Coral Reefs

Source: Section; Karki, et al., http://

  • Coral reefs are the most diverse coastal ecosystems on earth and of disproportionate ecological, economic and food security importance to the Asia-Pacific region which has an inordinate proportion of the world’s healthy coral reefs.  Coral diversity is highest in the Asia-Pacific region.
  • The death of reef-forming corals undermines resilience of coastal communities, and can lead to the collapse of important coastal ecosystems.
  • One third of reef- building corals in the region are threatened.
  • Loss of habitat quality, heavy damage to entire reefs are major threats in the region. In the case of El Niño event in 1998, 16 per cent of the world’s coral reefs and 50 per cent of those in the Indian Ocean were destroyed
  • Increase in sea temperature and ocean acidification have been projected as major drivers of change along coastal environments which may lead to decline in coral reefs.
  • Increasing outbreaks of crown of thorns starfish, a native predator that has boom bust cycles linked to environmental pollution from farm lined estuaries affected The Great Barrier Reef.
  • Coral bleaching events are also increasingly devastating to the northern two thirds of the reef over the last few years where coral-algae associations are disrupted by high sea temperature.  Habitats and communities in the Great Barrier Reef ranged from poor to worsening at the end of 2015, although some species like green turtle populations improved.
  • Among the most serious emerging threats to coral reefs are coral diseases, which have devastated coral populations throughout the Caribbean since the 1980s and accompanied the mass coral bleaching there in 2005 and 2006. Over 90 per cent of the main reef forming corals in the Caribbean have now died due to coral disease with the severity of disease outbreaks commonly correlated with corals stressed by bleaching.  Coral diseases are also being observed more frequently on Indo-Pacific reefs in heretofore unrecorded places such the Great Barrier Reef, areas of Marovo Lagoon in the Solomon Islands and the Northwestern Hawaiian Islands. The outbreaks seem to be related to bacterial infections and other introduced disease organisms, increasing pollution, human disturbance and increasing sea temperature, all of which have put reef-forming corals at serious risk.

Marine Ecosystems

Marine ecosystems, from coastal to deep sea, now show the influence of human actions, with coastal marine ecosystems showing both large historical losses of extent and condition as well as rapid ongoing declines (established but incomplete) {,}

  • Direct exploitation of fish and seafood has the largest relative impact in the oceans (well established) {}.
  • The direct driver with the second highest impact on the oceans is the many changes in the uses of the sea and coast land (well established) {} (See Summary of Observed Changes)

Distribution Of Changes In The Global Ocean

Source:  Halpern  (B. S., Walbridge, S., Selkoe, K. A., Kappel, C. V., Micheli, F., D’Agrosa, C., … Watson, R. (2008). A Global Map of Human Impact on Marine Ecosystems. Science, 319(5865), 948–952.doi:10.1126/science.1149345 

1.  Every square kilometer of the ocean is affected by some anthropogenic driver of ecological change

2.  Over 41% of the world’s oceans have medium to very high cumulative impacts:

  • North and Norwegian seas, South and East China seas, Eastern Caribbean, North American eastern seaboard, Mediterranean, Persian Gulf, Bering Sea, and the waters around Sri Lanka.

3.  Lowest impact areas include high-latitude Arctic and Antarctic poles, but this may change due to future polar ice loss

4.  Ecosystems with highest impacts include hard and soft continental shelves and rocky reefs, and coral reefs.

Summary of Observed Changes

Temperature Changes

  •  Rising sea surface temperatures; reduction in oxygen content; stratification; coral bleaching
  • Heat content increasing deeper into the ocean
  • Global ocean animal biomass consistently declines with climate change, and that these impacts are amplified at higher trophic levels (Source: Lotze, et al., Global ensemble projections reveal trophic amplification of ocean biomass declines with climate change. Ocean biomass declines with climate change
Source: Ocean biomass declines with climate change

Rising Sea Levels (“Looking past the horizon of 2100” Nature Climate Change)

  • Thermal expansion (warmer water expands)
  • Increased runoff from land-based ice sheets
    • Greenland
    • Antarctica
    • Mountain Glaciers

Changes in the Salinity of Seawater

  • Changes to the thermohaline circulation
  • Increased likelihood of seawater stratification

Threats to Coastal Populations and Infrastructure

  • Salt-water intrusion
  • Drowning forests
  • Inland migration of wetlands

Arctic Sea Ice is Shrinking

  • Warming air temperatures increase rate of melting
  • Positive feedback albedo loop

Increasing Ocean Acidity

  • Absorption of CO2 from atmosphere
  • Impacts to shell-forming species, including coralsAbsorption of CO2 from atmosphere

Land-Based Impacts

1.  Inorganic and organic pollutants

2.  Nutrient runoff (point and non-point) and associated hypoxia

Gulf of Mexico Dead Zone

3.  Aquaculture

4.  Invasive species

5.  Habitat destruction and alteration; e.g., coastal engineering; tourist development;  salt marshes, mangroves, seagrass beds, coral reefs

6.  Input and transfer of waterborne pathogens (runoff, aquaculture)

7.  Air pollution

a.  Toxic chemicals; e.g., POP’s over the Arctic zone by airborne volatilization;

b.    Mercury                          

Source:  Lavoie, R.A., Bouffard, A., Maranger, R., Amyot, M., 2018. Mercury transport and human exposure from global marine fisheries. Sci. Rep. 8, 6705.

