Human alteration of Earth is substantial and growing. Between one-third and one-half of the land surface has been transformed by human action; the carbon dioxide concentration in the atmosphere has increased by nearly 30 percent since the beginning of the Industrial Revolution; more atmospheric nitrogen is fixed by humanity than by all natural terrestrial sources combined; more than half of all accessible surface fresh water is put to use by humanity; and about one-quarter of the bird species on Earth have been driven to extinction. By these and other standards, it is clear that we live on a human-dominated planet.

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Human Domination of Earth's Ecosystems

A freely available dataset produced from three million Landsat satellite images reveals substantial changes in the distribution of global surface water over the past 32 years and their causes, from climate change to human actions. The distribution of surface water has been mapped globally, and local-to-regional studies have tracked changes over time. But to date, there has been no global and methodologically consistent quantification of changes in surface water over time. Jean-Francois Pekel and colleagues have analysed more than three million Landsat images to quantify month-to-month changes in surface water at a resolution of 30 metres and over a 32-year period. They find that surface waters have declined by almost 90,000 square kilometres—largely in the Middle East and Central Asia—but that surface waters equivalent to about twice that area have been created elsewhere. Drought, reservoir creation and water extraction appear to have driven most of the changes in surface water over the past decades. The location and persistence of surface water (inland and coastal) is both affected by climate and human activity1 and affects climate2,3, biological diversity4 and human wellbeing5,6. Global data sets documenting surface water location and seasonality have been produced from inventories and national descriptions7, statistical extrapolation of regional data8 and satellite imagery9,10,11,12, but measuring long-term changes at high resolution remains a challenge. Here, using three million Landsat satellite images13, we quantify changes in global surface water over the past 32 years at 30-metre resolution. We record the months and years when water was present, where occurrence changed and what form changes took in terms of seasonality and persistence. Between 1984 and 2015 permanent surface water has disappeared from an area of almost 90,000 square kilometres, roughly equivalent to that of Lake Superior, though new permanent bodies of surface water covering 184,000 square kilometres have formed elsewhere. All continental regions show a net increase in permanent water, except Oceania, which has a fractional (one per cent) net loss. Much of the increase is from reservoir filling, although climate change14 is also implicated. Loss is more geographically concentrated than gain. Over 70 per cent of global net permanent water loss occurred in the Middle East and Central Asia, linked to drought and human actions including river diversion or damming and unregulated withdrawal15,16. Losses in Australia17 and the USA18 linked to long-term droughts are also evident. This globally consistent, validated data set shows that impacts of climate change and climate oscillations on surface water occurrence can be measured and that evidence can be gathered to show how surface water is altered by human activities. We anticipate that this freely available data will improve the modelling of surface forcing, provide evidence of state and change in wetland ecotones (the transition areas between biomes), and inform water-management decision-making.

High-resolution mapping of global surface water and its long-term changes

Seventy-seven percent of the total water discharge of the 139 largest river systems in North America north of Mexico, in Europe, and in the republics of the former Soviet Union is strongly or moderately affected by fragmentation of the river channels by dams and by water regulation resulting from reservoir operation, interbasin diversion, and irrigation. The remaining free-flowing large river systems are relatively small and nearly all situated in the far north, as are the 59 medium-sized river systems of Norway, Sweden, Finland, and Denmark. These conditions indicate that many types of river ecosystems have been lost and that the populations of many riverine species have become highly fragmented. To improve the conservation of biodiversity and the sustainable use of biological resources, immediate action is called for to create an international preservation network of free-flowing river systems and to rehabilitate exploited rivers in areas that lack unaffected watercourses.

https://science.sciencemag.org/content/sci/266/5186/753.full.pdf

Fragmentation and flow regulation of river systems in the northern third of the world.

It has been frequently stated, but without provision of supporting evidence, that the world has lost 50% of its wetlands (or 50% since 1900 AD). This review of 189 reports of change in wetland area finds that the reported long-term loss of natural wetlands averages between 54–57% but loss may have been as high as 87% since 1700 AD. There has been a much (3.7 times) faster rate of wetland loss during the 20th and early 21st centuries, with a loss of 64–71% of wetlands since 1900 AD. Losses have been larger and faster for inland than coastal natural wetlands. Although the rate of wetland loss in Europe has slowed, and in North America has remained low since the 1980s, the rate has remained high in Asia, where large-scale and rapid conversion of coastal and inland natural wetlands is continuing. It is unclear whether the investment by national governments in the Ramsar Convention on Wetlands has influenced these rates of loss. There is a need to improve the knowledge of change in wetland areas worldwide, particularly for Africa, the Neotropics and Oceania, and to improve the consistency of data on change in wetland areas in published papers and reports.

How much wetland has the world lost? Long-term and recent trends in global wetland area

[1] Our understanding of the global dust cycle is limited by a dearth of information about dust sources, especially small-scale features which could account for a large fraction of global emissions. Here we present a global-scale high-resolution (0.1°) mapping of sources based on Moderate Resolution Imaging Spectroradiometer (MODIS) Deep Blue estimates of dust optical depth in conjunction with other data sets including land use. We ascribe dust sources to natural and anthropogenic (primarily agricultural) origins, calculate their respective contributions to emissions, and extensively compare these products against literature. Natural dust sources globally account for 75% of emissions; anthropogenic sources account for 25%. North Africa accounts for 55% of global dust emissions with only 8% being anthropogenic, mostly from the Sahel. Elsewhere, anthropogenic dust emissions can be much higher (75% in Australia). Hydrologic dust sources (e.g., ephemeral water bodies) account for 31% worldwide; 15% of them are natural while 85% are anthropogenic. Globally, 20% of emissions are from vegetated surfaces, primarily desert shrublands and agricultural lands. Since anthropogenic dust sources are associated with land use and ephemeral water bodies, both in turn linked to the hydrological cycle, their emissions are affected by climate variability. Such changes in dust emissions can impact climate, air quality, and human health. Improved dust emission estimates will require a better mapping of threshold wind velocities, vegetation dynamics, and surface conditions (soil moisture and land use) especially in the sensitive regions identified here, as well as improved ability to address small-scale convective processes producing dust via cold pool (haboob) events frequent in monsoon regimes.

