The Sea, The Sea

About a week ago I was browsing David MacKay’s excellent resource, “Sustainable Energy – without the hot air“. This, and a brief conversation earlier the same evening, had started me pondering (again) on the thorny topic of CO2 uptake by the oceans. Specifically, I wanted to make some progress towards answering the question:

“If we reduce the level of CO2 in the atmosphere from its present 390ppm or an even higher level in future, will the oceans release CO2 they are currently absorbing (about 2GtC/year)? And, if so, over what timescale?”

Professor MacKay includes a chapter (31, The last thing we should talk about) on geo-engineering. He notes:

“If fossil-fuel burning were reduced to zero in the 2050s, the 2Gt[/yr] flow from atmosphere to ocean would also reduce significantly. (I used to imagine that this flow into the ocean would persist for decades, but that would be true only if the surface waters were out of equilibrium with the atmosphere; but, as I mentioned earlier, the surface waters and the atmosphere reach equilibrium within just a few years.) Much of the 500Gt we put into the atmosphere would only gradually drift into the oceans over the next few thousand years, as the surface waters roll down and are replaced by new water from the deep.”

Now, the model I have in my head of CO2 uptake by the oceans is one of flows of CO2, rather than a chemical equilibrium. David MacKay’s comment caused some self-doubt on my part. The Professor is clearly not what we chess-players might refer to as a “rabbit”. Strong grand-master would be nearer the mark.

As regular readers will be aware, I’d reached a somewhat different conclusion to that of Professor MacKay. I concluded that the ocean will continue to helpfully take up 2GtC/yr from the atmosphere, on the basis that this may be the capacity of the processes to remove CO2 from the atmosphere.

I specifically doubted, though, that the oceans will continue to absorb a fixed proportion of our emissions, on the grounds that “the ocean ‘knows’ nothing about emissions – all it can possibly be affected by is the level of CO2 in the atmosphere.”

But this idea of “equilibrium” between the surface waters and the atmosphere suggests instead that the ocean can be considered as an extension of the atmosphere, so that if the total increase in CO2 in a year from fossil-fuel burning and terrestrial biosphere changes was (say) 6GtC, 4GtC would stay in the atmosphere and 2GtC would end up in the ocean; if it were 12GtC, 4GtC would end up in the ocean.

Now, undoubtedly there is an equilibrium between the waters at the very surface of the ocean and the atmosphere: that’s how these things work. Horrifically, I’m suddenly reminded of questioning on a very similar topic during a mock interview for university conducted by my school headmaster, who had himself written chemistry textbooks…

Anyway, undoubtedly, too, there are flows of carbon in various forms to and from the deep ocean.

The question is how we combine these idea of equilibrium and flows into a single model that will help us at least put a sign to flows of CO2 from atmosphere to ocean in various scenarios.

The consequences of a pure equilibrium would be that:

1. The ocean will continue to absorb a fixed proportion of net emissions, i.e. it will proportionally reduce the impact on atmospheric CO2 levels of future increases in atmospheric CO2.

2. As soon as atmospheric CO2 levels peak, the ocean will start to release a fixed proportion of any net reduction. i.e. it will be more difficult to get the atmospheric CO2 level back down, say to 350ppm.

On the other hand, if the true explanation is that in a (hypothetical) steady-state there is a balance between flows of CO2 from the ocean to the atmosphere and vice versa, then we need a different sort of explanation. We would have to conclude that processes that remove CO2 from the atmosphere are sensitive to a higher concentration of CO2 and are therefore proceeding more rapidly because CO2 is at around 390ppm compared to a historic level of 200-280ppm.

It’s likely that the processes controlling the interchange of CO2 between the air and the sea are sensitive to other factors, such as temperature and acidity (affected by the cumulative total of CO2 absorbed). But so far, these parameters have changed relatively little. When they do, all the evidence is that they will slow the rate of CO2 uptake by the oceans.

But the crucial point is that in a flow model, the oceans will continue to remove CO2 from the atmosphere as long as the atmospheric level is above the stable long-term level which prior to industrialisation was 200-280ppm.

