Mapping the global threat of land subsidence

Abstract

Subsidence, the lowering of Earth's land surface, is a potentially destructive hazard that can be caused by a wide range of natural or anthropogenic triggers but mainly results from solid or fluid mobilization underground. Subsidence due to groundwater depletion (1) is a slow and gradual process that develops on large time scales (months to years), producing progressive loss of land elevation (centimeters to decimeters per year) typically over very large areas (tens to thousands of square kilometers) and variably affects urban and agricultural areas worldwide. Subsidence permanently reduces aquifer-system storage capacity, causes earth fissures, damages buildings and civil infrastructure, and increases flood susceptibility and risk. During the next decades, global population and economic growth will continue to increase groundwater demand and accompanying groundwater depletion (2) and, when exacerbated by droughts (3), will probably increase land subsidence occurrence and related damages or impacts. To raise awareness and inform decision-making, we evaluate potential global subsidence due to groundwater depletion, a key first step toward formulating effective land-subsidence policies that are lacking in most countries worldwide.

Full text

By Gerardo Herrera-García1,2,3, Pablo Ezquerro1,4, Roberto Tomás2,5, Marta Béjar-Pizarro1**,
Juan López-Vinielles1,6, Mauro Rossi7, Rosa M. Mateos1,3, Dora Carreón-Freyre2,8, John
Lambert2,9, Pietro Teatini2,10, Enrique Cabral-Cano2,11, Gilles Erkens2,12,13, Devin Galloway2,14, Wei-
Chia Hung2,15, Najeebullah Kakar2,16, Michelle Sneed2,17, Luigi Tosi2,18, Hanmei Wang2,19, Shujun Ye2,20
Subsidence, the lowering of Earth’s
land surface, is a potentially destruc-
tive hazard that can be caused by
a wide range of natural or anthro-
pogenic triggers but mainly results
from solid or fluid mobilization un-
derground. Subsidence due to groundwater
depletion (1) is a slow and gradual process
that develops on large time scales (months
to years), producing progressive loss of land
elevation (centimeters to decimeters per
year) typically over very large areas (tens
to thousands of square kilometers) and
variably affects urban and agricultural ar-
eas worldwide. Subsidence permanently
reduces aquifer-system storage capacity,
causes earth fissures, damages buildings and
civil infrastructure, and increases flood sus-
ceptibility and risk. During the next decades,
global population and economic growth will
continue to increase groundwater demand
and accompanying groundwater depletion
(2) and, when exacerbated by droughts (3),
will probably increase land subsidence oc-
currence and related damages or impacts. To
raise awareness and inform decision-mak-
ing, we evaluate potential global subsidence
due to groundwater depletion, a key first
step toward formulating effective land-sub-
sidence policies that are lacking in most
countries worldwide.
A large-scale systematic literature review
reveals that during the past century, land
subsidence due to groundwater depletion oc-
curred at 200 locations in 34 countries [see
supplementary materials (SM)]. However,
subsidence extent is only known for one-
third of these records, information on the
impacts is scarce, and mitigation measures
were implemented only in a few locations. In
China, widespread subsidence affects cities
developed in the main sedimentary basins.
In Indonesia, coastal subsidence in Jakarta
is so severe that government authorities are
planning to move the capital to the island of
Borneo. In Japan, subsidence affected sev-
eral cities during the 20th century, includ-
ing more than 4 m of subsidence in Tokyo,
before groundwater management practices
mitigated further subsidence. Iran currently
hosts some of the fastest-sinking cities in
the world (25 cm year--1) because of unregu-
lated groundwater pumping. In Europe, the
greatest impact of subsidence occurs in the
Netherlands, where subsidence is primarily
responsible for placing 25% of the country
below the mean sea level and increasing the
flooding risk. Subsidence in the Po River
Plain in Italy started during the second half
of the 20th century and currently threatens
30% of the Italian population, contributing
to recurrent coastal flooding during extreme
high tides in Venice. In North America, in-
tense groundwater depletion triggers sub-
sidence from California’s Central Valley,
with as much as 9 m of subsidence in the
past century, to the Atlantic and Gulf of
Mexico coastal plains in the United States,
where subsidence is increasing flooding risk.
In México, subsidence rates are among the
highest worldwide (as much as 30 cm year-1),
affecting small structurally controlled in-
termontane basins where the main urban
centers developed, causing an important but
unaccounted economic impact.
