Ocean Salinity Part 7: Economic, Environmental, and Anthropogenic Impacts

Salinity as a Resource: The Economic Ocean

Desalination: Turning Salt Water into Fresh

What is the significance of salinity for the world's water supply: it is the one barrier between a thirsty world and the vast water of the sea, so the technology that removes the salt, desalination, has become a lifeline for dry coasts, islands and growing cities.

Desalination takes the salt out of sea water in two main ways. In reverse osmosis the water is forced under high pressure through a fine membrane that passes water but holds the salt back; in thermal distillation the water is heated so the fresh vapour rises and is condensed, leaving the salt behind. The figure below sets the two methods side by side.

India has built desalination into its coastal water plans. A low-temperature thermal desalination plant, which uses the small temperature difference between warm surface and cool deep sea water, was first built at Kavaratti in Lakshadweep by the National Institute of Ocean Technology, and large reverse-osmosis plants now supply cities such as Chennai. This is the subject of a well-known examination question.

The promise of desalination is matched by a price. It is energy-hungry and costly, and it returns a hot, very salty brine to the sea, so it solves the water problem only by creating an energy and a waste problem, a tension the environmental section below takes up.

• Reverse osmosis: water is pushed through a membrane that holds the salt back.
• Thermal distillation, including LTTD: the water is boiled off and condensed, leaving the salt behind.
• The trade-off: a drought-proof supply at the cost of energy and a salty brine.

Desalination has grown from a curiosity into a global industry. The dry, oil-rich states of the Gulf and the water-stressed coasts of the Mediterranean now draw much of their drinking water from the sea, and the technology has spread to every dry coast, so that desalinated water, once a luxury, is becoming a mainstay of the urban supply. Reverse osmosis, ever cheaper, now dominates the field.

India is building desalination into its coastal cities. Large reverse-osmosis plants supply Chennai from the Bay of Bengal, and others serve the industrial coast of Gujarat, so the answer to the recurring water crises of the peninsula is being sought, in part, in the salt sea that surrounds it. The choice ties the water supply to the price and the cleanliness of the energy that drives it.

The future of desalination is to marry it to clean energy. Because its great cost is the energy it eats, pairing desalination with solar or wind power turns it from a carbon burden into a sustainable source, and the falling price of both is making the salt sea a steadier well for the dry coasts of the world.

Salt and Minerals from the Sea

Sea water is a dilute ore as much as it is a drink withheld. Every kilogram carries about thirty-five grams of dissolved salts, and where the sun can evaporate it cheaply, that salt becomes a harvest, the oldest and largest of the resources won from the sea.

Common salt is gathered in the solar salt pans of the coast. Sea water is let into shallow ponds and the sun drives off the water until the salt crystallises, a method as old as civilisation, and India, with its long sunny coast, is among the world's leading salt producers, from the great pans of Gujarat and Tamil Nadu. The figure below shows what the sea yields.

The same water yields more than salt. Magnesium, a light, strong metal, is extracted from sea water on an industrial scale, and bromine, used in chemicals and flame retardants, is won from it too, so the dissolved minerals of the sea are a quiet but real part of the world's mineral supply.

Salt has shaped history as well as the economy. The control of salt has raised taxes and toppled rulers, and in India the salt of the sea carries a particular weight, for it was against the tax on salt that the freedom movement marched to the coast in 1930 to make salt from the sea. The salt pan is thus a place of memory as well as of industry.

The sea may yet yield rarer riches. Sea water holds, in faint trace, almost every element, and researchers work on winning lithium and even uranium from it as the demand for these grows, so the dilute mineral store of the ocean may matter more, not less, as the easy deposits of the land are exhausted.

The salt pan is an ecosystem as well as a works. The shallow, super-salty ponds draw flamingos and waders to feed on the brine shrimp and the algae that thrive there, so a working salt pan can also be a haven for birds, a reminder that even the hypersaline extreme has its life.

Energy from Salinity and the Sea

What is the significance of the salinity gradient as an energy source: where fresh river water meets salt sea water, the difference in their salt content holds usable energy, so the very mixing that this series has studied can, in principle, be turned into power.

Salinity-gradient power harvests the energy of mixing. When fresh and salt water are separated by the right membrane, water is drawn osmotically from the fresh to the salty side, and that flow can drive a turbine; this osmotic power, also called blue energy, is a clean if still costly source being trialled at a few river mouths.