To jump ahead a little, the question as to whether an equilibrium is dominant is likely to reduce to what we mean by the “surface waters”, since, at the limit, the surface of the ocean must be in equilibrium with the atmosphere next to it. In other words, how quickly does CO2 disperse away from the surface of the ocean?; and from power-station chimneys through the atmosphere to the surface of the ocean?

Looking at the rest of Professor MacKay’s chapter on geo-engineering, I couldn’t help reflect that there is a contradiction. If an equilibrium between the surface waters and the atmosphere is the dominant mechanism, then one would have thought there was little to be achieved by geo-engineering approaches to increase the absorption in a limited area of ocean (sprinkling them with calcium carbonate to absorb CO2 directly or with iron filings to encourage algal growth).

So, for the umpteenth time, I found myself referring to “the doorstop” – the AR4 IPCC Scientific report. And I can report that parts of the relevant sections of this document are virtually content-free. Now, I’ve been in situations when a lack of content has been highly desirable. The objective of some business communications, for example, is to say precisely nothing of any significance. I suggest, though, that the IPCC should not be playing this game.

Let’s turn first to section 6.4.1.4 on p.452. Here we learn that:

“There is evidence that terrestrial carbon storage was reduced during the LGM [last glacial maximum] compared to today. Mass balance calculations based on C13 [isotope] measurements on shells of benthic foraminifera yield a reduction in the terrestrial biosphere carbon inventory (soil and living vegetation) of about 300 to 700GtC…”

This doesn’t really tell us much about the mechanism of CO2 exchange between the oceans and the atmosphere, but is a rather scary fact. Warming leads to carbon leaving the oceans and being taken up by land flora. Ah, I hear you think, the trees take up carbon and the oceans release it to restore equilibrium. Sorry, Grasshopper. The trouble is that as the planet warms the level in the atmosphere goes up as well. This suggests to me that the oceans do indeed release carbon as the planet warms. It’s not pull by the “trees”, but push by the “seas”.

As I said, this is a rather scary fact. Given that the planet is warming rather rapidly. And that the exchange of carbon between atmosphere and oceans takes place at the surface. Where it’s warming. The fact that the deep ocean takes millennia to cool is not really relevant. Hmm, maybe I’ve jumped ahead again.

But back to the story.

Turn now to p.446 of the IPCC report, where we find Box 6.2: What Caused the Low Atmospheric CO2 Concentrations During Glacial Times? (Seems an odd way to phrase it, as glacial times are the norm, but let’s go on!). The answer is no-one really knows. (Actually, the answer to the IPCC’s question is easy: in glacial times the atmospheric CO2 level is so low it limits photosynthesis, so we should really be asking: What causes higher CO2 levels in interglacials?). Still, no-one really knows. Or as the IPCC put it:

“In conclusion, the explanation of glacial-interglacial CO2 variations remains a difficult attribution problem.”

There’s one proviso. There’s a speculative theory (no more than a hypothesis, really) that increased amounts of dust containing iron cause increased phytoplankton growth which causes the ocean to take up carbon from the atmosphere. I mention this because the complete line of reasoning is that colder conditions cause less plant growth, that is more deserts from where dust can blow… This would restore the idea of a “push” by the land – more trees, less dust leads to more carbon in the atmosphere. The trouble is that there’s no evidence that this mechanism could explain more than a small proportion (if any) of the observed changes in CO2.

So much for the top-down approach.

Is our understanding of the physical processes any better?

Let’s see how far we can get. The IPCC Science report notes in section 7.3.1.1 (p.514) that there are 2 “pumps”, i.e. processes that remove CO2 from the atmosphere:

1. The solubility pump – dissolving CO2, giving carbonic acid:
CO2 + H20 <—> HCO3+ + H+ (1)
buffered by carbonates (e.g. CaCO3, calcium carbonate):
CaCO3 <—> Ca++ + CO3– + HCO3+ + H+ <—> Ca++ + 2HCO3+ (2)
(see a previous post for how this might be helped along by dumping some more chalk in the sea).

