A beautifully illustrated guest post from alefnot, providing education about the carbon cycle.
A few days earlier this was air and water (and a trace bit of dirt). Is that amazing or what?!?
A couple of weeks earlier, so was most of this (except the woody bits). You can almost see CO2 being drawn down.
Young leaves have a different composition from older leaves — and a distinct color as well.
Dann, Blümlein alle, heraus! Heraus!
Der Mai ist kommen, der Winter ist aus!
If you listen very closely you can hear the breathing of the Earth in the miracle we call “primary productivity”:
CaSiO3 + 3 H2O + 2 CO2 –> Ca(2+) + 2 HCO3- + H4SiO4
Here CH2O is of course shorthand for carbohydrate, not formaldehyde. If you’re more quantitatively inclined you can plot it:
Figure from NOAA’s CMDL. There’s a lot going on in this figure. CO2 concentrations (color coded violet -> red = low -> high) are plotted against time and latitude. Time runs from 1997 to 2006, a short period but long enough to show some trends. The latitude runs from pole to pole, with the North pole farthest away from you. For reference, LA is at 34o N. Latitude; Boston 42o N. Latitude; Sydney, Oz 34o S. Lat.
First thing to note is that there are annual fluctuations over a decadal increase in CO2. In fact, if you extended this back 150 years you’d see pretty much the same trend, only that the decadal trend has accelerated in the last 50 years or so.
Second thing to note is that the fluctuations in the Northern Hemisphere are larger than those in the Southern.
Third thing to note is that the fluctuations are 180o out of phase between the hemispheres.
The annual fluctuations are driven by primary productivity: in the Spring plants put on new growth. Plants are actually sucking in CO2 and making leaves, flowers, wood, roots, fruit with the stuff. In temperate and colder regions in the Fall, plants shutdown, and much of this carbon gets released back into the atmosphere via the reverse reaction, respiration. The fluctuations in the hemispheres are out of phase because Spring comes in March-April in the North, September-October in the South. The fluctuations are larger in the NH because the NH has much greater landmass, at least in this geological age. Productivity on land is typically seasonal (except at the tropics, where you will note there is very little temporal variability): annual plants complete their cycle in one year, shrubs and trees typically shut down in winter. Productivity in the ocean, however, turns over much more rapidly — a few weeks — so annual fluctuations are much less marked.
Breathe in:
Breathe out:
This post is actually (a much belated) part 2 of a series on the carbon cycle: here we look at timescales (the first deals with reservoirs: you don’t have to read it, but I will be using terminology and information from it). Why are we interested in timescales? All sorts of reasons, including understanding how our planet evolved, why we have an oxygen rich atmosphere, why we have limestone and marble, coal and oil, etc, but probably the most pressing one is climate change, and how we might deal with it. So we’re going to focus on that one.
Here’s a figure of the carbon cycle from NASA:
Units are gigatons (= 1 petagram = 1015 g)carbon for the reservoirs, Gt/yr for the fluxes. Some of the numbers are not that well known, some are already outdated (the 750 Gt C in the atmosphere is closer to 830 Gt C today) so don’t put too many significant figures on them.
Let’s look at the marine biota reservoir, about 3 Gt C. Fluxes into and out of this reservoir amount to 50 Gt/yr C. From this we see that the marine biota reservoir turns over — that is, goes from CO2 -> CH2O -> CO2 — (50 Gt/yr)/(3 Gt) = 16.7 times per year, or that its turnover time is 3 Gt/50 Gt/yr = 0.06 yr = 22 days or about 3 weeks. Obviously kelp and blue whales and Mesonychoteuthis and Glory-of-the-Seas cones and all manner of other things live a lot longer than 3 weeks, but marine biota is dominated by plankton, which are very short-lived. Because this reservoir turns over 17 times in a year, annual fluctuations are strongly damped.
If, like Voltaire’s Greeks and Egyptians, you like to split hairs into four, we need to be careful about what we mean by timescale. You can compare simple scalings of reservoir/flux only if all of the flux mechanisms are the same. In this case, the implicit assumption is that the flux is linearly proportional to the size of the reservoir. This is not generally the case, but the assumption is still useful at least to first order and avoids getting bogged down in mathematical detail, so I will use it here. But keep in mind that the system is not a linear one, and if you want better numbers the details of the flux mechanisms become important.
On the other hand, the terrestrial vegetation reservoir is (by this reckoning) 610 Gt C, while fluxes out of it amount to 120 Gt/yr C, so it takes 5 years on average for this reservoir to turn over. Again, Sequoia sempervirens may live millennia, iceberg lettuce a few months before harvest, so there’s quite a range here, but overall this reservoir turns over slowly enough that annual fluctuations are quite marked.
