Original Image. And more than a few cubic inches per foot…
I was pondering a problem I’ve pondered before. How much wood sucks all the CO2 out of the air in a given area of surface? And how long does it take a given type of plant to do that?
This isn’t a trivial question. If the forests and swamps of the world can “suck the air dry” of CO2 in a year or so, then what does it matter if we add a bunch? Further, if plants can suck the air to a limit of their growth ability in a year or three, then the air WILL BE AT THAT LIMIT sans human action of burning old stored carbon. (Since plants have been around for a few years…) That would imply that historic low CO2 levels were not “optimal” but rather were just “plant starvation levels”…
So we have an interesting question: Can plants suck the CO2 out of the air in any reasonably fast way?
Plants. Potted or otherwise
Plants grow based on the availability of “stuff” they need. Water, minerals, sunshine. And yes, even CO2. For any plant, the question is “What is the limiting nutrient?” for any attempt to grow more.
Farmers try to answer this question all the time. No sense adding more Phosphorus if you are ‘rate limited’ on Potassium, for example. (That’s why fertilizers come in types with numbers like 10-10-10 or 20-10-5 so you can pick the ratio of Nitrogen to Phosphorus to Potassium that your plants need given how it grows and what your soil is like. http://www.thegardenhelper.com/fertilizer.html )
OK, so is CO2 the limiting nutrient? I’d assert that it often was. Yes, for some lands and crops adding Nitrogen or Phosphorus increases growth (so they are the limiting nutrients). And farmers have gone out of their way to assure that their lands are not rate limited on those common fertilizer components. But… Greenhouse operators regularly add CO2 to the space to increase production. That strongly implies that CO2 is a rate limiting nutrient in many / most cases of farm lands as well. The CO2 “bump” in growth tends to work up to about 1000 to 2000 ppm of CO2.
Sidebar on MiracleGrow: The “MiracleGrow” fertilizer works in cases when others don’t because it contains the trace minerals that are sometimes the rate limiting nutrient in a given soil. Especially the mineral deficient soils sold for potting plants. Iron, copper, zinc, manganese, etc. While clay and sand in most soils (along with decaying wood and plant parts) typically release enough trace minerals that it’s not rate limiting, there are regions where particular minerals ARE limiting. A farmer would be well served to get a trace minerals check done and apply the particular missing nutrient. (or at least get the county soils map and see if they are in an area known to have a particular issue). This is often done indirectly via simply spreading manure, compost, or other natural fertilizers (where the implication is that since the source was alive, everything needed for life is inside). Even kelp is harvested from the ocean and used as a fertilizer. So one person puts ‘kelp meal’ on and gets little gain, as their soil is already in good condition; while the next person puts it on and suddenly they get dramatic improvement. They declare it a miraculous fertilizer, when they would have been just as well served by applying the iron, copper, or other missing mineral. When I’m having problems with a new plant or potting soil, I’ll apply a bit of MiracleGrow. If the plant perks up, I know I’ve got a soil fertility issue. Usually blending a bit of ‘bunny poo’ in with the bagged potting soils ‘fixes’ it longer term. Also, instead of using rocks in the bottom of pots for drainage, I use sticks and twigs. These slowly decay and give up needed nutrients to the potted plants long after the commercial potting soil has been washed clean of fertilizers…
Back to CO2
We are presently at about 383 ppm and we were at 200+ not so long ago. A few hundred years.
In the more distant past of the planet (measuring in millions of years instead of thousands), we often had higher levels. 2000, even 3000 ppm and more.
(I’ve seen this chart in several places. In this case, I picked it up from this article:
http://www.junkscience.com/MSU_Temps/historical_CO2.htm but they reference this article as their source: http://www.geocraft.com/WVFossils/Carboniferous_climate.html (a nice article, BTW). So I’m not sure who to credit / get permission from.)
Clearly throughout much of evolutionary history, plants had a much higher level of CO2 to work with, punctuated by periods of depressed levels near our recent ‘historical’ starvation levels. In particular, during the Carboniferous Era when plants were making all our coal, they clearly were growing fast enough to suck CO2 down to growth limiting levels.
So: many plants evolved to expect easier access to CO2 than at present. (Open field trials of CO2 enrichment have also shown growth increases, so it’s not just a greenhouse thing.)
