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Human population growth, economic development, climate change, and the need to close the electricity access gap have stimulated the search for new sources of renewable energy. In response to this need, major new initiatives in hydropower development are now under way. At least 3,700 major dams, each with a capacity of more than 1 MW, are either planned or under construction, primarily in countries with emerging economies. These dams are predicted to increase the present global hydroelectricity capacity by 73 % to about 1,700 GW. Even such a dramatic expansion in hydropower capacity will be insufficient to compensate for the increasing electricity demand. Furthermore, it will only partially close the electricity gap, may not substantially reduce greenhouse gas emission (carbon dioxide and methane), and may not erase interdependencies and social conflicts. At the same time, it is certain to reduce the number of our planet’s remaining free-flowing large rivers by about 21 %. Clearly, there is an urgent need to evaluate and to mitigate the social, economic, and ecological ramifications of the current boom in global dam construction.
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RESEARCH ARTICLE
A global boom in hydropower dam construction
Christiane Zarfl Alexander E. Lumsdon
Ju
¨rgen Berlekamp Laura Tydecks
Klement Tockner
Received: 8 October 2014 / Accepted: 15 October 2014
Springer Basel 2014
Abstract Human population growth, economic develop-
ment, climate change, and the need to close the electricity
access gap have stimulated the search for new sources of
renewable energy. In response to this need, major new
initiatives in hydropower development are now under way.
At least 3,700 major dams, each with a capacity of more
than 1 MW, are either planned or under construction, pri-
marily in countries with emerging economies. These dams
are predicted to increase the present global hydroelectricity
capacity by 73 % to about 1,700 GW. Even such a dra-
matic expansion in hydropower capacity will be
insufficient to compensate for the increasing electricity
demand. Furthermore, it will only partially close the
electricity gap, may not substantially reduce greenhouse
gas emission (carbon dioxide and methane), and may not
erase interdependencies and social conflicts. At the same
time, it is certain to reduce the number of our planet’s
remaining free-flowing large rivers by about 21 %. Clearly,
there is an urgent need to evaluate and to mitigate the
social, economic, and ecological ramifications of the cur-
rent boom in global dam construction.
Keywords Biodiversity Energy River management
Sustainability Climate change
Introduction
Rapid growth of the human population and economic
development are tightly coupled with an increase in global
energy demand (UN 2012). Electricity production
increased by 72 % between 1993 and 2010 and is expected
to rise by an additional 56 % by 2040 (The World Bank
2014a; US Energy Information Administration 2014). At
the same time, more than 1.4 billion people remain dis-
connected from electricity supply, especially in rural Sub-
Saharan Africa and South Asia (UNEP 2012b). Securing
the future energy demand and closing the electricity access
gap are therefore paramount objectives set for the energy
sector (Crousillat et al. 2010; UN-Energy 2010).
Energy production and conversion account for 29 % of
global greenhouse gas emissions (UNEP 2012a). In addi-
tion, depletion of fossil energy resources as well as the
exploitation of uranium provide reasons for concern. Their
unequal global distribution leads to interdependencies
between countries and in the worst case to political con-
flicts, which are likely to increase as these resources
become further depleted (Asif and Muneer 2007).
Accordingly, renewable energy sources—geothermal,
C. Zarfl and A.E. Lumsdon contributed equally to the preparation of
the manuscript.
Electronic supplementary material The online version of this
article (doi:10.1007/s00027-014-0377-0) contains supplementary
material, which is available to authorized users.