Spatial analysis of subsidence locations
identified in our global database (see SM)
reveals that subsidence has preferentially
occurred in very flat areas where unconsol-
idated sediments accumulated in alluvial
basins or coastal plains, and where urban
or agricultural areas developed in temperate
or arid climates characterized by prolonged
dry periods. Land subsidence has generally
occurred in water-stressed basins, where the
combination of groundwater withdrawal and
natural groundwater discharge outpaced
groundwater recharge, resulting in ground-
water storage losses, groundwater depletion,
and compaction of susceptible aquifer sys-
tems. In the affected basins, land subsidence
mainly occurred in highly populated areas,
with half of documented occurrences in ar-
eas susceptible to flooding. In coastal zones,
the combined effects of absolute sea-level
rise and land subsidence contribute to rela-
tive sea-level rise (4). The contribution from
land subsidence may exceed the contribu-
tion from absolute sea-level rise by a factor
of 10 or more and could be especially critical
for 21% of the geographic locations identified
in our database, where land elevation is less
than 1 m above the mean sea level.
On the basis of the spatial analysis find-
ings, a global model is proposed to combine
the main variables influencing subsidence to
identify environmental settings favoring land
subsidence and the anthropogenic factors
leading to groundwater depletion (see SM).
Statistical analyses of lithology, land-surface
slope, land cover, and Koppen-Geiger climate
classes are used to predict global subsidence
susceptibility at a spatial resolution of 1 km2.
The probability of groundwater depletion
is estimated by identifying urban and irri-
gated areas suffering water stress and where
groundwater demand is high.
The analyses do not consider subsidence
magnitude and rate, owing to the lack of
this information at a global scale. Hence,
the combination of subsidence susceptibility
and the probability of groundwater deple-
tion is used to predict a “proxy” of subsid-
ence hazard, which permits identification of
exposed areas where the probability of land
subsidence occurrence is high or very high.
Even though these results do not necessarily
translate to direct impacts or damages, they
are useful for identifying potential subsid-
ence areas where further local-scale analysis
is necessary. The comparison of our model
predictions with an independent validation
dataset reveals a 94% capability to distin-
guish between subsidence and nonsubsid-
ence areas, according to the value of the area
under the receiver operating characteristic
curve (see SM). The global exposure to po-
tential subsidence is evaluated by calculating
the number of inhabitants living in potential
subsidence areas, i.e., subsidence hazard
proxy, and the equivalent gross domestic
product (GDP). This “proxy” of exposed
assets is calculated assuming that GDP per
capita is homogeneous within each coun-
try. Finally, the evolution of potential global
subsidence and the related exposure is pre-
INSIGHTS
34 1 JANUARY 2021 • VOL 371 ISSUE 6524 sciencemag.org SCIENCE
GEOSCIENCE
Mapping the global threat of land subsidence
Nineteen percent of the global population may face a high probability of subsidence
POLICY FORUM
See supplementary materials for author affiliations.
Email: g.herrera@igme.es

1 JANUARY 2021 • VOL 371 ISSUE 6524 35SCIENCE sciencemag.org
GRAPHIC: N. DESAI/SCIENCE
dicted for 2040 for a global change scenario
based on steady population growth and in-
creasing greenhouse gas emissions (Shared
Socioeconomic Pathways 2, Representative
Concentration Pathway 8.5), which accounts
for the greatest sea-level rise projections.
Our results suggest that potential sub-
sidence threatens 12 million km2 (8%) of
the global land surface with a probability
greater than 50% (MH to VH in the figure).
Potential subsidence areas are concentrated
in and near densely urban and irrigated ar-
eas with high water stress and high ground-
water demand, overlying some of the larg-
est and most depleted aquifer systems (5)
in Asia (e.g., North China Plain) and North
America (e.g., Gulf of Mexico coastal plain);
coastal and river delta areas worldwide (e.g.,
Vietnam, Egypt, or the Netherlands); and
inland sedimentary basins of México, Iran,
and the Mediterranean countries. Potential
subsidence is lower in Africa, Australia, and
South America, owing to the lower ground-
water depletion (6). In central Africa, poten-
tial subsidence only includes information on
the susceptibility, as groundwater depletion
is unknown. In this region, subsidence sus-
ceptibility (see fig. S6) could be useful to pre-
vent subsidence impacts on developing cities
that during the next decades could rely more
on the available groundwater resources.
To evaluate the exposure to potential sub-
sidence, we focus on areas where the poten-
tial subsidence probability is high or very
high (see the figure). The cumulative po-
tential subsidence area amounts to 2.2
million km2, or 1.6% of the land; includes
1.2 billion inhabitants, or 19% of the global
population; and has an exposed GDP of
US$ 8.19 trillion, or 12% of the global GDP.