Now the list of things to talk about concerning timescales in the carbon cycle is endless, so I’d like to look at just three examples. These are
- You as an organism
- Timescales of atmospheric carbon
- Sequestration, with a focus on plankton seeding
The human being as an organism
Many things have multiple timescales. You, for instance. There is a meme out there that we are CO2 sources just by breathing. That’s true on a daily timescale. Associating that with climate change is fatuous, of course, because that carbon comes from what we eat, which is carbon sequestered out of the atmosphere a few months to maybe three years earlier, depending on whether you’re eating green beans or beef. On that timescale you as an organism are carbon neutral. Moreover, if you’re gaining weight, you are sinking atmospheric CO2 over the time span while you are gaining weight; if you’re losing weight, you are a source of CO2 in that time span.
In fact, all of the carbon in your body ultimately came from CO2, so you are a CO2 sink while you are growing, and a carbon reservoir while you’re alive. After you die, well that depends: if you are cremated you turn into CO2 right quick. If you are adventuring in Borneo and touch something extremely toxic and die you will turn into CO2 probably within the year, a few at most. If you fall into a glacier, or are sacrificed and thrown into a bog, you might sequester carbon for millennia.
But this only refers to you as an organism, not to any possible lifestyle factors, like driving, burning coal, getting food from modern agriculture, etc. As a species, the story is very different.
Timescales of atmospheric carbon
Let’s look at that carbon cycle diagram again, this time looking at the atmospheric reservoir. About 99% of atmospheric carbon is CO2, nearly all of the remaining is methane. Methane has one dominant sink (loss mode), which is reaction with OH radical to form ultimately CO2. This happens on a timescale of about ten years, although as atmospheric methane concentrations increase that lifetime also increases as methane eats OH.
Now consider CO2. There are several sink terms here, some indirect, so we cannot really speak of a single timescale. The primary sinks are uptake by the biosphere, uptake into the ocean, and weathering of carbonate and silicate rock. There is also an indirect sink through seafloor subduction. We’ve seen that the marine biosphere has a turnover time of ~3 weeks. If we use the 750 Gt atmospheric reservoir number (you can update that if you like) we get a timescale of 6 years with respect to the terrestrial biosphere (750 Gt/121 Gt/yr — in fact the numbers are not known to great precision, so take it as an approximate number). We get about half of it back in 5 years; the rest of that carbon ends up in soils first, and will take ~25 yrs to return to the atmosphere.
How about ocean storage? If we use the numbers from the figure above (numbers from elsewhere are on the same order) we get 39000 Gt total C in the oceans and an outgassing flux of 90 Gt C/yr: the marine C reservoir turns over in ~400 yrs, using our simple scaling. We will revisit this point when we look at ocean sequestration.
Very long timescales in the carbon cycle
Rocks such as quartz, feldspar, granite, many sandstones, etc are silicates or have silicate components, and these weather in the presence of water and CO2 to form bicarbonate and silicic acid.
CaSiO3 + 3 H2O + 2 CO2 –> Ca(2+) + 2 HCO3- + H4SiO4
(There are many different silicates, all behave similarly) The silicic acid that runs off into the ocean provides the raw material for diatom tests, and eventually rain down to the sea floor to form a diatomaceous ooze. The HCO3- may precipitate out as CaCO3 under the right conditions (typically requiring calcareous phytoplankton or shelly things like molluscs); under acidic conditions it may return to the atmosphere as CO2; or it may run off into the ocean and add to the immense DIC (dissolved inorganic carbon) reservoir. This is an extremely s l o w process: timescales range from 105 – 108years, or even longer for longer for big batholiths. The Appalachians are some 480 million years old and still wearing down.
Breathe in:
This is the Sierra Nevada in Sequoia National Park. The chemical weathering of the huge exposed batholith that makes up the Sierra and includes Mt Whitney, El Capitan, and Half Dome is very slowly but surely sinking atmospheric carbon. On a somewhat faster timescale, carbon locked up in wetlands gets released when the wetlands dry up.
Ultimately the very long term cycle is closed when the marine carbon ends up as either limestone or organic muck that gets buried deep in the lithosphere. Uplifted limestone formations such as Mt Everest weather to release CO2, very very slowly. CO2 is also outgassed in volcanos and other magmatic events.