C. Zarfl (&)A. E. Lumsdon L. Tydecks K. Tockner
Leibniz-Institute of Freshwater Ecology and Inland Fisheries,
Mu
¨ggelseedamm 310, 12587 Berlin, Germany
e-mail: zarfl@igb-berlin.de
A. E. Lumsdon K. Tockner
Department of Biology, Chemistry and Pharmacy, Freie
Universita
¨t Berlin, Altensteinstraße 6, 14195 Berlin, Germany
J. Berlekamp
Institute of Environmental Systems Research, University of
Osnabru
¨ck, Barbarastraße 12, 49076 Osnabru
¨ck, Germany
Present Address:
C. Zarfl
Center for Applied Geosciences, Eberhard Karls Universita
¨t
Tu
¨bingen, Ho
¨lderlinstr. 12, 72074 Tu
¨bingen, Germany
Aquat Sci
DOI 10.1007/s00027-014-0377-0 Aquatic Sciences
123
solar, wind, waves, tides, biomass, biofuels, and hydro-
power—are rapidly gaining importance; their production
almost doubled between 1991 and 2011. Renewables cur-
rently account for 20 % of the global electricity
production, with hydropower contributing 80 % to the total
share (The World Bank 2014b,c). Worldwide, out of
37,600 dams higher than 15 m, more than 8,600 dams
primarily designed for hydropower generation are in
operation (International Commission on Large Dams
2011). Notably, 32 countries including Brazil, Mozam-
bique, Nepal, and Norway use hydropower to produce
more than 80 % of their electricity requirements (The
World Bank 2014b).
The Rio?20 targets require countries to meet their
growing energy demand through the use of Kyoto-com-
pliant energy resources (UNEP 2012b). This is an
additional major driver of investments in hydropower. At
present, 22 % of the world’s technically feasible hydro-
power potential [[15.6 million GWh (=10
6
kWh) per year]
is exploited (International Commission on Large Dams
2011). Following a period of relative stagnation during the
past 20 years, the current boom in hydropower dam con-
struction is unprecedented in both scale and extent (Poff
and Hart 2002; Fig. 1). The economic, ecological, and
social ramifications are likely to be major. However, the
spatial pattern of hydropower construction at the global
scale is unclear, as are the cumulated fragmentation impact
of the affected river systems, greenhouse gas emissions,
and social impacts (such as the relocation of people).
Here, we provide a comprehensive global inventory of
future hydropower dams with a capacity exceeding 1 MW.
We include dams that are both currently planned or under
construction. Information for each dam includes the project
name, geographical coordinates, river basin, hydroelectric
capacity, and construction timeline. The inventory is based
on information derived from more than 350 scientific ref-
erences, governmental and non-governmental sources, as
well as from other public databases, reports and newspaper
articles. When available, we used multiple independent
references and sources for cross-validation to reduce the
heterogeneity in data quality (Online Resource Table S1).
This compilation provides a conservative estimate because
it focuses on dams designed for hydropower production;
dams designed primarily for water supply, flood preven-
tion, navigation, and recreation are excluded. The
compilation also excludes very small hydropower dams
(\1 MW) that are currently under construction or planned;
their number is most likely very high but not documented
comprehensively over the world.
The data compilation enabled us to (1) identify future
hotspots in hydropower development in comparison to con-
temporary patterns (Online Resource Fig. S1), (2) calculate
the number of future hydropower dams related to river dis-
charge within major river basins, and (3) estimate the
cumulative future impact on the current state of river frag-
mentation (Poff and Hart 2002;Lehneretal.2011,G.Grill
et al. unpublished information). This information provides a
solid basis for future studies to identify regional conflicts and
the inevitable tradeoffs between the benefits of hydropower
generation and ecological, social and economic impacts.
Methods
Data collection on hydropower investments
A data search was conducted on the scope of investments
into the hydroelectricity sector to obtain an idea on its
Fig. 1 Global pace of
hydropower dam construction
of existing hydropower dams
(Lehner et al. 2011) and outlook
for hydropower dams which are
under construction or planned
C. Zarfl et al.
123
economic order-of-magnitude. Therefore, details on almost
500 investors since 1978 were collected including: name of
the investor, country and year of investment, name of
project, and amount spent (US$). The collection was not
restricted to investments in construction activities, but also
considered repair and maintenance activities as well as
expansion and improvement of hydropower infrastructure
in general. For the identified projects, more general over-
views on investors were provided by the World Bank and
International Rivers (The World Bank 2014f; International
Rivers, Banks and Financial Institutions 2010). However,
most of the data were derived from reports and web sources
of single investors.
