It’s a mantra repeated so often it is accepted as being true: wood sequesters carbon, concrete production emits carbon. As in most things, the truth is often lost in translation.
Living trees do sequester carbon. And long-lived, wood-based building products do sequester carbon. But only a small fraction of the carbon originally sequestered in the living tree. So too, producing cement is carbon intensive.
But the story does not end there. Recent research reported in Nature Geoscience shows that concrete reabsorbs much of the carbon previously emitted from cement production through a process called carbonation. Simply put, concrete might also serve as a carbon sink.
The study acknowledges the manufacture of cement produces roughly 5% of global CO2 emissions from all industrial process and fossil-fuel combustion in 2013. And while considerable attention has been paid to quantifying emissions from cement production, the natural reversal of the process—carbonation—has received little attention in carbon cycle studies.
Researchers examined regional and global CO2 uptake between 1930 and 2013 using an analytical model based on carbonation chemistry. The results suggest carbonation of cement materials over their life cycle represents a large and growing net sink of CO2, increasing from 0.10 GtC yr−1 (Gigatonnes of Carbon per year) in 1998 to 0.25 GtC yr−1 in 2013.
In total, it was estimated that a cumulative amount of 4.5 Gigatonnes of CO2 has been sequestered in carbonating cement materials from 1930 to 2013, offsetting roughly 43% of the CO2 emissions from production of cement over the same period. In general, the carbonation of cement products represents a substantial carbon sink not currently considered in emissions inventories.
So how does this compare with carbon sequestration in wood? Research reported in a study by the Pacific Northwest Building Resilience Coalition suggests that a relatively small amount of the carbon originally stored in a living tree makes its way into a long-lived building product such as dimensional lumber or cross laminated timber.
Estimates vary due to many factors, but in general only 18-30% of the carbon from the initial living tree ends up in a long-lived building product. Most of the originally sequestered carbon is lost during harvesting or is emitted when non-merchantable wood is burned for energy, disposed as waste, or used to produce It takes a long time before this carbon is reabsorbed by new forest growth.
Even at the 100-year mark, only half the original carbon lost is reabsorbed from the atmosphere by new forest growth. See chart.
A further complication arises with the end-of-life disposal or landfilling of long-lived timber (wood) products when the remaining sequestered carbon is released or converted to methane. Methane is known to have a Global Warming Potential (GWP) 28-36 times greater than CO2.
So, the oft quoted mantra – that wood is a sink and concrete a source of atmospheric carbon – noted above is rather more complicated than it first appears. In fact, while concrete continues to act as a carbon sink over time, wood after harvest, becomes a chronic CO2 emitter as well as a potential source of methane emissions.
Nothing is more effective in storing carbon than a living tree, but perhaps it is time to reconsider the value of concrete in the built environment as an important carbon sink as opposed to being cited as the villain in the global warming equation.
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