Depending on the type of metabolism (called C3 for plants that make a molecule with 3 carbons in the first step, or C4 for those that make a 4 carbon molecule first) you have some plants that start ‘gasping’ for more CO2 earlier than others. C3 plants are much more common, but need higher CO2 to function well. C4 plants can thrive at lower CO2 levels, but are a more recent evolutionary step. Plants are adapting to astoundingly low CO2 levels, compared to the past… at less than 1000 ppm CO2… So the evolutionary evidence is that CO2 is lower than in the past to the point where plants are trying to find ways to thrive, and not just survive, in such low levels.
The evidence is that plants suck the atmospheric level down to the point where they ‘rate limit’ on CO2. They set the long term lower bound via sucking the air ‘dry’ to the limits of their ability. And it looks like at about 100 ppm to 150 ppm plants just can’t pull any more CO2 out of the air at all. So the historic 200 ish ppm levels argue for us being at ‘starvation levels’ for plants back then, with just a bit extra in transit from volcanos and other natural sources into plants.
And the Math Says?
I know, most folks glaze over at numbers. But I like them. What can I say? I also like sauerkraut and wieners and India Pale Ale beer and even taking 25 VAC or so through the palms of both hands…. (The tingle is, well, hard to explain ;-) but I’m not alone; I saw other folks lining up at the ‘test your strength’ arcade electric gizmo too…. Though, come to think of it, I haven’t seen one of them in a decade or two. Wonder if they have been lawyered out of existance? – he googles – Not quite but close. These folks are making one for home use: http://www.boingboing.net/2010/04/21/toy-tests-your-abili.html but the original arcade games look to be collectables now.)
So, some numbers. (For those who don’t like them, just assume that they are here for other folks to double check and toss rocks at, if I’ve got anything wrong, then skip on down to where I say this shows that plants are dominant in the world CO2 balance.)
I’m going to use various numbers as I remember them. I’m also going to use 40 year old “standards” as that is how I learned to do this stuff. If you are an S.I. units fanatic, just suck it up and deal with it. I’m more fond of older units as, despite their ‘issues’ (that I’m quite aware of), they are designed for easy grasp by folks without calculators and computers. “Joule” just doesn’t speak to me but warm a pound of water by 1 degree F? That’s a BTU and when I want to warm a pound of milk from 40 F to 100F I want about 60 of them. Those of us who have warmed baby bottles will recognize this example… and recognize the utility of “A pint is a pound the world around”. Just because milk is the liquid in question does not make it any less valuable a rule than if it were water…
Back to the air.
So, we have a square foot of air column. Each square inch weighs about 14.7 lbs. So a square foot is about 12 x 12 x 14.7 or 2116.8 lbs. Or a bit over an American ton (2000 lbs) and a bit under a metric or long ton (2200 lbs more or less) and English Ton (2240 lbs). So we have about a ton per square foot of “air”. How much of that is CO2? 384 ppm, more or less. (It varies a bit with the year, the season, and the ocean surface temperature.) That number is “by volume”. You can also figure it out “by mass”.
Air is mostly Nitrogen (about 80%) as N2. Nitrogen is atomic weight 14, so a molecule is 28 units. It’s about 20% Oxygen. (the exact percentage changes over the years, and changes even more over geologic time scales). At an atomic wt of 16, that’s a 32 unit molecular weight. Average those together (and ignore the 1% or so of Argon and the other trace gasses) and you get about a 28.8 average molecular weight of the air. CO2 is 12+16+16 or 44 mol wt so it’s heavier. Take those weights and make a ratio ( 44 / 28.8 or about 1.527 ) that is how much the CO2 volume counts in mass. Or about 585 PPM Mass of CO2 (as it’s heavier, so the amount of mass is higher than the amount of molecules). I found one reference that claims it’s 583, so my estimate is somewhat close. (And that reference was not an official source either).
Now take that weight of air, at 2116.8 lbs, and adjust it for the CO2 fraction of .585 parts per thousand (notice how I took 1/1000 out there? ;-) and you get 1.2386 lbs per square foot of CO2 for ALL the CO2 in that air column.
So how big is a pound and a quarter of CO2 in wood? Got a chunk of wood that size?