Data collection and processing on hydropower dams
Geo-referenced data were collected for future hydropower
schemes that have a maximum design capacity of 1 MW or
greater. Information on future hydropower dams below this
capacity is only available sporadically, and often lacks
detail because of less onerous licensing requirements. For
this reason they were excluded from the study. Data for
dams that are under construction or at a late planning stage
were collected between August 2012 and February 2014
using different types of sources:
1. Peer reviewed literature
2. Government documents
3. NGO reports and publications
4. Newspaper articles
5. Commercial databases
6. Reports of energy producers
7. Reports of energy infrastructure engineers or
consultants
8. Other web sources
Dams were annotated in the database as planned if they
were described as such in the original data source, or if
they were reported as being at a feasibility stage where
social, cost-benefit and environmental aspects were under
evaluation. Dams at a pre-feasibility stage were not
included. About 80 % of the data contained spatial infor-
mation in various formats; these were converted for use
with the World Geodetic System 1984 (WGS 84). For the
remaining data, it was possible to geo-reference the dams
manually within a geographical information system (Arc-
GIS 10.1
TM
), using references reported in the original data
source literature and Google Maps
TM
or Google Earth
TM
.
All data were aligned to the HydroSHEDS (Lehner et al.
2008)15s(*500 m) global river network, except for 12
records that were beyond the extent of the HydroSHEDS.
Dams were snapped to the nearest river line within Hy-
droSHEDS. This approach relies on the accuracy of the
original coordinates and could introduce bias through
snapping to the incorrect river line. Therefore, all dam
locations were manually cross-validated wherever possible
by using additional data sources to ensure that they had
snapped to the correct river line.
The availability and accuracy of attribute data (including
spatial information) for each individual dam record was
determinedby the stage of the project. Consequently, projects
that were under construction invariably had more detailed
supplementary information. Furthermore, this information
was cross-referenced with the original objective noted in the
data source. Where possible, records were cross-validated
with multiple data sources to confirmthe status of the project,
or to provide attributes that were missing in the original data
source. Additional attributes were collected on the dam name,
continent, country, main river system, major basin (Food and
Agriculture Organisation 2011), sub basin (Food and Agri-
culture Organisation 2009), stage of construction, maximum
designed capacity (MW), dam height (m), start of construc-
tion, and planned date of completion. Discharge (m
3
s
-1
)
calculations were processed at a later stage.
To analyze the spatial distribution of future hydropower
dams, additional data were collected for those countries
where new hydropower dams are under construction or
planned. This included numbers for each country on the
size of the population without electricity access, which was
complete for all required countries in 2002 (Dorling 2007),
as well as GNI (gross national income; The World Bank
2014e) and GDP (gross domestic product; The World Bank
2014d) per capita (US$) in 2012. Both indices were related
to the expected future hydropower capacity per capita. In
addition, data on the technically feasible hydropower
potential (E
pot
in GWh year
-1
), the installed hydropower
capacity (K
inst
in MW) and the electricity production in
2011 (E
prod
in GWh year
-1
) were compiled (International
Journal on Hydropower and Dams 2012). Based on these
data, the potential electricity production E
future
(in GWh
year
-1
) by hydropower plants under construction or plan-
ned with their planned capacities (K
future
in MW) could be
conservatively estimated as (assuming the same average
efficiency as in existing large hydropower plants):
Efuture ¼Eprod
Kinst Kfuture
Combining information on the technically feasible
hydropower potential, the currently produced electricity,
and the potentially produced electricity, allowed us to
calculate the future exploitation of the remaining
technically feasible potential in each country.