Significance:  Human activities have increased the global circulation of mercury, a potent neurotoxin. Mercury can be converted into methylmercury, which biomagnifies along aquatic food chains and leads to high exposure in fish-eating populations.

Trends:  Mercury export from the ocean increased over time as a function of fishing pressure, especially on upper-trophic-level organisms. In 2014, over 13 metric tonnes of mercury were exported from the ocean. Asian countries were important contributors of mercury export in the last decades and the western Pacific Ocean was identified as the main source.

Estimates of per capita mercury exposure through fish consumption showed that populations in 38% of the 175 countries assessed, mainly insular and developing nations, were exposed to doses of methylmercury above governmental thresholds.

Given the high mercury intake through seafood consumption observed in several understudied yet vulnerable coastal communities, we recommend a comprehensive assessment of the health exposure risk of those populations.

Temporal trends (1950‒2014) of marine fisheries catches (dashed lines) and mercury exported (full lines). (a) Entire ocean (blue) and coastal (red) and high seas (orange) with breakpoint (circle) ± standard error (horizontal line). (b) Entire ocean (blue) and trophic levels (TL) rounded to the nearest integer: 4 (red), 3 (orange), and 2 (green).

Nutrients (N,P): Eutrophication, Dead Zones

Plastics (shipping, fishing, land-based sources)

  • Microplastics (nanoparticles; food chain)
  • Macroplastics (ingestion, entanglement, habitat loss (shorelines))
    • Great Pacific Garbage Patch

Source:   Lebreton, L, Slat, B, Ferrari, F, Sainte-Rose, B, Aitken, J, Marthouse, R, Hajbane, S, Cunsolo, S, Schwarz, A, Levivier, A, Noble, K, Debeljak, P, Maral, H, Schoeneich-Argent, R, Brambini, R & Reisser, J 2018, ‘Evidence that the Great Pacific Garbage Patch is rapidly accumulating plastic‘, Scientific Reports, vol. 8, no. 4666.

  • Ocean plastic can persist in sea surface waters, eventually accumulating in remote areas of the world’s oceans.
  • Our model, calibrated with data from multi-vessel and aircraft surveys, predicted at least 79 (45–129) thousand tonnes of ocean plastic are floating inside an area of 1.6 million km2; a figure four to sixteen times higher than previously reported.
  • Over three-quarters of the GPGP mass was carried by debris larger than 5 cm and at least 46% was comprised of fishing nets. Microplastics accounted for 8% of the total mass but 94% of the estimated 1.8 (1.1–3.6) trillion pieces floating in the area.
  • Plastic collected during our study has specific characteristics such as small surface-to-volume ratio, indicating that only certain types of debris have the capacity to persist and accumulate at the surface of the GPGP.

Finally, our results suggest that ocean plastic pollution within the GPGP is increasing exponentially and at a faster rate  than the surrounding waters.

Decadal evolution of microplastic concentration in the GPGP. Mean (circles) and standard error (whiskers) of microplastic mass concentrations measured by surface net tows conducted in different decades, within (light blue) and around (dark grey) the GPGP. Dashed lines are exponential fits to the averages expressed in g km−2: f(x) = exp(a*x) + b, with x expressed in number of years after 1900, a = 0.06121, b = 151.3, R2 = 0.92 for within GPGP and a = 0.04903, b = −7.138, R2 = 0.78 for around the GPGP.

Land Loss in Coastal Areas

Source:  Mentaschi, et al., Global long-term observations of coastal erosion and accretion, Scientific Reports volume 8, Article number: 12876 (2018)

On a global scale, between 1984 and 2015, the loss of permanent land in coastal areas amounts to almost 28,000 km2, roughly equivalent to the surface area of Haiti (Fig. a). This is almost twice as large as the surface of gained land (about 14,000 km2) over the same period. On the other hand, the overall surface of gained active zone (about 25,000 km2) is more than two times larger than the surface of lost active zone (about 11,500 km2). Overall, the gain of active zone roughly balances the loss of land, and the gain of land balances the loss of active zone. This translates into a net loss of approximately 14,000 km2 of surface for human settlements and terrestrial ecosystems (Fig. b).

Source:  Mentaschi, et al., Global long-term observations of coastal erosion and accretion, Scientific Reports volume 8, Article number: 12876 (2018)
On a global scale, between 1984 and 2015, the loss of permanent land in coastal areas amounts to almost 28,000 km2, roughly equivalent to the surface area of Haiti (Fig. a). This is almost twice as large as the surface of gained land (about 14,000 km2) over the same period. On the other hand, the overall surface of gained active zone (about 25,000 km2) is more than two times larger than the surface of lost active zone (about 11,500 km2). Overall, the gain of active zone roughly balances the loss of land, and the gain of land balances the loss of active zone. This translates into a net loss of approximately 14,000 km2 of surface for human settlements and terrestrial ecosystems (Fig. b).

