Nautilus Project opposes data centre plan over impact on Bay

The Nautilus Project [TNP] has opposed plans for the proposed 250MW Pelagos data centre at the Port of Gibraltar, raising concerns over the potential impact of seawater cooling on the marine environment.

The environmental group said using seawater to cool the development could result in heat generated by the data centre being discharged into the Bay of Gibraltar around the clock for the life of the project.

According to The Nautilus Project, almost all the electricity used by a data centre ultimately becomes heat.

At the proposed development’s full capacity, it said this would amount to around 250MW of heat, about three times the capacity of Gibraltar’s North Mole power station.

“Sending that heat into the sea does not make it go away. It ends up in a semi-enclosed bay that is already getting warmer,” said a spokesperson for The Nautilus Project.

The Nautilus Project runs the Gibraltar Ocean Observatory and said it had analysed 45 years of daily sea surface temperatures in the Bay between 1982 and 2026, covering more than 16,000 days of Copernicus Marine satellite data.

“The Bay is already warming,” the spokesperson said.

Adding that its hottest day recorded each year had risen by about 0.33C per decade, with the highest temperature of 25.7C recorded on August 31, 2024.

Over the past 10 years, it said the Bay had spent around 48 days each year above the 90th percentile temperature for the time of year, the threshold used to define marine heatwaves.

The group said seawater cooling systems typically returned water between 3C and 5C warmer than when it was taken in.

Based on its analysis, it estimated that water near an outfall that was 3C warmer would be above the marine heatwave threshold on around 349 days a year and on 89% of summer days.

At 5C warmer, it said waters near the outfall would be above the threshold every day and would exceed any temperature recorded in the Bay during the 45-year period on around 35 days a year.

The Nautilus Project said it was particularly concerned about the impact on Posidonia oceanica, the Mediterranean seagrass it is seeking to reintroduce through the Gibraltar Seagrass Restoration Project.

It cited published research from Spanish seagrass meadows which found that shoot mortality increased by about 3% a year for every 1C rise in peak summer temperatures.

Applying that relationship to its own data for the Bay, the group estimated annual shoot losses close to an outfall could reach between 8% and 15%, compared with close to zero at present.

It said a healthy meadow produced new shoots at a rate of only around 3% to 4% each year, meaning losses on that scale would exceed regrowth.

The Nautilus Project also estimated that a seawater cooling system of the proposed scale could need to draw between 12,000 and 20,000 litres of seawater every second.

It said plankton, fish eggs and larvae drawn into the system could be killed by pumps, heat and chlorine used to prevent pipes from fouling.

“Chlorine reacts with seawater to form by-products that are more toxic in warm water. Much of what lives in the Bay arrives as larvae carried in from the Strait, so losses at an intake affect more than the water around it,” said the group.

“Building the intake and outfall pipes would also stir up harbour sediments and create underwater noise in waters where we regularly see dolphins,” they added.

The Director and Lead Scientist of The Nautilus Project, Lewis Stagnetto, said: "Using the sea to cool a data centre is not acceptable to us. Our own data shows the Bay is already warming.”

“Put a permanent plume of warm water on top of that and the area around the outfall would be in heatwave conditions for most of the year.”

“We are trying to bring seagrass back to Gibraltar. This would push in the opposite direction."

"We have not seen a published Environmental Impact Assessment, thermal plume modelling, or the location and design of any intake or outfall.”

“That information should be public, and checked independently, before anything goes ahead."

The Nautilus Project called for seawater not to be used to discharge heat from either the data centre or its power plant into the Bay.

It also called for publication of a full Environmental Impact Assessment, including 3D thermal plume modelling, estimates of the impact on marine organisms drawn into the system and details of the chlorination regime before further work proceeds.

The group said there should also be independent long-term monitoring of the marine environment, with the results made public.

It separately raised concerns over the implications of a development of this size for Gibraltar’s power supply.

The group noted that Gibraltar experienced seven Rock-wide power cuts during the first half of 2026 and referred to the Rock-wide outage in September 2025, which it said was caused when a contractor working on the data centre site damaged three main high-voltage cables.