There are two themes in current “consensus” climate science. Perhaps it’s not apparent that they are contradictory..
- One side – climate is predictable
- The other – “tipping” points ahead, perhaps very close
This subject isn’t easy to untangle and no one really knows what the answer is.
What this post is about is one of those “tipping” points and how complex climate really is. This post is about the thermohaline circulation, also known in shorthand as the THC.
“Thermohaline” sounds like a tough concept to understand – but although the concepts involved don’t require any special science knowledge they aren’t immediately obvious.
Thermo - relates to temperature, and Haline relates to Saline, or Salt..
Energy Balance and the “Conveyor Belt”
When you consider the difference between the incoming solar radiation and the outgoing longwave radiation by latitude you start to realize how the earth’s climate moves heat around – more specifically how heat moves from the equator to the poles:
This graphic demonstrates the calculated solar radiation in – versus latitude, against the radiation out from the earth’s surface (longwave radiation).
In short, the equator receives a lot more energy compared with the poles because the sun is – comparatively – overhead a lot more. Therefore, the atmosphere and the oceans transport heat from the equator to the poles.
Here’s another graphic of the same imbalance:
Interestingly, the oceans and the atmosphere share the heat transfer more or less 50/50:
Now we see that the ocean takes a big role in moving energy to the poles, let’s look a closer look at what drives the ocean currents..
Temperature.. and Salt
Two obvious factors that push the ocean currents around are winds and the coriolis effect. The coriolis effect has to do with the fact that earth is rotating. Every explanation I have seen of it seems like a recipe for confusion if you haven’t already spent some time on it, so I’m not going to repeat that problem.. (take a look at the link above if you want to understand it better).
But the major factor that drives ocean currents is density. What determines density? Temperature and salinity:
The first time you see this kind of chart your eyes glaze over and you quickly move on to the next section.
But let’s try and make it easier to understand:
Here’s one example – cold water, almost freezing, becomes less saline. This might happen if a lot of ice was melting. See the change in density – 1.026 to 1.003kg/m3 – it doesn’t seem like much but it is very significant.
If you take the reverse direction, as water freezes it leaves most of its salt behind, so the water becomes much more saline and the density goes in the other direction.
The Thermohaline Circulation
Cold and high salinity water has a higher density than any other ocean water and so it sinks. There are two places that most ocean water sinks – around Greenland and at the Wedell Sea in Antarctica. Here is a simplistic representation of global ocean circulation:
One consequence of the THC is that warm surface water moves from the equator up to norther Europe. If that “global conveyor” didn’t exist then northern Europe would be much colder.
The driving force is the very cold very saline water that sinks rapidly just south of Greenland.
The Tipping Point
As the world heats up, which it is currently doing (in a broad sense, see Note 1), Arctic sea ice and the Greenland ice sheet are melting more rapidly.
At some point, the amount of melt water – low salinity water – will probably change the balance of the THC and send the system into reverse.
All the evidence is that this has happened before.
When – and if – it does happen, the heat conveyor will turn off, northern Europe will cool down and the Arctic and Greenland will refreeze.
As that happens, positive feedback from ice albedo and then from water vapor will keep driving the temperatures in the colder direction.
Well, who knows exactly what will happen? Or when.
In some follow on posts we will look at this in some more detail. Currently, GCMs (general circulation models) do not have a “tipping point” for the THC, they just show a weakening.
Our understanding of ocean dynamics do not require any new development of physics but we do require a lot more data. Temperature and salinity throughout the oceans has started to be provided through the Argo project. How much ice is melting is – surprisingly – a difficult subject.
Solving the equations of motion for the oceans requires temperature and salinity data as well as the meltwater component.
The possibility that the THC will change direction is a huge issue in the predictability of future climate.
If it does “switch” there will be significant climate effects. We can’t assume this effect will have a happy ending.
Note 1: The world has been heating up – broadly speaking, as there have been some ups and downs – since the end of the last ice age, 18,000 years ago. Sea levels have risen around 120m. And in the last 100 years the earth’s surface temperature has increased by around 0.7°C.