What is Thermal Pollution? Causes, Effects, And Solutions

Thermal pollution, also known as thermal enrichment, is a form of water pollution. It is a rapid change in water temperature. It could be a temperature increase or decrease.  This pollution occurs when industries take water out of a natural resource and discharge it to its source at a different temperature. 

In this article, we explore the detailed definition of thermal pollution. We will discuss its causes and effects. We will also examine examples worldwide and ways to reduce its occurrence.

Related Read: Environmental Impact of Geothermal Energy, Environmental Impact of Water Pollution.

What is thermal pollution?

Thermal pollution
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Thermal pollution refers to reducing water quality by any process that changes the ambient temperature of the water. A rapid change in water temperature occurs, whereby surface water becomes too hot for the marine ecosystem and the organisms. Pollution often occurs because of thermal discharges from nuclear power plants and factories like pulp and paper mills, chemical plants, and biochemical activities. 

As excess heat interacts with natural bodies of water, it becomes warm water. The saturation values of dissolved oxygen increase as they release warmer surface waters4, and the metabolism of aquatic life increases. These increases make the organisms use more oxygen to survive. 

The oxygen level needed by an organism increases with each 10C temperature increase. Furthermore, the biochemical oxygen increases, reducing oxygen in water bodies. It triggers changes in the aquatic life forms as their environment becomes uninhabitable. A marine animal’s natural body goes through changes. For instance, blue-green algae replace diatoms. 

Ambient water temperature is essential to the survival of aquatic life, but thermal pollution makes natural water bodies dangerous to them. It cuts off ample food supply, damages larvae and fish eggs, kills aquatic plants, and increases the migration of organisms to safer water bodies.

5 Causes of Thermal Pollution 

1. Cooling water used by power plants and industrial factories

power plants
Photo by Pixabay on Pexels,

Coal-fired power plants and nuclear plants are one of the major sources of thermal pollution. There are also power plants that use natural gas and biomass. These plants are constructed near large bodies of water to have access to constant water supply. 

They convert these bodies of water into steam, which drives the turbines to produce energy. They also use water to cool machines when they become very hot. The water absorbs the heat from the machinery. However, not all the water evaporates. The power plant usually returns what’s left to its source.

Other sources of thermal pollution that use nearby water bodies as cooling agents are steel mills, pulp and paper mills, refineries, and chemical plants. After using cool water to cool machinery, they discharge it to its source at elevated temperatures. Using bodies of water as cooling agents for machinery and releasing warm water back is known as once-through cooling. 

As of 2012, 425 power plants used once-through cooling systems to distribute water to cooling water. The warm water discharged from the cooling towers is usually up to 15C1, causing a local water temperature change.

2. Industrial wastewater  

Water discharged from a factory or nuclear power plant is an example of wastewater. The type of wastewater that causes thermal pollution is heated water. They can release heated wastewater in a process we discussed in the previous section, once-through cooling. 

These factories can release up to 500 million gallons of warm water. In addition, there is wastewater from other activities that occur in industrial factories. They don’t treat this wastewater before discharging it into natural water bodies. The temperature range in these bodies of water with untreated sewage often increases, leading to thermal pollution.

3. Urban runoff  

city street
Photo by Andrea Cau on Unsplash.

Urban runoff refers to the heated water from hot rooftops, parking lots, roads, and sidewalks. City streets, buildings, and other structures release excess heat that cools down from rainfall. The city becomes cold, while the heated water enters nearby water bodies, causing thermal pollution. 

Artificial ponds used to capture rainwater for flood control also experience thermal pollution. It happens when the wide and shallow water body collects rainwater. Its temperature increases. The water might flow into other cold water environments when it becomes full.

4. Soil erosion  and deforestation 

Photo by Andre Moura on Pexels.

Human activities on land are another significant cause of thermal pollution. As the topsoil gradually wears away, it erodes rivers and streams. It causes river beds to become broad and shallow, exposing more area to sunlight. It increases the water temperature of the river and warmer surface waters.

Another human activity causing the issue is deforestation, the felling of trees. The rapid development of civilization contributed tremendously to the constant feeling of trees for various construction purposes. 

