Saturday, December 22, 2012

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.

    13 comments:

    1. The list I used was from Wikipedia. I don't know why reforestation wasn't listed.

      I think the main problem with reforestation is, as you point out, it's slow.

      However in place of any conventional technological means for removing CO2 from the air, I think biological processes might be the best bet. I keep thinking of the Azolla event, which I'll be reading more about. If Azolla could take CO2 levels down from 3500ppm to about 650ppm, perhaps a biological technique would be the best approach. One we can leave to go, taking care of the need to maintain intervention for a long time.

      One thing I've not mentioned much is the availability of fossil fuels. I used a very pessimistic assessment of fossil fuel availability in my post on our re-running the PETM simply by doubling emissions so far. This was based on the assumption that we're at peak oil now, and the (rather unrealistic assumption) that the same could be true for gas and coal. Furthermore that additions of non-conventional sources, shale gas, tar sands, would be uneconomic as we run out of conventional FFs and the economy contracts.

      The problem I have with re-forestation is the feasibility in a world with:
      1) Increasing population, and need for food security to prevent civil unrest.
      2) Changes to the climate, we're already seeing large scale fires and droughts in the interiors of both N America and Eurasia - again think what that means for further warming and the transition of the Arctic to a seasonally sea ice free state.
      3) The possible impacts of Peak Oil and increasing FF costs. What will the poor revert to burning to keep warm?

      If only wood was denser than water we could grow forests and cut them down, then dump the wood over the abyssal deeps.

      Maybe we need to stop building in concrete, steel and brick, and move to timber buildings.

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      1. Forgot to add to point 2 - the situation where you've massively expended forests, only for them to burn en masse under future warming is rather analogous to my original problem with solar radiation management and its failure.

        I will provide some more comments on your previous post. However with Christmas, I have work on one side - trying to get things wrapped up for Monday, and family commitments on the other.

        However I do find myself uncomfortable arguing against you. I've been pessimistic about prospects for countering AGW for years, but have made it a rule to 'shut up' so as not to add to an atmosphere that would further decrease the chances of this civilisation surprising me and pulling off massive reductions. A surprise that would be very pleasant. I'd love to be wrong on this.

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    2. "Forgot to add to point 2 - the situation where you've massively expended forests, only for them to burn en masse under future warming is rather analogous to my original problem with solar radiation management and its failure."

      That is definitely a concern, especially at higher levels of warming. In my imaginary AGM scenario, we set a limit, such as 1.5C above preindustrial, and going over triggers SRM or similar. Hopefully that would render a massive forest die-back unlikely. If it didn't, then we have another problem: relatively modest warming would implicitly place the existing 300Gt carbon stored in forests at risk, even without a reforestation strategy.

      "Maybe we need to stop building in concrete, steel and brick, and move to timber buildings."

      Excellent idea. I don't know if you say this recent Science article on biochar, but I was encouraged: http://www.sciencemag.org/content/338/6110/1034.summary?sid=93e8c7c3-fd0c-453c-a5b5-7cd2436a83f6.

      Seems like it could potentially restore soils and therefore increase agricultural productivity, as well as trap some carbon: the article estimates 1Gt/yr with a realistic few percent of cropland dedicated to it (offset by increased productivity elsewhere).

      "The problem I have with re-forestation is the feasibility in a world with:
      1) Increasing population, and need for food security to prevent civil unrest.
      2) Changes to the climate, we're already seeing large scale fires and droughts in the interiors of both N America and Eurasia - again think what that means for further warming and the transition of the Arctic to a seasonally sea ice free state.
      3) The possible impacts of Peak Oil and increasing FF costs. What will the poor revert to burning to keep warm?"

      These are serious problems, but aren't they the same challenges we face in implementing mitigation as well? And as the world gets warmer, they're going to get more and more severe. Conflict over resources, social breakdown, even "successful" adaptation (like air conditioning) -- all threaten to accelerate climate change.

      I really don't have the answers here. Like you, I worry that our civilization just doesn't have it together enough to cope with this problem. But to invoke some anecdotal evidence, many of the very smart people who worked on the atom bomb believed beyond a shadow of a doubt that it would proliferate and be used in a world war, destroying civilization. Yet somehow we have managed to edge past that abyss for the last sixty years. I hope our response to AGW will similarly surprise.

      Happy Holidays!

