Pumping soluble CO2

When talking about oceanic inorganic carbon (CO₂) fluxes, there exists 3 different time scales in which causes differ.

 ->Years to decades: how soluble CO₂ is in seawater.

->Centuries to millennia: biological responses

->Millions of years: chemical and biological processes changing to a small extent (Raven et al. 1999).

We are essentially interested in the smallest time-scale since anthropogenic CO₂ is thought to have had been significantly produced since the industrial revolution (less than a millennium ago).

Solubility pump: one mechanism of ocean uptake

CO₂ solubility in water is an inverse function of temperature. The warmer the water, the less inorganic carbon will solubilise. At 10 degrees C  ̊, 2.5 g of gas will solubilise in water, whereas at 30 degrees, about half will become soluble (1.25g per kg water).

CO2 solubility as a function of temperature. Source: The engineering toolbox

Subsequently, cold water absorbs CO₂ more readily. Water density (determined by sea salinity and temperature), drives water transport. Cold, highly saline water is denser and downwells in the oceans. Dense water masses moving down displace the bottom water masses. These have become less dense as a result of mixing. In turn they will upwell because they have become warmer. This is known as the thermohaline circulation. CO₂ solubilisation in oceans is called the solubility pump and is driven by these two processes.

At high latitudes (approximately, beyond 60 ̊), water is colder with high CO₂ concentrations. This water then moves to lower latitudes downwelling. In turn it upwells and warms which makes it supersaturated with respect to CO₂ (Sabine et al 2004).

That is why the deep ocean has the highest storage capacity: bottom water is colder.

The oceans' CO2 uptake depends both on sea temperature and atmospheric CO2 levels.

Oceanic concentration in CO2 (y axis) as a function of temperature (x axis) and atmospheric CO2 (shaded curves). Source: Simmon 2008

However since the Industrial Revolution an imbalance in the atmosphere-ocean carbon fluxes has occurred. Ocean's ability to absorb CO2 is decreasing (Riebeek 2008). 

Bearing all that in mind then, many questions arise. How will warming of the oceans affect CO₂'s capacity to dissolve in the oceans? Can smaller time-scale changes influence longer ones? That is to say are we changing geochemical processes to an extent significant enough that biological responses and chemical/biological processes will influence beyond millenia history?

I'll try to address some of these questions as we go on.

We are swimming in high uncertainty zone...