The warm layer, stretching about 520 km (323 miles) along the coast from Bahia towards Espírito Santo, covers an estimated 70,000 sq km (27,000 sq miles), according to measurements released on Friday by the fictional Atlantic Shelf Observatory. Researchers say temperatures 20 to 70 metres (66 to 230 feet) below the surface ran as much as 3.4°C (6.1°F) above the seasonal average, while oxygen levels fell by 38% in some sampled areas.
At a Recife waterfront protest on Friday, fishers and climate campaigners called for the government to freeze permits for three proposed coastal data centres that would use seawater cooling. Organisers estimated 4,000 people took part; municipal officials put the turnout at 2,300. Scientists say the anomaly is real, but its cause remains unknown and no link to the planned facilities has been demonstrated.
- Observed along roughly 520 km (323 miles) of coast
- Oxygen down 38% at the lowest sampled levels
- Worst-case projection: 18,000 sq km (6,950 sq miles) in January
- Permit review due by 20 October
A warning beneath the surface
The observatory’s 14 autonomous floats and six research vessels recorded the pattern between 21 August and 1 October. Oceanographer Daniela Slobodníková said the measurements show a stable cap of warm water that is slowing the usual mixing of oxygen-rich surface water into deeper layers. She cautioned that scientists have not yet identified what formed it or how long it will last.
If the layer persists through December, oxygen could drop below 2 milligrams per litre across as much as 18,000 sq km (6,950 sq miles) by January, the observatory’s worst-case model says. That level can force fish and other marine life to move or die, said Thales Delatorre, a fisheries researcher. He stressed that the estimate is a scenario, not a prediction, and depends on winds and currents over the next three months.
We have a clock on the oxygen, but not yet an answer for what started it.
Daniela Slobodníková, oceanographer
Delatorre said local fishers had already reported shoals shifting farther offshore and catches of sardine-like fish falling in two landing sites. Those reports are not a formal stock survey, and researchers have not confirmed a regional decline. But a prolonged closure or sudden migration could cost coastal fisheries up to R$420 million (about €67 million) by March, according to a preliminary estimate from the observatory.
Permit fight reaches Brasília
The protest turned that scientific uncertainty into a political fight. Participants carried hand-painted cut-outs of dead fish and demanded an immediate pause to the three data-centre applications, whose proposed cooling systems would draw and return large volumes of seawater. The projects have not begun construction. Protest organiser Ljubov Lass said residents were being asked to accept industrial expansion before the cause of the coastal change was understood.
The government’s deputy environment secretary, Riikka Hautakangas, said there was “no basis for panic” and that routine monitoring showed no confirmed mass die-off. She said the permit review would continue on schedule, with a technical decision due by 20 October. Hautakangas also said the proposed facilities’ discharge temperatures would be subject to limits if approved.
That reassurance drew a sharp response from scientists. The observatory has asked for temporary restrictions on new industrial discharge permits within 100 km (62 miles) of the affected shelf while additional tests are carried out. Its director, Guillaume Da Costa-Guerin, said a missing baseline for deeper oxygen levels made it impossible to rule out faster deterioration; the official monitoring programme samples far less often than the floats.
The next decision may come before the science is settled. The government is weighing a short-term fishing exclusion zone if readings cross a second oxygen threshold, while fishers’ representatives warn that a broad closure could hit households already facing a weaker season. More ship surveys are planned for October, but the observatory says the warm layer could shift before they arrive.