Discharge data estimation
River discharge was calculated for 3,688 dam locations
within the available extent of HydroSHEDS. Therefore, a
A global boom in hydropower dam construction
123
global runoff raster grid was constructed to summarize
mean annual net cell runoff from 1980 to 2009. These
values were derived from the WaterGAP Global Hydrology
Model, and include runoff from land, lakes and wetlands,
but also consider evapotranspiration from open water sur-
faces (Do
¨ll and Fiedler 2008). Standard Arc Hydro Tools
(Maidment 2002) within the ArcGIS software using
unprojected data were used to delineate upstream drainage
basins for all dam locations. Discharge values could then
be calculated using an adapted zonal statistics tool (Clark
2012).
Terrain preprocessing
The HydroSHEDS 15-arc second GRID provided the input
for stream definition by applying the following imple-
mented procedures:
Terrain Preprocessing | Stream Definition
Terrain Preprocessing | Stream Segmentation
Terrain Preprocessing | Catchment Grid Delineation
Terrain Preprocessing | Catchment Polygon Processing
Terrain Preprocessing | Drainage Line Processing
Terrain Preprocessing | Adjoint Catchment Processing
River basin processing
For the river basin processing, the following procedure
within the ArcHydro Tool software was applied:
Watershed Processing | Batch Watershed Delineation
The global precipitation raster map from the WaterGAP
Global Hydrology Model (Do
¨ll and Fiedler 2008) was
converted to a new raster with the same cell size and extent
as the HydroSHED GRID inputs for each continent. To
calculate discharge values for each dam location, the
delineated dam basins were projected to continental Lam-
bert Conformal Conic projections, with the exception of
New Zealand, where the New Zealand map grid (NZMG)
was used.
Discharge of dam catchments
To calculate the sum of runoff for each delineated dam
catchment, a modified zonal statistics tool (Clark 2012)
was used to deal with overlapping areas of the derived
catchment polygons. The annual runoff raster grid (Do
¨ll
and Fiedler 2008) provided input values used in calculating
the output statistics for each dam catchment polygon. The
tool calculates the sum of runoff (mm year
-1
) as the sum
of zonal statistics based on delineated dam catchments,
which are based upon the values of the underlying annual
runoff raster. Taking the cell size (m
2
) into account, dis-
charge was calculated as follows:
Discharge ½m3s1
¼sum of runoff ½mm year1cell size ½m2
365 24 60 60ðÞ½s year11000 ½mm m1
To determine whether regional projections provided
sufficient resolution to process the discharge data
accurately, a sensitivity analysis was undertaken based on a
local projection (TUREF TM42) for the Coruh river
catchment in Eastern Turkey, which lies at the Eastern
extension of the European Lambert Conformal Conic
projection and thus experiences the largest distortions of all
grid cells when using a regional instead of a local projection.
Discharge values were calculated as described above, except
that the TUREF TM42 projection was used. A comparison of
the discharge data calculated based on the two projections
showed a mean relative difference of less than 2 %, indicating
that the regional projection could be used invariably.
Based on the discharge information for each hydro-
power dam, the number of dams in five different discharge
categories (\10, 10–100, 100–1,000, 1,000–10,000,
C10,000 m
3
s
-1
) was calculated for each major basin. This
provides distribution patterns of dam locations within a
river network and informs about the stream size classes
likely to be most influenced by hydropower dams.
Discharge of major basins
The total water resources (km
3
year
-1
) available within
each major basin were calculated to assess the extent to
which water resources are potentially exploited in major
basins. This was done by following the same procedure as
described above for the dam catchments using the major
basin catchment area instead of the dam catchment polygon
as the vector input. A density value, i.e. number of dams
per water resources availability (km
3
per year), was then
calculated for each major basin for both existing dams and
a scenario combining existing and future dams.