Size distribution of cross-shore transition length above 50 m, for erosion and accretion of land (a) and active zone (b). Lost (c) and gained (d) land around the world; lost (f) and gained (g) active zone; balance gained – lost land (e) and active zone (h). Maps show the length of cross-shore erosion and accretion aggregated on coastal segments of 100 km. In all the maps the 4 spots with the highest local transition along a 250 m transect are indicated.

Drivers of shoreline change around the world

List of local cases of erosion/accretion discussed in Mentaschi, et al The legend provides for each spot a brief summary of the drivers of shoreline change
Land Loss in Coastal Areas: Shoreline modification “masks” true rates of shoreline change along the Atlantic Seaboard

Source:  Armstrong, S. B., &  Lazarus, E. D. 2019).  Masked shoreline erosion at large spatial scales as a collective effect of beach nourishmentEarth’s Future,  7,  74– 84

 (R)ecent rates of shoreline change along the U.S. Atlantic Coast are, on average, less erosional than historical rates. This shift has occurred despite evidence of intensified environmental forcing, including acceleration in rates of relative sea‐level rise and increased significant wave height in offshore wave climates. We suggest that the use, since the 1960s, of beach nourishment as the predominant form of mitigation against chronic coastal erosion in the United States…could explain the unexpected reversal in shoreline‐change trends.

Using U.S. Geological Survey shoreline records from 1830–2007 spanning more than 2,500 km of the U.S. Atlantic Coast, we calculate a mean rate of shoreline change, prior to 1960, of −55 cm/year (a negative rate denotes erosion). After 1960, the mean rate reverses to approximately +5 cm/year, indicating widespread apparent accretion despite steady (and, in some places, accelerated) sea‐level rise over the same period.

Cumulative sediment input from decades of beach‐nourishment projects may have sufficiently altered shoreline position to mask “true” rates of shoreline change. Our analysis suggests that long‐term rates of shoreline change typically used to assess coastal hazard may be systematically underestimated. We also suggest that the overall effect of beach nourishment along of the U.S. Atlantic Coast is extensive enough to constitute a quantitative signature of coastal geoengineering and may serve as a bellwether for nourishment‐dominated shorelines elsewhere in the world.

Oxygen Minimum Zones

Declining Oxygen in the World’s Ocean and Coastal Waters

Source for the following material unless otherwise noted:   Breitburg, D., Grégoire, M. and Isensee, K. (eds.). Global Ocean Oxygen Network 2018. The ocean is losing its breath: Declining oxygen in the world’s ocean and coastal waters. IOC-UNESCO, IOC Technical Series, No. 137 40pp.  The Ocean is Losing its Breath


  1. Insufficient oxygen reduces growth, increases disease, alters behaviour and increases mortality of marine animals, including finfish and shellfish. The quality and quantity of habitat for economically and ecologically important species is reduced as oxygen declines.
  2. Finfish and crustacean aquaculture can be particularly susceptible to deoxygenation because animals are constrained in nets or other structures and cannot escape to highly-oxygenated water masses.
  3. Deoxygenation affects marine biogeochemical cycles; phosphorus availability, hydrogen sulphide production and micronutrients are affected.
  4. Deoxygenation may also contribute to climate change through its effects on the nitrogen cycle. When oxygen is insufficient for aerobic respiration, microbes conduct denitrification to obtain energy. This produces N2O – a powerful greenhouse gas – as well as N, which is inert and makes up most of the earth’s atmosphere.

Impacts of Excess Nutrients (eutrophication) on ocean oxygen

Nutrients – primarily nitrogen and phosphorus – from human waste, agriculture and industry, fertilize coastal waters. In a process called eutrophication, these nutrients stimulate photosynthesis, which increases the growth of algae and other organisms (See figure below). This results in more organic material sinking into deep water and to the sediment. Increased respiration by animals and many microbes eating or decomposing this organic material uses oxygen. The consequence can be oxygen concentrations that are far lower than those that would occur without human influence, and in some cases a complete lack of oxygen in bottom waters. Strong density differences between surface and bottom waters (referred to as ‘stratification’), due to temperature and salinity, can isolate bottom waters from the atmosphere and reduce or prevent re- oxygenation through ventilation. Semi-enclosed seas (e.g. the Black and Baltic Sea) can be sensitive to eutrophication and related deoxygenation because of their characteristic limited water exchange with the open ocean, and low ventilation rates.

Eutrophication:  nutrient flux of nitrogen and phosphorus from human waste, agriculture and industry, fertilize coastal waters

Impacts of excess nutrients (eutrophication) on ocean oxygen. (Figure modified from wikipedia/commons/d/dd/Scheme_eutrophication-en.svg)

Deoxygenation Facts

Oxygen Minimum Zones (OMZs)

OMZs are places in the world ocean where oxygen saturation in the water column is at its lowest, are shown in blue.  Areas with coastal hypoxia are shown in red.   Hypoxic conditions are often defined as 2 mg/L O2.

The number of water bodies in which hypoxia associated with eutrophication has been reported has increased exponentially since the 1960s; hundreds of systems worldwide have been reported with oxygen concentrations <2 mg L-1, lasting from hours to years. The increasing severity and prevalence of this problem reflects the invention and increasing use of synthetic fertilizers and the growing human population. Global fertilizer use shown includes data of fertilizers with nitrogen, phosphate and potash.