Got Wood? About 1 1/4 pounds of CO2 worth?
This site has wood densities:
They range from 7 lbs / ft^3 for balsa to 83 lbs / ft^3 for Ebony, more or less, but with 50 or so being very common. (These are for dry weights).
But that’s a carbo-hydrate. How does CO2 relate to carbohydrate? Well, pretty directly.
Take the molecular weights and compare them. 12+16+16= 44 vs 12+16+1+1 or 30 for carbohydrate (COH2 more or less – yes, I know I’m ignoring the ends of the chains, we’re not working out to 4 decimal places here…) So we have about 30/44 of mass to hold the same amount of carbon… so that 1.25 lbs of CO2 takes about 0.8445 lbs of wood to store it.. because we throw away one of the “O” Oxygens in the process of making wood (and we are all glad for that as we like to breath!)
OK, so we don’t have 1 1/4 lbs of wood, only 0.85 lbs. But it still makes me happy ;-)
How big is that, in volume? Using a 50 lbs / ft^3 figure, we get about 3.1 inches on a side. Call it 7.8 cm on a side. Pretty darned small. ( or a cylinder of about 1 1/4 inches in radius and 6 inches long. Did I say “darned small”? Sorry, I meant large. Very large. Gigantic. Enormous even!…)
So let me get this straight: A 12 x 12 inch square of dirt with a 3.1 x 3.1 x 3.1 inch cube of wood on top of it has removed ALL the CO2 from the air column above it? Or a cylinder of, er, “enormous proportions” has, er, um, spent it all too? Yup.
(I’ll be making pictures of this in the next couple of days… The cube, damn it, the cube!)
So what is that in tons per acre?
About 16.7 tons.
(43,560 square feet per acre x 0.85 lbs per square foot / 2200 lbs/ton)
[ There are three common tons. The "short" at 2000 lbs, the "metric" at 2205 lbs, and the "long" at 2240 lbs. To be precise in the 4th decimal place, one would need to use the specific ton in question. But since we're just looking for "about how many whole tons", you can ignore the 4th and even the 3rd decimal point. It works out to 18.5 short tons, 16.5 long tons, and 16.8 metric tons. In all cases, about 17 tons +/- a bit. ]
But don’t we get about 50 tons / acre of wet wood or about 25 tons dry? (E. Grandis in the 2nd year of growth produces 63 tons / acre at http://www.treepower.org/yields/main.html )
Yes, we do, for very fast growth species like poplar and eucalyptus. So a fast species completely drains the air above it of all CO2 in one year AND most of the acre next to it. Given that plants can’t suck CO2 out below about 100 ppm, it’s more like they drain twice the area they occupy down to the limit of survival. In one year.
Ordinary hay runs about 2 tons dry weight per acre. Switchgrass runs 10 to 15 tons dry mass per acre. http://ipst.gatech.edu/faculty_new/faculty_bios/ragauskas/technical_reviews/Bioethanol%20from%20Wood%20Facts.pdf
Bamboo? A bit more at 15-50 tons / acre commonly and up to 100 tons / acre for a 5 year stand for some species with special care.
The yield of dry wood varied from 17 to 54 tons per acre, depending on the species.
Cutting in 10-foot strips every 5 years produced a yield of 18 to 45 tons of dry wood per acre. The 4-year average yield was 28 tons per acre.
That 28 tons per acre (dry tons, given the context) would be about 56 wet tons / acre / year.
I have a ‘timber bamboo’ in my back yard and each year I must cut down a bunch of stems to keep it in check. I’ve now got about a cord ( 4 foot x 4 foot x 8 foot ) of old stems that have dried but not yet composted back into the soil. And that is with me trying to make them go away… When it starts to push up a new stem, it comes out of the ground at full diameter (about 3 to 4 inches for this species) and rises at an astounding rate. To about 40 feet tall in one season. Then it grows all the side shoots and leaves. An amazing growth factory. If I needed fuel from a small area, I’d get a chop saw and plant bamboo.
So a stand of bamboo will take ALL the CO2 out of 2 to 4 times it’s surface area. In one year.
And algae (pond scum) can grow even faster, so it’s not just a ‘land’ thing…
(For more, and for reference links on things like the logarithmic growth rate of plants with CO2, see the earlier posting:
So we’ve got examples from wood to grasses that all drain all the CO2 from the air above them inside a season or two. And stuff grows all over the world.
