Uncharted Territory

April 12, 2009

Ocean Carbon Uptake: Further Reflections

Filed under: Global warming, Science — Tim Joslin @ 5:05 pm

In my previous 2 posts, The Sea, The Sea and How To Freeze A Mammoth, I have argued – nay, stronger than that, pointed out – that the 2GtC/yr of carbon that the oceans are helpfully taking up from the atmosphere is due largely to a reduction in the amount of carbon released annually as currents exchange deep with surface water.  The deep sea has a higher carbon content than the surface waters because of the “biological pump” whereby organic material (krill poo, dead whales etc) descends through the water column.

Towards the end of my last blog entry, I discovered that the quantification I had previously sought in vain in the 1000 odd pages of the IPCC’s latest Science report does in fact exist in their carbon cycle diagram, Fig. 7.3 on p.515.  These figures broadly support the guesstimates I made towards the end of my initial blog entry of the series.

In case you don’t have a copy of the IPCC report to hand, let me explain what Fig. 7.3 tells us.  It notes that, of total anthropogenic carbon emissions during the industrial era, 18Gt remains in the surface waters and 100Gt is now in the intermediate and deep ocean.  The diagram even includes a flow of 1.6GtC/yr from the surface to the deeper ocean.

The ocean “surface” is that part which, moreorless by definition (perhaps the scientists could make this explicit sometime), is in equilibrium with the atmosphere.  Now, CO2 in the atmosphere is at roughly 390ppm, against a preindustrial level of 280ppm.  1 ppm ~= 2GtC (~ means approx.).  So, the surface layers of the ocean represent an “extension” of the atmosphere for the purposes of holding carbon dioxide, of, according to the IPCC, only 18/220, i.e. very ~ 10%.

That is, as we increase the atmospheric CO2 by 2ppm/year, 4GtC, the part of the ocean in equilibrium with the atmosphere is helping us out by dissolving an additional 0.4GtC.

Add 0.4GtC to the 1.6GtC “removed” annually by turnover of the surface waters (and, I suppose, diffusion) – though in fact only removed in the sense that the turnover of the surface waters results in the emission to the atmosphere of 1.6GtC/yr less carbon than would be the case without the elevated atmospheric level of CO2 caused by industrial carbon emissions – and we get the observed 2GtC/yr total net uptake by the oceans compared to the rough equilibrium between the atmosphere and the oceans in the few thousand years prior to the industrial era.

Until changing conditions (e.g. rising temperatures) affect the relevant processes, the consequences are:

1. The oceans will continue to reduce any increase (or increase any reduction) in atmospheric CO2 by about 10% due to the reasonably fast process of chemical equilibration.

2. The oceans will continue to take up around 1.6GtC/yr whilst atmospheric CO2 levels remain at their current elevated level.  This level of uptake will only increase slowly if our annual CO2 emissions continue to increase – i.e. as I discussed some time ago, in this scenario, the oceans will take up a declining proportion of our annual emissions and more will remain in the atmosphere.   In fact, there’s no direct relation between our annual emissions and the airborne fraction (AF).  It is daft to suppose there would be.

2A. On the other hand, if our annual emissions decline, we will still get the benefit of the 1.6GtC net removal from the atmosphere attributable to oceanic circulation.

3. Ocean CO2 uptake is not very sensitive to geo-engineering interventions to increase the amount of CO2 that dissolves in it, e.g. by dumping calcium carbonate in the sea (though this might eventually be worth doing – expensive though it would be because of the mass that would have to be transported – in order to preserve shelled creatures, corals etc).  The problem is that the surface waters only turn over about once every ~10 years on average (18Gt extra carbon held in total divided by 1.6Gt transported to the depths each year – my previous guesstimate was once every 20 years).

4. Ocean CO2 uptake is very sensitive to changes in the circulation of the oceans. Since such circulation is more likely to lessen than to increase, we really are getting ourselves in deep water!
[Note (12/6/09): this is a potentially misleading throwaway comment – as explained previously a reduction in the rate of oceanic circulation would, assuming the biological pump is unaffected and atmospheric CO2 levels remain elevated, lead to a reduction in the rate of release of carbon by the oceans, i.e. overall the oceans would take up even more atmospheric carbon].
[Note (18/11/09): Did I write this? The only problem is that I no longer believe the ocean circulation is likely to lessen over the next century or so. As it is thermally driven it must increase in strength, a positive feedback – see later post – since this will bring more carbon to the surface from the deep ocean].

5. Ocean CO2 uptake is very sensitive to changes in the biological pump, which removes 11GtC (according to Fig.7.3) each year.

I hardly ever keep my promises for future blog entries, but in the unlikely event that I do on this occasion, next time I’ll discuss what factors could affect the biological pump…

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2 Comments »

  1. […] a year ago, I professed myself “baffled” by the BERN carbon cycle model.  Since then, I’ve finally twigged how the oceans will behave over the next century or so.  This post aims to further clarify my current […]

    Pingback by Still Baffled by BERN, but a little wiser « Uncharted Territory — April 20, 2009 @ 6:49 pm

  2. […] — It turns out that in fact sink (1), dissolution of CO2, the only one that clearly does depend on the annual […]

    Pingback by The Guardian on Chinese Coal (including the Strangely Deserted City of Ordos) and the Airborne Fraction, plus some more comments on the Ocean Circulation « Uncharted Territory — November 19, 2009 @ 8:26 am


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