Great green sinks (pt. 2)

During and following the COP21 conference, REDD+ schemes received a lot of attention. They've been hailed as perhaps necessary to avoid a global mean temperature rise of 2 degrees.

Also spoken about at Paris was afforestation - growing forests in novel areas. A little more complicated by nature, this has not received quite the same glowing response as REDD+.

Forests sequester carbon from the air, leading to a negative radiative forcing (having a negative effect on surface energy balance, resulting in a lower temperature). But forests also lead to a set of complex, nonlinear biogeophysical feedbacks. These may act to enhance the negative radiative forcing, or offset it.

Perhaps the most important influence on the net radiative forcing is the location of the forest, and numerous studies warn that afforestation won't act to mitigate climate change everywhere on the planet.  


Figure 1. Dark boreal forests have significantly lower albedo than surrounding snow. Source: State of Alaska Fish and Game Dept.

For example, at boreal regions prone to (white) snow cover, creating (dark green) tree cover means a substantial decrease in surface albedo (Fig. 1 and 2B). By simulating the changes in albedo and atmospheric carbon dioxide, and the associated radiative forcings that would result if boreal forests were planted, Betts (2000) showed that the carbon dioxide soaked up by new forests was not enough to offset the decrease in albedo, resulting in net positive radiative forcing. This essentially equates to a rise in surface temperature.

At tropical latitudes the decrease in albedo is offset by cooling caused by increased evapotranspiration and precipitation compared with pastureland (Bonan, 2008). Case studies for this include Amazonia, where deforested areas for cattle grazing produce a much drier, hotter climate than forest-covered areas (Bonan, 2008). Similar results have been seen in Africa and Asia.

Figure 2. Geographic, biogeochemical and biogeophysical properties of nonforest (green) and forest (blue). The forests have also been split into tropical (pink), temperate (yellow) and boreal (purple). A: Geographic extent of forest vs nonforest. B: Total carbon stored as nonforest vs forest. C: Net ecosystem production (NEP) for tropical, temperate and boreal forest. D: Satellite-derived direct-beam albedo for snow-covered and snow-free forests. E: Evapotranspiration normalized by equilibrium rate in relation to canopy resistance for: wheat (green), corn (blue), deciduous forest (pink), jack pine (yellow) and oak savanna (purple). Individual data points plotted as points and means as bars. Source: Bonan, 2008. 

But there are other important considerations. For example, with a population projected to reach nearly 10bn by 2050, it's important that sufficient arable land is left free for agricultural use.

The Great Green Wall

The world’s largest tree planting project is already underway in China, and has seen over 66 billion trees planted since 1978. The Three-North Shelterbelt Project, dubbed the Great Green Wall, will stretch 4,500km by its completion in 2050, increasing global forest cover by over a tenth.

When completed, the wall will act as a protective shield from dust storms, which are becoming increasingly frequent and extreme as a result of growing desertification. 

Figure 3. Thin green line: the trees will form a barrier across dusty desert regions. Source: Imagine China via The Economist

The logistics of the project are huge: people living in regions being afforested must undertake a certain number of mandatory tree-planting days per year. In rural areas this often means asking people to leave farmland untended and plant trees instead. But the trees are being planted in dry, arid regions, and many are not surviving. The project is also raising concerns among rural communities of potential groundwater depletion as a result of the new green wall.

GCM studies have also shown that the surface temperature will be raised as a result of the project, and precipitation in China will increase over most locations (Fan et. al., 1998; Gao et. al., 2003). The project has received heavy criticism, and must serve as a lesson to other countries considering afforestation programmes: future afforestation programmes must carefully consider the environment of the intended forest, and be certain of the success of such a large project before embarking upon it. There must also be careful research to model regional and global effects on climate by newly planted forests.