We have historical levels of CO2 that are only modestly above the level where a plant simply can not pull any more CO2 out of the air, which strongly implies that the 200 ish level of CO2 is at an equilibrium level where plant growth and death balance (and not an optimal level, a starvation level).
Plant growth enhancement with CO2 up to 1000 ppm is strong, and up to 2000 ppm is measurable, meaning that plants know how to use more, expect to have more, and are rate limited at the low levels of today.
And perhaps most importantly, even a small standing mass of wood represents more CO2 than in all the air above that chunk of ground. So a standing forest will not only pull a great quantity out of the air, but will hold a great quantity for as long as the forest stands. We don’t need to cover the whole planet with trees, only a small part of it, to have pulled CO2 levels down to that starvation balance level.
(A corollary to this would be that cutting down the forests of Europe and North America did a heck of a lot to the CO2 balance of the planet and the continued burning and felling of forests in South America, Asia, and Africa continue to have this leveraged impact. It would be interesting to calculate the tons of CO2 released into the air from all that forest destruction and compare it to the growth rate of CO2. Perhaps it’s not the oil that matters most after all…)
Basically, the rate of growth of wood makes it pretty clear that the extractive side of the ledger is able to beat the generation side while the logarithmic growth curve of plants says that as CO2 increases, their ability to capture it increases for at least 5 times the historic levels of 200+ ppm. The plant balance is very important to the CO2 level.
That same log curve says that as CO2 levels drop, plants rapidly reach a point where they survive, but do not thrive, and growth is limited. A stabilization level is reached.
My assertion is that the historic levels of CO2 represent that lower bound of growth, and not some optimal level for life on the planet.
Sidebar on Degree Days: Plants need a certain number of days above a lower bound of temperature to grow. Each plant reaches maturity and maximum growth based on the number of ‘degree days’ of warmth it gets. To the extent that the planet warms up, more plants get more ‘degree days’ faster, and so grow more quickly. Any ‘global warming’ is balanced by ‘global greening’ and ever faster rates of CO2 extraction into wood. So if you are worried, go plant a tree or some bamboo. If you are not worried, harvest some corn to go with your grass fed beef steak and fire up the BBQ by the pool; taking pleasure in the fact that you are feeding the next crop. The plants will find a balance point in either case.
Update: Well mixed?
In response to a comment from Malaga View about the degee of mixing of the CO2 in the air, here is a NASA image of CO2 via the infrared sounder:
It’s pretty easy to see that the major forested areas of the world are busy pulling CO2 out of the air. The south pole and Greenland being deep blue leads me to think there is some ‘cold water stripping’ of CO2 from the air via rain and snowfall.
OK, I’ve got a couple of pictures for “illustration purposes”. In each picture, there is a 1 foot square white ceramic tile. That is the area of ‘air column’ that would be depleted by the object on the square. Behind each target item is a tree, just to give a bit of perspective… There are three pictures. In more or less reverse order, they are:
One is an oak block of 13 ounces. The amount of CO2 over a square foot ought to be about 13.6 ounces, so you will need to imagine the oak about 5% larger. (In other words, it would look exactly like it does now ;-)
There is also a “stick” from some kind of fast growth light weight weed like bush that keeps taking over part of my front yard. I don’t know what it is, but it goes from “nothing” to 6 foot tall 2 inch diameter in a couple of years. I keep chopping it down, it keeps coming back. The stick is very low density. Maybe 1/2 that of pine. So for pine, imagine it about 1/2 size, and for oak, figure about 1/3 or less. The stick is 13.6 ounces, so darned close to exact.
The third picture I took is of a corn stalk. It’s a scrawny one that didn’t make hardly an ear at all (and the possums got that…). It was about 18 ounces when I picked it up, (it’s been pulled for a couple of days now and had already dried off the cob and the leaves). I pulled off just about all the leaves and cobs I could and got it down to 15.8 ounces. Then again, it’s probably still got a couple of ounces of water in it. At any rate, you could have two of them in the picture and still not have anything.
So, have a look. This is the CO2 over the square, when sucked out by a plant.
A corn stalk
How about a soft light wood stick?
An Oak Plank