Article: Paul J Durack, Susan E Wijffels, Peter J Gleckler. Long-term sea-level change revisited: the role of salinity. Environmental Research Letters, 2014; 9 (11): 114017 DOI: 10.1088/1748-9326/9/11/114017
Sea level rise is becoming a more familiar concept to people around the world. Some measurements and estimates put the rate of sea level rise in the last 20 years at roughly 3.2 mm a year, which is almost twice the rate of the 80 years prior to that. Coast lines along the eastern seaboard of the United States (among many other regions of the world) are shrinking with rising sea level, threatening coastal inhabitants from wetland species to humans. Sea level rise also sets up coastal areas for more damaging storms, making coastal habitats and communities even more vulnerable to physical phenomena like storm surges and tidal waves.
While sea level rise is on the minds of most coastal residents (who happen to make up nearly 50% of the world’s population), most people are not aware of the complex, multifaceted mechanism driving it. Most people are now aware that climate change has greatly reduced snowpack and glacier ice reservoirs on land, leading to increased runoff and freshwater contribution to the ocean. This repartitioning of water from a solid, land-based reservoir to the ocean has driven much of the sea level rise observed around the world.
In addition to the reduction in water storage on land, climate change has also contributed to sea level rise through the thermal expansion of the ocean. Temperature has a great influence on the density of fluids, where warmer temperatures make fluids like water expand and colder temperatures make fluids contract. While small changes in water density does not seem like a major issue, less dense fluids occupy more space. So increasing global temperatures decreases sea water density, and this less dense sea water now occupies more space than it did before, resulting in an upward expansion.
What has received less attention and is generally not as well understood is the impact of salinity changes on sea level. Much like thermal expansion and contraction, changes in salinity can also alter sea water’s density, where more dissolved solutes (increased salinity) makes sea water denser and thus more compact. Conversely, decreasing the solute content of sea water (decreased salinity), reduces sea water’s density, leading to expansion. Salinity, though, is generally thought of as a fairly constant feature of the ocean that does not change much on the short times scales relevant to most people (oceanographers refer to stable features in the ocean, like salinity, as conserved). Thus, if salinity has not changed as drastically as temperature has in the last couple of decades, then it does not likely have a major contribution to sea level rise.
However, recently published research conducted by scientists at Lawrence Livermore National Laboratory in California has flipped this argument on its head – salinity has changed in the world’s ocean in recent years and their research suggests that this change has an important contribution to sea level rise.
Researchers Paul Durack, Susan Wijffels, and Peter Gleckler used observational data sets and model outputs to study the contribution of both thermal and salinity contributions to sea level rise. They used two independent, long-term data sets and compared them to each other as well as to a model output. The authors used temperature and salinity data spanning from1945-2012 and calculated ocean volume (which is directly related to sea level and density) using a widely used set of equations (the thermodynamic equations of state, circa 2010 – or TEOS-10). The authors compared these maps to those generated from the Coupled Model Intercomparison Project Phase 5 (CMIP5), which is a widely used global model in climate change studies (learn more about CMIP5 here). The team was able to use these maps to analyze global and regional scale trends in sea level rise and the different contributions that thermal and salinity expansion possibly make.
The research team found complex interactions between salinity and temperature-driven density changes on both global and regional scales. Globally, the ocean has experienced mostly thermal expansion as temperatures increase throughout the Earth (though the temperature changes are not uniform and may vary in importance regionally). Globally, salinity changes are seem to have less of a contribution (Figure 2a).
Regionally, however, the researchers found that salinity has a much larger contribution to specific ocean basins. For instance, the Atlantic and Indian oceans have increased salinity, leading to halosteric contraction (increased density from increased salinity) while the Pacific Ocean has responded oppositely, and has decreased in salinity, leading to halosteric expansion (Figure 3). Interestingly, the researchers found a complex interaction between thermosteric and halosteric changes in ocean volume. Halosteric contractions observed in the Atlantic and Indian oceans (dark colors in Fig 2C and 2D) are compensated with thermosteric expansion (light colors in Fig 2C and 2D). This compensation mechanism is not entirely surprising as many mechanisms that change temperature also change salinity through processes like evaporation (a warming Atlantic will experience thermosteric expansion but will lead to increase evaporation which increases salinity, leading to halosteric contraction which may reduce the severity of volume changes). However, the Pacific Ocean was seen in both the observed studies and the CMIP5 model output to have undergone both thermosteric and halosteric expansion, which may augment and worse sea level rise.
This study by Durack et al. has furthered our understanding of sea level rise mechanisms. Before, salinity was not thought to be a contributor to sea level rise but now has been shown to be an important component that needs to be considered. Furthermore, the researchers have shown that salinity might play a very large role in explaining regional scale differences (i.e. between the Atlantic and Pacific basins). The possible compensatory or synergistic interactions between temperature and salinity changes complicates out understanding of how sea level rise acts on smaller scales but ultimately we are better equipped now to understand any changes that we are observing now, and will observe in coming years.
Sea level rise, as it turns out, is more complicated than most people had originally thought. While we are increasing our understanding of its drivers with studies like these, we still need to learn more if we are to apply this knowledge on a local scale where coastal inhabitants can directly benefit from this knowledge.