World’s two largest dead zones

Source:  European Environment Agency Ocean Oxygen Content Indicator Assessment 

Distribution of oxygen-depleted ‘dead zones’ in European seas

Oxygen-depleted zones in the Baltic Sea have increased more than 10-fold, from 5 000 to 60 000 km2, since 1900, with most of the increase happening after 1950. The Baltic Sea now has the largest dead zone in the world. Oxygen depletion has also been observed in other European seas in recent decades.

The Gulf of  Mexico dead zone is the world’s second largest in the world

Source:  EPA Gulf of Mexico Dead Zone

(The Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, a group working to reduce the Gulf dead zone through nutrient reductions within the Mississippi River watershed, has set a 5-year average measured size target of 1,900 square miles.)

2019 Forecast: Summer Hypoxic Zone Size, Northern Gulf of Mexico

NOAA and the United States Geological Survey (USGS) released their 2019 forecast for the summer hypoxic zone size in the Northern Gulf of Mexico on June 10, 2019. Scientists are expecting the 2019 area of low oxygen, commonly known as the ‘Dead Zone,’ to be approximately 7,829 square miles, or about the size of Massachusetts. This prediction is large primarily because of high spring rainfall and river discharge into Gulf.

Biogeochemical Flows: Nitrogen

Planetary Boundary

  • Global boundary: 62 Tg N/year
  • Source: *Steffen, W., K. Richardson, J. Rockström, S.E. Cornell, 2015. Planetary boundaries: Guiding human development on a changing planet. Science 347: 736, 1259855
  • Overall, the fixation of nitrogen through Haber–Bosch (120 Tg N yr−1) in 2010 was double the natural terrestrial sources of Nr (63 Tg N yr−1).
  • The overall magnitude of anthropogenic relative to natural sources of fixed nitrogen (210 Tg N yr−1 anthropogenic and 203 Tg N yr−1natural) is so large it has doubled the global cycling of nitrogen over the last century. 
  • The overall magnitude of anthropogenic relative to natural sources of fixed nitrogen (210 Tg N yr−1 anthropogenic and 203 Tg N yr−1 natural) is so large it has doubled the global cycling of nitrogen over the last century.
  • Source: Fowler, et al., The global nitrogen cycle in the twenty-first century, Philos Trans R Soc Lond B Biol Sci. 2013 Jul 5; 368(1621): 20130164.


The supply of Nr–reactive nitrogen (NH3, NH4, NO, NO2, HNO3, N20, HONO, PAN and other organic N compounds) is essential for all life forms, and increases in nitrogen supply have been exploited in agriculture to increase the yield of crops and provide food for the growing global human population. It has been estimated that almost half of the human population at the beginning of the twenty-first century depends on fertilizer N for their food. (Source: Fowler, et al.)

As nitrogen is a major nutrient, changes in its supply influence the productivity of ecosystems and change the competition between species and biological diversity. Nitrogen compounds as precursors of tropospheric ozone and atmospheric particulate material also degrade air quality. Their effects include increases in human mortality, effects on terrestrial ecosystems and contribute to the radiative forcing of global and regional climate. (Source: Lee, et al., see figure below)

Global N budget. Numbers represent global land N storage in TgN or annual N exchange fluxes in TgN yr−1 for contemporary (1991–2005 average) and preindustrial (1831–1860 average in parenthesis) times. See notes for this figure and the table below in:  Lee, et al., Prominence of the tropics in the recent rise of global nitrogen pollution.  Nature Communications (2019)10:1437  and Supplementary Table 1 (modified below) and Supplementary Note 1 ( 019-09468-4.)
Nitrogen Stores & Fluxes Published Estimates 1990’s Published Estimates 2000’s
Biological N Fixation 112; 139  
Agricultural 32 50-70
Preindustrial 58; 195  
Non-agricultural NA  
Natural 107; 128  
Atmospheric Deposition 59  
Haber-Bosch (Synthetic fetilizers) 100 120
Fluxes to the ocean 48 45
Fluxes to the atmosphere 189  
Denitrification N2 115 96
Other emissions 74 70
Fluxes to the land storage 60 27
Soils/litter storage 95,000  
Fluxes:  TgN/year    
Storage:  TgN    

Biogeochemical Flows: Phosphorus

Planetary Boundary

  • Global P Boundary: 11 Tg P/year*
  • Regional (watershed) P Boundary: 6.2 Tg P/year*
  • Current global rate of P fertilizer to croplands* (primary source of P to regional watersheds): 14.2 Tg P/year*
    • Total P flow through international agricultural trade increased from 0.4Tg to 3.0 Tg between 1961-2011**
    • The fraction of P taken up by crops that is subsequently exported increased from 9% to 20% between 1961 and 2011**
    • Global P flows through international trade of agricultural products have become an important feature of the global P cycle, accounting for 20% of the P in global crop production, 17% of the P globally used as mineral fertilizer, and 27% of the P that was traded as mineral fertilizers in 2011.**
  • (Sources: *Steffen, W., K. Richardson, J. Rockström, S.E. Cornell, 2015. Planetary boundaries: Guiding human development on a changing planet. Science 347: 736, 1259855, **Nesme, T.,  G.S. Metson, and E.M. Bennett. 2018. Global phosphorus flows through agricultural trade. Global Environ. 50:133–141.Change 50:133–141. doi:10.1016/j.gloenvcha.2018.04.004