Fragmentation of large river systems
Locations of future hydropower schemes were assigned to
catchments of 292 large river systems (LRS) classified by
Nilsson et al. (Nilsson et al. 2005) as ‘‘not affected’’,
‘moderately affected’’ or ‘‘strongly affected’’ according to
their classification of river channel fragmentation and
water flow regulation by dams. In total, 2611 hydropower
dams out of our database are located within 108 of the
LRS. Numbers of future dams were then summarized for
each of the LRS to investigate which LRS might undergo
initial or further fragmentation by future hydropower dams.
C. Zarfl et al.
123
Maps
All maps were drawn using the Mollweide projection,
which provides a global representation of the major river
basins that is accurate in area and true to scale along the
equator and the central meridian.
Results
As of March 2014, a total of 3,700 hydropower dams with
a capacity of more than 1 MW each were either planned
(83 %) or under construction (17 %). These dams are
predicted to increase global hydropower electricity capac-
ity from 980 GW in 2011 (International Journal on
Hydropower and Dams 2012) to about 1,700 GW within
the next 10–20 years. Although small and medium-sized
dams (1–100 MW) will dominate in number ([75 %),
93 % of the future hydropower capacity will be provided
by 847 large dams with a capacity of more than 100 MW
each.
Future hydropower development is primarily con-
centrated in developing countries and emerging
economies of Southeast Asia, South America, and
Africa. The Balkans, Anatolia, and the Caucasus are
additional centers of future dam construction (Fig. 2,
Online Resources Fig. S2, Table S2). More than 40 %
of the hydropower capacity under construction or
planned will be installed in low and low-middle income
countries (GNI \$4,085 per capita; The World Bank
2014e), which excludes China ($5,720 GNI) and Brazil
($11,630 GNI) but covers hotspots such as the Demo-
cratic Republic of Congo ($230 GNI), Pakistan ($1260
GNI), and India ($1580 GNI).
The Amazon and La Plata basins in Brazil will have the
largest total number of new dams in South America,
whereas the Ganges–Brahmaputra basin (mainly India and
Nepal) and the Yangtze basin (China) will face the highest
dam construction activity in Asia (Fig. 3; Online Resource
Tables S2, S3). Very large dams, each with a capacity of
more than 1 GW, will primarily be located in Asia, espe-
cially in the Yangtze basin, and in South America, mainly
in the Amazon basin (Online Resource Fig. S3). The Xi
Luo Du dam in the Yangtze basin (14.4 GW) and the Belo
Monte dam on the Xingu
´River in the Amazon basin (11.2
GW) are examples of very large dams already under
construction.
Assuming the current efficiency of electricity pro-
duction (GWh per year) per installed dam capacity
(GW) and that all dams will be realized, China will
remain the global leader in hydropower dam construc-
tion because it still has a remaining technically feasible
potential of more than 1.8 million GWh per year.
Nevertheless, China’s share of total future global
hydropower production will decline from currently 31 to
25 % because of a disproportionate increase in new
hydropower dam construction in other parts of the
world. In Africa, for example, out of the technically
feasible hydropower potential (1.5 million GWh per
year) less than 8 % are currently exploited (International
Commission on Large Dams 2011). There, the focus is
on the construction of large dams, each with a capacity
of more than 100 MW (Online Resource Table S4).
Construction of the 3,700 dams worldwide may increase
global hydropower production by 73 %, corresponding to
an increase in the exploitation of the technically feasible
hydropower potential from a total of 22 % today (Inter-
national Commission on Large Dams 2011)to39%.
Fig. 2 Global spatial
distribution of future
hydropower dams, either under
construction (blue dots 17 %) or
planned (red dots 83 %)
A global boom in hydropower dam construction
123
However, the share of hydropower in total global elec-
tricity production will rise only slightly from 16 % in 2011
to 18 % until 2040 because of the concurrent increase in
global energy demand.