Cutting down trees exposes more area to sunlight, further contributing to soil erosion. Deforestation also contributes to warmer water temperatures by removing the shade covering lake shores and river banks and exposing them to sunlight. 

5. Natural Causes: Volcanoes and other geothermal activities

Natural causes are also responsible for warmer surface waters in the natural environment. These natural causes are volcanoes, underground geothermal vents, and lava. These structures produce a lot of heat that increases nearby water temperature. 

Also, heat from wildfires, global warming, and climate change can cause thermal shock. Thermal shock is a rapid change in water temperatures. Climate change is part of the causes of cold water thermal pollution of glaciers as they melt quickly, resulting in warmer water.    

6 Effects of Thermal Pollution 

body of water
Photo by Sophia Simoes on Unsplash.

1. Decrease in dissolved oxygen levels.  

One of the effects of thermal pollution is the reduction of oxygen levels in warm water. The molecules in the water move faster when cold water bodies start getting warmer temperatures, allowing more oxygen to escape. 

Research shows less oxygen in warm surface waters and deep water habitats. It is more noticeable in freshwater habitats as oxygen in lakes reduces 9.3 times more than that of the world's oceans. Less oxygen in natural bodies of water threatens the whole ecosystem. 

The loss of deep water dissolved oxygen ruins water quality by releasing accumulated nutrients from sediments into the water3. This release often triggers the phytoplankton biomass and facilitates the growth of algae blooms.

2. Increase in the presence of toxins in water. 

Urban and agricultural runoff and wastewater from industries can contain toxic chemicals, fuel, solvents, and heavy metals. Also, thermal discharge from nuclear power plants can be radioactive. 

All these pollutants can harm aquatic animals and plants. These additional pollutants are always rich in nutrients, increasing the growth of algae blooms. Furthermore, the pollutants can make marine species sterile. 

3. Destruction of aquatic habitats.  

Many aquatic organisms are sensitive to changes in water temperature. They express intense stress when water temperatures suddenly become hot or cold. Organisms suffer ill health and lose their lives to thermal pollution. 

Those who survive the onslaught become deformed in several ways. For instance, these organisms could become less fertile or reproduce offspring with developmental delay. Eventually, the population of these aquatic animals will be reduced. 

The loss of many aquatic species scatters the food chain, upsetting the ecosystem's balance. The absence of some plants and animals to feed the remaining survivors means they still suffer more because their metabolism becomes faster.

As their metabolism speeds up, they eat a lot more than usual. It further disrupts the balance of the ecosystem. Another effect of thermal pollution on aquatic ecosystems is coral bleaching. The warm temperature of the sea surface causes thermal stress, leading to the breakdown of coral-algal symbiosis. 

Symbiosis is essential for the survival of coral reefs, but thermal pollution forces the corals to expel the algae in them. Corals lose their color and become completely white. The more coral bleaching occurs, the weaker coral reefs get.

Thermal pollution doesn’t only occur when cold water rapidly becomes hot water. Sometimes, cold water mixes with hot water bodies like hot springs. Cold water pollution also occurs when large dams release water during warmer months. Plankton, turtles, fish, and fish eggs cannot survive cold water temperatures. It reduces the growth of fish and breeding activities.  

4. Harmful algal blooms 

Algal blooms are groups of algae plants that grow speedily. They are dangerous to humans and biodiversity. They can become too dense and kill the oxygen in the water. Algal blooms can also produce poison and release harmful gases. 

You'll find them growing in freshwater, saltwater, and brackish waters. They bloom when thermal pollution increases the water temperature range. Also, the presence of nutrients, nitrogen, and phosphorus aids the bloom's fast growth.

Algal blooms change the look and smell of the water when they die and decay. They release gases that smell like rotten eggs or rotting plants as they decompose. Also, blooms change the color of the water to red, green, blue, brown, and yellow. 

Blooms harm the ecosystem by using all the oxygen, leaving the organisms and fauna in the water to choke. The blooms can grow so dense and block sunlight from reaching the organisms that need it. It can clog the gills of fish, shellfish, and other animals, disrupting their breathing abilities. It also absorbs a lot more sunlight, increasing the temperature of the water.

