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.

Does geoengineering have to continue for thousands of years?

Cloud whitening, one geoengineering strategy

Chris Reynolds at Dosbat expresses the basic pro-science critique of geoengineering:

With the seriousness of the situation regards AGW becoming more clear, talk of geo-engineering has increased. I think that rather than try to stem population increase, examine our economic system and expectations, and reduce CO2 emissions, this will be seen as a viable option in the years to come.

However geo-engineering will, I am confident, be used as an excuse to carry on emitting CO2 and avoiding dealing with the fundamental flaw in our civilisation; exponential growth in a finite world. It is dangerous and is a recipe for disaster.

He may be absolutely correct on all points; the only parts I take issue with are the bolded ones (and the finite growth thing, a bit.)

The first point, the "rather than," is one we have to confront on a regular basis with the adapt-nik ("Don't mitigate -- adapt!) subspecies of lukewarmer. To wit: we can (and must) do more than one thing at a time. The resources required to investigate and prepare for the possibility that we may need to geoengineer are miniscule relative to mitigation, adaptation, or even population control.

If we were only going to do one thing, it sure as hell wouldn't be geoengineering. But we're going to have to do more than one thing.

The second point, that geoengineering may be used as an excuse to defer action, is a serious concern. Geoengineering would only work as a temporary bridge to allow intensive mitigation to bear fruit. But would it, actually, become an excuse for inaction?

This is a different, and slightly more upbeat, counterargument compared to the related riposte: "We don't need an excuse for inaction; we're excusing it fine as it is." In that fatalistic outlook, geoengineering becomes like a clean needle program for heroin addicts: we wish we could fix the underlying issue; we can't; we're going for damage control.

I am not such a fatalist; and I do not necessarily think that the availability of geoengineering will make mitigation less attractive. Consider, for example, how the adapt-vs-mitigate debate has unfolded (or failed to unfold) after Hurricane Sandy. Experts looking at the flood surge have suggested we could have prevented a large portion of the roughly $50 billion damages using $10-$15 billion dollars in floodgates. So my question is: Where are the adaptniks screaming for these new defenses?

I was able to find a tepid endorsement from Bjorn Lomborg which radically lowballed the cost of said adaptation.

Much of the risk could be managed by erecting seawalls, building storm doors for the Subway, and simple fixes like porous pavements – all at a cost of around $100 million a year.

If you follow the link, it leads to an article from Popular Science which includes this:

If New York—part of the Northeast megaregion—suffers a direct hit, workers will spend weeks pumping a billion gallons of brackish water out of its subway and train tunnels. The salt will corrode power lines, transformers and thousands of brakes and switches that control the trains. Some subsystems could take a year or more to restore.

To avoid such a scenario, New York state recommends the city invest well over $100 million a year in storm protections. City planners are already experimenting with dozens of low-tech fixes, says Adam Freed, deputy director of the Mayor's Office of Long-Term Planning and Sustainability.

 Note "well over $100 million" not "around $100 million." And while the original source describes these "dozens of low-tech fixes" as merely able to mitigate the nightmare scenario, in Lomborg's retelling they eliminate "much of the risk."

Lomborg also repeats the fallacy that the risks of a damage storm surge have nothing to do with climate change -- even though sea level rise, by definition, makes storm surges more destructive.

And that tepid, dishonest, weasel-worthy endorsement was really about all we heard from the "adapt, don't mitigate" crowd after Sandy. Judith Curry had a guest poster make the pitch for more weather satellites. Lucia Liljegren, not a word. So why not pitch adaptation in this brave new world?

My theory is, actual real-world adaptation makes the problem of global warming too real. It's one thing when "adaption" is an abstract concept describing something we may do in the future. But when it actually comes down to spending tens of billions of dollars on flood defenses, planned retreat from parts of the coastline, restoring wetlands, lowering levees, hardening the power grid, and beefing up the first responder network -- well, if you start spending that kind of money (hundreds of billions for starters, talking about the US alone), people might get to wondering why this global warming stuff is so gosh darned expensive and getting more so. And that might lead them to ask when we are going to stop adding to the bill by spewing billions of tons of CO2 into the air.