In regard to environmental impacts, our analyses show
that the re-accelerating construction of hydropower dams
will globally lead to the fragmentation of 25 of the 120
large river systems currently classified as free-flowing
(Nilsson et al. 2005), primarily in South America (Online
Resource Table S5). Worldwide, the number of remaining
free-flowing large river systems will thus decrease by about
21 %.
We also gave a special focus to the evaluation of
environmental consequences of dam building in basins that
will experience high levels of water resource exploitation
in relation to the discharge volume available (Fig. 4). In
only a few cases, future dam building activities will con-
centrate on the high-discharge river segments
([100 m
3
s
-1
), i.e. on large lowland river segments and
main tributaries (Online Resource Table S7). The majority
of basins will experience the exploitation of river segments
with low discharge and high gradient, which goes along
with the future high global share of small and medium-
sized dams (\100 MW).
Fig. 3 Number of future
hydropower dams per major
river basin. Red [100, Orange
26–100, Yellow 11–25, Green
B10, Gray no data available
Fig. 4 Number of future
hydropower dams in relation to
major river basin discharge
(dams per km
3
year
-1
). Red[1,
Orange [0.25–1, Yellow
[0.05–0.25, Green [0–0.05,
Gray no data available. For
comparison with existing
hydropower dams please see
Online Resource Fig. S10
C. Zarfl et al.
123
Earlier studies show that average CO
2
and methane
emission rates amount to 85 g and 3 g per kWh (with an
uncertainty factor of 2) of produced hydropower electricity
(Barros et al. 2011; Hertwich 2013). This means, that
future hydropower plants may add 280–1,100 Tg
(1 Tg =10
12
g) CO
2
and 10–40 Tg methane to the
atmosphere, which corresponds to 4–16 % of the global
carbon emissions by inland waters (Raymond et al. 2013).
An economic perspective reveals that the 3-year-average
investment in hydropower has increased more than sixfold
in 2010–2012 in comparison to the 3-year average a decade
ago (Maeck et al. 2013; Online Resource Fig. S4). Esti-
mated investments total about 2 trillion US$ for all future
hydropower dams currently under construction or planned,
assuming average construction costs of large dams at 2.8
million US$ per MW (Ansar et al. 2014). With an average
construction time of 8.6 years for a dam (Ansar et al.
2014), the annual investments for future hydropower dams
may thus be as high as 220 billion US$.
Concerning the involvement of investors, about 35
investors contributed, for example, to the Brazilian
hydroelectricity industry between 2010 and 2012, seven of
which were investors from the USA, Spain, France, and
Switzerland. In Africa, the main investors have been Hy-
dromine (USA) and Sinohydro (China) with more than 1
billion US$ of investments contributing to the hydropower
sector in Cameroon and Zambia, respectively (Online
Resource Table S8).
Similarly, there is no correlation between future
hydropower dam construction and the economic condition
(GNI) of a country (Online Resource Fig. S5). Neverthe-
less, the future hydropower capacity per country increases
with increasing rates of GDP growth (Online Resource Fig.
S6), as well as with the technically feasible hydropower
potential remaining (Online Resources Figs. S7, S8). The
correlation with economic growth rates corresponds to the
high industrial share (90 %) of future energy demand
(OECD 2012).
In contrast, we found no correlation between the per-
country estimate of the planned increase in hydropower
capacity and the number of people lacking access to
electricity. In India, for example, almost 300 million peo-
ple lacked access to electricity in 2009, but the low
technically feasible potential for hydropower in most parts
of the country will prevent narrowing the electricity gap
substantially, even if the entire potential were exploited.
