Reforestation as carbon sequestration

Source.

In my previous post, I suggested that geoengineering might not have to persist continuously for thousands of years, if solar radiation management were used, not as a "destination therapy" but as a bridge, in combination with intensive mitigation, until a realistically slow program of carbon sequestration could take effect. How the carbon might be sequestered wasn't discussed. Chris Reynolds offered offers some options (from the relevant Wikipedia page):

Creating biochar (anaerobic charcoal) and burying it to create terra preta

Bio-energy with carbon capture and storage to remove carbon and simultaneously provide energy

Carbon air capture to remove carbon dioxide from ambient air

Ocean nourishment including iron fertilisation of the oceans

All of which have land use or energy input requirements. Clearing more land would place more pressure upon ecosystems, or food prices. Any energy used would have to be non fossil fuel, which would eat into whatever offsets could be made to fossil fuel burning reductions. Overall the whole process would have to not impact the poor (food prices), and would have to be substantial.

Obviously a tall order. But is there perhaps a way around some of these requirements, a way to sequester carbon without clearing more land, putting more pressure on ecosystems, or investing a lot of (our own) energy? It turns out there is!

Simple reforestation could sequester 3Gt/year of CO2. In fact, simply halting deforestation (18% of current emissions * 33Gt = about 6Gt of CO2 per year) would more than do the trick. But since we are eventually going to have to push our emissions near net zero anyway, the gain from halting deforestation is already "baked in" to the mitigation scenario (which was to leave us with 1,200Gt in net emissions including carbon-cycle feedbacks.) But what is not baked in is adding back forest cover.

Obviously this would be a slow and difficult process. It would involve increasing housing density, abandoning uneconomic farms and ranches (a disastrous hobby of rich governments the world over; the price tag for agricultural subsidies in 2011: $252 billion)) and growing food more efficiently on the land that remains. It might mean more expensive meat, tilting our diets towards more grains and pulses. But the potential gains are substantial. In fact, they are sufficient:

"The woody biomass of forests is estimated in
this paper to contain 300 × 10^9 tons carbon. For
comparison, the cumulative emissions from the
combustion of fossil fuels in the 19th and 20th
century were about 280 × 109 tons. In 2000, the
atmosphere contained about 790 × 109 tons in
CO2 (Enting et al. 2001, Marland et al. 2002)."

-- "New, Low Estimate for Carbon Stock in Global Forest Vegetation Based on Inventory Data"

About 30% of the world's forest cover has been lost in the last two centuries, since the Industrial Revolution. Taking the conservative estimate of the forest biomass quoted above, restoring the world's forests to their extent a few centuries ago could trap at least 126Gt of carbon, which is the equivalent of about 462Gt of CO2 -- a hair more than I said we needed to get back to a reasonable atmospheric CO2 level and stop solar radiation management.

I'm not sure why reforestation didn't make Chris' list, above. It does have a prominent place on the wikipedia page for carbon sequestration, but not for carbon dioxide removal. Just speculating, I think one might overlook the obvious potential of reforestation because it so obviously is very slow, and would be completely unable to cope with the BAU emissions expected in the 21st century. Once again, there is a critical difference between looking at geoengineering as cure (hopeless and stupid) and looking at it as one element of an intensive program to keep the world under 2C, with its specific role being to buy a little time.