Burying Trees to Sequester Carbon

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Burying biomass to fight climate change

* 03 May 2008 by Richard Lovett

A few years ago, Ning Zeng began to wonder about the hidden potential of landfill sites. He had been discussing a mystery with his students: for some reason, North America's carbon dioxide emissions are not quite as high as they "should" be. Perhaps, one student suggested, America's huge landfill sites were acting as carbon sinks. After all, a lot of what is thrown away does not decompose: even 50-year-old newspapers can be perfectly legible.

Zeng, an atmospheric scientist at the University of Maryland in College Park, later calculated that the amount of carbon sequestered in this way is actually tiny, but it gave him an idea. What if we could sequester the carbon locked up in trees in such a way that it doesn't get released back into the atmosphere? Could we store enough of it to offset a meaningful amount of emissions?

It sounds like a long shot, but Zeng is convinced it could work. In a recent paper in the journal Carbon Balance and Management (vol 3, p 1), he calculated that if we buried half of the wood that grows each year, in such a way that it didn't decay, enough CO2 would be removed from the atmosphere to offset all of our fossil-fuel emissions. It wouldn't be easy, but Zeng believes it could be done.

Zeng's is not the only proposal of its kind. Other researchers are totting up the amount of carbon that could be sequestered in various kinds of biomass and are finding that it is a surprisingly large amount. Not enough to halt climate change on its own, perhaps, but enough to make a sizeable dent in atmospheric carbon and to buy us the time we need to sort out the mess we've made.

The idea of burying carbon in biomass makes sense: plants remove CO2 from the air to produce carbohydrates by photosynthesis. The carbon is returned to the atmosphere when the plant dies and decays. Planting trees to sequester carbon is approved under the Kyoto protocol, but critics of this approach point out that the carbon locked up in forests is only kept out of the atmosphere for as long as the tree is alive, and that older trees start emitting more carbon than they take up as they reach old age. Recent studies have also suggested that warmer temperatures and higher atmospheric CO2 may eventually kill trees, casting doubt on the use of forests as long term carbon sinks (New Scientist, 27 October, 2007, p 42).

There is a lot of interest in the possibility of sequestering CO2 in disused gas and oil wells or porous rocks, or even in ocean beds. Trouble is, finding viable sites for this kind of project is tricky and the technology needed is far from ready. Burying biomass, say enthusiasts, has none of these problems.

Wood burial is perhaps the simplest of the ideas. Zeng's proposal is to thin forests regularly, and to bury excess wood, forestry waste and even trees that have been grown specifically to be buried in trenches between the remaining trees. To prevent the wood decomposing and the carbon being released, it would need to be buried deep enough to avoid being broken down by soil fauna and fungi, or stored above ground in watertight shelters. Zeng gives an example of a plot of 1 square kilometre (100 hectares), with the excess wood from 1 hectare of woodland buried deeper than 5 metres and down to 20 metres. He calculates that this could sequester 1 tonne of carbon per hectare - using that land to grow trees would sequester 1 to 5 tonnes, depending on the age of the forest and the type of tree. Burying wood sounds like a lot of trouble for a small gain, but Zeng insists that, unlike simple growing, this is a long-lasting and perhaps permanent carbon sink. He estimates that offsetting all of the world's current emissions would be achievable with a workforce of one million people - substantially fewer than those already employed in the forestry industry in the US alone. Even so, to offset all our emissions, most of the world's forests would have to run a wood burial scheme.

Continue reading: http://www.newscientist.com/article/mg19826542.400-burying-biomass-to-fight-climate-change.html