Salinity has a direct effect on sea levels as salt increases the density of water. Nonetheless, its role in long-term estimates of globally averaged sea-level rise has been largely ignored, says Paul Durack at Lawrence Livermore National Laboratory, US, together with colleagues from there and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Marine and Atmospheric Research in Australia. One reason, says Durack, is that many previous sea-level studies have focused on global average changes, and global salinity changes are very small.

In previous studies, Durack and colleagues found that the pattern of salinity in the ocean – the ocean’s so-called salinity field – has been changing over the course of historical observation. The researchers analysed computer simulations that suggest a strong link between patterns of evaporation and precipitation – that is, the atmospheric water cycle – and ocean salinity, with the ocean in places where it rains more becoming fresher and areas of sea that experience more evaporation becoming saltier. In other words, the results showed that climate change does not just affect the atmospheric water cycle, it affects ocean salinity too. “Salinity observations provide us with another way to observe ongoing climate change,” said Durack.

Now, Durack and colleagues have investigated salinity and temperature data from the 1950s using two independent observational datasets. The researchers looked at the changes in observations and in climate-change models from the Coupled Model Intercomparison Project phase 5 (CMIP5), which is used by the Intergovernmental Panel on Climate Change. They found that the regional sea-level changes due to salinity were much larger than had been previously assumed – up to one-quarter of the size of the coincident sea-level changes due to temperature variations and the resulting thermal expansion of the water.

The effect of salinity is not the same everywhere, however, the observations and models revealed. Whereas in the Pacific Ocean a freshening, or salinity reduction, has led to a density drop that augments thermally driven sea-level rise, in the Atlantic an increase in salinity has boosted density, acting against the sea-level rise due to temperature. The work shows that “salinity-driven changes should not be neglected in future regional sea level studies,” Durack said.

Durack adds that the​ study has not attributed the salinity changes to any direct cause. “While previous work has shown that surface salinity changes are driven by changes to the water cycle (evaporation and precipitation), other aspects may also be playing a role in the three-dimensional changes we’ve uncovered in this study,” he said. “Further work currently underway aims to explicitly diagnose what processes are causing these coherent changes in the observations and models, and whether all models show the same processes driving these changes.”

The findings are published in Environmental Research Letters (ERL).

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