The sea offers other forms of energy beside it. The rise and fall of the tides, the march of the waves, and the temperature difference between warm surface and cool deep water in ocean thermal energy conversion all draw power from the sea, so the ocean is an energy frontier as well as a mineral and a water one.

These ocean energies remain mostly a promise. Tidal power works only where the tidal range is great, wave power is hard to build to last in the rough sea, ocean-thermal conversion needs a deep tropical site, and salinity-gradient power awaits a cheaper membrane, so the energy of the sea, vast on paper, is still won only in pilots and a few special places. The frontier is real but not yet open.

The Ocean's Resources and the Resource Crisis

The Four Great Resources of the Ocean

What is the significance of the ocean for a crowded, resource-hungry world: as the land's reserves are strained, the sea is looked to as a vast, scarcely tapped store of minerals, energy, food and fresh water, and weighing those resources is a recurring task of the examination.

The resources of the ocean fall into four great families. There are the minerals, from the salt and metals of the water to the polymetallic nodules of the floor; the energy, from offshore oil and gas to tides, waves and the salinity gradient; the food, from fisheries and aquaculture; and the fresh water of desalination. The figure and table below set them out.

Each of these is real, and each is bounded. The sea can ease the world's resource crisis, but only at a cost in energy, in money and in harm to the ocean itself, so the resources of the sea are a promise to be weighed, not a windfall to be seized. The next section sets the balance.

The deep floor holds a mineral prize and a hard question. Across the abyssal plains lie polymetallic nodules, lumps rich in manganese, nickel, copper and cobalt, and India has been granted a site to explore them in the Central Indian Ocean Basin. Yet mining the deep sea would scar a habitat barely understood, so the prize comes wrapped in a warning.

The living sea is a resource of a different kind. Beyond the minerals and the energy, the genes of marine life, the enzymes of deep-sea microbes and the compounds of corals and sponges are a store of medicines and materials only beginning to be tapped, so the biodiversity of the ocean is itself a resource to be used with care.

Critically Evaluating the Ocean's Resources

To evaluate the ocean's resources critically is to set their promise against their price. On the side of promise, the sea is immense, much of it is renewable, and it lies close to the coastal cities where most of humanity now lives, so it can genuinely relieve the pressure on the land.

On the side of price stand cost, harm and governance. Winning minerals and fresh water from the sea is energy-hungry and dear; fishing and mining can wreck the very ecosystems they draw on; and much of the deep ocean lies beyond national borders, so its use raises hard questions of law and equity that the world is only beginning to answer.

• Promise: the sea is vast, partly renewable, and close to the coastal cities.
• Price: high cost and energy use, environmental harm, and weak governance of the deep sea.
• Verdict: the ocean can ease the resource crisis, but only if used within limits and shared fairly.

The honest verdict is one of cautious hope. The ocean can be a major part of the answer to the resource crisis, but only if it is used within limits, with the harm counted and the benefits shared, which is the case for a sustainable blue economy that the final section makes.

The deep ocean beyond national borders is governed in common. Under the law of the sea, the floor beyond any country's zone is the common heritage of humankind, and an International Seabed Authority licenses and regulates its exploration, so the question of who may take the deep ocean's wealth, and on whose behalf, is being worked out in international law even as the technology to take it matures.

The wise course is to move with caution. Because the deep sea is so little known and so slow to heal, many argue for a precautionary pace, taking only what can be taken without lasting harm, so that the evaluation of the ocean's resources ends not in a simple yes or no but in a call for restraint.

The Environmental Cost: Pollution and the Brine Problem

Marine Pollution: Oil, Plastic and Nutrients

What is the significance of pollution for the salty sea: the same coasts that draw water, salt and food from the ocean pour their waste back into it, so the working of the sea has a shadow, the steady poisoning of the very resource it depends on.

Three pollutants weigh heaviest on the sea. Oil spilled from tankers and rigs smothers the surface and the shore; plastic waste breaks into microplastics that climb the food chain into the fish and the people who eat them; and the nutrients of fertiliser and sewage feed the algal blooms that breed the dead zones an earlier part described. The figure below lists the chief threats.

These harms fall hardest on the coast and its people. The pollution gathers where the rivers meet the sea, in the estuaries and the shallow shelf that are the nurseries of the fisheries, so the damage to the coastal sea reaches straight back into the food and the livelihoods of the millions who depend on it.