Geoengineering might similarly offer the public some clarity on this issue. I assume it will be far more expensive that it currently seems, it will be highly controversial on the world stage, it will have unwanted side effects and limited efficacy. Researching and preparing such a system might have the opposite of the effect Chris expects; it might focus the public's mind on what a god-awful problem this is and how we need to get busy fixing it.

If we research and prepare this tool (not deploying it until/unless we win an international consensus and after warming has crossed a specific threshold or we see evidence of rapid catastrophic feedbacks) the debate which ensues may, as Sandy has, stimulate the public and international debate on mitigation, so as to prevent or minimize the use of such desperate measures.

Another major concern with geoengineering schemes is the impracticality of keeping them running for a long, long time:

Why do I say we would need to keep up SRM for millennia?

Archer & Brovkin's 2006 paper "The Millennial Atmospheric Lifetime of Anthropogenic CO2", PDF, shows that the emissions of CO2 will remain in the atmosphere/ocean system for thousands of years. Their abstract sums up their findings perfectly:

The notion is pervasive in the climate science community and in the public at large that the climate impacts of fossil fuel CO2 release will only persist for a few centuries. This conclusion has no basis in theory or models of the atmosphere/ocean carbon cycle, which we review here. The largest fraction of the CO2 recovery will take place on time scales of centuries, as CO2 invades the ocean, but a significant fraction of the fossil fuel CO2, ranging in published models in the literature from 20–60%, remains airborne for a thousand years or longer. Ultimate recovery takes place on time scales of hundreds of thousands of years, a geologic longevity typically associated in public perceptions with nuclear waste.

So if we take any geo-engineering scheme that doesn't involve massive emissions reductions or active draw-down of CO2, we need to keep it up for at least 1000 years, the more CO2 we emit the longer the recovery of CO2 back to pre-industrial will take.

And if we falter...

That's obviously a legitimate concern. There isn't a single government on the face of the Earth that has maintained its present form of government for even a single millennium. If we are expecting them to maintain a stable geoengineering scheme for thousands of years, that's obviously impractical. And since solar radiation management strategies mostly poop out within a few years of stopping, you confront the possibility of decades or centuries of global warming hammering the world in the space of a few years.

Brrrrrrrrrr. I've scared myself. But perhaps the picture is not so dire. What if we look at geoengineering not as a mono-strategy, but, as I suggest, as one component of a threefold strategy of adaptation, mitigation, and geoengineering?

Let's say we get serious about mitigation and end up with 1,200Gt of CO2 equivalent added to the atmosphere (either it took too long to forge agreement, or the cuts could not be made fast enough, or the carbon feedbacks hit us too hard; we missed the 1,000Gt goal for 2C, but only just.) About 60% of that shows up in the atmosphere; the rest is immediately taken up by the carbon cycle. 720Gt. That's us, permafrost melting, forest dieback, what have you.

Let's say 50% of that remains in the atmosphere 300 years later. That's 360Gt. Meanwhile we are practicing some solar radiation management with cloud whitening and contrails and aerosols injected into the stratosphere. But we also have been doing some carbon sequestration.

Carbon sequestration is hard: suppose we don't get it off the ground for 20 years and it then takes us 30 years to ramp up sequestration to a grand total of 3Gt/year (that's about 9% of current emissions). We then practice that for about 150 years. That would take out 450Gt, but some of that would have been sequestered anyway -- we will only count 2/3 of the 450Gt as actual sequestration -- 300Gt. 

That leaves us with 60Gt above preindustrial -- about 330ppm of CO2 -- and we can probably stop spewing stuff into the sky at that point, the 200-year mark. Two hundred years is a very long time, but it is a lot less than thousands of years. Many governments have been more or less stable for 200 years, including the United States.

The exact figures are subject to debate, but the basic thrust is clear: two ideas that seem impractical on their own (solar resource management and carbon sequestration) get much more reasonable if you intelligently combine them with each other and intensive mitigation. Without mitigation, none of this works: you're continuing to shoot holes in the bottom of the boat as you're bailing it out.