Unexpected Benefits and Impacts of Ocean Warming - Can Heat Pumps Save the Seas? Heat Circulation Shift in the Era of Marine Heatwaves

1. The Ocean as a "Massive Buffer"

As a result of global warming, the ocean has absorbed more than 90% of the excess heat, with an average heat absorption of 200 TW from 1971 to 2016—ten times the scale of humanity's total primary energy demand. The temperature increase pace of the surface 75 m is 0.11 °C per decade. The heat accumulated beneath the sea surface frequently causes marine heatwaves (MHW), severely impacting ecosystems and fisheries.

2. Is It All Bad News?

The German newspaper WELT (dated July 26, 2025) presented the perspective in "Wärmepumpen – Das Meer erwärmt sich – das hat auch eine gute Seite" that "warming seawater can be utilized for urban heating." Seawater has a higher specific heat than air even in winter, maintaining a stable 2–10 °C. Using it as a heat source results in a higher COP (Coefficient of Performance) than air-source heat, with low noise, making it easier to install in urban areas. Stockholm's Löpsten seawater HP, operational since 1986, runs six units totaling 180 MW, with intake at 15 m and return water at 0.5 °C, continuing for over 30 years.

3. Latest Projects and Technology Trends

• Esbjerg (Denmark): Operational from June 2025, a 70 MW CO₂ refrigerant SWHP. It provides heating for 25,000 households in a hybrid system with a wood biomass furnace and electric boiler.

• Gothenburg (Sweden): A 160 MW system includes a single HP of up to 50 MW, capable of supplying 80 °C hot water even with seawater at 3 °C.

• Using deep seawater for cooling, DWS–C: In Toronto and Hawaii, offices are directly cooled with 4 °C deep water during summer, reducing electricity by 90%.

In recent years, the adoption of natural refrigerants like CO₂ and ammonia has become mainstream. To counter seawater corrosion, titanium heat exchangers and backwashing systems are used, with maintenance costs for salt damage estimated at 1–2% of annual power generation.

4. Voices on Social Media

• The Reddit r/climate automoderator emphasizes the importance of individual actions, urging the replacement of fossil fuel heating with electric heat pumps.

• On the RealClimate forum, there is positive discussion about accelerating SWHP as a technology to extract heat, even though the abnormal warming of the upper ocean is a warning sign.

• Technical blogs point out the scale needed, stating that several hundred GW would be required to lower seawater temperature by 0.1 °C, yet they also recognize the potential for mitigating urban heat islands in coastal cities on a regional scale.

5. Ecosystem Impact and Regulations

To avoid the suction of microorganisms and changes in local salinity due to water intake and discharge, Europe mandates

• 5 mm mesh at intake points

• suction speed ≤0.15 m/s

• and ΔT≤3 K at discharge locations.
  In eco-monitoring at the demonstration stage, no significant decrease in plankton populations has been observed, and careful scaling is recommended.

6. Economic and Policy Outlook

The IEA estimates that increasing the global large-scale water heat source HP capacity from 1 GW to 8 GW by 2030 could reduce CO₂ emissions from urban heating by 1.4 Mt annually. The European Green Deal has established a **"Blue Heat" subsidy**, covering 25% of the installation cost (average 130 €/kW). Its role as a "large-scale heat storage" to absorb off-peak renewable energy is also emphasized.

7. Applicability to Japan

Tokyo Bay and Osaka Bay have a high winter water temperature of about 10 °C, with a low risk of freezing even in shallow areas. Combining with sewage heat utilization in coastal areas could lead to the decarbonization of gas boilers in data centers and apartment complexes in waterfront subcenters. Challenges include ① obtaining sea area occupancy permits ② fouling of heat exchangers due to seawater turbidity ③ synchronization with the expansion of renewable energy sources. Movements to incorporate this into the Ministry of Land, Infrastructure, Transport and Tourism's port renewable energy fund and GX League demonstrations are beginning.

8. Future Research Themes

1. Integration of seawater HP and desalination (RO/MEH) plant waste heat

2. Hybrid base load heat supply with small modular reactors (SMR)

3. Simultaneous optimal operation of marine heatwaves and heat demand using AI predictions

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