Applying the concept of "energy return on investment" to desert greening of the Sahara/Sahel using a global climate model

摘要

Altering the large-scale dynamics of the Earth system through continual anddeliberate human intervention now seems possible. In doing so, one shouldquestion the energetic sustainability of such interventions. Here, from thebasis that a region might be unnaturally vegetated by employingtechnological means, we apply the metric of "energy return on investment"(EROI) to benchmark the energetic sustainability of such a scenario. We dothis by applying EROI to a series of global climate model simulations wherethe entire Sahara/Sahel region is irrigated with increased rates ofdesalinated water to produce biomass. The energy content of this biomass isgreater than the energy input rate for a minimum irrigation rate of about200 mm yr?1 in the winter and 500 mm yr?1 in the summer,thereby yielding an EROI ratio >1 : 1, expressing energeticsustainability. Quantified annually, the EROI was >1 : 1 for irrigationrates more than 500 mm yr?1, progressively increasing to a maximum of1.8 : 1 with 900 mm yr?1, and then decreasing with further increasesin the irrigation rate. Including the precipitation feedback arising fromchanges in moisture recycling within the study region approximately doublesthese EROI ratios. This overall result varies spatially and temporally, sowhile the entire Sahara/Sahel region is irrigated equally, the westerncoastal region from June to August had the highest EROI. Other factors wouldcomplicate such a large-scale modification of the Earth system, but thissensitivity study concludes that with a required energy input, desertgreening may be energetically sustainable. More specifically, we have shownhow this type of EROI analysis could be applied as a metric to assess adiverse range of human alterations to, and interventions within, the Earthsystem.