Breathe out:
This is Mt Erebus. Frequently ash plumes are seen emanating from the top (though not, unfortunately, when I took the picture) as the volcano peacefully closes the long-term carbon cycle. Less peaceful closures include Tambora, Krakatau, more recently Pinatubo and Mt St Helens. Every so often the outgassing is so huge and so rapid that lifeforms struggle to adapt: the Siberian Traps are the primary culprit in the greatest mass extinction of all time, the end-Permian event, and the Deccan Traps, which might have done the dinosaurs in before the bolide provided the coup de grace. The long term cycle of weathering, burial, and uplift takes on order 108 years, and is responsible for the compositional stability of the atmosphere.
Marine carbon sequestration
A few years ago the company Planktos made a splash by proposing to sequester CO2 by adding iron to areas of the ocean that were high in nutrients but low in productivity (HNLC — high nutrient, low chlorophyll), and then sell the carbon credits. HNLC regions have low productivity for their levels of nitrogen and phosphorus because they are missing one or more critical nutrients, typically iron. The region they chose was the HNLC region off the Galapagos. Last year they put the project off indefinitely. According to Planktos, it was because of “a highly effective disinformation campaign waged by anti-offset crusaders”. Environmentalists were worried about unintended consequences. “Scientists questioned whether the firm would be able to quantify carbon sequestration from its efforts”.
Let’s think about what those scientists were questioning using the carbon cycle diagram above (at this point you should be able to work out the numbers yourself). Essentially Planktos were proposing to increase the marine biota reservoir by iron addition, in so doing they would increase the uptake flux into the surface ocean from the atmosphere. Plankton grows very quickly, but it also doesn’t live very long, so it will release that carbon back to the surface ocean, through respiration, as food for predators such as zooplankton, and through scavenging by microbes. Of the 50 Gt/yr leaving that reservoir, 40 Gt/yr stay in the surface ocean, the other 10 Gt/yr sinks to the deep ocean, either by mixing as dissolved inorganic carbon (DIC) or by sinking of organic carbon. In other words, 80% of the carbon taken out of the atmosphere in this way remains in the surface ocean, which equilibrates with the atmosphere on a ~decade timescale (1020 Gt/90 Gt/yr using the numbers from the diagram) Not very efficient. In fact, while their justification for their choice of the Galapagos is that it is an HNLC region, the region is also an upwelling region, which makes transport into the deep ocean even less efficient. And even the fact that it is HNLC means that iron will be scavenged in the surface ocean particularly efficiently, which will tend to reduce the carbon export flux to the deep ocean.
What happens to the 20% (or less) that makes it into the deep ocean? The turnover time of the oceans is a few centuries (381 years by the figure). It will reequilibrate with the surface ocean and the atmosphere in that time. Maybe we’ll have gotten our ducks in a row and will have found ways to deal with the excess CO2 that we loaded into the deep ocean by then and everything will be hunky-dory. Or maybe we’re just loading a time-delayed charge of CO2 that will add stress both to climate and to ocean surface pH when it comes back up.
Ideally, the excess carbon would drift down to the seafloor and end up as sediment. This eventually gets subducted into the lithosphere and comes back up as CO2 in volcanos. That happens on a ~100M year timescale, the longest one available on the planet, so that would be permanent, for all intents and purposes. However, that flux is only on order 0.2 Gt/yr (carbon is efficiently recycled in the oceans), which is 0.4% of the 50 Gt/yr going in and out of the marine biota reservoir. IOW, 99.6% of the carbon Planktos takes out of the atmosphere will not be sequestered for the long haul. Almost certainly at least 80% would not be sequestered even for the short haul. As an absolute minimum, schemes such as that Planktos is proposing must quantify carbon flux to the deep ocean.
If you want to sequester carbon, what you want is a lithospheric reservoir, preferably lithified as carbonate rock, or as buried organic sediment. Next best would be estuarine wetlands. Reforestation only sequesters for a century or so, but that is still something, and healthy forests provide many other benefits (which not the case for more CO2 in the ocean). Freshwater wetlands sink a lot of carbon but release methane; OTOH they provide a great many ecological services. There are many excellent reasons to protect and reconstruct freshwater wetlands, but climate mitigation is not high among them.
Relative timescales
One last note about timescales. Relative timescales are key. If we used fossil fuels at the same net rate as they are being formed, then fossil fuels would be renewable. And we’d be riding horses, using oxen as tractors, burning wood to cook and keep warm, and have a much lower population. If the Siberian Traps had not erupted so quickly (probably over in no more than a million years) perhaps more lifeforms could have adapted and there might not have been a catastrophic extinction. If we did not put out so much CO2 so quickly, mitigation might be possible, adaptation would be cheaper, and we might not be in the middle of the 6th Great Extinction. Change is inevitable, and life is resilient enough to adapt. But the rate of change determines whether the adaptation is graceful or catastrophic.