On the other hand, countries like the Democratic Republic
of Congo and Brazil, where large proportions of the pop-
ulation lack access to electricity as well, exhibit an
enormous potential for hydropower development (Online
Resource Fig. S9). With the expansion of their hydropower
capacity, these countries might seek to close their elec-
tricity access gap, which would require developing a
national electricity grid. The expansion, however, might
also be driven by private economic interests in exploiting
the hydropower potential to export power or develop the
industrial sector. Countries with a very low electrification
rate (\20 %), such as Kenya and Tanzania, could already
supply the whole population with electricity by their
hydropower capacity installed at present, if it were not used
by industry, for example, for mining operations. Pakistan
and Nigeria are other examples for which the expansion of
hydropower could allow the electricity access gap to be
closed in the future. This assumes that the demand caused
by rapid population growth in these countries (UN
Department of Economic and Social Affairs, Population
Division 2013) and increasing industrial requirements will
consume only part of the expected electricity surplus
generated by new hydropower dams.
Discussion
Our results show that hydropower will not be able to
substitute non-renewable electricity resources such as coal,
oil, and uranium. Even if the entire technically feasible
hydropower potential will be exploited, which would cor-
respond to a dam construction boom almost five times that
currently estimated, hydropower would contribute less than
half of the global electricity demand projected until 2040.
Without any additional hydropower dam construction,
however, its share in electricity production would drop to
12 %.
Although being a renewable electricity source, hydro-
power is also accompanied by significant environmental
impacts on free-flowing rivers, ranging from fragmenta-
tion, which prevents free movement of organisms, to
severe modification of river flow and temperature regimes
and to dramatic reductions in sediment transport (Vo
¨ro
¨-
smarty et al. 2010; Liermann et al. 2012). Future
hydropower dam construction may affect some of the
ecologically most sensitive regions globally. For example,
the Amazon, Mekong, and Congo basins, which will be
heavily impacted by future hydropower dams, jointly
contain 18 % of the global freshwater fish diversity (http://
www.fishbase.org). Similarly, the Balkan region, a hot spot
area in hydropower development, is a key freshwater bio-
diversity region in Europe (Griffiths et al. 2004). Notably,
hydropower dams under construction or planned within the
currently free-flowing large river systems will contribute
less than 8 % to the planned global hydropower capacity,
ranging from 3 % in Africa to 10 % in Asia, and from
0.3 % of the total planned capacity in Brazil to more than
80 % in Malaysia, Papua New Guinea, and Guyana (Online
Resource Table S6). This suggests that fragmentation
impacts on the remaining free-flowing rivers in the world
A global boom in hydropower dam construction
123
could be reduced by evaluating (transboundary) construc-
tion options in river systems already strongly fragmented to
date. In East Africa, for example, fragmentation impacts
could be reduced by abandoning hydropower dams in the
Rufiji River, the last remaining large free-flowing river
network in this region, while implementing compensatory
capacities in the Nile and Zambezi Rivers, which are
already heavily fragmented today. Of course, given the
clustering of new dams in specific areas of the world, this is
not a generally feasible strategy but should be considered
in some regions.
It is known that hydropower is not a climate neutral
electricity source (Wehrli 2011). Depending on the envi-
ronmental and technical conditions, reservoirs can be
important emitters of greenhouse gases (Maeck et al.
2013). Our estimations for methane and carbon dioxide
emissions by future hydropower dams are a rough estimate
because emissions depend on the location and morphom-
etry of the reservoir but also on how the stored water is
released from the reservoir (e.g. deep water or surface
water release). Future hydropower plants will primarily be
constructed in the subtropics and tropics, where greenhouse
gas emission from reservoirs is estimated to be high, par-
ticularly during the first years after completion (Barros
et al. 2011). According to IPCC (2014), estimated maxi-
mum emissions may even exceed by up to a factor 10 the
emissions avoided by refraining from burning fossil fuel.
On average, however, lifecycle GHG emissions of hydro-
electricity are more than 30 times lower than that of coal
(IPCC 2014), which underlines the need for attention to be
paid to how to weigh the greenhouse gas emissions against
the damage to water resources, biodiversity, and ecosystem
processes and services.