5. Disruption of the biodiversity food chain.  

Another effect is the disruption of the biodiversity food chain. Amphibians and fish migrate when the water is not at the particular temperature range required for survival. 

Their migration reduces the bird's food supply. Eventually, they also migrate away from the area. The remaining surviving organisms struggle to stay alive and reproduce. 

6. Adapted animals experiencing thermal shock. 

Thermal shock is a reaction to the abrupt change in water temperature- from hot to cold and vice versa. Fish and other organisms adapted to a particular temperature range can die immediately from a rapid increase or decrease in water temperature.    

Examples of thermal pollution around the world 

mississippi river
Photo by Dirk DBQ on Wikimedia Commons licensed under CC BY 2.0 (Cropped from original).

Mississippi River has the highest heat emissions, with 62% coming from coal-fired plants and 28% from nuclear plants in 20016. Other sources of thermal pollution are wastewater and agricultural runoff2.

The Middle Eastern world is experiencing water scarcity, so they use desalination to improve their water security. However, a study conducted in 2020 showed the cooling water used in Ashkelon and Hadera desalination plants created a heat plume 25% warmer than natural seawater temperature. The thermal pollution in these bodies of water stresses the benthic organisms living close to the seafloor.

There are concerns regarding California's last operating nuclear power plant, Diablo Canyon, near San Luis Obispo. The complaints were about the effects of thermal pollution on the Pacific Ocean from the cooling water discharged from the plants.

In 2021, the owner of Diablo Canyon signed a $5.9 million settlement agreement with the state of California over violation of permits meant to limit thermal pollution.

Solutions To Thermal Pollution 

Industries and factories should join hands to solve this issue. The solution requires a switch in their industrial practices. They should implement the following: 

Cooling Ponds 

Factories can use cooling ponds to reduce the waste heat from their cooling water before discharging it into a natural body of water. Cooling ponds are shallow reservoirs that allow water to release heat waste naturally. 

Cooling towers are more efficient ways to solve thermal pollution because they maximize water's exposure to the air by spraying water through a tower. Some heat evaporates and cools down as the water passes through the air. 

Simply put, cooling towers reduce heat by passing the water through the atmosphere. They can reuse the remaining water as a cooling agent for the factory.

Recycling Wastewater

Another solution to thermal pollution is to reuse wastewater instead of discharging it into the environment. Factories can cool it in a cooling tower or pond before reusing it for cooling processes. 

They can also use waste heat to provide heat through the power plant or factory. Factories can circulate the hot water through the pipes to provide heat to nearby houses and farms. It is helpful during the cold and winter seasons. 


Thermal pollution is a major environmental problem that affects the marine ecosystem and biodiversity. Human industrial activities and natural phenomena like underwater thermal vents and hot springs contribute to it. 

As the temperature increases, it kills most aquatic animals in their natural environment. It changes the whole water ecosystem, from taste to smell to oxygen composition. 

It would be best if industries around the world took measures to prevent the occurrence of thermal pollution in the environment. They should avoid releasing water with different temperature values into its source. They can also find ways to reuse wastewater and the heat that accompanies it. 

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Madden, N., Davis, M., & Lewis, A. (2013). Thermal Effluent From the Power Sector: An Analysis of Once-through Cooling System Impacts on Surface Water Temperature. IOP SCIENCE.


Raptis, C. E., Van Vilet, M. T. H., & Pfister, S. (2016). Global Thermal Pollution of Rivers From Thermoelectric Power Plants. IOP SCIENCE. 


Jane, S. F., Hansen, G. J. A., Kraemer, B. M., Leavitt, P. R., Mincer, J. L., North, R. L., Pilla, R. M., Stetler, J. T., Williamson, C. E., Woolway, R. I., Арвола, Chandra, S., DeGasperi, C. L., Diemer, L. A., Dunalska, J., Erina, O., Flaim, G., Grossart, H. P., Hambright, K. D., Rose, K. C. (2021). Widespread Deoxygenation of Temperate Lakes. Nature. 


Brenniman, G. R. (2006). Thermal Pollution. Kluwer Academic Publishers eBooks.

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