My previous post attempted to answer the question as to whether the oceans would continue to take up CO2 if the level of the gas in the atmosphere started to decrease. To sum up, I concluded that, in fact, the oceans would continue to help us out. The reason is that different mechanisms dominate the exchange of CO2 between the sea and the air over different timescales:
- over short timescales – years – the surface layers of the oceans are in equilibrium with the atmosphere. The oceans (to a limited degree) buffer changes in atmospheric CO2.
- over intermediate timescales – decades to centuries – the turnover of the surface waters of the oceans dominates the chemical equilibrium of the surface waters with the atmosphere. The ocean will continue to remove carbon whilst the level in the atmosphere declines over decadal timecales, whilst this level remains greater than the equilibrium with the oceans as a whole, that is (arguably) while it is greater than around 280ppm. Over time, though, this equilibrium point will shift (upwards) as ocean warming and acidification reduce the capacity of the processes controlling the net annual removal of CO2, notably the “solubility pump”. [I've now noticed that the IPCC implicitly support this conclusion - in their carbon cycle diagram (Fig 7.3, p.515), they show that 18Gt of anthropogenic carbon has ended up in the "surface ocean", available to bubble back out if the atmosphere level of CO2 decreases suddenly, but 100GtC has ended up in the "intermediate and deep ocean", from where it can't easily be re-released. The proportions are roughly what I assumed in my previous post, too].
- over very long timescales – millennia – the entire ocean is in equilibrium with the atmosphere. Though the effect on this equilibrium of relatively small changes in the processes driving the exchange of CO2 is very high. In particular, the “biological pump” removes 10GtC/yr (I explain below where this figure comes from). A 10% change in efficacy, sustained for a millennium, therefore represents around 1000GtC, somewhat more than is present in the atmosphere.
Whilst writing the previous post, I came across what appears at first glance to be a bit of a howler by the IPCC. In section 18.104.22.168.1 Robust findings (no less), and elsewhere, they claim that:
“A potential slowing down of the ocean circulation and the decrease of seawater buffering with rising CO2 concentration will suppress oceanic uptake of anthropogenic CO2.” (my emphasis).
I beg to differ. I don’t understand how a “slowing down of the ocean circulation” would have this effect.
Here’s a carbon cycle diagram, quite similar to (though rather simpler than) the one the IPCC include as Fig. 7.3 on p.515 (this one’s thanks to NASA via Wikipedia and has no copyright restrictions, though the IPCC stuff might not have any either). Blue numbers represent annual carbon flows, black ones carbon stores:
Now, what’s important is that the “solubility pump” returns an annual net 100 – 91.6 = 8.4GtC to the atmosphere from the oceans. The solubility pump would be directly affected by a slowing of the oceanic circulation.
The 8.4GtC is counterbalanced by 10GtC removed from the atmosphere by “marine biota”. This “biological pump” would not be directly affected by a slowing of the oceanic circulation (though might be affected indirectly, by a reduction in available nutrients).
The IPCC seems to think that blocking a process – the solubility pump – with the net effect of adding carbon to the atmosphere will “suppress oceanic uptake of anthropogenic CO2″. This conclusion seems more than a little fishy to me!
At first glance the IPCC seem to be confused by their deltas. They have analysed the solubility pump in terms of the difference between the pre-industrial state and the present, with lots of nice diagrams showing where the “anthropogenic carbon” has ended up. The pump is, it seems, putting 2GtC less carbon into the atmosphere than before. But the solubility pump used to be balanced by the biological pump, which takes carbon out of the atmosphere. If we stop the solubility pump, we’ll still be left with the biological pump! If this happened (and it’s quite a big “if”), more carbon would be removed from the atmosphere each year!!
Why is this important?
Well, I think it would be a good idea to really understand the ice age cycle before trying to predict what will happen to the carbon cycle over the next century or two as we warm the planet. The point is that the carbon cycle plays a large part in reinforcing the Milankovitch cycles which change the pattern of warming of the Earth over thousands of years.
One puzzle that it seems to me should be resolved is why the planet does not just keep on warming as it comes out of an ice age. It was warmer than it is now during the last interglacial 120,000 years ago (120 kya) and at the end of the last ice age 10 to 5 kya (IPCC p.460 ff) (though unless we act now, in 50 to 100 years it will be significantly warmer than during those periods). Warming during an interglacial is fast: strong positive feedbacks are in play – warming causes increased CO2 release from the oceans (see previous post) which causes more warming.
We need some negative feedbacks! The obvious one is that in a wetter and warmer world, land uptake of CO2 starts to exceed oceanic release (which is why it might be a good idea to allow reforestation so that nature can help solve the problem for us). Another negative feedback, I suggest, may be a slowing of the oceanic circulation – driven as it is by the cooling of poleward currents (IPCC Box 5.1, p.397). Since the poles warm faster than equatorial regions this switch-off of ocean circulation is likely to happen as the world warms, as often discussed in the media, such as the film The Day After Tomorrow. (Though real life would be nowhere near as dramatic!).
Such a sudden cessation of warming could help explain how mammoths are found so well-preserved in permafrost! More to the point, it could help explain how CO2 stops rising at the end of interglacials. The situation is complex, since instability is produced, as cooling caused by slowing of the ocean circulation would tend to cause the circulation to restart. Not only that, the sudden cooling would reduce CO2 uptake in high northern latitudes in particular (by inhibiting plant growth). [This points to a problem with a mechanism that just relies on slow cooling to explain the turning point in the ice age cycle - the land (taking up carbon at this point) cools faster than the oceans (which are releasing carbon). This would surely cause a net increase in CO2, tending to reverse the cooling.].
I therefore speculatively hypothesise that the peak warming in (natural) interglacials is caused by a reversal of rising CO2 caused by a stop-start sequence in the ocean circulation. This may act together with the Milankovitch cycles to tip the Earth back into a cooling phase leading to the next ice age. Also, of course, if the cooling freezes the vast northern wetlands (which we’re now melting), e.g. in Siberia, it very quickly removes a large source of methane, which, because methane breaks down in the atmosphere relatively quickly to less powerfully warming CO2, would very quickly produce more cooling.
Has the IPCC got it wrong? And missed part of the explanation for the ice age cycle?
Afterword: It occurs to me that some people might think that increased CO2 uptake due to a slowing of the ocean circulation might represent something of a get out of jail card. On the contrary. It would surely result in even worse climate instability than we’re already heading for. We need to reduce GHG levels before we get to The Day After Tomorrow point.