High-income countries account for 62%
of the global exposed GDP but only 11%
of the global exposed population, whereas
low-income countries account for 54% of
the global exposed population and 12% of
the global exposed GDP. It is expected that
the capability of low-income countries to
implement the political, regulatory, and
socioeconomic measures necessary to pre-
vent and mitigate subsidence impact will
be less than that for high-income countries.
Potential global subsidence
The color scale indicates the probability intervals classied from very low (VL) to moderately low (ML) to moderately high (MH) to very high (VH), for every 30-arcsec
resolution pixel (1 km by 1 km at the Equator). The white hatched polygons indicate countries where groundwater data is unavailable, and the potential subsidence only
includes information on the susceptibility. For details on data and methods, and for maps of other regions, see supplementary materials.
60ºN30ºN 0º
30ºS
120ºW 60ºW 0º 120ºE60ºE
45ºN40ºN 35ºN
30ºN
110ºE 120ºE 130ºE
30ºN 40ºN 35ºN
115ºW120ºW
East Asia
North America
110ºW
VL L ML MH H VH
Potential global subsidence
The color scale indicates the probability intervals classied from very low (VL) to moderately low (ML) to moderately high (MH) to very high (VH), for every 30-arcsec
resolution pixel (1 km by 1 km at the Equator). The white hatched polygons indicate countries where groundwater data is unavailable, and the potential subsidence only
includes information on the susceptibility. For details on data and methods, and for maps of other regions, see supplementary materials.
60ºN30ºN 0º
30ºS
120ºW 60ºW 0º 120ºE60ºE
45ºN40ºN 35ºN
30ºN
110ºE 120ºE 130ºE
30ºN 40ºN 35ºN
115ºW120ºW
East Asia
North America
110ºW
VL L ML MH H VH
Potential global subsidence
The color scale indicates the probability intervals classified from very low (VL) to very high (VH), for every 30-arcsec resolution pixel (1 km by 1 km
at the Equator). The white hatched polygons indicate countries where groundwater data is unavailable, and the potential subsidence only includes information
on the susceptibility. See maps of other regions in supplementary materials.

INSIGHTS |
PERSPECTIVES
36 1 JANUARY 2021 • VOL 371 ISSUE 6524 sciencemag.org SCIENCE
Potential subsidence threatens 484 million
inhabitants living in flood-prone areas, 75%
of whom live in fluvial areas and 25% of
whom live near the coast. This number of
threatened inhabitants corresponds to 50%
of the global population exposed to flooding
hazards according to previous estimates (7),
demonstrating the importance of consider-
ing potential subsidence in global flooding
risk analyses.
Most of the global population exposed
to potential subsidence live in Asia (86%),
which is about 10 times the combined ex-
posed population of North
America and Europe (9%).
The results indicate that 97%
of the exposed global pop-
ulation is concentrated in
30 countries (see SM). India
and China share the top two
rankings of potential subsid-
ence in terms of spatial ex-
tent and exposed population.
Egypt and the Netherlands
have the largest populations
living in potential subsid-
ence areas that are below
the mean sea level. The greatest population
densities in potential subsidence areas oc-
cur in Egypt and Indonesia, whereas the
relative exposure per country, measured as
the exposed population normalized by the
total population, is greater than 30% for
Egypt, Bangladesh, Netherlands, and Italy.
The United States ranks first in terms of
GDP exposed to potential subsidence, ow-
ing to its high GDP per capita.
Combination of the aforementioned
metrics permits derivation of a potential
subsidence index ranking (see SM). Seven
of the first ten ranked countries have the
greatest subsidence impact, accounting for
the greatest amount of reported damages
(Netherlands, China, USA, Japan, Indonesia,
México and Italy).
During this century, climate change will
cause serious impacts on the world’s wa-
ter resources through sea-level rise, more
frequent and severe floods and droughts,
changes in the mean value and mode of pre-
cipitation (rain versus snow), and increased
evapotranspiration. Prolonged droughts
will decrease groundwater recharge and in-
crease groundwater depletion, intensifying
subsidence. The global potential subsidence
is predicted for 2040 using the same sub-
sidence metrics and available global pro-
jections of water stress, water demand var-
iations, climate, and population (see SM).
