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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