2. The biological pump whereby phytoplankton (algae) takes up carbon as it grows.

The IPCC note that:

“Together the solubility and biological pumps maintain a vertical gradient in CO2… between the surface ocean (low) and the deeper oceans (high)…”

[my emphasis]

This is where this whole topic starts to do my head in. How can it be that there is less CO2 at the surface, yet the oceans are taking up the CO2 we’re emitting through burning fossil fuels and forests?

Obviously there is a circulation in the oceans. The IPCC note (we’re still on p.512) that:

“In winter, cold waters at high latitudes, heavy and enriched with CO2… because of their high solubility [sic, I don’t know what they’re trying to say either], sink from the surface layer to the depths of the ocean. This localised sinking, associated with the Meridional Overturning Circulation (MOC)… is roughly balanced by a distributed diffuse upward transport of [CO2] primarily into warm surface waters.”

This exchange of dissolved CO2 – lots coming up, rather less going down – constitutes the “solubility pump”, but the biological pump, which, remember, involves organisms taking up CO2 near the ocean surface – effectively from the atmosphere – only operates downwards.

So here’s what I think is happening: there is still a net release of CO2 from the solubility pump, but less CO2 is released now that atmospheric CO2 is around 390ppm compared to when it was lower (280ppm say), because of simple equilibrium chemistry. This assumes there is plenty of carbonate about to stop, through equilibrium (2), the oceans becoming more acidic, reducing CO2 uptake by pushing equilibrium (1) to the left.

So whereas previously with CO2 at 280ppm, the solubility pump would have released (say – these are hypothetical figures) 4 GtC/yr and the biological pump taken 4GtC/yr back to the ocean depths, now, with CO2 at 390ppm, the solubility pump might be releasing only 2GtC/yr but the biological pump is still taking up 4GtC/yr. Hence the net 2GtC/yr uptake by the oceans which is in large part saving us from ourselves.

Digression: I have to say that I can’t help making the observation that the solubility pump depends on the MOC, and that there are those who think the MOC might eventually fail, driven as it is by the cooling of surface waters flowing from low to high latitudes (the IPCC discusses this in Box 5.1, p.397). This would, according to my reasoning, lead to a decrease in the release of CO2 via the solubility pump, increasing the net uptake of CO2 by the oceans, though this may be offset if the biological pump is also weakened (by a reduction in nutrient upwelling, say). I am therefore hypothesising a mechanism (a negative feedback) helping to cause interglacial warming periods to be self-limiting. I should point out, though, that this is completely the opposite of what the IPCC say (e.g. sections 7.3.4.1 and 3 to 5, p.530 and 532-3 and 7.3.5.4 on p.536). Digression over.

Let’s summarise where we are: I am suggesting that the equilibrium between CO2 in the atmosphere and in the oceans is potentially important. Even though the oceans release CO2 through this mechanism, the equilibrium chemistry means they release less as atmospheric CO2 rises.

But how much less?

I mentioned at the outset that it is not in dispute that CO2 is in equilibrium between the air and the water at the surface of the ocean. But how deep is the surface? What is the gradient in CO2 concentration away from the surface of the ocean? How much extra CO2 can be taken up (or as we have seen how much less released) in a year? Is the mechanism saturated at 2GtC/year as I assumed when I reported on my home-made carbon-cycle model?

It’s when we try to answer these questions that the IPCC Science report becomes – how shall I put it? – a little disappointing.

We turn now to Chapter 7: Couplings Between Changes in the Climate System and Biogeochemistry. In section 7.3.4.1 (p.528) we “learn” that: “Equilibration of surface ocean and atmosphere occurs on a time scale of roughly one year.” My school headmaster would have a fit! This sentence is indeed content-free. There is no definition of what is meant by “surface ocean”. Is it 1mm, 1m or 100m? Until we can answer this question we are unable to quantify the effect of the “solubility pump”.

Back to chapter 5. Section 5.4: Ocean Biogeochemical Changes includes some interesting diagrams (p.405) showing how “anthropogenic carbon” is dispersed in the oceans. These show that carbon levels are most elevated, compared to pre-industrial levels, in the top 200m or so of the oceans – “more than half of the anthropogenic carbon can be found in the upper 400m” (p.404) – and in the North Atlantic.

