford Institute of Oceanography, Canada and David Jackett also of CSIRO.

In the old definition, salinity, which is a measure of the salt content in the oceans was determined by electrical conductivity measurements and assumed that the composition of salt in seawater is the same in all the world’s oceans.

“The new approach, involving Absolute Salinity, takes into account the changes in the composition of sea salt between different ocean basins which, while small, are a factor of about ten larger than the accuracy with which scientists can measure salinity at sea,” explains McDougall.

Once the new definition is adopted, climatologists will have a better variable to plug into their models for understanding temperature effects due to differences in salt types and concentrations. The difference will be less than 1 degree Celsius at the sea surface, but it is nevertheless important to correct for such biases in climate models.

IOC site on ocean salinity