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    3. Here is the conceptual (I'm not a scientist) problem that I have with the whole, "Let's just plant forest idea": If we recover the 30% of the forest that has been lost, we are only back to the amount of CO2 that we lost when we cut down the forests and haven't recovered any of the fossil fuel CO2. The 30% of CO2 from the cut down forests is still sitting out there somewhere waiting to re-emerge when we replant those forests. The CO2 in the atmosphere today is the net effect of fossil fuels and land use changes combined minus some small amount of geological weathering that occurred in the past thousand years. Fixing land use changes cannot in the long run fix fossil fuel CO2. We could make land use changes and stop warming for today, but the CO2 will come back out of the oceans and where ever else it hides to haunt our ancestors for a very long time. The only caveat is that since forests in conjunction with fungi appear to be responsible for most of the geological weathering in the biosphere it is a good idea to plant them.

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      1. "Fixing land use changes cannot in the long run fix fossil fuel CO2."

        Again, if you're trying to solve the whole problem with one tool, your mental model is too simple. A basic outline of one possible solution that includes reforestation might be:

        Reduce GHG emissions:
        -- more clean energy production
        -- more efficiency
        -- more conservation

        Increase uptake:
        -- Reforestation
        -- Restore wetlands
        -- Biochar
        -- No-till agriculture

        Temporary SRM:
        -- cloud whitening
        -- aerosols

        As for the fact that the CO2 will come out of the oceans, that's going to take a very, very long time to happen. The oceans are nowhere near equilibrium with the CO2 we have put into the atmosphere so far and are likely to continue to whisk away C to the deep ocean until and unless AGW disrupts the mixing currents. One of the reasons I am willing with great reluctance to contemplate a little geoengineering is to avoid running headlong into tipping points like that.

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    4. Just turned up this abstract from the AGU FM, from Christina Ravelo et al.:

      "The response of climate to past changes in atmospheric greenhouse gas composition can be used to assess Earth System sensitivity. Unlike Charney climate sensitivity, which is related to the strength of feedbacks involving short timescale climate processes such as those involving clouds and water vapor, Earth System sensitivity also integrates feedbacks involving long timescale changes in the cryosphere, terrestrial vegetation, and deep ocean circulation.

      "We show that paleoclimate data from the Miocene, Pliocene and Pleistocene epochs provide evidence for changing sensitivity through time, probably due to changing boundary conditions due to tectonics. In the middle Miocene (~18-12 Ma), major climate change trends appear to be coupled to pCO2 changes; however, in the late Miocene (~12-5 Ma), climate was warmer than today even while pCO2 was similar to today, indicating a decoupling between long-term climate evolution and pCO2 change.

      "In the Pliocene and Pleistocene, there appears to have been strong coupling between climate and pCO2 changes; there is a wide range of Earth System sensitivity values, all of which exceed or are at the high end of Charney and Earth System sensitivity estimates derived from climate models. In the early Pliocene warm period, pCO2 was 350-400 ppm, implying an Earth System sensitivity (temperature change for a doubling of pCO2) of 7-9C.

      "Subsequently, large Northern Hemisphere glaciations began to occur when pCO2 dropped below about 300 ppm in the middle Pliocene. For the Late Pleistocene ice ages, estimates of tropical sensitivity are approximately 3-4C for a doubling of pCO2, which we suggest represents a minimum value for global sensitivity during the last ~500,000 years.

      "Overall, paleoclimate data have important implications.

      "First, tectonic changes in basin shape impacted ocean circulation and climate sensitivity through the Miocene and possibly the earliest Pliocene, indicating that the initial conditions of the mean oceanic state plays a role in the climate response to pCO2 change.

      "Second, to explain why early Pliocene estimates of Earth System sensitivity are so high, it may be important to improve how the large observed changes in upper ocean circulation are simulated by models.

      "Third, in a period when ocean basins were similar to modern, ice age climate sensitivity to pCO2 changes is underestimated by climate models even when long term changes in solar forcing and ice sheet size and distribution are taken into account, implying that internal positive feedbacks are stronger than previously thought."

      The idea that there might be a major change in ocean circulation induced by increased CO2 is more or less what I've been expecting, but that it would result in a doubling of sensitivity is a new one on me. There's much more to say, but the point it raises pertinent to the immediate conversation is, if it's the case that we're already committed to the change, would we, after (or perhaps during) the long rough ride of the transition, then want to try to drop CO2 levels far enough to reverse the change, thus signing us up for even more disruption?

      With that happy thought, to bed.

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    5. Steve,

      I caught your link to Blocking activity data over at Neven's. Very Very Interesting!!! I'm working on it.

