Each shade consisted of pipes with nylon cloth attached, fixed temporarily to the seabed with cord. It floated 30 centimeters (about 12 inches) above the coral and cut incoming light by 60%.

Divers visited every two weeks to track color changes and take tissue biopsies. The biopsies let the researchers monitor the microscopic algae living inside the corals' tissues. This partnership matters: the coral shelters the algae, and the algae supply most of the coral's nutrition. The algae also give corals their color. When corals turn white under heat stress, the partnership has collapsed and the coral is starving.

What they found

Despite the intense heat, shaded corals paled less than expected, and the gap between shaded and control corals was widest during the most severe thermal stress. Shaded corals even recovered some color after installation.

But the study comes with caveats. The team saw less bleaching overall than anticipated, and no coral died — not even the controls. One possible reason: the record 2023 heat wave had left most corals occupied by a heat-tolerant alga called Durusdinium, which tolerates high temperatures but is linked to slower coral growth and possibly greater disease susceptibility.

"Shades reversed the observed bleaching, but the event wasn't severe enough to cause mortality on control or shaded corals," Neely said. "However, we know that even bleaching that doesn't cause mortality can impact corals' future disease susceptibility and reproductive capacity. It would be interesting to quantify if and to what extent shade structures benefited these corals after their removal by tracking disease and reproductive output."

Limits of the approach

The biopsies showed no differences in algae density or algae species between shaded and control corals. The sampling location may explain this: permits required the team to sample from coral edges, which receive less light and bleach less — meaning even control samples got some shade.

Permit constraints also delayed installation until well after water temperatures had risen enough to bleach coral. Installed earlier, the shades might have worked better. Even so, the researchers measured a significant positive effect.

The bigger question is scale. "Shading is not a cure-all for coral reefs, and its use would require hard decisions about what corals are worth saving," Neely said. "We found it was effective on two highly susceptible coral species, so it is likely to be useful for most others, but additional studies would be needed to confirm that. It is also likely that thermal stress would reach a point at which no amount of shading could help a coral, but only future experimentation would be able to define the bounds of that."

In short: underwater umbrellas won't save reefs from climate change. But as a stopgap for precious individual corals during the next killer heat wave, they showed real promise — and could buy targeted corals time while the world grapples with the underlying problem.