The silver bullet that twinkles green

I find that there's something incredibly brazen in naming a journal article Irrigated afforestation of the Sahara and the Australian Outback to end global warming. I think it's because, in my head, the word 'to' in the title reads as is going to, rather the likely intended sense of so as to. Ornstein, Aleinov & Rind's 2009 paper proposes a massive biogeoengineering scheme: the afforestation of currently desert land in order to sequester ~8 Pg C per year, enough to effectively halt anthropogenic climate change.

Ornstein et al suggest the use of Eucalyptus grandis, a fast growing species native to Australia that is commonly used in commercial forestry, such as this Kenyan timber plantation.

Initially, sea water desalination could supply the enormous irrigation necessary to support subtropical forests in an arid desert such as the Sahara. Eventually, however, the climatic feedback mechanisms that contributed to previous green Sahara states would come into effect (if you don't know about this then you clearly haven't been reading my blog), with the amplified monsoon rains sustaining these new plantations. Since forests tend towards CO₂ neutrality as they mature, sections could be felled and replanted periodically, with the harvested trees being used for timber or to produce biofuel. Alternatively, this biomass could be pyrolysised to provide a renewable source of energy as well as produce biochar, a charcoal which can be buried to fertilise soil and store significant quantities of carbon in the ground.

To support these assertions, the study describes a GCM experiment in which the land surface of the Sahara and Australian Outback has been changed to represent a tropical forest, while soil moisture is artificially topped up at each time step so as to simulate irrigation.

Precipitation anomaly with an afforested Sahara and Outback. Image generated from supplementary video provided by authors.  

The resultant rainfall intensification shown above is very large, although it is not sufficient to completely supplant irrigation. Ornstein et al acknowledge that expense of this water supply, as well as those involved in the physical infrastructure necessary to establish a continent-spanning forest plantation, could be prohibitive. It is unlikely that the monetary value of the forest biomass (either as biofuel or more abstractly in the carbon trading market) would ever become economically competitive compared to fossil fuels, though even if this did happen it would still require a massive initial investment with a decades-long return period. With technological feasibility assumed, these costs, as well as the immense organisational difficulties involved, are identified by the authors as the most significant obstacles to such a desert afforestation scheme.

Like this, but in reverse. © John Holcroft

The Irrigated afforestation of the Sahara... paper provided the inspiration for this blog's name, so I'd really like to hear your opinions on the idea; whether from a modelling perspective and a more general one. Does the afforestation of the Sahara desert seem feasible to you? It amounts, essentially, to terraforming — are there any issues that you could see arising from this? If money wasn't an issue, would you support it?