Causes and Effects of Thermal Pollution

on August 2, 2017

Simply put, thermal pollution comes from hot water or cold water being dumped into a body of water. Bodies of water naturally tend to dissipate the heat gained from warm currents, underwater hot springs, and from the sun. Thermal pollution is called that because it overwhelms the natural temperature control mechanisms that work in the water. The sudden temperature change poses a health risk to a wide range of aquatic and amphibious creatures.

This article is an overview of the problem, as it relates to water supplies. Thermal pollution is usually a product of dumping hot water into cooler water, but cold water in a warm body of water can also cause problems. This article focuses on the sources of hot water and the consequences of introducing it into bodies of water.

The Main Cause of Thermal Pollution:

Many human and natural factors contribute to the problem of thermal pollution. The single biggest cause of thermal pollution is probably cooling for industrial machinery and power plants. Water is an excellent, and free, cooling agent. This is why many industrial operations pull in relatively cool water to cool their machinery and let the relatively warm water flow back into the river or lake or sea.

Thermal pollution also has some natural causes. Geothermal vents and hot springs introduce excess heat into bodies of water. Soil erosion, deforestation, and runoff from paved areas are other artificial sources of hot water. Deforestation eliminates shade, which exposes the water to sunlight. Water on hot paved surfaces gets hot, then runs off into nearby bodies of water, raising the water temperature. Retention ponds can also be a source of thermal shock because the relatively small and shallow bodies of water can absorb quite a bit of heat energy from the sun. Pumping that water directly into a river, lake, or bay causes a significant temperature increase, just like pouring a hot pitcher of water into a bathtub full of water causes the water to jump a few degrees Fahrenheit.

The Effects of Thermal Pollution:

The effects of thermal pollution are diverse, but in short, thermal pollution damages water ecosystems and reduces animal populations. Plant species, algae, bacteria, and multi-celled animals all respond differently to significant temperature changes. Organisms that cannot adapt can die of various causes or can be forced out of the area. Reproductive problems can further reduce the diversity of life in the polluted area.

However, thermal pollution can be beneficial to some species. Bacteria and algae tend to benefit from the excess heat. Some larger animals also benefit from the warmer water. In Florida, manatees spend the winter near power plants, where the cooling water they use warms up the shallow salt water.  On balance, thermal pollution is a negative force for many reasons.

Decreased Dissolved Oxygen:

Warm water holds less oxygen than cool water. If the oxygen level drops animals that cannot move to another area may begin to die. In deeper bodies of water, the injection of warm water can keep oxygen from dispersing into deep water, which is potentially good for bacteria but dangerous for aquatic animals. The decreased oxygen can cause algae blooms that pose a threat to aquatic plants and animals. This algae bloom problem is probably the most common and best-known side effect of thermal pollution.


Fish and amphibians may move away from the warm water to a more-suitable location, disrupting the ecosystem for animals that remain. Birds may also be forced to leave in search of areas with more food. Plants and certain animals will be stuck in the area, which can lead to huge losses. Migration away from the polluted area contributes to a dramatic loss of biodiversity at sites where thermal pollution happens.

Increased Toxins:

Toxins in the water are more a side effect of dumping waste water than a direct effect of thermal pollution. Chemical pollution is an almost inevitable side effect of using water for cooling. Solvents, fuel oil, and dissolved heavy metals end up in the lake or river where the cooling water gets dumped. Nuclear power plants can also release slightly radioactive cooling water. The chemicals may have a range of toxic effects on plants and animals, from fatal poisoning to mutations and sterilization.

Loss of Biodiversity:

The sudden heating can kill off vulnerable organisms or drive them away. This is one of many serious issues for threatened and endangered animal species. This loss can come from organisms dying from the hot water, being unable to reproduce as effectively as before, or simply leaving the area. We usually think of animals as casualties of water pollution, but multi-celled aquatic plants are also at risk when thermal pollution changes the local aquatic ecosystem.

Ecological Impacts:

The local aquatic ecosystem can be damaged by thermal pollution, especially if it is dramatic, as in copious amounts of warm water being dumped into a chilly pond or bay or river. “Thermal shock” can kill off insects, fish, and amphibians. This sudden loss of life causes further issues with the ecosystem. Key food sources are no longer adequate. A threatened or endangered local population may be wiped out or put under even more pressure. Coral reef bleaching has also been observed when a power plant or factory is dumped into coastal water. Coral bleaching happens when the coral organisms die.

Reproductive Effects:

A significant temperature increase in the water can cause reproductive problems. Warmer water can reduce the fertility of some organisms. Other species may suffer birth defects or lay deformed eggs because of chemical changes in the body caused by warmer water. Defective eggs and birth defects hurt the overall reproductive fitness of the animal population and can reduce the population. Thermal pollution can change the biology of aquatic organisms in a variety of ways.

Increased Metabolic Rate:

Warmer water may be good for cold-blooded fish and amphibians, but only for a limited time. One of many real problems that warm water may cause is faster metabolism, which means animals need more food. The local ecosystem may not be able to support a significant increase in food consumption. Worse still, the warmer water gives an advantage to certain organisms while it puts stress on others. The more-adaptable organisms may unbalance the ecosystem simply by out-competing other organisms and by eating them or driving them to starvation.