Any facility that cools with seawater affects the sea in two places. At the intake, small fish, eggs and larvae can be drawn in or trapped against screens. At the outfall, warmer water can change the conditions for life on the seabed nearby. Research shows these effects are real, especially in warm tropical waters. That is why regulators set limits on how much warmer the returned water may make the sea, and why intakes and outfalls are designed with care.
At the intake
United States regulations define the two main effects. Entrainment means any life stages of fish and shellfish in the intake water flow entering and passing through a cooling water intake structure and into the cooling system. Impingement means the entrapment of any life stages of fish and shellfish on the outer part of an intake structure or against a screening device. The US Environmental Protection Agency explains that intake structures pull large numbers of fish and shellfish or their eggs into a cooling system, and that larger organisms may be killed or injured when they are trapped against screens. Reusing the same cooling water many times, in a closed cycle, reduces how much seawater has to be drawn in.
At the outfall
The EPA calls temperature a master environmental variable for aquatic ecosystems. A 2024 systematic review of power plant discharges, published in Frontiers in Marine Science, found that thermal discharge generally led to reduced benthic diversity and abundance, meaning fewer kinds and numbers of creatures on the seabed, and that warm discharges generally coincided with a decline in dissolved oxygen. It also notes that in tropical regions even a temperature increase of less than 5°C can adversely affect certain organisms.
Regulators allow a limited area near the outfall, called a mixing zone, where the warm water dilutes. The EPA describes it as a limited area or volume of water where initial dilution of a discharge takes place, and says it must not impair the use of the water body as a whole.
The limits regulators set
| Where | Rule | Source |
|---|---|---|
| India, new coastal power projects using seawater | Rise in temperature of the receiving water must not exceed 7°C over the ambient temperature | Ministry of Environment and Forests, Environment (Protection) Rules, notification of 22 December 1998 |
| United States | Intakes must reflect the best technology available for minimizing adverse environmental impact; thermal limits are set to protect a balanced, indigenous population of shellfish, fish, and wildlife | Clean Water Act, section 316 |
| International guidance cited in the 2024 review | Temperature differences near plants should not exceed 7°C in subtropical and 5°C in tropical waters | GESAMP, 1984, as cited in the review |
What reduces the effects
Design choices matter: drawing water from deeper and further offshore, low intake speeds and fish-friendly screens, reusing cooling water in a closed loop, and discharging through diffusers at a depth and place where the warm water mixes quickly. A 2002 study for the State of Hawaii, for example, proposed discharging at a depth of at least 150 feet. Monitoring before and after operation shows whether the design is working.
What is not certain
- Most evidence comes from power stations, which discharge far more heat than a data centre. Effects at data centre scale are less studied.
- Effects depend on the local sea: depth, currents, temperature and the species present.
- Indian limits cited here are those notified for thermal power plants; the rules applied to any other facility are set in its own approvals.
Sources and method
- US Code of Federal Regulations, 40 CFR 125.92, Special definitions. Up to date as of 24 September 2026. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-D/part-125/subpart-J/section-125.92
- US Environmental Protection Agency, Cooling Water Intakes, updated 3 September 2026. https://www.epa.gov/cooling-water-intakes
- US Environmental Protection Agency, Thermal Discharges in NPDES Permits: Overview of Resources and Tools, June 2023. https://www.epa.gov/system/files/documents/2023-07/Thermal-Discharges-Report.pdf
- US Environmental Protection Agency, Water Quality Standards Handbook, Chapter 5: General Policies, September 2014. https://www.epa.gov/sites/production/files/2014-09/documents/handbook-chapter5.pdf
- Clean Water Act section 316, 33 U.S.C. 1326, as reproduced by the Cornell Legal Information Institute. https://www.law.cornell.edu/uscode/text/33/1326
- Ministry of Environment and Forests, Government of India, Environment (Protection) (Second Amendment) Rules, 1998, G.S.R. 7, 22 December 1998. https://cpc.parivesh.nic.in/writereaddata/ENV/envstandard/envstandard49.pdf
- Leng and others, Impacts of thermal and cold discharge from power plants on marine benthos and its mitigation measures: a systematic review, Frontiers in Marine Science, volume 11, 2024. https://frontiersin.org/journals/marine-science/articles/10.3389/fmars.2024.1465289/full
- State of Hawaii, Department of Business, Economic Development and Tourism, with Makai Ocean Engineering and the University of Hawaii, Sea Water District Cooling Feasibility Analysis for the State of Hawaii, October 2002. https://energy.hawaii.gov/wp-content/uploads/2011/10/SeaWaterDistrictCoolingFeasibilityAnalysisfortheStateofHawaii2002.pdf
Method: each link was opened on 29 October 2026 and every figure was taken from the page cited beside it. No figure was estimated.
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This article provides general information, not investment advice or an offer of securities.