AskPablo: Land Use Changes

tree.jpgThis week Tyler asks: “What is the climate change impact from deforestation, land-use changes, and reforestation?” This topic is quite relevant these days because many carbon offset organizations offer to plant trees to mitigate your personal carbon emissions. Are you really getting what you pay for? And what is the impact of cutting down a forest? These are the questions we will explore in this week’s AskPablo.

Many of us know of the experience that Coldplay had with tree-based carbon offsets. In an effort to do the right thing they offset the carbon emissions from their second album by paying for 10,000 mango trees to be planted. Unfortunately most of the trees died, making this story cannon fodder for carbon offset opponents and critics. Let’s look a bit deeper into the issues and realities behind land-use changes and their impact on the global carbon dioxide surplus.
Any so-called land-use change has an impact on the global climate through an increase or decrease in net carbon emissions and sequestration (absorption of CO2). Green plants are made of cellulose, or C6H10O5, which is assembled by the plants from CO2 and H20. For each cellulose compound formed six CO2 compounds are sequestered and six oxygen molecules are released. This process, called sequestration, slowly removes CO2 from the atmosphere and accumulates it in biomass. When this biomass is burned the carbon rejoins the oxygen to release CO2. Biomass can also decay an release methane, CH4, which has a global warming potential that is 21 times higher than CO2. The theoretical problem then, is that the making of a cellulose compound sequesters six units of CO2, but can create two or more units of methane. Two units of methane are as potent as 42 units of CO2, so clearly the net carbon balance does not look good (42-6 = 38).
Researchers at the Max Planck Institute for Nuclear Physics determined in a study that plants release 10-30% of the world’s methane emissions, a number that has been increased by industrial agriculture and the “green revolution.” While methane released from biomass decay requires an anaerobic (no oxygen) environment (like in land-fills), it was discovered by these researchers that live plant material emits methane at rates between 10-1000 times that of dead biomass, even in an oxygen rich (aerobic) environment. As always, these scientific results have been disputed by other scientists and have been confirmed by yet more scientists so we can’t really know what to think just yet. But I think we need to understand that some plants may not have the beneficial impact that we would expect.
So what about tree farms? Well, in general the accumulation of biomass is good, since it represents the sequestration of CO2 from our already saturated atmosphere. Young trees grow rapidly and sequester CO2 most effectively in their first 20 or so years. As they matures their rate of sequestration slows, and when they die and decay a great deal of that CO2 enters the atmosphere again. When we burn biomass such as trees we are, in essence, liberating flammable gases from the cellulose. The smoke released contains a lot of soot and is generally bad for local air quality but, since there is a pile of ash left behind, the net effect of burning biomass is actually negative. This is because that ash represents carbon that was sequestered from the atmosphere that did not become CO2 or CH4 again.
Does this mean that we should rapidly cultivate all available land with trees and burn them in order to save the world? Yes and no. If we can effectively harness the energy contained in the biomass, through gasification and production of liquid fuels like cellulosic ethanol, while preventing the carbon from being released into the atmosphere, we could eliminate a great deal of oil. Since oil is just biomass that decayed millions of years ago it represents CO2 that was sequestered and locked away. The less of this carbon that we reintroduce into the atmosphere, the better. So cellulosic ethanol, unlike corn-based ethanol, can actually remove CO2 from the atmosphere while partially suppling our thirst for liquid fuels.
Forest product that end up in durable goods, like furniture and flooring, essentially store their carbon until it is released as methane in a landfill or CO2 in a fire. The longer we can prolong the return of that carbon to the atmosphere, the better. If we had massive underground caverns to store biomass in it would effectively prevent that carbon from returning to the atmosphere, but this solutions is not very feasible. Paper products have an even shorter lifespan, especially if not recycled, so their carbon return much quicker.
So, our conclusions are:

  • Growing trees is great, although some can release methane
  • Young trees remove more CO2 since their growth rate is higher
  • Cellulosic ethanol sequesters CO2 and fuels our need for liquid fuels
  • Burning biomass may be better than letting it decay anaerobically into methane

I look forward to reading a great discussion on this topic. What do you think about it?
Pablo Päster, MBA
Sustainability Engineer
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7 responses

  1. Mike,
    Biomass as a building material is essentially the same as other durable goods, such as furniture. A building will last between 60 and 100 years, at which point the building materials may be sent to the landfill or may have been consumed by fire or termites, all of which will release some carbon dioxide and/or methane.

    Here is an article that, based on a recent study published in Science magainze, points out that all forests are not created equal. This study shows that (unlike previously thought) tropical forests are larger carbon sinks than their temperate counterparts. So, if you’re buying carbon offsets based on reforestation or well-managed forests, perhaps it would be better to know that those forests are in the tropics.
    However, since sequestering carbon in trees is temporary no matter how you look at it, would it be better to offset your CO2 emissions with actual reductions like those traded on the Chicago Climate Exchange?

