Great green sinks

Forests gained a lot of attention during the Paris climate talks recently.

The world's largest greenhouse-gas emitter, China, has not pledged to peak its annual emissions until 2030, noted Houghton et. al., (2015) in the latest Nature Climate Change, which coincided with COP21. What's more, very few developing countries expect to peak theirs until 2050. It means that removing carbon dioxide from the atmosphere may be essential if we are to avoid a 2 degree rise in global temperatures (Fig. 1; Houghton et. al., 2015).



Figure 1. Historic carbon emissions 1850 - 2015 (solid lines): annual carbon emissions from fossil fuels (solid grey line) and tropical forest management (solid green line). Total carbon emissions (solid black line). From 2015, the total emissions to ensure a 75% probability of avoiding a rise of over 2 degrees C (dashed black line) are shown. Total fossil fuel emissions to avoid a rise of 2 degrees C if the only changes are made to fossil fuel usage (dashed grey line). Total emissions from fossil fuels (orange line) if successful changes to forest management are implemented simultaneously (dashed green line; negative emissions show CO2 removal from atmosphere). Effect of tropical forest management on carbon emissions mitigation (hatched grey area). Source: Houghton et. al., 2015.

So it is no coincidence that plans for reductions in deforestation, reforestation (replacing trees that have been cut down), and afforestation (growing trees in novel areas) all featured in many national pledges at COP21. The role of forests as carbon sinks was explicitly mentioned in the final agreement, which encouraged countries to conserve and sustainably manage their forests.

Though it doesn't involve science-fiction-worthy technology, large-scale forest projects are a form of carbon-dioxide-removal geoengineering (Caldeira et. al., 2013). Such programmes also provide employment for locals and promote wildlife conservation.




Tropical Amazon rainforest. Source: American University

REDD+

Deforestation and forest degradation is responsible for 1 - 2 Pg (petagram) of carbon emissions every year (Houghton et. al., 2015). However, Reducing Emissions from Deforestation and forest Degradation (more snappily known as REDD+) projects have gained considerable momentum in the past few years, culminating in their formal inclusion in many pledges at the COP. The final agreement text promised funding for REDD+ projects from the Green Climate Fund (which will total at least $100bn/year).

In a case study of the tropical Colombian Andes, Gilroy et. al. (2014) described REDD+ as a convergence of carbon storage, biodiversity protection and economic viability - especially in areas where cleared forest is used for cattle grazing, which yields low economic returns. Abandoned deforested areas have shown promising rates of recovery with minimal human intervention (Sanchez-Cuervo et. at., 2012), suggesting that wider-scale forest regeneration could be seen on a large scale, with little human effort.

The German, Norwegian and UK governments all pledged to provide funding for global REDD+ schemes during the Paris climate negotiations. On the other hand, some critics warn that REDD+ projects should not be implemented by developing nations instead of transitioning away from dirty fossil fuels - rather, they should complement other emission-reducing policies.

Carbon locked in trees is vulnerable to wildfires, like those in Indonesia earlier this year. These came at a devastating cultural, human, environmental and economic cost and saw thousands of tonnes of carbon being rapidly released into the atmosphere. Such risk of catastrophic wildfires like this somewhat undermine forests' potential as a permanent carbon sink; the images (Fig. 2) provide a dramatic reminder of just how temporary carbon storage as trees can be. Worryingly, scenes like these may become more frequent as global mean temperatures rise. Such risks mean that carbon storage as trees is classified as "decadal" (as opposed to "centurial" - "millenial" in geological stores) (Caldeira et. al., 2013).

Figure 2a. A fire fighter extinguishes a blaze in Indonesia. Source: The Guardian.

  

Figure 2b. A plane flies over a burning Indonesian forest. Source: mb.com

But the benefits of forest conservation and regeneration do still outweigh the potential risks - indeed they will likely form a necessary role in avoiding a 2 degree temperature rise. Plus, the forests with the most scope for significant REDD+ projects are mainly tropical, rich in biodiversity and home to some of the highest concentrations of threatened species on the planet (Myers et. al., 2000); protecting the forests will also mean saving many species from extinction. 

At the moment, Houghton et. al., (2015) speculate that the biggest challenge for mass REDD+ projects is economic and political - not least given that current forest management is producing net positive emissions rather than negative. But now with the full backing of the UNFCCC, including substantial Green Climate Fund sponsorship for projects, hopefully these challenges can be overcome.