The plastic has gathered into vast ocean garbage patches. Carried by the currents, floating plastic collects in the calm centres of the great gyres, the largest known as the Great Pacific Garbage Patch, while along the Indian coast the rivers carry a heavy load of waste to the sea. The microplastic they shed is now found in the deepest trenches and in the bodies of fish.

• Oil: tanker and rig spills smother the surface and the shore and kill marine life.
• Plastic: floating waste forms ocean garbage patches and sheds microplastics into the food chain.
• Nutrients: fertiliser and sewage run-off drive the eutrophication that breeds dead zones.

The Brine Problem: Desalination's Salty Shadow

Desalination carries a salty shadow of its own. For every litre of fresh water it makes, it returns to the sea a smaller volume of brine, water far saltier and often warmer than the sea it came from, and where this brine pools near the outfall it can harm the life of the sea bed.

The brine sinks and lingers. Because it is dense, the salty discharge sinks and spreads slowly along the bottom, raising the salinity around the plant above what the local plants and animals can bear, so the very salinity this series has studied becomes, at the outfall, a pollutant in its own right.

• Brine is more saline and often warmer than the sea it came from.
• Being dense, it sinks and pools along the bottom near the outfall.
• It can be blunted by diffusers, dilution and careful siting, or recovered for its salt and minerals.

The scale of the brine grows with the thirst for water. As the world builds more and larger desalination plants, the volume of hot, salty brine returned to the sea rises with it, concentrated along the very coasts where the plants cluster, so the brine problem is not a curiosity but a growing stress on the coastal sea that careful design must answer.

Some plants now turn the brine to use. Rather than dumping it, a few works recover the salt and the minerals from the brine or blend it with other flows before release, so that the waste of one process becomes the feedstock of another, a small sign of the circular thinking a sustainable blue economy will need.

The remedy is in the design. Spreading the discharge through diffusers, diluting it before release and siting plants where currents disperse it can blunt the harm, so the brine problem is a reminder that no use of the sea is free, and that managing salinity is part of managing the resource.

Ocean Acidification: The Linked Chemistry Change

Beside the changes in salinity runs a change in the sea's chemistry. As the ocean absorbs the carbon dioxide that human activity pours into the air, the gas reacts with sea water to make it slowly more acid, a change known as ocean acidification that runs alongside the warming and the freshening this series has traced.

Acidification threatens the builders of the sea. The more acid water makes it harder for corals, shellfish and the tiny calcareous plankton to build their shells and skeletons, so the base of the marine food web is put at risk, a concern the examination has tested directly. It is a separate chemical change from salinity, but it shares the same human cause.

The change is already measurable. The surface ocean has grown about a tenth of a pH unit more acid since the industrial age began, a large change on that scale, and the depth at which shells begin to dissolve is rising towards the surface, so the cold, productive seas, including the waters off India, are among the first to feel the squeeze on their shell-building life.

Acidification and salinity change press on the same creatures. The corals, the shellfish and the plankton that a warming, freshening, acidifying sea threatens are the same builders of the marine food web, so the several human changes to the ocean compound one another, and the cost falls, again, on the life that the coast depends on.

The Human Hand on Salinity

Dams, Diversions and the Starving Delta

What is the significance of the human hand for salinity: human works on the rivers and the climate are now changing the salinity of the coast directly, so the salt of the sea is no longer set by nature alone but in part by the choices of people upstream and worldwide.

Dams and diversions cut the fresh flow that holds the sea back. As rivers are dammed for power and drawn off for farms and cities, less fresh water reaches the coast, so the salt presses further into the estuaries, the soils and the groundwater of the deltas, the salt intrusion an earlier part described. The starving delta is a human creation.

Over-pumping of coastal wells deepens the harm. Where coastal communities draw groundwater faster than the rain can refill it, the sea seeps into the emptied aquifer, salting the wells, so the management of fresh water inland and at the coast is, in the end, the management of salinity on the land.

The world's great deltas show the pattern. Where the Nile is held behind its dam, the Colorado is drawn off before it reaches the sea, and the rivers of the Indian and the Bangladeshi Sundarbans run thinner than before, the salt has pressed inland, spoiling soils and fisheries. The starving delta is a problem of the whole world, and acute on the coasts of India.

