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226 Managing Global Warming

from landfill gas. Some agricultural and forestry wastes can be used for bioenergy,
although most will need to be retained in situ to maintain soil fertility and reduce wind
or water erosion. Finally, biomaterials, particularly construction timber, after the end
of its useful construction life—which may involve its reuse in some other construction
project—can be combusted for energy.

6.3 Hydroelectricity

6.3.1 Introduction

In the early years of electricity, most was produced from hydro, but was soon over-
taken by fossil fuels burnt in power stations. Today, despite rapid growth in recent
decades of first wind, then solar electricity, hydro still dominates RE electricity pro-
duction [5]. The hydro potential in OECD countries is now largely exploited; most of
the remaining potential is in Asia, Africa, and South America [21]. Hydropower is a
mature technology, and little in the way of technical advances can be expected. It also
has EROEI values greater than other RE sources [22]. There are even indications that
the energy ratio for new hydroconstruction is falling globally, in that the annual elec-
tricity output per megawatt of installed power over the period 1994–2011 was <40%
of its value in the year 1993 [21].

6.3.2 Hydroelectricity in 2050

Hydroelectricity cannot be readily stored. But unless the hydroplant is a simple run-of-
the-river installation, electricity can be generated on demand because of the gravita-
tional energy of the water stored behind the dam wall.
It is likely that by 2050, most of the world’s remaining technical potential for
hydropower will be utilized. But some potential will remain unexploited—and so
annual electric output will still be well below 30EJ for several reasons. First, ongoing
climate change will add to uncertainty about future catchment precipitation levels and
the season distribution of annual river flows, although some areas (such as Arctic-
draining rivers in northern Europe) will see conditions favorable for further hydro out-
put. In some mountainous areas, such as the Himalayas, hydropotential could show a
temporary increase, fuelled by continuing loss of glacier mass [6,23]. This uncertainty
will impact on the economics of hydrodevelopment, given that dam structures can
have an expected lifetime of 100 years or so.
Second, extreme rainfall events are expected to increase in frequency. The soil ero-
sion potential varies nonlinearly with rainfall intensity (and so will catchment area
landslides into the reservoir), so that sedimentation rates of hydroreservoirs will
increase, shortening their service lives. In the Amazon basin, another factor could seri-
ously affect the basin’s vast hydropotential. Deforestation, which is still occurring in
the Amazon, initially increases river flows, and thus hydropotential, because of less
transpiration from trees. But beyond a certain level of basin forest loss, further defor-
estation reduces hydropotential. The modeled results of Stickler et al. [24] showed that
hydropotential could be reduced to only a quarter of its full potential if 40% of forest