*Phosphorus and Agricultural Trade

Global phosphorus fertilizer application to cropland


  • Critical element for all living organisms
  • Availability drives the productivity of many aquatic and terrestrial ecosystems worldwide
  • In agricultural systems, additional P can be supplied to soils as mineral fertilizer or manure to support crop growth and sustain high yields
  • Mineral P fertilizer production is dependent on the physical and economic availability of mined rock phosphate resources (non-renewable, diminishing, geopolitically concentrated)
    • The P cycle has been greatly transformed since the pre-Industrial era through increased agricultural mineral P fertilizer use
  • P losses to water bodies through runoff and erosion from fertilized agricultural soils and from the inadequate management of animal manure or human excreta has led to aquatic eutrophication
  • International trade of agricultural products (food, feed, fiber and fuel) are a key component of the global phosphorus cycle; agricultural flows of P are driven by trade of cereals, soybeans, and feed cakes
  • 28% of global P traded in human food, 44% in animal feed and 28% in crops for other uses in 2011

Water Supply and Sanitation

Binh thanh district, Ho Chi Minh, Vietnam  Photo by Anh Vy on Unsplash
  • The percentage of global population using at least a basic drinking water service rose from 81 to 89% between 2000 and 2015
  • 3 out of 10 (2.1 billion; 29% global population) did not have a safely managed drinking water service in 2015
  • 844 million still lacked even a basic drinking water service
  • Water-related deaths impact thousands and costs billions

Average annual impact from inadequate drinking water and sanitation services, water-related disasters. Adapted from WWAP (UNESCO World Water Assessment Programme). 2019. The United NationsWorld Water Development Report 2019: Leaving No One Behind. Paris, UNESCO

In 2015, an estimated 2.1 billion people lacked access to safely managed drinking water services and 4.5 billion lacked access to safely managed sanitation services. (WWAP)

Almost half of people drinking water from unprotected sources live in Sub-Saharan Africa, where the burden of collecting water lies mainly on women and girls, many of whom spend more than 30 minutes on each trip to collect water. (WWAP)

Proportion of population using at least basic drinking water services, 2015
Source: WWAP (UNESCO World Water Assessment Programme). 2019. The United NationsWorld Water Development Report 2019: Leaving No One Behind. Paris, UNESCO


  • Worldwide, only 2.9 billion people (39% of global population) used safely managed sanitation services in 2015.  40% of these people lived in rural areas.
  • 2.1 billion people had access to “basic” sanitation services.
  • 2.3 billion (one out of every three people) lacked even a basic sanitation service—nearly 1 billion people (892 million) still practiced open defecation.
Photo: SuSanA Secretariat. Creative Commons BY (cropped)

Global and regional sanitation coverage, 2015.  Source:  The United Nations World Water Development Report 2019: Leaving No One Behind. Paris, UNESCO, 2019.


World’s largest distributed store of freshwater


  • Total groundwater volume in the upper 2 km of continental crust is approximately 22.6 million km3 of which 0.1-5.0 million km3 is less than 50 years old1
  • The volume of modern groundwater is equivalent to a body of water with a depth of about 3 meters spread over the continents2
  • Groundwater replenished over a human lifetime of 25-100 years is a finite, limited resource with a spatially heterogeneous distribution dependent on geographic, geologic and hydrologic conditions3
  • Accounts for as much as 33% of total global water withdrawals4
  • Over 2 billion people rely on groundwater as their primary water source5
  • 50% or more of the irrigation water used to grow the world’s food is supplied by groundwater6

            Source:  1,2,3–Gleeson, T., Befus, K. M., Jasechko, S., Luijendijk, E., & Cardenas, M. B.   (2015). The global volume and distribution of modern groundwater. Nature           Geoscience, 9(2), 161–167.doi:10.1038/ngeo2590 

            4,5,6–Famiglietti, J. S. (2014). The global groundwater crisis. Nature Climate Change,   4(11), 945–948.doi:10.1038/nclimate2425 

Groundwater Use by Country, in cubic kilometers per year

Source:  Fienen M.N., Arshad M. (2016) The International Scale of the Groundwater Issue. In: Jakeman A.J., Barreteau O., Hunt R.J., Rinaudo JD., Ross A. (eds) Integrated Groundwater Management. Springer, Cham, DOI The International Scale of the Groundwater Issue