  3. Hi all –
    Couple of keys here.
    1. As far as the impact of land use and deforestation, it is huge. Indonesia is now I believe the third greatest emitter of greenhouse gases. Mainly attributable to its high levels of tropical deforestation.
    2. If young trees are fast growing and absorb more CO2 than old trees, should we cut older trees down and plant tree farms? The answer reflects the reality that the forest is, indeed, greater than the sum of its trees!
    For example, one problem with using tree farms to sequester CO2 is the CO2 that comes out of the soils following logging, even if there are young trees growing there.
    In many cases, this outflux of CO2 from soils will be even greater than the amount being taken up by young trees!
    Here is a nice little summary of a related study from New Scientist:
    Another issue is into which part of trees the extra absorbed CO2 is being allocated. Scientists are finding that a lot of it is going below ground into fine roots, which are very short lived, and quickly return the carbon to the soil when they die…
    While these kinds of complexities might leave one’s head spinning when pondering how we can slash human impacts to the global carbon cycle, there’s plenty of things we need to do that are plain as day.
    When it comes to land use, I’d say THE major take home is (as somebody noted above) that we REALLY REALLY need to find ways to protect the world’s tropical forests. Both for their CO2 uptake capacity, and for the many other environmental services they provide to people and planet (climate regulation, precipiation…) That means finding a solution to the problem that right now, there are lots of people who need to use the forest to feed their families, and the only way they know is logging – for wood or to clear farmland.
    We need to find new ways for these folks can to support themselves and their families. The carbon offsets market is, of course, one such mechanism that has huge conservation development potential – to generate lots of funds to pay people to leave tropical forests standing (even if the whole carbon offsets thing still requires some serious standards and regulation to get it working right). Certified and properly monitored sustainable forestry is another, though even FSC still needs much better compliance monitoring.
    For a good summary of how to manage ecosystems to maintain their resilience to climate change, check out WWF’s Buying Time manual:
    Cheers all – great topic!

  4. I find it hard to believe that managed forests with the reduced or eliminated understory is a beneficial land use. Reduce genetic diversity is only one of the detrimental impacts. Of course it is better than most uses but natural reforestation is certianly the best alternative. Of course natural reforestation will not allow for economical harvesting of product whether it be fiber, fuel or food. Lets face it, biomass has an impact and can it really be managed?

  5. The best site for a managed forest may be land that was previously biologically unproductive, such as previously developed areas or brownfields. Former agricultural land would also work well, unless you cut down an existing forest to replace the lost agricultural area.
    This column looks only at the climate change impact of land-use. Biodiversity and a multitude of other issues further complicate the matter and must be evaluated for individual locations to determine the best land-use policy.

  6. Pablo Hi.
    This is a very timely discussion bearing in mind the current controversy that surrounds forestry based offsets.
    As you point out, one of the most valid criticisms of carbon offset forestry is that it only provides a short-term store for the absorbed C.At Treeflights we have spent a lot of time trying to improve the ‘permanency’ of the offset that our trees will provide.
    We have set up a trust to hold ownership(in perpetuity) of the land on which the trees stand so that if the company fails the trees will still be safe.
    We have a long-term management plan for our forests that specifically aims to reduce and delay the flow-back of absorbed Carbon to the atmosphere. The trees we grow are all hardwood species chosen for their future value as timber. When hardwood timber is kept dry it may retain its carbon for thousands of years. Ely Cathedral in England has an oak roof built 1000 years ago and it is still in very fine shape, holding on to its carbon very well, thank you.The oak trees used in its construction were themselves 300 years old when the cathedral was built. This means that they were were sequestering their carbon around 1300 years ago.
    When timber is kept saturated in an anaerobic environment, such as under water, it can last even longer. There are Viking long ships in museums in York that had oak keel timbers. These ships were sailing up the east coast of England in the 7th century and 80% of the oak (and their carbon) is still there.
    Yes, Methane is a far more destructive ghg than CO2. When we change land-use from methane generating ruminant pasture to carbon hungry forest we have the added benefit of a reduced methane footprint. The land we plant trees on, hitherto was grazed by cows each of which release around 500 litres of methane a day. Some estimates suggest that the worlds cows are more destructive to our climate than all the worlds cars put together.
    The best solution all round is to convert pasture to forest then once the trees have done the bulk of their absorption work, we should harvest them and use in ways that delay the inevitable flowback of carbon to the air and then replant the same land with new c-hungry saplings.
    Our problem is very simple. We have too much carbon in our atmosphere. Trees have evolved over millions of years to become immensely powerful and efficient at withdrawing carbon from our atmosphere, using only natural processes and with myriad collateral benefits. This is precisely the job that we need doing right now.
    The answer is staring us in the face.

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