Thermal Expansion of the Oceans

Time to start investigating why sea level changes. I’m going to begin with thermal expansion of water - the biggest cause of observed global mean sea level change (GMSL) since 1971 (IPCC).

 

 

The proportion of sea level rise attributable to thermal expansion or contraction of sea water is called the thermosteric change. According to the recent IPCC report, between 1971 and 2010 thermal expansion caused on average 0.8 mm/yr rise in GMSL, but satellites observations between 1993 and 2010 show a 1.1 mm/yr rise in GMSL as a result of thermal expansion. So observations over the last fifty years suggest that the rate of thermosteric rise is increasing.

So, how does this work?

It’s all to do with the heat content of the oceans. Ocean water changes density with temperature and salinity – colder, more saline water is denser than warmer, fresher water. As the heat content of the oceans increases, the density decreases, and the water takes up more space causing a rise in sea level (e.g. Johnson and Wijffels, 2011). So it is important to know how much heat the oceans are accumulating in order to work out how this can affect sea level.

 Figure 1: Comparing changes in global ocean heat content for the top 700 metres, and the top 2000 metres (NOAA).

Figure 1 shows that heat content is increasing, but also that the oceans aren't warming uniformly - there is a higher rate of heat content rise in the top 700 metres. Heat content of the top 2000 metres has increased by 24.0 x 10²² J from 1955 - 2010, and of this 16.7 x 10²² J is in the top 700 metres alone (Levitus et al, 2012). This is because the ocean is warmed at the surface where it interacts with the atmosphere and absorbs heat from the sun.




Figure 2: Ocean heat content change as a function
of depth 1955 - 2010 (Levitus et al, 2012)

 

Unsurprisingly heat content change in the oceans also varies with location. Figure 2 shows the Pacific Ocean  has the highest  heat content change for surface waters in the last 50 years. But deeper than 200 metres the Atlantic Ocean has the highest change in heat content of all the oceans. There has also been warming in the deepest parts of the oceans - of the waters below 3000 metres depth, the Southern Ocean has shown the highest heat content change (IPCC).


So it is clear the oceans are warming but how exactly does this translate into sea level rise?


Well, it all depends on the original temperature of the water, and the pressure it is under. Colder water at lower pressures will expand less than warmer water under higher pressure when warmed by the same amount.


So pressure and temperature...


Thermosteric sea level change is affected on decadal timescales by pressure-changing climatic phenomenons such as El Nino, the Southern Oscillation, and the North Atlantic Oscillation (Ishii et al, 2006). Atmospheric temperature affects thermal expansion in the oceans on seasonal and longer term timescales (Wigley et al, 1987). This is what we are observing right now with greenhouse gases increasing atmospheric temperature.


All this means that, while thermal expansion is the biggest contributor to sea level rise, it does not have an even contribution across the world (see Figure 3). This map actually surprised me - I was expecting highest thermosteric sea level rise around the equator where the water is warmer. In fact, while there is is some rise near the equator above Australia, other equatorial regions are experiencing thermosteric sea level fall and the main areas of thermosteric sea level rise are actually in the north Pacific. I'm not actually sure why this is - I will have to do some more reading and get back to you - in the meantime if you have any ideas (or indeed the answer!) let me know. 

Figure 3: Estimated thermosteric sea level change in the top 700 metres, 1993 - 2003 (Church et al, 2008)