Increasing carbon sinks: a success story?

In this post I am going to focus on mitigation strategies aiming to reduce atmospheric concentrations of carbon dioxide through land use change.  In a previous post I explained how agricultural expansion has caused land use change namely through deforestation, on a huge scale, which in turn leads to increased concentrations of carbon dioxide in the atmosphere. Here I am going to discuss a few mitigation strategies that aim to revert these effects by increasing carbon sinks.  

The first strategy, which the IPCC suggests to be one of the most effective methods of reducing emissions is to allow or encourage cropland to revert to another land cover that is similar to the native vegetation of an area.  This will increase carbon storage (as cropland does not store much carbon), for example converting arable land to grassland results in the accrual of soil carbon because of lower soil disturbance and reduced carbon removal in harvested plants. Lal 2004 suggests that restoring land use to as it was before clearance, especially on marginal cropland (as shown below) and using recommended management practices, will have significant effect on reducing the rate of enrichment of atmospheric carbon dioxide.  Another benefit is that an alternative land cover to cropland have lower nitrous oxide emissions due to few nitrogen inputs as fertiliser.  

It is also possible to convert drained croplands back to wetlands, this can result in the rapid accumulation of soil carbon, removing it from the atmosphere.  However a downside to this is methane emissions can increase.  

Another strategy put forward to lessen emissions from land use change is to prevent deforestation and protect forests keeping them intact.  Soares -Filho et al (2006), wrote a paper specifically looking at conservation in the Amazon Basin. They state that by 2050, following current trends of agricultural expansion in that area, 40% of Amazon forests will be destroyed.  Not only will this significantly reduce biodiversity, but also they estimate this will release 32 ± 8 Pg of carbon in to the atmosphere.  Consequently they suggest that a network of protected areas is necessary to prevent the destruction.  However implementing policies of protected areas to stop deforestation, is difficult.  As Sathaye et al (2006), suggest there are many economic incentives behind deforestation, and consequently location of these protected areas is very important as economic alternatives need to be found to implement these polices. 

The final strategy I looked at, was afforestation, where forests are replanted.  Articles show this to have mixed success rates.  In some areas it can yield considerable soil carbon accumulation rates for example Post and Kwon (2000) have found afforestation to be successful at absorbing carbon from the atmosphere in the northern hemisphere over the few decades.  On the other hand, Tate et al (2005), found the opposite in New Zealand, that after afforestation the soil absorbed less carbon, than it did before.  In Richards and Stokes (2004) paper, which reviews recent afforestation studies, found hat afforestation has good potential at reducing atmospheric carbon dioxide but it is a very complex process, where location and tree specie were very important in determining the success of carbon accumulation.  However a major downside to afforestation is the economic cost of it, it requires high investment and will be several decades before revenue can be generated.

From looking at mitigation strategies that have been implemented, trying to protect or increase the size of carbon sinks, it seems clear that different policies work better in different places.  I think it important these policies are implemented, however some economic incentive will probably have to be found to encourage governments to put them in place, especially those whose main income comes from agriculture and logging.