Global Scale Attribution of Anthropogenic and Natural Dust Sources and their Emission Rates Based on MODIS Deep Blue Aerosol Products

The Aral Sea is a huge terminal lake located among the deserts of Central Asia. Over the past 10 millennia, it has repeatedly filled and dried, owing both to natural and human forces. The most recent desiccation started in the early 1960s and owes overwhelmingly to the expansion of irrigation that has drained its two tributary rivers. Lake level has fallen 23 m, area shrunk 74%, volume decreased 90%, and salinity grew from 10 to more than 100g/l, causing negative ecological changes, including decimation of native fish species, initiation of dust/salt storms, degradation of deltaic biotic communities, and climate change around the former shoreline. The population residing around the lake has also been negatively impacted. There is little hope in the foreseeable future to fully restore the Aral Sea, but measures to preserve/rehabilitate parts of the water body and the deltas are feasible.

The Aral Sea Disaster

The Aral Sea in the Soviet Union, formerly the world's fourth largest lake in area, is disappearing. Between 1960 and 1987, its level dropped nearly 13 meters, and its area decreased by 40 percent. Recession has resulted from reduced inflow caused primarily by withdrawals of water for irrigation. Severe environmental problems have resulted. The sea could dry to a residual brine lake. Local water use is being improved and schemes to save parts of the sea have been proposed. Nevertheless, preservation of the Aral may require implementation of the controversial project to divert water from western Siberia into the Aral Sea basin.

https://science.sciencemag.org/content/sci/241/4870/1170.full.pdf

Desiccation of the Aral Sea: A Water Management Disaster in the Soviet Union

The Aral Sea in 1960 was a huge brackish water lake (4th in the world in surface area) lying amidst the deserts of Central Asia. The sea supported a major fishery and functioned as a key regional transportation route. Since 1960, the Aral has undergone rapid desiccation and salinization, overwhelmingly the result of unsustainable expansion of irrigation that dried up its two tributary rivers the Amu Darya and Syr Darya and severely damaged their deltas. The desiccation of the Aral Sea has had severe negative impacts, including, among others, the demise of commercial fishing, devastation of the floral and faunal biodiversity of the native ecosystems of the Syr and Amu deltas, and increased frequency and strength of salt/dust storms. However, efforts have been and are being made to partially restore the sea’s hydrology along with its biodiversity, and economic value. The northern part of the Aral has been separated from the southern part by a dike and dam, leading to a level rise and lower salinity. This allowed native fishes to return from the rivers and revitalized the fishing industry. Partial preservation of the Western Basin of the southern Aral Sea may be possible, but these plans need much further environmental and economic analysis. This paper, mainly utilizing hydrologic and other data as input to spreadsheet (Microsoft Excel)-based hydrologic and salinity models, examines the current efforts to restore the Aral and looks at several future scenarios of the Sea. It also delineates the most important lessons of the Aral Sea’s drying.

The future Aral Sea: hope and despair

The Aral Sea, a once vast brackish terminal lake in the heart of Central Asia, has been rapidly drying since the 1960s. It had separated into four separate waterbodies by September 2009. The maximum water level decline was more than 26 m, whereas the lake surface area decreased 88% and the water volume 92%. The lake salinity increased by more than 20-fold. Prior to the modern recession, the Aral Sea experienced a number of water level declines and subsequent recoveries over the last 10 millennia. The main causative factor until the 1960s was the periodic westward diversion of the Amu Dar’ya, the main influent river, towards the Caspian Sea by both natural and human forces. The post-1960 recession, however, was overwhelmingly the result of unsustainable irrigation development. The lake’s modern recession has caused a broad range of severe negative ecological, economic and human welfare problems. To restore the Aral Sea to its 1960s’ size and ecological condition would be very difficult, if not impossible, in the foreseeable future. The plight of the Aral Sea, however, is far from hopeless. Partial restoration of portions of the lake is still feasible. A project to raise the Small (northern) Sea was completed in Fall 2005, raising its water level by 2 m, and lowering its salinity to a level not much higher than the early 1960 levels. Its ecological recovery has been dramatic, and a new project to improve further the Small Aral was recently announced. Improving the Large (southern) lake would be much more difficult and expensive. A project to save the deep Western Basin partially is technically feasible, however, and should be given careful evaluation. It is important to repair and preserve what is left of the deltas of the two tributary rivers, Syr Dar’ya and Amu Dar’ya, as these two rivers are of great ecological and economic value, and act as biological refugia for endemic species of the Aral Sea.

The past, present, and future Aral Sea

The Aral Sea basin, located in the central Asian part of the former Soviet Union, is undergoing dramaticallly rapid and intense environmental change. Pervasive human misuse and overuse of its water, land and other critical natural resources have led to severe degredation of key ecological systems. This work analyzes the environmental, human and economic problems that have arisen and presents recommendations for future research need. The primary focus is on the drying of the Aral Sea, but related issues of diminished river flow, land and water pollution, and degredation, ecosystem deterioration and adverse effects on humans are also examined.

Philip Micklin

Papers

The Aral Sea Disaster

Desiccation of the Aral Sea: A Water Management Disaster in the Soviet Union

The future Aral Sea: hope and despair

The past, present, and future Aral Sea

The Aral Sea Basin