37 Largest Aquifer Systems

Source: Map and table):  Richey, A. S., B. F. Thomas, M.-H. Lo, J. T. Reager, J. S. Famiglietti, K. Voss, S. Swenson, and M. Rodell (2015), Quantifying renewable groundwater stress with GRACE, Water Resour. Res.,51, 5217–5238, doi:10.1002/2015WR017349
Aquifer Map # Aquifer Name
1 Nubian Aquifer System (NAS)
2 Northwestern Sahara Aquifer System (NWSAS)
3 Murzuk-Djado Basin
4 Taoudeni-Tanezrouft Basin
5 Senegalo-Mauritanian Basin
6 Iullemeden-Irhazer Aquifer System
7 Lake Chad Basin
8 Sudd Basin (Umm Ruwaba Aquifer)
9 Ogaden-Juba Basin
10 Congo Basin
11 Upper Kalahari-Cuvelai-Upper Zambezi Basin
12 Lower Kalahari-Stampriet Basin
13 Karoo Basin
14 Northern Great Plains Aquifer
15 Cambro-Ordovician Aquifer System
16 Californian Central Valley Aquifer System
17 Ogallala Aquifer (High Plains)
18 Atlantic and Gulf Coastal Plains Aquifer
19 Amazon Basin
20 Maranhao Basin
21 Guarani Aquifer System
22 Arabian Aquifer System
23 Indus Basin
24 Ganges-Brahmaputra Basin
25 West Siberian Basin
26 Tunguss Basin
27 Angara-Lena Basin
28 Yakut Basin
29 North China Aquifer System
30 Song-Liao Basin
31 Tarim Basin
32 Paris Basin
33 Russian Platform Basins
34 North Caucasus Basin
35 Pechora Basin
36 Great Artesian Basin
37 Canning Basin

Global Aquifer Depletion Hotspots in Arid and Semi-arid Zones

Aquifer Country
Northwest Sahara      #2 Algeria, Libya, Tunisia

California Central Valley 
High Plains (Ogallala) #17 USA
Guarani                    #21 Argentina, Brazil, Paraguay, Uruguay
Northern Middle East Iran, Iraq, Syria, Turkey
Arabian                      #22 Iraq, Jordan,Oman,Qatar,Saudi Arabia,
UAE, Yemen
Northwestern India #23 India, Pakistan
North China Plain      #29 China
Canning Basin         #37 Australia

Adapted from:  Famiglietti, J. S. (2014). The global groundwater crisis. Nature Climate Change, 4(11), 945–948.doi:10.1038/nclimate2425 

Highlights from Global Aquifer Hotspots: Central Valley, California (#16)

  • Most productive agricultural area in the United States
  • 40%+ of Central Valley’s water supply comes from gw, primarily to meet agricultural demand
  • Groundwater use increased from 0.75 to 2.5 km3 per year between 1962-2003
  • 2007-2010 severe drought caused a permanent aquifer system storage loss of ~2%, due to irreversible compaction of the aquifer system
    • Source: Ojha, et al., Sustained Groundwater Loss in California’s Central Valley Exacerbated by Intense Drought Periods, Water Resources Research, June 2018, 54(5575). Doi:  10.1029/2017WR022250)Most productive agricultural area in the United States

May 7, 2015 – September 10, 2016
Several trouble spots that were identified in 2015 have continued to subside at rates as high as 0.6 meters (2 feet) per year. Significant subsidence was measured in subsidence bowls near the towns of Chowchilla, south of Merced; and Corcoran, north of Bakersfield. These bowls cover hundreds of square kilometers and continued to grow wider and deeper between May 2015 and September 2016. Subsidence also intensified near Tranquility in Fresno County, where the land surface has settled up to 51 centimeters (20 inches) in an area that extends 11 kilometers (7 miles).  NASA Earth Observatory map by Joshua Stevens, using modified Copernicus Sentinel SAR data (2016) courtesy of Tom Farr and Cathleen Jones, NASA Jet Propulsion Laboratory. Caption by Alan Buis (Jet Propulsion Laboratory) and Ted Thomas, California Department of Water Resources; edited by Mike Carlowicz.

Declining groundwater storage appears to be part of the Central Valley’s future

Central Valley groundwater storage trends and simulations
The GWD Model simulations for both the past and future years. The simulated groundwater storage changes for the years 1980–2014 (Blue line) are compared with observations (red icons). In future years, several precipitation scenarios are examined.
Source:  Massoud, E., et al., Projecting groundwater storage changes in California’s Central Valley, Scientific Reports 8(1), August 2018. Doi:10.1038/s41598-018-31201-1
Highlights from Global Aquifer Hotspots: High Plains Aquifer (United States) #17 (The International Scale of the Groundwater Issue)
  • One of the largest gw aquifers in the world (8 states; 450,000 km2)
  • Provides drinking water for 2.3 million people
  • Extensively used for agricultural production
  • Storage decline from 1950-2007 accounts for 36% of total gw depletion during 1900-2008
  • Depletion is highly variable spatially; very high depletion rates in the southern and central HP, large areas of Texas and Kansas. Low/no depletion in the northern HP aquifer (#14)

