Halosteric Sea Level Rise

Sort of scraping the barrel here - the salt barrel that is (are salt barrels even a thing?), time for salinity induced sea level rise aka halosteric rise. This together with thermosteric rise that we discussed way back in October is the steric component of sea level. Basically meaning that heat and salt effect how much space the water in the oceans actually takes up.

Just in case you weren't sure ... here is some salt (PlanetScience)

But how on Earth does salt effect sea level?


It's the same as thermal expansion - it's all to do with changing density of the oceans. The saltier the oceans the more desnse they are and the less space they take up. But as we are seeing at the moment the oceans are getting fresher as ice melts around the world and effectively dilutes the oceans. So they are less dense, and this means they take up a little more space, so sea level is higher (Antonov et al, 2002).


But it's actually quite a bit more complicated than this...


Because the oceans are so so so big, they can pretty much be assumed to have constant salinity even over hundreds of years (Antonov et al, 2002). This means that halosteric sea level rise is nowhere near as important for global sea level as thermosteric rise. Ishii et al (2006) have estimatic that halosteric sea level rise accounts for only 0.04 mm a year between 1955 and 2003, compared to the 0.31 mm a year they attributed to thermal expansion for the same period. 


Infact, the interesting thing about halosteric rise is that only ~1% of the halosteric expansion actually adds to the global sea level. This is because when freshwater is added to the oceans, while this does decrease the salinity and density, this is counteracted by the freshwater getting denser because it has mixed with the salty oceans. This basically means that you cannot really look at the two processes separately - the sea level is mainly rising because water has been added to the oceans, rather than the salt concentration decreasing (IPCC AR4). As you can see in this graph from NOAA, the halosteric component of sea level rise is minimal:

A very small upwards trend is observed in the halosteric component of sea level change (NOAA)

But don't disregard it so quickly, salt is still a really important control on regional sea level - the oceans aren't the same saltiness everywhere. As salt content is redistributed through the various ocean basins there can be an major impact on local sea level. At the moment the salt content of the Pacific Ocean is decreasing, while the Atlantic is getting saltier. Modelling suggests that in the Atlantic the high salt content is strongly counteracting the effects of thermal expansion, so sea level is not rising as fast as it could be (Durack et al, 2014).


So less salt on your chips are more in the ocean? Might help the decrease the sea level! On second thoughts - probably there isn't enough salt in the world to account for all the other causes of rising sea level that I have previously discussed.