Oceans' response to climate change

Last week I showed a mechanism through which the oceans could absorb CO₂. Since it follows a cycle, however, carbon’s motion is circular. In that sense, the oceans in turn release CO₂ into the atmosphere. I will go through the major implication of global warming in relation to the solubility pump.

There exists an atmosphere/ocean pressure balance. If the partial pressure of CO₂ increases in the atmosphere, the gas will disperse into the water body below it. That is because the air-water boundary’s partial pressures must be in equilibrium (NOAA).

The more CO₂ in the atmosphere, the more oceans will uptake carbon dioxide to compensate. This induces us to think that yes, oceans are a reliable CO₂ sink.

Then, we can count on the oceans to absorb the CO₂ we release, can’t we?

The solubility pump is all very nice and comforting for future climate change.

Well, it’s not that easy. To sum up in Riebeek’s poetic words: “carbon dioxide leaks out of the ocean like a glass of root beer going flat on a warm day” as a result of rising temperatures (2008).

The portion of unabsorbed atmospheric CO₂ leads to global warming. Rising atmospheric temperatures warm the oceans over time. As we have seen last week, warmer temperature decreases CO₂ solubility in water. Conclusively, an increase in CO₂ released through fossil fuel combustion, decreases oceans' ability to uptake CO₂.

This is called a positive feedback effect where the consequence of CO₂ release into the atmosphere (rising ocean temperature) increases the cause.

Le Quere modelled and measured the change in CO₂ sink (in PgC/year) over the Southern Ocean between 1981 and 2004 (2008).

Figure 1 Expected CO₂ uptake variability (blue) plotted with the observed changes (in red) over time. The y axis represents the change in carbon dioxide sink ranging from -2 to 1.5 PgC/year. Source: Le Quere 2008

We can see that the modelled ocean uptake did not fit observations. What was expected was a steady increase in CO₂ uptake. Instead, there is annual variability but “no long term increase” (Riebeek 2008). We can even suspect a slight overall decrease in ocean ability to absorb CO₂.

We are not fully understanding ocean carbon mechanisms. Quantifying atmospheric, oceanic exchanges has proved even more difficult. Although it is hard to predict absorption variability, evidence suggests we cannot count on oceans’ uptake to increase. Scientists even suspect an imminent decrease (2008). Obviously, this decrease is very alarming. Oceans have been mitigating effects of fossil fuel combustions so far. If carbon cannot be transferred to the oceans, global warming is thought to intensify in the near future. For these reasons, a lot of debate is going on as to how oceans will respond to climate change.

Measuring anthropogenic CO2

Oceans are intricate system, interrelated with other earth systems. And the biological, chemical and physical levels all play a role in the oceans carbon cycle. Therefore it is a difficult task to identify and isolate one variable to make future predictions. NASA concedes that there is much unknown about the dynamics in carbon uptake and release.

What complicates things further is that natural variability is also occurring (Riebeek 2008).

There is also geographic variability. In the Northern Hemisphere, oceans uptake carbon at all latitudes. In contrast, only the mid-latitudes in the Southern Hemisphere uptake carbon and there is outgassing from the oceans in the high latitudes (Gloor et al. 2003).

 

Estimated anthropogenic CO2 in the oceans. Source: Sabine et al. 2004

 

Besides, CO2 is cannot be traced, since it is transformed biologically (Gloor et al. 2003).

In such a context, how can we assess human-induced changes? Can we make reliable predictions?

Release of fossil fuel combustion to the atmosphere decreases the ratio of oxygen over nitrogen. This process is balanced by photosynthesis, which increases the ratio. Oceans, on the other hand, do not influence the ratio. Land storage of CO₂ then, is determined by the change in O₂/N₂ ratio. Ocean carbon storage is inferred from terrestrial storage: we subtract land storage from total CO2 uptake  (Sabine et al. 2004).

We can see that oceanic storage was not directly measured but inferred from terrestrial storage (which itself is a result from different equations). Does this induce a higher uncertainty in the amount of oceanic uptake? The less direct the measurement process the more likely errors will arise.

An alternative approach is to apply atmospheric, ocean models. They account for changes associated with photosynthesis processes (Gloor et al. 2003).

Although accurate precise results are hard to obtain, we can be confident in the long-term trends. Alternative methods confirm the fact that oceans are overall a net carbon sink although absorption is spatially distributed.

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...

Change of horizon

Hey everybody,

So after realizing I would not find substantial literature about my previous subject I decided to change topic. One thing that I learnt from this is that it is a largely uncovered matter. Psychologists talk about how time affects our way of living, and ecologists talk about how our way of living affects our environment. It would be interesting to try and link the two. But this task was paramount for me. An idea for a phD anyone?

So now I am shifting my discussion to oceans. They seem fascinating. Probably the least known area on our planet although it covers more than half of it. Being in the deep oceans is like exploring undiscovered landmass or another planet. We realize how tiny and insignificant we all are in the immensity of water.

I could go on about how indispensable they are to life on earth. Oceans are closely intertwined with the atmospheric system. They bear a crucial role in climate variability. For instance effects of the Southern Oscillation of El Niño are felt globally. Under El Niño conditions (warmer sea surface temperature in the Pacific), Northern Europe experiences colder, drier winter (Welt et al 2011).

A role oceans have been playing that has been crucial in climate change is in relation to CO₂. Indeed, oceans act as CO₂ sinks.

Before all, we can explain how the CO₂ cycle including land biomass, atmosphere and oceans works.

We are only focusing on anthropogenic carbon exchanges. Fossil-fuel combustion releases carbon to the atmosphere. Upon entering the atmosphere carbon is transformed into CO₂ . Some of this CO₂ remains in the atmosphere, thereby causing global warming (BBC). The rest is taken up by carbon sinks. Terrestrial plants absorb some of this carbon through photosynthesis. And the oceans suck in the rest in ways I will explain in depth later.

Carbon fluxes and reservoirs. Black arrows represent naturally-occurring CO₂. The red arrows represent anthropogenic carbon dioxide (Source: IPCC AR4WG1) 

These two sinks, then, have mitigated the consequences of industrialisation. It is necessary to understand how they work, and how they vary for environmental change predictions. I chose to focus on oceans. I had heard of photosynthesis but I did not know the oceans acted as sinks as well. And upon looking at the ocean reservoir, it absorbs 100 GtC (Gigatons of carbon) yearly on average which is a significant portion.

Disagreements on the amount and proportion of CO2 exist. There is much debate about how much carbon oceans absorb. In terms of portions it varies from a third (ACE, Wada et al. 2011) to a quarter of CO₂ anthropogenic emissions (NOAA).
It is extremely difficult to assess how much carbon is absorbed. Oceans remain largely unknown systems (NOAA). What is unknown spurs a lot of debate.


I will try to follow a pattern:

->Mechanisms of CO2 ocean uptake

-The solubility pump

-The biological pump

->measuring CO2 concentration

-tools

-geographic variability

-temporal variability

-> consequences of climate change

-in water chemistry

-in water biology

->the future