These also include the direct and indirect consequence
of relocating or displacing humans, especially of indige-
nous people, the loss of access to natural resources, and a
highly disproportional distribution of economic benefits
and costs for (international) companies, local governments
and populations. Our results also demonstrate that the
expected huge expansion of hydropower capacity will most
likely fail to close the global electricity access gap. In
addition, the increase of transboundary hydropower pro-
jects creates potential for conflict similar to that
experienced among interdependent consumers of fossil and
nuclear energy resources (Stone 2010; Online Resource
Table S3). Thus, dams could intensify the complexity of
resource demands for energy, water, flood prevention, and
food supply (Vo
¨ro
¨smarty et al. 2010; Costanza et al. 2014),
which emanate from users and stakeholders with multiple
social backgrounds and interests.
Hydropower dam construction and related investments
are also a ‘‘transboundary’’, i.e. international, business. Our
analysis on involved investors underlines a shifting
geopolitical situation, with an increasing number of pro-
jects financed by internationally operating companies
based in foreign countries. In general, these parties only
seize investment opportunities but are not involved in
project development or dam operation (McDonald et al.
2009). Nevertheless, most of the global investors follow the
so-called equator principles, a credit risk management
framework to ensure internationally-agreed minimum
standards for social and environmental risk assessments
(Equator Principles Association 2013). Concerning the
estimated annual investment for the future hydropower
dams of 220 billion US$, it must be noted that these costs
include neither the operational costs of hydropower dams
nor the gains through electricity production. Potential
social and environmental costs are not included either.
Based on our analyses, it is evident that hydropower will
not be a general or the only solution (1) to tackle the
problems of growth in energy demand and climate change,
(2) to close the electricity access gap, or (3) to erase in-
terdependencies in electricity production. Indeed, we
urgently need to advance existing regulatory guidelines and
standards to create synergies, rather than trade-offs, among
the different water users, including ecosystems. The
Hydropower Sustainability Assessment Protocol (Interna-
tional Hydropower Association 2010) for evaluating the
impacts of hydropower dams to be built provides a first
step towards encompassing the environmental and social
aspects of sustainability during the planning, implementa-
tion, and operation stages of hydropower dams. However, a
participatory approach that includes the affected people is
still missing in the protocol. The global database of future
hydropower dams we present here may form a valuable
basis to evaluate where to build hydropower dams, and
how to improve the dam building management to support a
systematic planning approach that includes environmental
and social costs as well as the consultation of stakeholders
and affected people.
Conclusion
Global population growth and increasing electricity demand
on the one hand, and the urgent need to decrease greenhouse
gas emissions on the other hand, lead to a new boom in the
construction of hydropower dams worldwide. Despite the
renewable nature of hydroelectricity, this technology also
comes along with severe social and ecological adverse
effects, e.g. relocation of people and transboundary conflicts,
fragmentation of free-flowing rivers, and habitat changes,
thus further threatening freshwater biodiversity. This does
not necessarily need to be the case since we can develop
sustainable ways of implementing and operating hydro-
power dams to optimize the production of renewable
C. Zarfl et al.
123
electricity while minimizing negative consequences. With
our comprehensive synthesis of effort to map current and
planned hydropower dam construction, we provide the basis
to quantify and localize future hydropower dams on a global
scale. This allows for a systematic management approach
that takes network effects and cumulated impacts of multiple
dams within a river basin into account.
Acknowledgments This research has been partially carried out
within the Erasmus Mundus Joint Doctorate Program SMART (http://
www.riverscience.eu) funded by the EACEA and the EU-funded
project BioFresh (http://www.freshwaterbiodiversity.eu). Dr. Ulrich
Schwarz provided data for the Balkan region. William Darwell, Mark
O. Gessner, Christopher Kyba, Bernhard Lehner, LeRoy Poff and
Emily S. Bernhardt provided helpful comments. Madeleine Ammar
collected data on worldwide hydropower investments.
Conflict of interest The authors declare that they have no conflict
of interest.
Compliance with ethical standards This article does not contain
any studies with human participants or animals performed by any of
the authors.
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