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Abstract:
Altering the large-scale dynamics of the Earth system through continual and deliberate human intervention
now seems possible. In doing so, one should question the energetic
sustainability of such interventions. Here, from the
basis that a region might be unnaturally vegetated by employing
technological means, we apply the metric of “energy
return on investment” (EROI) to benchmark the energetic
sustainability of such a scenario. We do this by applying
EROI to a series of global climate model simulations
where the entire Sahara/Sahel region is irrigated with increased
rates of desalinated water to produce biomass. The
energy content of this biomass is greater than the energy input
rate for a minimum irrigation rate of about 200mmyr−1
in the winter and 500mmyr−1 in the summer, thereby yielding
an EROI ratio >1 : 1, expressing energetic sustainability.
Quantified annually, the EROI was >1 : 1 for irrigation rates
more than 500mmyr−1, progressively increasing to a maximum
of 1.8 : 1 with 900mmyr−1, and then decreasing with
further increases in the irrigation rate. Including the precipitation
feedback arising from changes in moisture recycling
within the study region approximately doubles these EROI
ratios. This overall result varies spatially and temporally, so
while the entire Sahara/Sahel region is irrigated equally, the
western coastal region from June to August had the highest
EROI. Other factors would complicate such a large-scale
modification of the Earth system, but this sensitivity study
concludes that with a required energy input, desert greening
may be energetically sustainable. More specifically, we have
shown how this type of EROI analysis could be applied as a metric to assess a diverse range of human alterations to, and interventions within, the Earth system.