November 1, 1998-October 331, 2016
The High Plains Aquifer, also known as the Ogallala Aquifer, is under stress. Farmers today have to drill ever deeper wells in order to pump water for irrigation, and one recent study found the aquifer to be under more strain than any other in the United States. About 30 percent of the water once stored beneath Kansas is already gone, and another 40 percent will be gone within 50 years if current trends continue.  The map at the left of this page shows irrigation frequency in the basin between 1999 and 2016. Areas watered nearly every year are purple; those watered only rarely are yellow. The extent of the Ogallala Aquifer is shown with gray. The second image highlights the variability in irrigation between center-pivot irrigation fields in an area along the Colorado-Nebraska border. The most widely grown crops in the basin are corn and wheat.  NASA Earth Observatory maps by Lauren Dauphin, using data from Deines, Jillian, et al. (2017). Story by Adam Voiland.
Highlights from Global Aquifer Hotspots: Northwest India (#23) The International Scale of the Groundwater Issue
  • India is the world’s largest consumer of gw
  • 1/3 of India’s aquifers are overexploited, semicritical, or critical
  • 70% of India’s agricultural production depends on gw
  • GW is the primary source (85%) of drinking water for rural areas
  • GW is typically extracted by tubewells (~15 million)
  • In Northwestern India (Rajasthan, Punjab, Haryana, including Delhi) gw has been unsustainably depleted between 2000-2008 in an amount equivalent to double the capacity of India’s largest surface-water reservoir (Rodell, et al., 2009)
  • This region has a population of approximately 114,000,000.
Highlights from Global Aquifer Hotspots: North China Plain (#29) The International Scale of the Groundwater Issue
  • Region supplies nearly half of China’s wheat and one-third of other cereal grains
  • Population is more than 200,000,000
  • Rapid groundwater depletion since the 1980’s
  • Saltwater intrusion into the freshwater aquifer due to groundwater depletion has salinized 44% of the total area between the coastal plain and Laizhou city.

Endorheic Basins

What remains of the large inland lake is a fraction of what it was in the 1950s and 60s. In those years, the government of the former Soviet Union diverted so much water from the Amu Darya and Syr Darya—the regions’ two major rivers—to irrigate farmland, that it pushed the hydrologic system beyond the point of sustainability. During subsequent decades, the fourth largest lake in the world shrank to roughly a tenth of its former size and divided into several smaller bodies of water.

Source: Endorheic global decline

Definition:  surface flow is landlocked from the ocean


  • Cover 20% of the Earth’s land surface, and nearly 50% of its water-stressed regions
  • Decreases appear to be related to recent climate conditions in conjunction with direct human activities

Terrestrial Water Storage Changes in Endorheic Basins April 2002-March 2016

Adapated from Table 1 Endorheic global decline
*Terrestrial Water Storage = Groundwater + soil moisture + surface waters + snow + ice

River Basin Modifications

Aswan High Dam, April 12, 2015  NASA Earth Observatory.  Astronaut photograph ISS043-E-101953 was acquired on April 12, 2015

Environmental Significance

Source:  Grill, et al., Mapping the world’s free-flowing rivers, Nature 569(7755):215-     221, doi: 10.1038/s41586-019-1111-9

  • Destruction/separation of floodplains from rivers alters ecosystem services such as natural flood storage, nutrient retention and flood-recession agriculture
  • Built river infrastructure has been linked to declines in terrestrial and freshwater species
  • Sediment capture by dams may cause alteration of the geomorphic dynamics of rivers and the shrinking of river deltas worldwide
  • Inland fisheries provide the equivalent of all dietary animal protein for 158 million people worldwide
  • Only 37% of rivers longer than 1,000 kilometers remain free-flowing in their entire length
  • Only 23% of rivers longer than 1,000 kilometers flow uninterrupted to the ocean
  • Very long free-flowing rivers are largely restricted to remote regions of the Arctic and of the Amazon and Congo basins
  • In densely populated areas, only a few very long rivers remain free-flowing, such as the Irawaddy and Saleen

Dams and Reservoirs

  • the leading contributors to the loss of river connectivity
  • There are approximately 2.8 million dams (with reservoir areas >1000 cubic meters) regulating and creating over 500,000 kilometers of rivers and canals for navigation and transport and building irrigation and water-diversion schemes
  • More than 3,700 hydropower dams (>1MW) are currently planned or under construction worldwide.
2019 Hydropower Status Report
More than 21.8 Gw of hydroelectric capacity was put into operation in 2018
2019 Hydropower Status Report
Electricity generation from hydropower projects achieved a record 4,200 terawatt hours (TWh) in 2018, the highest ever contribution from a renewable energy source, as worldwide installed hydropower capacity climbed to 1,292 GW, according to the 2019 Hydropower Status Report
China added the most capacity with the installation of 8,540 megawatts, followed by Brazil (3,866 MW), Pakistan (2,487 MW), Turkey (1,085 MW), Angola (668 MW), Tajikistan (605 MW), Ecuador (556 MW), India (535 MW), Norway (419 MW) and Canada (401 MW).
Brazil has now overtaken the United States as the second largest producer of hydroelectricity by installed capacity, after 3,055 MW was put into operation last year at the 11,000 MW Belo Monte complex in the country’s northeast.