Although predicted potential subsidence ar-
eas increase only by 7% globally, the threat-
ened population is predicted to rise by 30%,
affecting 1.6 billion inhabitants, 635 million
of whom will be living in flood-prone ar-
eas. These changes will not be homogene-
ous. Between 2010 and 2040, the predicted
population exposed to potential subsidence
increases more than 80% in the Philippines,
Iraq, Indonesia, México, Israel, Netherlands,
Algeria, and Bangladesh. The increase will
be moderate, less than 30%, for China, the
United States, Italy, and Iran. Potential sub-
sidence is forecasted to decrease in Japan
and Germany, owing to effective ground-
water management policies and population
declines. Finally, potential subsidence is pre-
dicted to emerge in high-latitude northern
countries like Canada and to
increase in extent in Russia
or Hungary, where climate
change will favor longer dry
seasons.
Further advancements in
the global evaluation of sub-
sidence can be made when a
global historical database on
subsidence rate, magnitude,
and extent has been com-
piled, which could be largely
sourced from continental
monitoring of surface dis-
placements using satellite radar imagery (8).
Widespread continuous monitoring of sub-
sidence will permit better evaluation of the
potential impact of land subsidence, espe-
cially in countries like Indonesia, México, and
Iran, where local studies revealed the highest
subsidence rates worldwide, but the national
dimension of subsidence is still unknown.
Further research also is necessary to evalu-
ate the cost of damage caused by current and
historical subsidence worldwide. The combi-
nation of damage information with hazard
estimates will permit improved assessments
of potential loss and design of cost-effective
countermeasures. Presently, annual subsid-
ence costs are only published for China (US$
1.5 billion) and the Netherlands (US$ 4.8
billion) (9). The greater subsidence costs in
the Netherlands owe to the exposed popula-
tion below the mean sea level and the large
investments made to prevent flooding. Our
model, which does not yet consider mitiga-
tion measures, likely overestimates potential
subsidence exposure in the Netherlands and
Japan, where groundwater management has
effectively controlled subsidence over the
past decades (10).
Our results identify 1596 major cities, or
about 22% of the world’s 7343 major cities
that are in potential subsidence areas, with
57% of these cities also located in flood-
prone areas. Moreover, subsidence threatens
15 of the 20 major coastal cities ranked with
the highest flood risk worldwide (11), where
potential subsidence can help delimit areas
in which flooding risk could be increased
and mitigation measures are necessary.
Overall, potential global subsidence re-
sults can be useful to better define the spa-
tial extent of poorly documented subsidence
occurrences, discover unknown subsiding
areas, prevent potential subsidence impacts
wherever groundwater depletion occurs,
and better identify areas where subsidence
could increase the flooding risk. In any
of these scenarios, an effective land-sub-
sidence policy should include systematic
monitoring and modeling of exposed ar-
eas, evaluation of potential damages, and
cost-benefit analyses permitting implemen-
tation of adequate mitigation or adaptation
measures. These measures should consider
groundwater regulation and strategic long-
term measures, such as the development
of alternative water supplies and the pro-
tection and (or) enhancement of natural or
artificial recharge of aquifers.
Considering that the potential subsidence
may affect 635 million inhabitants living in
flood-prone areas in 2040, it is of prime im-
portance that potential subsidence is quanti-
fied and systematically included in flood risk
analyses and related mitigation strategies.
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ACKNOWLEDGMENTS
Four anonymous peer reviewers and S. E. Ingebritsen (U.S.
Geological Survey) helped to improve the manuscript.
Funding for this study was provided partly by the Spanish
Research Agency (AQUARISK, PRX19/00065, TEC2017-
85244-C2-1-P projects) and PRIMA RESERVOIR project, and
by all the institutions represented in the Land Subsidence
International Initiative from UNESCO. G.H-G., P.E., R.T., M.B.-P,
and J.L.-V. designed the study, performed the analysis, and
wrote the initial manuscript with input from all other authors.
R.M.M., E.C.-C., and M.R. advised on the susceptibility
analysis. R.M.M., J.L., P.T., and G.E. advised on hazard analysis.
D.C.-F., J.L., P.T., E.C.C., G.E., D.G., W.C.H., N.K., M.S., L.T., H.W.,
and S.Y. advised on global exposure analysis. R.T., M.B.P.,
R.M.M., J.L., P.T., W.-C.H., N.K., L.T., H.W., and S.Y. contributed
essential data for the analysis. All the authors edited and
revised the manuscript through the different reviews. Any use
of trade, firm, or product names is for descriptive purposes
only and does not imply endorsement by the U.S. government.
The authors declare no competing interests. All data included
in this study are available at figshare (12).
SUPPLEMENTARY MATERIALS
science.sciencemag.org/content/371/6524/34/suppl/DC1
10.1126/science.abb8549
“Subsidence
threatens 15
of the 20 major
coastal cities
ranked with
the highest flood
risk worldwide.”