Climate Change and the Humanised Water Cycle

The widest human hand of all is on the climate. By warming the planet, human activity is speeding up the global water cycle, sharpening the salinity pattern that this series began with, so that the salty seas grow saltier and the fresh ones fresher across the whole ocean.

For the coast the effects compound. A warmer sea raises fiercer cyclones, a rising sea pushes salt further inland, and a more variable monsoon strains the fresh-water supply, so the human changes to the water cycle meet the human changes to the rivers at the coast, where they fall together on the same people. Managing this is the task that the blue economy must take up.

The poorest coasts are the most exposed. The low, crowded deltas and the small islands that have done least to warm the world stand to lose the most from a rising, saltier, stormier sea, so the human reshaping of salinity and the climate is a question of justice as well as of geography, one that the world's response must hold at its centre.

Toward a Sustainable Blue Economy

Balancing Use and Conservation

What is the significance of the blue economy: it is the idea that the wealth of the sea can be used to lift human welfare only if it is used sustainably, so that the resource survives the using, and it gathers the economic and the environmental threads of this part into one aim.

A sustainable blue economy uses the sea within its limits. It means fishing within what the stocks can bear, desalinating with clean energy and careful brine disposal, mining the floor only where the harm is understood, and protecting the estuaries and reefs that the whole coastal economy rests on. Use and conservation are made to serve each other.

For India the stakes are large. With a long coast, a vast exclusive economic zone and a coastal population in the hundreds of millions, India has named the blue economy a pillar of its development, so the wise use of the sea, and of its salinity, is a national as well as a global task.

India has set out a programme for its blue economy. Through the Deep Ocean Mission, which explores the deep sea for minerals and life and develops ocean technology, the Sagarmala programme for ports and the coast, and the drive for marine renewable energy, the country is trying to turn the ocean into a managed source of growth. Its long coast and its exclusive economic zone of more than two million square kilometres are the canvas.

• Sustainable fisheries and aquaculture for food and livelihoods.
• Clean shipping, ports and coastal infrastructure, including the Sagarmala programme.
• Marine renewable energy and responsible deep-sea mineral exploration through the Deep Ocean Mission.
• Coastal and marine tourism, with the protection of reefs, mangroves and estuaries.

Here the economic chapter of the series closes. From the fresh water wrung out of the salt sea to the brine returned to it, salinity has run as a thread of use and cost through the human ocean, and the final part draws the whole series together for the examination.

UPSC Relevance and Exam Focus

Where the Economic and Environmental Ocean Fits in the UPSC-CSE Syllabus

This topic spans General Studies Paper I and Paper III, the geography of resources and the economy and environment, and it is heavily examined because desalination, ocean resources, the blue economy and marine pollution are live questions of policy as well as of geography.

The questions most often test the uses of the sea, the methods and the place of desalination, the resources the ocean offers a resource-hungry world, and the environmental costs of using them, from pollution to the brine of desalination.

Several linked points recur and are worth holding in working memory:

• Desalination: reverse osmosis and thermal distillation; the LTTD plant at Kavaratti, Lakshadweep, by NIOT.
• Salt and minerals: solar salt pans (India a leading producer); magnesium and bromine from sea water.
• Ocean energy: salinity-gradient (osmotic) power, tidal, wave and ocean-thermal energy conversion.
• Four ocean resources: minerals, energy, food and fresh water, each with promise and limits.
• Marine pollution: oil, plastic and nutrient run-off; the dead zones that follow eutrophication.
• The brine problem: desalination returns a hot, extra-salty brine that harms the sea bed.

A 2008 Prelims question asked where India's first low-temperature thermal desalination plant, making one lakh litres a day, was commissioned, the answer being Kavaratti in Lakshadweep; a reader who has fixed that NIOT pioneered LTTD on that island can answer it directly.

A 2014 Mains question asked for a critical evaluation of the ocean's resources for the world's resource crisis; this part supplies both the resources, the minerals, energy, food and fresh water, and the critical balance, the cost, the environmental harm and the governance gaps, that a strong answer must weigh, closing on the case for a sustainable blue economy.

Sources and Further Reading

• Press Information Bureau (ocean technology and desalination)
• Ministry of Earth Sciences: Ocean technology and the blue economy
• FAO: The State of World Fisheries and Aquaculture
• NOAA: Marine debris and pollution
• Wikipedia: Desalination
• Wikipedia: Osmotic power
• Wikipedia: Blue economy
• NCERT Class 11, Fundamentals of Physical Geography