      Taken as a whole Earth System sensitivity (not Charney) could be substantially affected by ocean circulation. Research I've blogged on recently looks at Hyperthermals in the Eocene. They find that part of the explanation that best fits observations is that deep ocean ventilation shut down under warming. This would make sense, with the surface of the oceans warming it would cut down on deep ocean ventilation. An outcome of this would be that CO2 from the atmosphere would saturate the upper layers instead of being subducted into the abyssal deeps. So any inputs of CO2 to the atmosphere would build more rapidly in the atmosphere as the ocean drawdown of CO2 would lessen substantially. On top of this subduction of sensible heat would stop.

      Tracker,

      I hope you're right about both geoengineering being used intelligently, instead of in a thoughtless panic, and about us having the ability to 'get it together' to deal with the problem.

      I was a free market type. Having been a Thacherite, and being influenced by people like Fukyama. My realisation about climate change robbed me of politics. That's because I have concluded that the free-market has no answer to a problem like this, not one that will really address the problem.

      The problem is we're stuck with free market theories, which result in, if not demand, continual growth. Our civilisation is one based on fossil fuels. Having looked at the numbers previously I'm not convinced there is a serious alternative to provide the Terawatts we use. My best hope previously had been Fusion, probably based on the torroidal containment systems. Let's say that's cracked this decade, some team run one for a year and have a workable means of getting power from it and onto the grid. Let's be more optimistic, within 20 years due to massive investment the G8 nations are moving over from fossil fuels to fusion, the prospect of a non carbon hydrogen economy becomes a dream within our grasp.

      What effect does this have globally? Fossil fuel prices drop. Nations seeking to develop now see an option. Africa for example - they use the cheaper fossil fuels and resources they already have together with cheap labour to undercut the developed world. And we have a second industrial revolution. At least with that scenario the tar sands and very hard to get at oil should be costed out of the equation.

      The problem with fossil fuels is they are so energy rich that it's hard to see how, economically speaking, they won't be burnt. And the game of the age is free market economics. Neven's said over at his blog that we need to find another way. I agree that it's the only option that will avert disaster. Over Xmas I've seen various people and have discussed climate change occasionally. Things have changed - I now know nobody who doesn't accept that its happening - most think we're going to get our assess kicked.

      But they still drive, they still keep their houses warm, they're still jetting off abroad for their holidays. What's amused me is that of the three people I know of (self included) who've given up driving and put extra layers on rather than the heating - none of us have kids. It's the people fretting about their children's future who emit the most.

      I'll let the matter drop now. Rather than snipe at the prospects for solutions I think it best to shut up so as not to deter others from trying.

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      1. Thank you, Chris, for your thoughts. I share your concerns about our civilization and the way the existing market system will tilt towards fossil fuels until the price fairly represents the externalities (climate change, air pollution, slag, water consumption, damage to the landscape -- all of which are heavily subsidized by the non-Exxon public.)

        The tricky political problem is that we need to get on this boat together. We cannot have half the world cutting greenhouse emissions and the other half burning coal. Getting the world to move as a unit without free riders is a daunting prospect, but not, I think, totally impossible. The most realistic scenario, I think, is for the US, India, China, and the other big fish of the developed world to agree on a plan -- and agree that they will crush the economies of any country that fails to cooperate. No trade, no aid, economic sanctions.

        This is, in effect, how we've survived sixty-seven years without any further use of nuclear weapons. The big people recognize the threat. The little governments include some crazies, but they know the heavies are going to come down on them like a ton of bricks if they go there.

        You can make similar arguments for biological weapons, chemical weapons, slavery and genocide, although the record gets more mixed as you go down the line. Some societies abstain, some get bullied.

        To bully the world towards action, of course, the first step would have to be our consciousness and determination leading the world, instead of following it. It shames me as an American to see the Irish using pollution taxes and India taxing every ton of coal sold and realize how far we are from that here.

        Al Gore is right -- we need collective political action that regulates the market. I think a lot of people look at the heavily collaborative model of international cooperation, the UN/EU/World Trade Association model, and say "We'll never get everyone on board." I think they're probably right, though I would be happy to be wrong. But there is nothing to say you can't achieve reluctant cooperation through more or less brutal power politics one the world powers truly recognize climate change as a threat to their way of life. That is my primary hope.

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    6. I think you're right that action by trade blocs may be part of the solution. Don't play ball and you don't gain access to our markets.

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