I really don't get carbon trapping

Maybe it’s just a concept, a construct for other future actions. But can insignificant man really dent the CO2 levels in the atmosphere and seas through wholly artificial means? Some of the proposals are valid (pumping CO2-rich water into limestone formations among others). But all methods look crazy expensive compared to just nudging mother nature.

How much better are the methods of artificially trapping CO2 compared with, say, planting or just leaving thousands of hectares of land to forest? A three-foot diameter tree must store more than a tonne of carbon. Or how about just protecting swamps and marsh land to allow the black goo to accumulate en route to becoming lignite and coal?

Planting to forest has other advantages too. Whether or not it is ever a major part of atmospheric carbon reduction, it should be pursued anyway.

The exact numbers depend a lot on exactly what you’re comparing it to.

But biosequestration isn’t exactly simple. Land is valuable, forests are valuable, and you can’t easily acquire large swaths to just set aside forever. And even if you do, the geopolitical climate 50 years from now might mean that some other nation will fight you for the land and free up the carbon again.

The US EPA, for example, says that a 500 MW coal plant would require 62,000,000 trees over 10 years to sequester what a CCS underground pool could store.

Another example, this time Australian, estimates that forest-sequestering about half of their power plant CO2 emissions (200 Mt) would require a 10x expansion of their current forestry lands. “This is ten times larger than current Australian forestry operations. Increase in forests in Australia of this scale would also have an unknown but likely beneficial effect on the sub-surface soil storage of endogenous soil carbon. The availability of land and water for this purpose and the need for alternative uses for the land could, however, provide a significant barrier to this expansion.”

As with most things environmental, CCS shouldn’t stand alone. It’s just another tool in the toolbox…

Glad the numbers are coming in, thanks. So biosequestration won’t cut it for present level of emissions. But has anyone considered my own version?

Collect sewer muck and other biodegradable trash into a huge catchment, forming either a giant swamp, or sanitary landfill? The landfill will be the size of a small country (or a principality.)

Unless you seal that somewhere, biodegradation isn’t really the same as carbon sequestration. Decomposition releases methane, an even more dangerous greenhouse gas (per volume, though I’m not sure how much you’d get per volume of muckity-muck… it depends on what’s in your sewer muck). See this example article. Trees are sort of a special case because they store all that carbon in static tissue for decades. Other living things tend to die sooner and release at least some of their organic compounds sooner. More on landfill carbon sequestration.

I am not an expert at this, but leaving sewage in a big pool by itself won’t result in oil anytime soon; the process also requires heat and pressure, both of which will generate additional CO2e if artificially provided.

Also, municipal biosolids typically have some value as fertilizer or fuel. Our city captures all the solid waste coming from sewage and uses it as compost for city parks and forests, for example. Other places in the world just burn their trash for energy. Others still compost their stuff in biodigesters that capture waste methane and then burn that (more cleanly than would occur in a landfill) for energy. IOW you wouldn’t necessarily want to just store all that stuff away.

Most CO2e emissions come from power generation and transportation anyway, not municipal solid waste… meaning even if you captured all sewage, you’d not really be solving the problem. (Unless you pumped CO2e from power plants and cars into some sort of biomedium, but that’s still not the same thing as just having a big pool of ready-made CO2 sludge to begin with)

I said I wasn’t an expert, and I’m happy to provide another perspective: At least one article suggests that artificial wetlands (not just ponds of sludge, but with marsh plants growing and sucking up carbon) may be net carbon sinks even with the decomposition byproducts factored in. I did not read the study itself, but I wouldn’t be too surprised if that were the case.

However, that still suffers from the land-use problems: whose district are you going to flood to make giant marshes? How much habitat will you destroy in the process? How many mosquitoes will you spawn, and how will they affect the birds in the area, and how will those birds affect other vegetation, etc.? How do you prevent somebody else from filling in your marsh 15 years later, causing a huge release of decomposition gasses? Etc.

Swamps are really net carbon traps, though they might release a lot of methane and CO2. They’re nature’s septic tanks.

How 'bout this. You know the story of Ramree island in Burma where 900 Japanese soldiers were supposed eaten by crocodiles? Japanese figures were less than 400 but anyway, Ramree island is 10 to 16 miles off the Burmese mainland, separated by swamps (and croc-infested.) Here in the Philippines, I can point out a number of similar spots where dotted islands can, with major (but not too major) earthworks, be turned into a giant swamp and wetlands, still leaving “fast” land for adequate human access and habitation. The Chesapeake eastern shore would be similar.

Ok, let’s do some rough math. This article says 1.5 tonnes CO2e/hectare of coastal saline (or brackish?) wetland, less for other types of wetlands. This other article says 9.7 billion tonnes of CO2e globally produced in 2012.

So 9.7 billion / 1.5 hectares = 65 million km^2, or basically 40% of the Earth’s surface… something like Africa plus North America would have to be turned into coastal marsh.

…I should point out that I’m usually pretty terrible with numbers, so if that’s wrong, feel free to correct me.

Hmm, looking into those figures more, I don’t quite trust them – specifically the 1.5 tonnes per hectare number. Let me look into it further.

6.4 billion hectares needed (assuming the time frame for both the 1.5t/ha is equal to the 9.7bn t world-wide) or 43% of earth’s land area, ONLY if the marsh is your sole sequestration mechanism.

For the 1.5 tonnes a year, I can just estimate. It takes about 20 feet of soft vegetation to make a 1-foot coal seam, or maybe 1.5-2.0 feet of swamp muck. Annual undergrowth that falls to the ground doesn’t look like it can reach 20 feet. I’d put it at 3 inches (7.6mm) compressed a year. So that’s just 4mm of new muck which, assuming it all turns to pure coal, is 38 cu meters for 1.0 hectare. That’s 72 tonnes of coal per hectare-year! But that assumes the plant matter is wholly converted.

I think a more workable model is to put back what fossil fuel has been exploited since time immemorial, coupled with the natural process. You can’t do this in one year, of course. It’ll take longer than clearing all the land mines that have been planed.

(Edit: Simulpost) I checked three other sources (1, 2, 3) and I think 1.5 T/ha is too low; the other sources suggest 6 to 9 per hectare. Not sure why there’s such a big difference. Maybe they meant acres, but even that’s only like 3.7 T/ha, still low.

Anyway, let’s assume the more reasonable 6 or so T/ha/yr. And then say that the Earth can herself sequester some amount of that without major issues – in my light reading, it seems like maybe up to 60% or so of that, leaving about 4 billion tonnes / year left to deal with. 4 billion / 6 T/ha/yr = 6.7 million square km, or a bit smaller than the size of the continental US. Still a pretty big area, and nowhere near practical. Part of the solution, sure, but I don’t think it’s the entirety of it.

Living wetlands are one thing, but if you want to make coal from that, wouldn’t it take quite a bit of energy and time? (too tired to do the math right now)