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Global renewable energy resources and use in 2050 225

(e.g., biodiversity, pest control). Axel Kleidon [19] estimated present HANPP as 40%,
and based on a model of the vegetation-climate system, argued that for HANPP values
beyond about 45%, any percentage rise would produce feedbacks that would reduce
global NPP, and eventually, the absolute level of HANPP measured in energy or
mass units.
The world population in 2015 stood at 7.35 billion; the UN [20] expect this to rise to
9.73 billion by 2050. If present trends for per capita income growth also continue,
there will evidently be increased demand for all uses of biomass, raising the question
of which ones should get priority. Clearly, from an ethical viewpoint, food should
come first: we should produce enough food for the likely expanding human family.
Nevertheless, this does not necessarily imply that present dietary trends are followed.
If instead of expanding animal products as at present [18], the world moved toward a
more vegetarian diet, agricultural production could be reduced, given that animal
products need far more inputs (and produce more GHGs) per kilojoule of food energy
than nonanimal products. Nevertheless, the FAO [19] anticipate that output of both
grain and animal products will continue to rise, as will agricultural land area. Increas-
ing agricultural yields can reduce the inputs of land needed for food production, but
the FAO saw yields only growing slowly. Future productivity gains will be more dif-
ficult to make, particularly in the face of on-going climate, uncertainty about future
phosphorus availability, biodiversity loss, and other changes. And although land pro-
ductivity (output per hectare) has risen, the same is not true for energy productivity.
Historically, food was produced with a much lower ratio of energy inputs to energy
output—this ratio declined by more than an order of magnitude in the 20th
century [20].
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In 2014, although the nonfuel production of timber was 1836Mt (1836 10 t), up
from 1700Mt in 1990 [19], timber is losing share as a construction material, as are
natural fibers like cotton [18]. Yet in terms of meeting a given building function, such
as the framing for a four-storey apartment block, timber has a much smaller carbon
footprint than steel or reinforced concrete. This suggests that for climate mitigation,
biomaterials should be expanding, rather than contracting as at present, their share
of the market. Such a biomaterials expansion would lower the global potential for
bioenergy, assuming that all biomass uses together face a global upper limit—that
for HANPP.
Clearly, there is no simple answer to the question: What is the global technical
potential for bioenergy? It all depends on future consumption of biomass for bioma-
terials and food. The input resources necessary to meet food needs in 2050 involve
both ethical and technical questions. The huge range of estimates in the literature
for 2050 bioenergy technical potential reflect this uncertainty: values vary from a
low of 10–60EJ to an upper value of over 1500EJ [10]. Since this upper value is
75% of the entire global terrestrial NPP, it is clearly unrealistic. Given the superior
GHG savings possible with using biomaterials as a substitute for more energy- and
GHG-intensive materials, as well as rising food needs, values toward the lower end
of the range are more likely.
However, the three human uses for biomass are not fully independent of each other.
Human wastes can be used to produce methane from both sewage treatment works and