The trouble is, we’re no nearer answering the question as to how long we can consider it takes to renew the active layer of the oceans that exchanges CO2 with the atmosphere.

Let’s try another tack. Let’s say (generously) that the layer is 100m, on average, based on inspection of CO2 diffusion diagrams in the IPCC report. Let’s say it takes 1000 years for the oceans to completely turn over – a figure noted a few times by the IPCC. If the oceans are 5000m deep (on average) as shown in the IPCC figures, then the 100m “surface layer” is renewed every 50th (100/5000) of 1000 years, that is every 20 years.

Now we can try to answer the question posed at the start:

“If we reduce the level of CO2 in the atmosphere from its present 390ppm or an even higher level in future, will the oceans release CO2 they are currently absorbing (about 2GtC/year)? And, if so, over what timescale?”

The answer depends on the timescale we are looking at:

1. If we reduced the level of CO2 in the atmosphere overnight (more realistically by say 1ppm from one year to the next), then the surface layers of the ocean will release some carbon as it re-equilibrates with the atmosphere.

2. But if, more realistically, we reduce the level of atmospheric CO2 from one 20 year period to the next, we can consider the outcome as follows:
– in both 20 year periods the ocean will outgas the same amount of CO2 from the deep;
– in the first period the ocean will carry away more carbon (or release a little less) than in the second period.
There is no correlation between what happens in the second period and in the first.

3. After a millennium or so, the ocean might release more carbon because of the extra carbon it is absorbing now. On the other hand, more carbon may simply end up in sediments.

Conclusion: The oceans will not release a significant proportion of the anthropogenic carbon they have absorbed since industrialisation if we reduce the level in the atmosphere back to 280ppm over a century or two.

“Equilibrium” and “flow” models of oceanic carbon uptake are relevant over different timescales. The flow model is applicable to decades and centuries, the equilibrium model to years and (possibly) millennia.

I believe it is inaccurate to say, as David MacKay does, that:

“If fossil-fuel burning were reduced to zero in the 2050s, the 2Gt[/yr] flow from atmosphere to ocean would also reduce significantly.”

The increase in annual oceanic CO2 uptake due to the difference between CO2 levels in the atmosphere and the ocean is partly due to the difference between CO2 levels now and when the current surface waters were last exposed to the atmosphere, that is, the difference between 390ppm and 280ppm approx. and partly due to the difference between the CO2 level compared to the previous year – about 2ppm. If (as I’ve assumed) 1/20th of the surface waters are renewed each year, we should allow 1/20th of (390-280)ppm, that is 5.5ppm as the comparable CO2 concentration difference. 5.5 is several times 2, so the dominant cause of net oceanic CO2 uptake at present is the renewal of oceanic surface waters, not annual increases in atmospheric levels of CO2.

In other words, when Professor MacKay goes on to say:

“Much of the 500Gt we put into the atmosphere would only gradually drift into the oceans over the next few thousand years, as the surface waters roll down and are replaced by new water from the deep.”

he is correct – this process is going on. But, I suggest, it accounts for at least 75% of the 2GtC/yr of our CO2 pollution that the oceans are helpfully soaking up for us.

And if we were to reduce atmospheric CO2 levels by say 1ppm/year (e.g. by ceasing fossil-fuel burning and enacting a programme of worldwide reforestation), oceanic surface re-equilibration would reduce the annual decrease by only about 10%, and with atmospheric CO2 at its current level, and all else being equal (unfortunately it probably won’t be), the solubility pump performance attributable to oceanic surface water turnover will continue to remove around 1.5GtC/year (about another 0.75ppm).

To go on, reducing atmospheric CO2 concentrations at a rate of 1.65ppm (based on the above figures), reducing to 1ppm as we approach the pre-industrial equilibrium, would allow us to return from 450ppm to 280ppm in around 170/1.325 [(1.65+1)/2] or around 130 years.

[Though, as I said, all else is not equal and positive feedbacks due to warming of the oceans and decreased albedo because of loss of ice-cover, etc. will most likely increase this timescale significantly.  On the other hand, if we do it before the deep ocean has warmed, we might just save the planet!].