Small Hydropower Plants (SHP)

Source:  Couto, T. B., & Olden, J. D. (2018). Global proliferation of small hydropower plants – science and policy. Frontiers in Ecology and the Environment, 16(2), 91–100.doi:10.1002/fee.1746 

Definition:  Considerable variability in definition across countries; refers broadly to facilities that produce less electricity and operate in smaller rivers as compared to large hydropower plants.  Vary greatly in level of flow control, storage capacity, diversion structures.  70% of countries classify as installations with less than 10 MW capacity


  • There are close to 11 SHPs for every large hydropower plants (LHPs) = 11% of global hydropower electricity generation
  • 82,891 SHPs are operating or under construction in 150 countries
  • 181,976 new plants may be installed if all potential capacity were developed
  • 10,569 new projects appear in national plans
  • China has the world’s largest number of SHPs—47,073; 57% of the world’s SHPs
  • Europe has 26,877 SHPs
  • Future plans for SHPs are concentrated in Asia, the Americas, Southern and Eastern Europe and East Africa

            Environmental Impacts

  • Similar to LHPs; but SHPs generally occur in smaller rivers, which is significant given the ecological importance of headwater streams
  • Cumulative ecological impacts of SHPs (multiple installations in the same river basin) appears to be an underappreciated issue

Freshwater Planetary Boundary

Freshwater consumption and the global hydrological cycle

“The freshwater cycle is strongly affected by climate change and its boundary is closely linked to the climate boundary, yet human pressure is now the dominant driving force determining the functioning and distribution of global freshwater systems. The consequences of human modification of water bodies include both global-scale river flow changes and shifts in vapour flows arising from land use change. These shifts in the hydrological system can be abrupt and irreversible. Water is becoming increasingly scarce – by 2050 about half a billion people are likely to be subject to water-stress, increasing the pressure to intervene in water systems.  A water boundary related to consumptive freshwater use and environmental flow requirements has been proposed to maintain the overall resilience of the Earth system and to avoid the risk of ‘cascading’ local and regional thresholds.”
Stockholm Resilience Planetary Boundaries


  • Two control variables:
    • Global–Maximum amount of consumptive blue water use
      • 4000 cubic km/year (4000-6000 km3/yr)
    • Regional (River Basin)–Blue water withdrawal as % of mean monthly river flow
  • Current Global value (river basin not determined): 2600 km3/year

Hydrological Cycle in the Anthropocene

Source:  (Global Hydrological Cycle in the Anthropocene) This commonly reproduced image from the USGS of the averaged depiction of the hydrological cycle does not represent important seasonal and interannual variation in many pools and fluxes.
A hydrologic cycle in the Anthropocene should include:

  • Include anthropogenic influences
  • Include global teleconnections
  • Multiple catchments
  • Endorheic basins
  • Estimates of green, blue, and grey water use

Estimates of Global Pools and Fluxes of Water

Estimates of pools and fluxes are based on a synthesis of approximately 80 recent and global-scale studies; volumes represent the central point of the most recent or comprehensive individual estimates. Adapted from Abbott, et al., Nature Geoscience 12, 533–540. 2019
  • Based on the figures in the previous table, human appropriation (~24,000 km3yr-1) redistributes the equivalent of half of global river discharge or double global groundwater recharge
  • Irrigated agriculture is the largest water consumer, accounting for ~85-90% of water consumption, followed by industrial, and domestic water use
  • Global water consumption rose 40% between 1980 (~1,200km3) and 2016 (~1,700km3).

Global and Sectoral Water Consumption 2016

Adapted from: Qin, et al., Flexibility and intensity of global water use,Nature Sustainability 2, 515-523 (2019)

Top Six Water Stressed River Basins by Continent 2012-2016 5-year Index

In each continent, the basin is selected as the one having the largest water stress index among the top 10% basins that have substantial electricity generation, crops production, human population, livestock and dam capacity in each continent.

Source:  Source:  Qin, et al., Flexibility and intensity of global water use, Nature Sustainability 2, 515-523 (2109)

Global Demands on Freshwater

Source:  Wu, N., Wang, C., Ausseil, A. G., Alhafedh, Y., Broadhurst, L., Lin, H. J., Axmacher,J., Okubo, S., Turney, C., Onuma, A., Chaturvedi, R. K., Kohli, P., Kumarapuram Apadodharan, S., Abhilash, P. C., Settele, J., Claudet, J., Yumoto, T., Zhang, Y. Chapter 4: Direct and indirect drivers of change in biodiversity and nature’s contributions to people. In IPBES (2018): The IPBES regional assessment report on biodiversity and ecosystem services for Asia and the Paci c. Karki, M., Senaratna Sellamuttu, S., Okayasu, S., Suzuki, W. (eds.). Secretariat of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, Bonn, Germany, pp. 265-370.

Over 2 billion people live in countries experiencing high water stress. Recent estimates show that 31 countries experience water stress between 25% (which is defined as the minimum threshold of water stress) and 70%. Another 22 countries are above 70% and are therefore under serious water stress.

It has been estimated that about 4 billion people, representing nearly two-thirds of the world population, experience severe water scarcity during at least one month during the year.

Global overview of countries experiencing different levels of water stress
(the ratio of total freshwater withdrawn annually by all major sectors, including environmental water requirements, to the total amount of renewable freshwater resources, expressed as a percentage). Source: WWAP (UNESCO World Water Assessment Programme). 2019. The United NationsWorld Water Development Report 2019: Leaving No One Behind. Paris, UNESCO

Trends in Terrestrial Water Storage (TWS) April 2002-March 2016

Adapted from Rodell, et al. 2018; based on Figure 1 and Table 1
Terrestrial Water Storage = Groundwater + soil moisture + surface waters + snow + ice