On Thermal Expansion & Thermal Contraction - 35

Fig. 1

I. Got Pure Water?

One thing that is never missing in the text containing the proclamation that "thermal expansion is the major cause of sea level rise" is the dogmatic statement that "water expands when heat is added to it."

Likewise, in the similar proclamation that "thermal expansion is a major cause of sea level rise" is the dogmatic statement (or mantra?) that "water expands when heat is added to it."

The only problem with that is the scientific fact that water does not always expand when heat is added to it.

The full truth is that water expands (increases in volume) OR contracts (decreases in volume) when heat is added to it.

Fig. 2

So, what controls whether water expands or contracts when heat is added to it?

The answer is that it depends on the temperature of the water at the time the heat is added (Fig. 1).

Why is that?

That is because when the water is below its maximum density temperature (4 deg. C) the addition of heat to that water will cause it to become more dense (contraction, less volume).

Fig. 3

But, when the water is above its maximum density temperature (4 deg. C) the addition of heat to that water will cause it to become less dense (expansion, more volume).


Sea water, being water and all that, holds to the same principle (Fig. 2), except that, unlike pure water, its temperature of maximum density depends on its degree of "impurity" and the pressure upon it caused by its depth.

Impurity, then, in this case means that there is more in it than just water.

Yes, the characteristic of salinity ("saltiness"), alters the temperature of sea water's maximum density, depending on degree of salinity.

It has a complicated "personality," because depth can also play a part.


As regular readers know, I have the TEOS-10 toolkit (TEOS-10 software).

I have finally developed a module (which uses that toolkit) to calculate the maximum density temperature of sea water at various depths and at various degrees of impurity.

Fig. 4

As it turns out (at least for the Greenland ocean areas) the maximum density temperature is so low that it does not impact upon thermal expansion and contraction in that geographical area.

What I mean is that when heat is added to sea water in that area it will only expand, not contract.

The only way the sea water in that ocean will contract (lose volume) is if cold water from melting glaciers flows into it to cool it further.

That is happening at the 0-2000 meter depth levels in that area (Fig. 4).

Compare the graph at Fig. 3 (a hypothetical maximum density of sea water at 4 deg. C), and the graph at Fig. 2 (a hypothetical maximum density of sea water at about 2 deg. C), with the graph at Fig. 4 (the actual TEOS-10 calculated temperatures and maximum densities at three depth levels).

Fig. 5

I did the Antarctica module that focuses on the WOD zones there.

The same maximum density conditions exist there as they do in Greenland (Fig. 5).

That is, the only thermal expansion and contraction that takes place is at densities above the maximum density of the sea water there.

The reason I have ventured into the sea water density issue is because of what is written in the TEOS-10 manual:

"Since the density of seawater is rarely measured, we recommend the approach illustrated in Figure 1 as a practical method to include the effects of composition anomalies on estimates of Absolute Salinity and density. When composition anomalies are not known, the algorithm of McDougall et al. (2012) may be used to estimate Absolute Salinity in terms of Practical Salinity and the spatial location of the measurement in the world oceans."

(TEOS-10 Manual, p. 14, p. 24 PDF).The toolkit has this to say about the C++ function that calculates the maximum density temperature:

!==========================================
elemental function gsw_ct_maxdensity (sa, p)
!==========================================
!
! Calculates the Conservative Temperature of maximum density of seawater.
! This function returns the Conservative temperature at which the density
! of seawater is a maximum, at given Absolute Salinity, SA, and sea
! pressure, p (in dbar).
!
! SA = Absolute Salinity [ g/kg ]
! p = sea pressure [ dbar ]
! ( i.e. absolute pressure - 10.1325 dbar )
!
! CT_maxdensity = Conservative Temperature at which [ deg C ]
! the density of seawater is a maximum for
! given Absolute Salinity and pressure.
!----------------------------------------------------------------------------------------------

Imagine the difficulty in calculating actual thermal expansion and contraction if the maximum density fell at 4 deg C or 2 deg C as shown in Fig. 2 and Fig. 3.

As we now know, the only thermal expansion to be calculated is based on an increase in the Conservative Temperature (CT) while the only thermal contraction to be calculated is based on a decrease in the CT.

That simplifies things, but changes nothing concerning the Dredd Blog hypothesis that, all things considered, thermal expansion is a minor factor in sea level change.

The previous post in this series is here.